United States Office of Research and EPA/600/R-92/036

Environmental Protection Development February 1992 Agency Washington, DC 20460

GIS

Technical Memorandum 3:

Global Positioning Systems

Technology and its

Application in Environmental

Programs
EPA/600/R-92/036

February 1992

GIS Technical Memorandum 3:

Global Positioning Systems Technology

and its Application in

Environmental Programs

by

Robert Puterski1

Jerome A. Carter1

Mason J. Hewitt, III2

Heather F. Stone2

Lawrence T. Fisher, Ph. D.1

E. Terrence Slonecker3

1

Center for Spatial Analysis, Environmental Programs Office Lockheed Engineering & Sciences Company 1050 E. Flamingo Road, Suite 126, Las Vegas, Nevada 89119

2

Remote and Air Monitoring Branch Advanced Monitoring Systems Division Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency P.O. Box 93478, Las Vegas, Nevada 89193-3478

3

Environmental Photographic Interpretation Center Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency P.O. Box 1575 V.H.F.S., Warrenton, Virginia 22186

Project Officer

Mason J. Hewitt, Ill

ENVIRONMENTAL MONITORING SYSTEMS LABORATORY

OFFICE OF RESEARCH AND DEVELOPMENT

U.S. ENVIRONMENTAL PROTECTION AGENCY

LAS VEGAS, NV 89193-3478

Printed on Recycled Paper
NOTICE

The information in this document has been funded (wholly or in part) by the United States Environmental

Protection Agency under contract 68-C0-0050 to Lockheed Engineering and Sciences Company, Inc. It has

been subjected to Agency review. Mention of trade names or commercial products does not constitute

endorsement or recommendation for use.

ii
Foreword

The U.S. Environmental Protection Agency (EPA) has long employed data of spatial orientation in pursuit of

its mission to understand and protect the environment. For years, these data were applied in standard

cartographic presentation techniques, either via hand-drawn or digital transposition from a source map. In

either case, the map developer or analyst had the ability during this transposition to apply decision rules of

logical consistency to make the map “right,” shift and offset map elements so that their relationships to each

other did not violate inherent rules of consistency (e.g., streets do not cross buildings, city boundaries follow

the delimiting streets). These adjustments and the inaccuracies introduced in the transposition process itself

may or may not be considered viable, depending upon whether these adjustments and errors exceeded

traditional map accuracy standards.

Regardless of the acceptability of these errors, they are virtually undetectable to the decision maker or

technical analyst, who is presented with a map product. The nature of these errors in hard copy maps is

attributable to the medium itself, which is not amenable to overlay and comparison analysis. More often than

not, mapped data is presented in its own singular context, with few other types of spatially-correlated data

simultaneously presented. However, with the advent of Geographic Information Systems (GIS), digital spatial

data sets are generated and stored independently and then combined in analysis, making differences in

resolution and accuracy of spatial data visually detectable. Although each separate data set may not violate

its own accuracy standard, the use of these differing maps may produce a composite map that is perceived

as being flawed. Recognizable inconsistencies may or may not detract from the accuracy of the spatial

analysis of interest, depending upon the nature of the analysis. At a minimum, they possibly detract from the

credibility of the analysis product.

Global Positioning Systems (GPS), originating from the U.S. military programs, have great potential for

ameliorating these types of problems as well making errors easier to detect. With the ability to locate features

with an accuracy of a few meters, this technology essentially lowers the detection limit for positional accuracy

at low cost. Indeed, the U.S. National Geodetic Survey acknowledges that the accuracy of GPS positioning

may exceed the accuracy of some benchmarks in the National Geodetic Reference System (NGRS) (National

Geodetic Survey, 1990). Previously, cadastral surveys, which are relatively expensive, were considered to

be the only highly accurate positioning system.

GPS technology is now enjoying many civilian sector applications. With this increasing demand, not only is

the cost of units going down, but a tremendous amount of development effort is being applied toward

increasing their portability, accuracy, and ease of data integration with popular mapping system applications.

Proposals have been made within EPA to establish networks of survey base stations that would offer complete

coverage of a Regional jurisdiction for roving GPS units. Such proposals shed light on the potential future for

the use of GPS in EPA. By integrating GPS into the Agency’s regulatory data collection efforts, benefits in

improved spatial analysis resolution could result in ‘lighter” solutions in protection, causation, and resortation

decision making. Eliminating the range of uncertainty in positional data may offer opportunities for cost

savings.

In comparison to highly accurate GPS data, the relative acceptability of EPA’s existing spatial data, in terms

of resolution and accuracy, diminishes. Locational methods previously employed may no longer be suitable

for applications and analysis that require the most rigorous spatial data quality available. The EPA has been

iii
aware of the locational errors in Agency-sponsored data collection efforts. And as the Agency seeks to

integrate geographic analysis as part of its operations, these locational errors become apparent and many

times embarrassing. In order to address this error source the EPA has adopted a Locational Data Policy

(LDP). The purpose of the LDP is to “ensure the collection of accurate, consistently formatted, fully

documented locational coordinates in all relevant data collection activities pursuant to EPA’s mission

(LDPGD, 1991 ).” In order to support the 25 m accuracy target specified within the LDP, the policy endorses

GPS as the technology of choice. Collecting highly accurate GPS data requires careful planning. Once

collected, consideration and thoughtful treatment of the data must be given vis-a-vis its use with data of

substantially lower accuracy. This report seeks to provide EPA personnel information and guidance on this

technology and its potential use in Agency applications. It reflects the best available information at the time

of publication. GPS is a rapidly changing technology that will require constant attention if we are to achieve

maximum benefit.

The GIS Technical Memorandum series is produced by the Geographic Information Systems Research and

Development Program at EPA’s Environmental Monitoring Systems Laboratory in Las Vegas, NV. The

purpose of this series is to disseminate information on procedures, applications and the results of applied

research in GIS and allied technologies. For more information contact:

GIS R&D Program Manager

U.S. EPA

EMSL-LV

P.O. BOX 93478

Las Vegas, NV 89193-3478

iv
Abstract

Global Positioning Systems (GPS) area location determination technology that offers significant opportunities

for obtaining highly accurate locational data at low cost. In order for the technology to perform up to its

capabilities in Agency applications, Environmental Protection Agency (EPA) staff will need to develop a

greater understanding of the technology itself, coordinate systems, surveying, and basic geodesy. EPA has

been collecting expertise in the use of this technology over the last 3 years via pilot use of GPS systems to

enhance locational control in Agency projects. In order to operationalize the use of this technology within EPA,

there also exists a need to develop concise standard operational procedures and methodologies for its use.

This document is a beginning toward fulfillment of these needs. It is intended to be an introductory reference

that describes the technology and how it could be employed in EPA work. It provides an overview of survey

methods from initial planning to data reduction and postprocessing. Ancillary but important issues such as

reference datums and use with geographic information systems are covered in order to provide the reader

additional context regarding the use of this spatial information in a project environment. Case studies

performed by the Environmental Monitoring Systems Laboratory, Las Vegas, are also included in this

document as auxiliary background that may provide helpful techniques.

v
Contents

Page

Foreword

Abstract

Figures

Tables

Acknowledgements

1. Introduction

A. Overview

B. Geopositioning

2. Conclusions and Recommendations

3. Global Positioning Systems

A. Components of a GPS

B. Fundamentals of Satellite Positioning

C. GPS Hardware Features and Options

D. Factors Affecting GPS Accuracy

E. Future Use of GPS Technology

4. Use of GPS for Environmental Applications

A. GIS Applications

B. Field Sampling

C. Remote Sensing

D. Real-time Attribute Coding Software

5. Performing a GPS Survey

A. Survey Planning

B. Reconnaissance

C. Survey Execution

D. Data Reduction and Processing

E. Integration of GPS Data into a GIS Data Base

References

Appendixes

A. Sources of Additional Information on GPS

B. Glossary of GPS Terms

C. EPA Locational Data Policy

D. EPA Case Studies

E. PLOTSSF and CLEANSSF Processing Software

F. Field Charts and Forms

iii v

ix

xi

xiii1125778

15

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23

23

23

24

24

25

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27

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vii
Figures

Number Page

1

2

3

4

5

6

7

8

9

10

11

Three Major Components of the Operational GPS Complex

GPS Receiver Configuration

Pseudo-range Time Shift

Carrier Beat Phase

Point Positioning

Relative Positioning

GPS Operating Modes and Accuracy Potential

Differential GPS

Proportional Error

Confidence Regions

Error Sources

7

8

10

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11

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12

13

17

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18

ix
Tables

Number Page

1 GPS Data Message Content 10

2 Error Budget for Conventional GPS vs. Differential 14

3 Typical Range Vector Measurement Errors 19

xi

Acknowledgements

The authors wish to acknowledge contributions and assistance, received in the research and development

of this document, made by the following persons:

For assistance with pilot surveys in California and Connecticut, Dr. Arthur Lange of Trimble Navigation

Systems and Mr. David Wolf of the U.S. Geological Survey.

For technical assistance, Mr. Donald Bundy and Mr. William H. Aymard of the U.S. EPA Environmental

Monitoring Systems Laboratory at Las Vegas (EMSL-LV).

For editorial assistance, Mr. Douglas Elliot of the EMSL-LV and Ms. Donna Sutton of Lockheed Engineering

& Sciences Company.

For external peer review and recommendations, Asta Miklius of the U.S. Park Service, Hawaii National Park,

James R. Lucas of the National Geodetic Survey, and Dr. Arthur Lange of Trimble Navigation Systems.

For field form development, Mr. Greg Charles of EPA Region I and Mr. Terrence Slonecker of EMSL-LV.

xiii

Section 1

Introduction

A. Overview

Geographic Information Systems (GIS) technology has evolved rapidly in recent years to become

a valuable tool in the analysis of environmental

problems. As with any new technology, the need

persists to continue research and develop methods to

optimize the use of GIS by environmental scientists

and managers. Toward this end, the Spatial Analysis

Team (SAT), a component of the U.S. Environmental Protection Agency (EPA) Environmental Monitoring Systems Laboratory in Las Vegas, Nevada

(EMSL-LV), has been conducting research since

early 1989 into the use of Global Positioning Systems (GPS) technology as a source of locational data

and as a quality control mechanism for GIS applications under the Superfund program.

This document is intended for use by personnel

in all EPA Regional and Program Offices who need

to improve locational information in environmental

data bases. It provides fundamental information

about the use of GPS and how they can be employed

to support Agency activities. It is intended to supply

the reader with information about geopositioning

technology and methodologies, particularly for users of GIS, to ensure efficient and effective implementation of the technology and optimization of

geopositional accuracy, and to understand how GPS

can aid in establishing increased spatial accuracy in

environmental data bases. To facilitate survey mission planning, software has been identified by this

document that will enable the user to project satellite

configurations at any given future date.

It should be recognized that GPS represents a

rapidly growing and expanding technology, undergoing a seemingly continuous series of changes and

improvements. While this document depicts the

state of the technology at this time, it is possible that

the science of GPS will have advanced technologically soon after the publication of this document.

Therefore, this document provides a basic level of

understanding of geopositioning and GPS technology and will be augmented with additional documentation as the technology continues to grow.

This section provides historical background

information on geopositioning, describing (1) the

technology from which GPS has evolved, (2) conventional surveying techniques that have traditionally been used in geopositioning, and (3) emerging

technologies which, along with GPS, are redefining

geopositioning accuracy standards.

Section 2 (Conclusions and Recommendations)

summarizes the issues that the authors consider most

important in implementation of the technology within

EPA.

Section 3 (Global Positioning Systems) provides a discussion of all aspects of GPS technology,

beginning with descriptions of the fundamentals of

geopositioning from space. Information regarding

the nature of GPS receivers will help readers understand the varied features and options that are available

as suitable hardware components for global positioning. Also reviewed in this section area number

of topics related to GPS spatial accuracy, factors

affecting GPS positional accuracy, and the effect of

GPS receiver dissimilarities.

Section 4 (Use of GPS in Environmental Applications) describes how GPS will continue to provide

a valuable geopositioning resource for EPA activities. Included are descriptions of applications as a

data collection and quality assurance tool for GIS

projects, quickly and accurately collecting and recording, or verifying, positional information in a

common geographic reference system. Other Agency

applications are discussed as well, including the use

of GPS as a positioning system in field sampling and

remote sensing efforts.

Section 5 (Performing a GPS Survey) provides

an outline for the planning and management of a GPS

survey. All phases of conducting a GPS survey are

included, from planning and reconnaissance, through

the actual GPS survey methodology, to the processing

1
of satellite transmitted positional data and integration and water and power lines. The U.S. Geological

of the data into a digital data base. Survey (USGS) has historically conducted topoAppendix A (Sources of Additional Informaion on GPS) is a summary of sources that can be

sought out for further information about GPS technology and for specific information on the status of

the NAVSTAR constellation.

Appendix B (Glossary of GPS Terms) defines

some common terms used in the GPS technical

community.

Appendix C (Locational Data Policy) is the

current official EPA policy on locational data

requirements.

Appendix D (EPA Case Studies) details surveys that have been conducted on Superfund sites on

the east and west coasts by EMSL-LV to explore the

utility of the technology to EPA programs and,

generally, for positioning of regulated facilities and

monitoring stations.

Appendix E (PLOTSSF and CLEANSSF Processing Software) summarizes the capabilities of

specific GPS processing software available from

EMSL-LV.

Appendix F (Field Charts and Forms) is a

collection of sample forms useful in conducting and

documenting a GPS survey.

B. Geopositioning

Conventional Surveying

Geopositioning techniques and technologies

have undergone considerable evolution in recent

years, as evidenced by the emergence of systems like

GPS. Many of the older, traditional positioning

survey methods are being replaced by advanced

geopositioning systems. These older survey methods tend to require more field time, are highly labor

intensive, and are costly per feature identified. Although conventional surveying is still appropriate

for high accuracy requirements in localized, accessible study areas, as employed today, these older

methods are best used in concert with other more

advanced and cost-effective techniques. Topographic, cadastral, and geodetic surveys are perhaps

the three types of conventional surveying most impacted by advanced technologies like GPS.

Topographic surveys determine the elevation

heights and contours of land surfaces. These surveys

also serve to locate buildings, roads, sewers, wells,

graphic surveys in order to produce thousands of

topographic maps at a scale of 1:24,000. These maps

provide an excellent base for much of the EPA

environmental analyses. As these maps become

available in digital form, they are providing an important source of locational and geographic data for

the Agency. GPS is currently being used to obtain

more accurate positional information for many of

these land features.

Cadastral surveys are performed to establish

legal and political bounties, typically for land

ownership and taxation purposes. A boundary survey

is a type of cadastral survey which is limited to one

specific piece of property. The U.S. Bureau of Land

Management (BLM) relies on cadastral surveying to

determine the legal boundaries of public lands.

Published cadastral surveys are of importance to

EPA, as for instance, where Potentially Responsible

Parties (PRPs) and impacted parties on Superfund

enforcement cases are concerned. GPS already can

provide a highly accurate means of conducting

boundary surveys of these types of facilities.

Geodetic surveys (i.e. control surveys) are global surveys made to establish control networks (comprised of reference or control points) as a basis for

accurate land mapping. Geodetic surveys provide

quantitative data on the absolute and relative accuracy of reference positions or physical monuments

on the earth’s surface. The U.S. National Geodetic

Survey (NGS) is responsible for establishing a national geodetic control network for the entire country

referenced to a national horizontal Datum. NGS also

establishes the vertical Datum, the location of mean

sea level from which most elevation data are determined or referenced. Highly accurate EPA

geopositioning requirements should be attained with

reference to a well defined geodetic survey or network. In many parts of the country, the NGS is

employing GPS to establish or correct geodetic survey networks.

In the event that EPA will require or avail itself

of conventional surveying to meet its geopositioning

needs, it is important to understand these techniques,

their strengths, and their limitations. EPA does and

will continue to use products derived from conventional surveying methods. The geopositional accuracy of the products generated by EPA, USGS, NGS,

and BLM can usually be obtained from these Agencies at the time of product acquisition.

2
Methods of Point Surveying

Global positioning systems technology is but

one of a number of positioning techniques that have

been developed since the late 1950’s and are being

used for establishing positions of points on or near the

Earth’s surface. Besides GPS, some of the advanced

technology systems being utilized for geopositioning

and navigation today include OMEGA, Loran-C,

Transit, Inertial Survey Systems (ISS), VHF Omnidirectional Range/Distance Measuring Equipment

(VOR/DME), Tactical Air Navigation (TACAN),

Instrument Landing System (ILS), and Radiobeacons.

The US Department of Transportation (DOT), primarily through the US Coast Guard (USCG) and the

Federal Aviation Administration (FAA), is responsible for the application of these technologies for

civil navigation, whereas the US Defense Department (DOD) oversees the use of these systems for

military users.

OMEGA and Loran-C are two navigation systems that can be used for surveying and

geopositioning. OMEGA uses ultra-low frequency

transmissions from eight transmission stations. It is

sensitive to changes in season and time of day due to

the propagation characteristics of the low frequency

signal. It is used primarily in marine and submarine

navigation across most of the earth’s surface. Its

accuracy is considered to be 2 to 3 nautical miles

(Ackroyd and Lorimer, 1990).

Loran-C is a navigation system that is generally

unaffected by diurnal disturbances and is used for

navigating in either sea or air. It employs radio

signals centered at 100 kHz, and is pulsed to allow for

time-difference measurement. In general, these two

technologies are used mainly for navigational purposes, although Loran has been tested, with variable

success, within the EPA for use as a geopositioning

tool. Its effectiveness has depended upon a number

of variables, including type of Loran receiver used,

proximity to Loran transmitting stations, and proximity to high elevation or other obstructive landforms. Although military use of Loran-C and

OMEGA will be phased out early in the 1990’s,

operation of these systems for use by civilians is

guaranteed until at least the year 2000 (Wells et al.,

1988). Of the two, Loran-C is considered the preferred option for EPA applications and only for

coastal areas and along the mid-continent.

Transit is a satellite positioning technology,

similar to GPS, which generally provides a high

degree of accuracy at a reasonable unit cost. It

generates positions based on measurements of the

Doppler effect produced as the satellites move in

their orbits. Although less accurate than GPS, Transit is still considered a useful technology because of

its speed and accuracy of operation as compared to

conventional surveying, and its affordable cost.

Military use of Transit is scheduled to be replaced by

GPS by 1994. At that time, U.S. government operation of the Transit system will cease (Wells et al.,

1988).

Inertial Survey Systems are self-contained and

highly mobile systems which detect relative compass

direction by using gyroscopes. The most effective

application of ISS is to measure unknown points that

are located between known control points. ISS may

serve some EPA needs where EPA and/or contractual

personnel are in the field with vehicles which could

potentially be equipped with ISS. When ISS is

combined with GIS, they are mutually supportive.

VOR/DME and TACAN provide the basic

guidance for enroute air navigation in the U.S. Military and civil aviation use of both systems will be

phased out by 1997. Long-term continued operation

of VOR/DME for civilian use remains uncertain,

though the system will continue to be available until

at least 2000.

ILS is a passive system used commercially for

precision aircraft radar approach navigation. The

Federal Aviation Administration is currently investigating the continued use of this geopositioning

technology and may recommend transitioning to a

system based on GPS. A similar active system is

PAR, or precision radar, used mostly by the military.

Both systems have a very short range.

Radiobeacons for military applications will be

discontinued by 1997. A decision on continued

operation of these widely used facilities by the civilian community will not be made for some time

(Wells et al., 1988).

Of all the advanced geopositioning technologies, GPS holds the greatest promise for EPA due to

its relative flexibility, accuracy, ease of use, cost

benefits, and longevity.

Section 2

Conclusions and Recommendations

With proper planning GPS can provide accurate coordinates sufficient to meet the requirements

of the Locational Data Policy. While receiver technology and software is still rapidly evolving, the

technology itself is stable and proven. The U.S.

Government will continue to invest in GPS architecture and infrastructure for some time to come. As

discussed in the findings of the Locational Accuracy

Task Force (LDPIG, 1991), there are a number of

issues associated with the GPS-based accuracy standard that need to be addressed in order to undertake

an Agency-wide implementation. This section will

discuss these issues and offer some recommendations based on the topics examined in this report.

Issues/Concerns

Cost of implementation

The costs of an Agency implementation are

unknown at this time and will only become clear after

a robust requirements analysis. There are many

unknowns related to the numbers and types of GPS

units required at the Regional level. It is safe to say

that with complete implementation of the Locational

Data Policy, the need for receivers could be quite

high. A national procurement vehicle could be an

option for cost containment.

Another factor that will influence costs is community-based networks. A community network will

assist with the correction of GPS data after field

collection by providing a correction data stream

from a fixed station receiver. The use of a C/A code

receiver for the fixed station is expensive and it is

unknown how many community networks are needed

to service the needs of the EPA. Costs could be

controlled by sharing equipment with other agencies

or investing in correction data broadcasts offered by

some satellite communication companies.

Selective availability

The intentional degradation of satellite signals

by the Defense Department to inhibit real-time use is

the principal criticism levied against proponents of

GPS for use in navigational applications. Selective

availability is an intentional error that can be corrected using postsurvey processes in the office. The

section “Fundamentals of Satellite Positioning,” explains the technical aspects of selective availability.

For most field collection projects this is not a problem. If selective availability is implemented, realtime use may not be possible.

Interoperability of vendor equipment

With the variety of equipment available, each

manufacturer has developed different methods of

capturing and relaying data such as phase measurements, time, and station information. The vendor

community has not offered any data exchange standards that would allow the interoperability of P-code

receivers. As a potentially large user of GPS data

from many sources, the Agency should become

active in the development of GPS exchange

standards.

Recommendations

In order to integrate GPS into the operations of

the EPA it is recommended that implementation be

supported by a program which provides for training,

methods development and applications research along

with an aggressive program to influence the vendor

community.

A robust methods and applications development program is necessary because the technology

has not been explored fully. New and more efficient

5
applications could be developed with dedicated ef- educational outreach so that consistent methods and

fort. Pushing the technology toward greater user procedures are employed.

utility will require concerted effort to pioneer new This implementation will have to be accompa- products and product add-ens in conjunction with

the vendor community. This dedicated effort will tied by funds designated to support these functions.

require a well designed training effort to provide

6
Section 3

Global Positioning Systems

The Navigation Satellite Time and Ranging

Global Positioning System (NAVSTAR GPS) has

been under development by the DOD since 1973 as

a result of the merger of the U.S. Navy’s TIMATION

Program and the U.S. Air Force’s 621B Project.

Both of these programs were initiated in the mid1960s to develop a passive navigation system using

measured distances to orbiting satellites. Although

the original goal of GPS was to provide ground, sea,

and air units of the United States military and its

NATO allies with unified, high-precision, allweather, instantaneous positioning capabilities, in

to civilians, perhaps with certain restrictions, once

the system is fully operational, affording 24-hour,

three-dimensional positioning, by mid-1993 (Wells

et al., 1988).

A. Components of a GPS

There are three major elements to the operational GPS complex: the control segment, the space

segment, and the user segment (Figure 1). All three

of these segments are required to perform positional

its present phase GPS is freely available for anyone determination.

to use. The technology will continue to be available

SPACE SEGMENT

• 24 Satellites

• 12 Hour Orbital Period

• 12,000 mi. Altitude

USER SEGMENT

• Track Code and Phase

• Extract Satellite Message

• Computer Position

CONTROL SEGMENT

• Time Synchronization

• Orbit Prediction

• Satellite Health Monitoring

Figure 1. Three major components of the operational GPS complex.

7
Control Segment

The Control Segment consists of five monitoring stations (Colorado Springs, Ascension Island,

Diego Garcia, Hawaii, and Kwajalein Island). Three

of the stations (Ascension, Diego Garcia, and

Kwajalein) serve as uplink installations, capable of

transmitting data to the satellites, including new

ephemerides (satellite position as a function of time),

clock corrections, and other broadcast message data,

while Colorado Springs serves as the master control

station. The Control Segment is the sole responsibility of the DOD who undertakes construction, launching, maintenance and virtually constant performance

monitoring of all GPS satellites.

The DOD monitoring stations track all GPS

signals for use in controlling the satellites and predicting their orbits. Meteorological data also are

collected at the monitoring stations, permitting the

most accurate evaluation of tropospheric delays of

GPS signals. Satellite tracking data from the monitoring stations are transmitted to the master control

station for processing. This processing involves the

computation of satellite ephemerides and satellite

clock corrections. The master station controls orbital

corrections when any satellite strays too far from its

assigned position, and necessary repositioning to

compensate for unhealthy (not fully functioning)

satellites.

phase. Current plans call for these to replace the

Block II generation, beginning in the late 1990’s.

A slightly confusing situation exists in that

there are two satellite reference numbering systems.

The NAVSTAR or space vehicle numbers (SVN) is

the older of the two and is based on launch sequence.

The second numbering system, known as the pseudorandom number (PRN) or space vehicle identity (SV

ID), is more commonly used and is based upon

orbital arrangement.

User Segment

The User Segment consists of all Earth-based

GPS receivers. Receivers vary greatly in size and

complexity, though the basic design is rather simple.

The typical receiver is composed of an antenna and

preamplifier, radio signal microprocessor, control

and display device, data recording unit, and power

supply (Wells et al., 1988) (Figure 2). The GPS

receiver decodes the timing signals from the “visible” satellites (four or more) and, having calculated

their distances (refer to discussion below), computes

its own latitude, longitude, elevation, and time. This

is a continuous process and generally the position is

updated on a second-by-second basis, output to the

receiver display device, and, if the receiver provides

data capture capabilities, stored by the receiver logging unit.

Space Segment

B. Fundamentals Of Satellite Positioning

The Space Segment consists of the constellation of NAVSTAR earth orbiting satellites. In November of 1991, there were 16 satellites in orbit, 5 of

the original prototype (Block I) design, and 11 the

operational (Block II) design. The current Defense

Department plan calls for a full constellation of 24

Block II satellites (21 operational and 3 in-orbit

spares) to be deployed by 1993. The satellites are

arrayed in 6 orbital planes inclined 55 degrees to the

equator. They orbit at altitudes of about 12,000 miles

each, with orbital periods of 12 sidereal hours (i.e.,

determined by or from the stars), or approximately

one half of the Earth’s period of rotation. The current

GPS constellation provides approximately 12 hours

of 3-D position fixes per day in North America. As

more satellites are launched on the planned 60- to 90-

day interval, the GPS window will expand until a full

constellation will provide users with the ability to

obtain three-dimensional positional information for

any point on the face of the Earth, 24 hours a day.

Block III GPS satellites are presently in the design

Satellite positioning operates by measuring the

time delay of precisely transmitted radio signals

from satellites whose position can be very accurately

determined. Furthermore, with the help of a few

Figure 2. GPS receiver configuration.

8
fundamental laws of physics, the positions of these

satellites as a function of time (their ephemerides)

can be rather easily predicted. By measuring the

distances (or range vectors) between a survey point

of unknown location on the surface of the earth and

the predicted positions of a number of orbiting satellites, it is possible to derive the position of the

unknown survey point.

GPS Signal Structure

There are two radio frequency bands available

for transmission of satellite positioning signals; they

are centered on frequencies of 1575.42 and 1227.6

MHz. The two frequencies, or “carriers”, are called

Link 1 and Link 2 (L1 and L2), respectively, and

carry a number of modulated signals. These signals

require a wide band width of 2 MHz because there

are so many signals being simultaneously transmitted. The high frequency of the carrier bands is

essential to avoiding significant ionospheric effects.

The modulating signals appear random but in

fact are carefully chosen sequences of binary values

(zeros and ones), using a mathematical algorithm

which generates the sequences that repeat during a

fixed time interval. Both the satellites and GPS

receivers generate the same code sequences at precisely the same intervals. The three types of code

(described below) have varied periods, and repeat as

frequently as every millisecond or as infrequently as

every 267 days. The codes are referred to as pseudorandom noise (PRN) codes. The satellites transmit

three different types of PRN codes.

Since the pattern of electronic pulses in the

PRN code is known, the matching or aligning of

receiver-generated and satellite-generated PRN codes

provides the means by which satellite distance can be

derived. If the comparison of PRN codes is conducted over a period of time, the number of signal

“matches” increases, thereby providing a means of

clearly recognizing even very faint signals.

C/A-code and P-Code

One PRN code, known as the Coarse/Acquisition code (C/A-code), consists of a sequence of 1,023

binary values or “chips”, generated at a rate of 1.023

million chips per second. This results in a sequence

repetition every millisecond. The C/A-code is considered the standard GPS code and is sometimes

referred to as the “civilian code.” Compared with the

P-code described below, the C/A code is considered

to be less precise, ±100 meters vs. ± 16 meters

without correction.

A second PRN code, known as the Precise or

Protected code (P-code), consists of another sequence of binary values, and repeats itself only after

267 days. Each 1-week segment of this code is

unique to one GPS satellite and is reset each week.

The P-code is carried at 10 times the frequency of the

C/A-Code, and can be encrypted to enable exclusive

military use. In actuality, with newest receiver

designs, there is practically no difference in the

accuracy of measurements made with either C/A or

P-code (Hum, 1989). However, the P-code is not

entirely secure; the equation that generates the Pcode (and, thus, the structure of the code itself) is

now well known (Ackroyd and Lorimer, 1990).

Y-Code

Another PRN code, the Y-code, is now being

transmitted on Block II satellites. It is also for

military use and its encryption is more secure than

the P-code. Y-code works by adding a mask to the Pcode that is then removed by the receiver. Decryption

requires use of a special code key which is printed on

a microprocessor.

While PRN code may seem an overly sophisticated approach to transmitting a radio signal, it in

fact enables GPS to be the practical and relatively

inexpensive utility that it is. Without PRN, each GPS

receiver would require its own large parabolic satellite dish, much like those needed for satellite television reception. By matching and “amplifying” the

PRN codes generated by receiver and satellite, GPS

satellites do not need to be highly powerful and

expensive, and receivers with very small antennas

are sufficient to pick up the signals.

It is through access to the Y-code and the means

to compensate for manipulation of orbit data and

clock frequency that permits selective availability.

Currently, the standard positioning service (SPS),

available to everybody, provides access to the C/A

code, both L-codes, P-code, and the navigation message. The precise positioning service (PPS) provides

access to these codes plus the Y-code used to

operationalize selective availability.

Navigation Message Contents

The carrier frequencies and codes are modulated to carry numerous messages that are necessary

9
to perform positioning calculations. The code is

broken into 1500-bit frames, transmitted over 30

seconds. Each frame contains five subframes. Each

subframe is made up often, 30-bit words. Some of

the subframe information does not vary from frame

to frame (subframes 1,2, 3). Subframes 4 and 5 will

“page” data from frame to frame, with up to 25 pages

in a master frame which will take 12.5 minutes to

transmit (Wells et at., 1986). Table 1 summarizes

subframe message content.

Measuring Range Vectors

A variety of techniques can be used to measure

earth-to-satellite range vectors. GPS utilizes what is

referred to as one-way radio ranging to determine

satellite distances. With radio ranging, every GPS

satellite and all earth-based GPS receivers simultaneously generate an identical PRN signal. The

timing of the satellite radio signal transmissions,

traveling at the speed of light, is calibrated by atomic

clocks aboard each satellite. Each satellite transmits

the coded signal towards earth, where it can be

captured by the user’s receiver.

When a satellite signal arrives at the earthbound receiver, its signal is simply matched to that

Table 1. GPS Data Message Content

Subframe Content (*)

1

2

3

4

5

Flags (L2 code&data, week, satellite accuracy&

health)

Age of data

Coefficients for clock corrections

Satellite ephemeris

Satellite ephemeris

25 pages with:

Satellite almanac

Ionospheric model coefficients

Clock data

Antispoof flag

Satellite configuration

Satellite health

Other special messages and reserved space

25 pages with:

Satellite almanac

Satellite health

*(all frames include handshaking telemetry)

generated by the receiver. The receiver measures the

time difference between identical segments of satellite-generated and receiver-generated signal in order

to determine the length of time needed for the signal

to travel from satellite to receiver.

A GPS receiver is capable of making only two

types of measurements: pseudo-range and carrier

beat phase (Wells et al., 1988).

Pseudo-range

Pseudo-range is conceptually quite simple and

is the measurement made by most GPS receivers.

Pseudo-range is the time shift required to line up

matching segments (called code epochs) of satellitegenerated and receiver-generated and receiver-generated code (Figure 3), multiplied by the speed of

light. Ideally, the time shift is the difference between

the time of signal transmission and the time of signal

reception. In fact, the relative motion of the satellite

with respect to the receiver (the Doppler effect)

causes the two time frames to differ, which introduces a bias into the measurement. Lack of precise

synchronization between the satellite and receiver

clocks also can create clock bias, affecting all measurements equally while using a specific receiver.

These biased, time-delayed measurements are thus

referred to as pseudo-ranges.

A rule of thumb for estimating the precision of

pseudo-range measurements is 1 percent of the period between successive code epochs. For the Pcode, successive epochs are 0.1 microsecond apart,

implying a measurement precision of 1 nanosecond.

When multiplied by the speed of light, this implies a

range measurement precision of 30 centimeters. For

the C/A-code, the numbers are 10 times less precise,

or a range measurement precision of 3 meters.

Figure 3. Pseudo-range time shift.

10
Carrier Beat Phase

The received frequency of a GPS satellite signal is different than the frequency transmitted by the

satellite, and is continually changing due to the

Doppler effect. Carrier beat phase is the phase of the

signal which remains when the satellite carrier frequency (L1 or L2) is different or “beat” with the

constant frequency generated in the receiver (Figure

4). To make this measurement, the receiver must be

able to determine the difference in carrier wavelengths (or cycles) between the satellite and receiver

signals. Because the wavelength of the carrier is

much shorter than the wavelength of the PRN codes,

the precision of carrier beat phase measurements is

much higher than the precision of code pseudoranges. For the GPS L1 carrier signal, the wavelength is about 20 centimeters. Using the rule of

thumb that phase measurements can be made to

about one percent of the wave-length, this implies a

precision of 2 millimeters, permitting very high

precision positioning for certain applications.

Obtaining the initial number of integer cycles

of the carrier between satellite and receiver is very

difficult, if not impossible. Realistically, an assumption must usually be made about the initial (unknown) cycle ambiguity. Once this assumption is

made, it is critical that an integer cycle count be

maintained as the satellite-to-receiver range changes

with time. When an interruption occurs, for any of a

variety of reasons, in reception of the satellite carrier

frequency (“loss of lock”), the receiver effectively

“loses count” of the number of cycles between satellite and receiver signals. This is known as cycle slip.

Overcoming initial cycle ambiguity and minimizing

cycle slip problems typically requires a higher quality GPS receiver, meticulous antenna placement, as

well as lengthy and uninterrupted ranging sessions.

These requirements restrict the use of carrier beat

measurements for real-time applications.

The position of a point on or above the Earth’s

surface can best be described in terms of one or two

general reference frames. In the first case, the

position is defined with respect to a well-defined

coordinate system, commonly a geocentric system

(i.e., a system whose point of origin coincides with

the center of mass of the Earth). This is known as

point positioning and is illustrated in Figure 5. Alternatively, the point can be described using relative

positioning, in which another surface point of known

location, such as a benchmark or control monument,

serves as the origin of a local coordinate system

(Figure 6), such as a State Plane system.

Figure 5. Point positioning.

Figure 4. Carrier beat phase. Figure 6. Relative positioning.

11
Positioning from satellite ranges is based on the

same principle used in traditional terrestrial surveying methods. Simply stated, by measuring the distances to three noncoincident points of known positions, a triangulated solution can be attained. GPS

extends this general concept to a space-based system, measuring the distances to three or more nonplanar satellites and triangulating the position of a

survey point accordingly. Analogous to the point

and relative position modes described above, there

are two modes of satellite positioning: absolute and

differential. The distinctions between these two

operational modes of GPS are very significant in

terms of methodology and accuracy (Figure 7).

Absolute positioning

This mode of positioning relies upon a single

receiver station. It is also referred to as “stand-alone”

GPS, because, unlike differential positioning, ranging is done strictly between the satellite and the

receiver station and not using a ground-based reference station to assist with the computation of error

corrections. As a result, the positions derived in

absolute mode are subject to unmitigated errors

inherent in satellite positioning (see section below,

Factors Affecting GPS Accuracy). Overall accuracy

of absolute positioning is considered to be no greater

than 50 meters at best by Ackroyd and Lorimer, and

±100-meter accuracy by the U.S. Army Corps of

Engineers.

Differential positioning

Relative or Differential GPS carries the triangulation principle one step further, with a second

receiver at a known reference point (Figure 8). To

further facilitate determination of a point’s position,

relative to a known earth surface point, this configuration demands collection of an error correcting

message from the reference receiver.

Differential-mode positioning relies upon an

established control point. The reference station is

placed on the control point; a triangulated position

from the satellites is derived, and then compared it to

the control point coordinate. This allows for a

correction factor to be calculated and applied to other

roving GPS units used in the same area and at the

same time. Inaccuracies in the control point’s

coordinate are directly additive to errors inherent in

the satellite positioning process. Error corrections

Figure 7. GPS operating modes and accuracy potential.

12
Figure 8. Differential GPS.

derived by the reference station vary rapidly as the for widely separated stations, the direction cosines

factors propagating position errors are not static over

time. This error correction allows for a considerable

amount of error to be negated, potentially as much as

90 percent. A summary of the impact differential

positioning has on the overall error budget is presented

in Table 2.

The assumption made when operating in differential mode is that bias and random error factors

affecting the reference station are equally affecting

roving units operating off the station. For this

fundamental assumption of common errors to be

true, the units must be tracking the same satellites

and must be within a range of approximately 2,000

kilometers of the reference receiver (Denaro, 1990).

Other sources indicate that the effective range for

separation of reference and roving stations may be

less, on the order of 500 km (Ackroyd and Lorimer,

1990).

The range restriction is imposed due to the two

factors which contribute to the “spatial decorrelation”

of the errors. The frost range-reducing factor is that,

from satellite to receiver may differ, causing a differing observation of satellite ephemeris error. Second,

the error produced by atmospheric disturbances to

the C/A code transmission is dependent upon the

path the transmission takes through the atmosphere.

The variability of this error at geographically separated units may be significant because atmospheric

propagation errors account for as much as 60 percent

of total positioning error. Given the current conditions of selective availability (SA), the intentional

degradation C/A code accuracy by DOD, the reference receiver station data should be recorded and

transmitted at least every 20 seconds to update the

error correction (Kruczynski, 1990) (Ackroyd and

Lorimer, 1990).

For static or stationary mode of positioning,

data from the roving station should be collected for

a period of 3 to 5 minutes: additional time spent at a

location beyond this period will not enhance positional accuracy significantly. According to testing

done by the U.S. Air Force at the Yuma Proving

13
Table 2. Error Budget for Conventional GPS vs. Differential

Conventional C/A Code Accuracy Differential C/A Code Accuracy

1 sigma error (meters) 1 sigma error (meters)

Error source Bias Random Total Bias Random Total

Ephemeris data 3.5 0.0 3.5 0.0 0.0 0.0

Satellite clock 1.5 0.7 1.7 0.0 0.7 0.7

Ionosphere 4.0 0.0 4.0 0.0 0.0 0.0

Troposphere 0.0 0.5 0.5 0.0 0.5 0.5

Multipath 0.0 1.0 1.0 0.0 1.0 1.0

Receiver 0.0 0.05-3.5 0.05-3.5 0.0 0.05-3.5 0.05-3.5

Calibration site residual 0.0 0.0 0.0 0.0 1.9 1.9

UERE (RMS) 5.5 3.7 6.7 0.0 4.2 4.2

Filtered UERE (RMS) 5.5 0.9 5.6 0.0 1.1 1.1

1 sigma vertical axis error VDOP = 2.5 14.0 2.6

Maximum vertical error (SFO) VDOP = 15.6 87.4 16.4

Source: Dr. Bradford Parkinson, Stanford University, 1991.

Grounds, differential positioning yielded horizontal

accuracies of 2 to 5 meters (Kruczynski et at., 1986)

(Kruczynski, 1990). Vertical errors in differential

mode will be larger than horizontal error.

Techniques of Correction

Pseudo-Range vs. Positional Corrections

There are two types of positional connections:

corrections to the computed latitude, longitude, and

elevation, and corrections to the pseudo-ranges between the satellites and the roving station. The latter

method is more complicated in that a range correction factor for each satellite must be computed (four

or more ranges to be corrected), and the correction

must compensate for clock bias which will differ

between each receiver station.

There are significant differences between the

two correctional methodologies that have implications on the utility of all data collected. Receiver

units, while operating in either static or dynamic

mode, are constantly scanning available satellites

and locking onto the best configuration for accuracy.

Reference stations are operating in the same manner.

However, they may not necessarily always lock onto

the same set of satellites at any given moment. The

method for pseudo-range correction is more flexible

because it does not require that both receivers be

locked onto the same satellites; the reference station

will provide corrections for all satellites in view. The

positional correction methodology requires that positioning be based on the same set of satellites at both

the reference and roving stations. Receivers record

information on the constellation in use along with the

positional coordinates derived; this allows for a

comparison of constellations used during post-processing. With positional corrections, if the constellations used do not match, that coordinate must be

thrown out and not used in final coordinate computation. Given that as many as 200 coordinate fixes

may be collected for a point position over a 3- to 5-

minute recording session, some loss of individual

data points may not affect the feasibility of deriving

a coordinate solution for a point.

Real time vs. postsurvey correction

processing

Differential positions can be derived either in

real time, or via postsurvey processing. Real time

differential positioning requires a dedicated communications link, such as VHF-FM radio across which

the correction signal can be transferred, and a radiowave receiver hardware option on the roving unit.

The differential correction data is transmitted at a

low data rate in a standard data format termed RTCM

SC-104, defined by the Radio Technical Commission for Maritime Services.

An alternative real-time correction scheme

transmits data from the reference station in the same

frequencies and formats as the satellites themselves;

14
this is called a “pseudo satellite” approach. Realtime positioning has advantages for applications that

involve use of GPS for guidance to a given coordinate, such as in tightly designed environmental data

collection (derivation of positions postsurvey is not

applicable). However, postprocessing-derived positions tend to be more accurate because the correction

process can be supervised and enhanced via sophisticated processing techniques that require greater

computer processing capabilities than are available

in the roving units. Supervised processing might

include editing of outliers (possibly due to swings in

SA error and multipath) and adding missing data

(particularly applicable for mobile positioning which

might involve positioning in areas where the satellites may have been intermittently masked). In

fact, postprocessing affords such greater relative accuracy that real-time positioning should only be

considered for applications that need to be determined while in the field (Wells et al., 1988). However, some environmental monitoring that requires

sampling at a predetermined position will demand

real time capabilities. So far, EPA has not performed

any GPS surveying that involved real-time positioning. The accuracy of this mode of operation has not

yet been explored by this Agency.

Postprocessing involves the use of data filtering and smoothing routines. The Kalman filter and

smoothing routines are applied recursively to the

data forward and backward, reducing the variability

of positions at a given collection point. The filtering

and smoothing routines are used on both the roving

and receiver station data streams. The filtering

reduces the affect of signal noise and multipath.

Using differential positioning in this way, the standard deviation of the error for a point location can be

reduced to 1.5 meters, from typical stated accuracy

on the order of 3 to 5 meters (Lange and Kruczynski,

1989).

Static and Kinematic Positioning

GPS can be used to position both stationary and

moving objects. If the receiver is stationary (static

positioning), multiple range vectors to each of several satellites are calculated. Such redundant observations provide a higher level of accuracy for the

determined position. When using static positioning,

the GPS operator typically has a choice of real time

or postprocessed results. With real-time processing,

each successive observation at the same location is

processed so as to provide an improvement over the

previously determined position, whereas in

postprocessing mode the data is simply stored for

later refinement and generation of data of potentially

higher quality.

When the receiver is moving (kinematic positioning), instantaneous positions or “fixes” are made,

ideally from four range vectors observed simultaneously. There is generally no redundancy in the

data and normally a real-time solution is sought,

consisting of one fix at a time. The resulting string

of fixes can usually also be postprocessed using one

of a number of existing smoothing operations, as a

means of improving the quality of the positional fix

(Wells et al., 1988).

Static and kinematic positioning techniques

can be used in either absolute or differential mode.

Of the various conjurations possible, “relative

semi-kinematic positioning” has proven to be a very

efficient and accurate means of obtaining positional

information. The concept calls for one stationary

receiver serving as a base station, preferably but not

necessarily upon a known survey marker, and a

second roving receiver.

C. GPS Hardware Features and Options

While GPS is becoming a basic utility that can

be used in a wide range of applications, it is important

to recognize the rapidly expanding nature of the

technology and the broad spectrum of GPS hardware

available. Proper selection of the appropriate equipment requires a careful analysis of the intended

application of the device. The main factor in this

selection is accuracy requirements, although other

factors of importance are cost, ease of operation, and

the amount of data to be collected.

There are two broad groups of receivers: those

that track satellites sequentially, and those that can

track four or more satellites simultaneously. Sequencing receivers must sequence through four different satellites before they can calculate a position.

Continuous receivers have multiple channels and

can devote one channel to each of four satellites

simultaneously, permitting continuous, real-time

position measurements.

Sequencing Receivers

Sequencing receivers, which share one channel

with several satellites, usually have less circuitry,

and are less expensive, requiring less power to operate. Unfortunately, the sequencing can interrupt

15
positioning and can limit overall accuracy. Subspecies of sequencing receivers include starved power

single channel receivers, single-channel receivers,

fast-multiplexing single-channel receivers, and twochannel receivers. Multi-channel receivers have

distinct advantages with respect to canopy cover,

buildings, and other obstructions.

Starved Power Single-Channel Receivers

These receivers are designed for portability

and are usually powered by small batteries. In order

to limit power consumption, they may only take a

position reading once or twice per minute and turn

themselves off between readings. Low cost means

they are well suited as a personal locational device

for hikers or weekend sailors. Their accuracy is

typically better than most LORAN systems and,

unlike LORAN, they work anywhere in the world.

The main disadvantages of this type of receiver are

degraded accuracy, limited user interfacing, and,

unlike more sophisticated devices, their inability to

measure vehicle or vessel velocity with any precision.

Single-Channel Receivers

Standard single-channel receivers remain on

continuously. Consequently, these devices use more

electric power but are slightly more accurate and can

measure velocity as long as there are no significant

accelerations or course alterations.

Fast-Multiplexing Single-Channel Receivers

More complex circuitry enables this type of

device to sequence between satellites much more

quickly. As a result, the device can make ranging

measurements while it is also monitoring a satellite

data message. It can function continuously, but the

enhanced circuitry results in the device costing as

much as a two-channel sequencing receiver, which is

much more flexible and more accurate.

Two-Channel Sequencing Receivers

The addition of a second channel to a GPS

receiver increases its capabilities significantly. A

second channel immediately doubles the system

signal-to-noise ratio, meaning it can lock onto signals under more adverse conditions and can track

satellites closer to the horizon. A two-channel receiver never has to interrupt its navigation functions

because, while one channel is continuously monitoring positioning data, the other is acquiring the next

satellite. In addition, velocity measurements are

typically much more precise.

The disadvantages of a two-channel design are

that it has been historically more costly to construct

and requires more power to operate. Since the advent

of large-scale integrated circuits, however, the cost

of adding a second channel has been substantially

reduced. Two-channel receivers do remain typically

more expensive than their single-channel counterparts, largely because most users who seek the higher

accuracy and continuous functions of a two-channel

device usually also want a more sophisticated package of user controls and displays.

Continuous Multichannel Receivers

Continuous multichannel receivers are presently the receiver of choice for most applications

requiring high accuracy. These are capable of simultaneously monitoring four or more satellites and can

give instantaneous position and velocity. They are

designed in 4, 5, 6, 8, and even 10 and 12 channel

configurations, and are often used in surveying and

scientific applications. With four channels, a receiver can double the signal-to-background noise

ratio of a two-channel receiver and quadruple that of

a single-channel system.

With current technology, receivers with a minimum of six channels mark a significant threshold

with respect to functionality and performance. Five

of the channels can be used to continuously track 4+

satellites and the sixth channel can be devoted to

collecting data messages. Receiver manufacturers

can use data messages to track such things as carrier

signal, ephemeris reports and other differential correction information.

Besides the obvious advantage of being able to

continuously measure a position, multichannel receivers can also reduce the Geometric Dilution of

Precision (GDOP) problem (see section below, Factors Affecting GPS Accuracy). Instead of relying on

a calculation of the four best positioned satellites,

some of these systems are capable of tracking all

satellites in view, in order to get the absolute minimum GDOP. Multichannel receivers are specified to

have an accuracy of 1 centimeter ±1 millimeter for

each kilometer of baseline. For example, two survey

grade receivers separated by 10 km will compute a

16
separation of 10 km ±21 cm (Lange and Kruczynski,

1989).

D. Factors Affecting GPS Accuracy

The concept of accuracy is central to any method

of spatial measurement and is essential to understanding GPS technology. The level of positional

accuracy possible in a given positioning exercise is

highly variable and is a function of the chosen means

of communicating the concept of spatial accuracy, as

well as the different survey and positioning techniques employed.

Expressing Positional Accuracy

Positional accuracy is usually expressed in one

of two ways. The proportional error method is

expressed as the position error divided by the distance to the origin of the coordinate systems used

(Figure 9) and is usually expressed in parts per

million (ppm). Proportional accuracy can be defined

for point positioning as well as for differential positioning. In point positioning (referenced to the

geocentric center of the earth), a 10-meter en-or in the

position of a point represents an error of 10 m divided

by the radius of the earth (6.371 x 106m) and is equal

to 1.6 parts per million (1.6 ppm). In a differential

positioning scenario using two earth surface points

100 km apart, a proportional accuracy of 1.6 ppm

would require that the relative position of the points

be known to within 16 centimeters (0.16 m position

error/100,000 m baseline length = 1.6 ppm).

A second method of expressing positional accuracy which is more commonly used is confidence

regions, either ellipses (two-dimensional cases) or

Figure 9. Proportional error.

ellipsoids (three-dimensional cases). Confidence

regions are areas or volumes, physical confidence

regions, that will contain the true location at a

preselected level of probability, as shown in Figure 10 (Wells et al., 1988).

Figure 10. Confidence regions.

Classes of Surveys

Classes of surveys are prepared by the Federal

Geodetic Control Council and provide a measure of

the accuracy required and used within the U.S. The

advent of GPS technology has resulted in a reassessment of the applicability of these classes. In many

cases, GPS exceeds the standards established by

these survey classes.

A first order survey yields position or closing to

1 part in 100,000 (10 ppm) and is used for applications such as national geodetic network control and

the study of small earth crustal movements. A

second order survey has two classes, the first class

producing results between first order accuracy and

accuracy of 1 part in 50,000 (20 ppm), the second

class accurate to 1 part in 20,000 (50 ppm). Second

order surveys are used for coastline, inland waterway, and interstate highway surveys. A third order

survey also has two classes with class one being

accurate to 1 part in 10,000 (100 ppm), used for local

surveys, while third order class two is the lowest

class, accurate to one part in 5,000 (200 ppm). Third

order surveys are used for establishing control for

local developments, topographic and hydrographic

surveys, and other similar projects.

GPS provides a tool for surveying to 1 part in

100,000 in 45 minutes of satellite tracking and 1 part

in 500,000 for 2 hours tracking. Even greater accuracy exceeding 1 part in 1,000,000 is possible using

more advanced continuous, or “survey-grade,” receivers which are described earlier in this document.

17
Factors Affecting Positioning Accuracy

The accuracy of GPS positioning depends upon

the satellite geometry constellation, biases or errors

in the range measurements. Geometric “strength”

varies with satellite visibility and positions and,

therefore, it is a consideration in survey planning.

Biases influencing GPS measurements are primarily

related to either inherent imperfections in the receiver or satellite, or represent observation dependent distortions, such as variations in signal propagation. Biases are typically removed or at least

suppressed in differential-mode operation or in data

averaging. The sources of bias and error include

satellite clock errors, satellite ephemeris errors,

unmodelled atmospheric delays, multi-path signals,

receiver internal noise, and selective availability y

(Figure 11).

Satellite Geometric Strength

The effect of satellite geometry is expressed as

the geometric dilution of precision (GDOP). There

are several components of GDOP, though of primary

concern here is the positional dilution of precision

(PDOP). Simply stated, the PDOP is a measure of

the geometrical strength of the GPS satellite configuration. The level of accuracy associated with positional measurements will vary depending upon the

relative angles between the range vectors of two or

more satellites. Generally, the higher the value of the

PDOP the greater the uncertainty in the position of

the receiver. Conversely, the lower the GDOP and

PDOP, the more accurate the instantaneous point

position may be. The best dilution of precision

occurs with one satellite directly overhead and three

others equally spaced around the horizon, as low in

the sky as possible without risking obstruction by

terrain or other obstacles.

The GDOP can be considered a scalar factor

that represents the contribution of the constellation

geometry to potential positioning accuracy. This

factor could be multiplied by the composite pseudorange measurement error to get an overall error

estimate. If the pseudo-range measurement error is

10 meters and the GDOP is 3, an overall accuracy of

30 meters is potentially achievable (Ackroyd and

Lorimer, 1990).

The PDOP value changes with time, as the

satellites travel along their orbital paths, and with

survey point location, since the satellite configuration is dependent upon the position from which it is

Figure 11. Error sources.

18
observed. PDOP is also affected by topographic

relief, because the satellite and receiver require direct line-of-sight for the satellite signals to be received, and otherwise desirable satellites may be

blocked from view. Receivers are usually set to

reject satellites closer to the horizon than a specified

angle called the mask angle.

Related dilution of precision concepts include

horizontal dilution of precision (HDOP), and vertical dilution of precision (VDOP). The HDOP relates

simply to the two dimensional fix (which requires

one fewer satellite), and the VDOP reflects the

potential to measure elevation accurately.

Range Measurement Accuracy

Range measurement accuracy is affected by a

combination of considerations. Table 3 shows some

of the range measurement biases: small variations in

the accuracies of the satellite atomic clocks; slight

uncertainties regarding the actual satellite ephemeris; errors due to the electronics within the receiver;

and ionospheric and atmospheric delays in the propagation of the GPS signal. One of the more important

of these considerations is cycle slip, instances where

satellite signal reception was interrupted.

Together, these sources of error give each GPS

measurement a variable level of uncertainty. Fortunately, the assorted sources of uncertainty combined

do not typically add up to a very significant measurement error and, for the most part, can be predicted

and removed mathematically, by the receiver or

during postprocessing via data averaging or removal

of outlier data.

Clock error

Post-modeling timing difference between receiver clocks and the more reliable satellite clocks is

Table 3. Typical Range Vector Measurement Errors

Error Source Error

Satellite Clock Error 2 feet

Ephemeris Error 2 feet

Receiver Errors 4 feet

Atmospheric Propagation 12 feet

an error intrinsic to the passive nature of the receiver

units. If the receiver units were to transmit clock

information to the satellite, clocks could be synchronized and clock errors overcome. However, as GPS

was developed as a military application, transmission of a signal from a receiver unit possibly would

betray the location of the user. Precise calibration of

these clocks is critical to the accurate determination

of a satellite’s range vector. Clock error or time

offset is multiplied by the speed of light to derive the

user equivalent range error. Thus, an error in calibration of 1 microsecond creates an error in range of

approximately 300 meters (Wells et al., 1988). Since

prohibitive cost precludes outfitting every GPS receiver with its own atomic clock, slight variations in

clock calibrations become an inherent part of determining range vectors. In order to offset this timing

problem, it is necessary to determine the coincident

range vector to a fourth satellite. Ranging to a fourth

satellite permits correction of any errors related to

residual clock bias within the receiver.

Ephemeris error

Errors in the satellite orbit data or ephemeris as

transmitted in the C/A code lead to positioning errors

that can be as much as 20 meters off. Enhanced

ground tracking capabilities are to be developed that

will reduce this source of error to around 5 meters in

the future (Wells et al., 1988). However, again by

operating in differential mode, much of this source of

error can be removed. Selective availability (see

discussion below) contributes significantly to inaccuracies in the orbital data broadcast.

Ephemeris error may also result from orbital

adjustments made to the constellation. It is a slow

process to move the satellites because maneuvers are

performed in such a way as to minimize the expenditure of on-board propellant. During the period of

movement, which may extend over a few months, the

satellite is still sending navigation messages, but its

utility in positioning is degraded. It is possible to

obtain the precise ephemeris for the day of a survey

after the fact from the NGS. Currently, you must

wait up to 2 months before this information is available and it is provided at a fee. However, for highest

accuracy positioning during periods of repositioning, it may be necessary to recalculate positional

solutions with the precise rather than the orbital

information contained in the navigation broadcast

message at the time of the survey.

19
Atmospheric delays

The ionosphere is that area of the atmosphere

where free electrons exist, generated by the ultra

violet radiation from the sun. In this region, approximately 50 to 1,000 kilometers above the Earth’s

surface, the electromagnetic GPS signal is dispersed

to some degree by the ionized gas molecules. Tropospheric interferences vary according to meteorological conditions (temperature, water vapor content,

and atmospheric pressure). These factors cause

delays in signal reception that correlate to positional

inaccuracy. For absolute mode operation, atmospheric delays are a significant source of error. It is

possible to model atmospheric delay simulating its

contribution to the error budget. However, when

operating in differential mode, it is not considered to

be a necessary error suppression mechanism

(Kruczynski, Abby, and Porter, 1985).

Cycle slips

One other type of range measurement error

occurs when continuous satellite signal reception is

interrupted. For an instant, the receiver experiences

ambiguity as to which cycle of the C/A code it is

receiving. This will lead to what is known as cycle

slip. The best way to deal with significant slip

problems is to examine and edit data, removing

positions obtained immediately after signal breaks.

Multipath

Another factor affecting positioning accuracy

is the phenomenon of multipath, wherein a satellite

signal arrives at a receiver via two or more

different paths. This is usually observed when operating near large reflecting obstacles, which act in

effect as extensions of the receiver’s antenna. These

problems can be minimized or avoided through careful survey preparation and receiver/antenna positioning; in particular, avoidance of proximity to

metal buildings and bodies of water is recommended.

It should be noted that interference might be encountered from reflective objects within buildings that are

not visible to the field crew performing the survey.

Selective Availability

Since March 25, 1990, the accuracy of position

frees obtainable by civilians has been intentionally

degraded by the DOD. Where employed, this degradation will utilize an operational mode called selective availability (S/A). S/A will affect the accuracy

of the C/A code by artificially creating a significant

clock and ephemeris error in the satellites. When S/

A is operating, positional data via absolute mode

positioning may be so inaccurate as to be completely

inappropriate. Since S/A has been in effect, absolute

positional error has increased an additional 50 to 200

meters. Ironically, the need to deploy large numbers

of commercial grade GPS units in the Persian Gulf

War prompted DOD to turnoff selective availability

during the hostilities. It appears likely that it will be

reintroduced soon.

To get a good position fix in the presence of

S/A, differential GPS can be used. As described

earlier, differential GPS utilizes a stationary reference GPS receiver, set up on a known location, to

determine exactly the degree of error contained in the

satellite data. The reference receiver is set to record

a reference file, within which any deviations from

“truth” will be logged. Consistently anomalous

departures from the actual known position of the

reference receiver betray the presence of S/A. The

reference file generates a time-tagged position correction that can be applied to a remote GPS data file

collected using another receiver, at an unknown

location, within a few hundred kilometers from the

reference location.

Because some of this error can be effectively

overcome by operating in differential mode, the rate

at which signal degradation varies is of concern. The

clock “dither” is more rapid and changing than

ephemeris degradation. Because the change of rate

direction can be at approximately 3-minute intervals,

differential position correction codes must be recorded at the reference station with greater frequency (Ackroyd and Lorimer, 1990).

The continued access to useful and reliable

GPS signals and related information by the civilian

community rests in the hands of the DOD. The

existing arrangement originated in 1983 and has

spawned the development of an expansive civilian

GPS industry. The arrangement, allowing civilian

access to the capabilities of the GPS Standard Positioning Service (SPS) has been confirmed in every

edition of the Federal Radionavigation Plan (FRP).

The FRP is a Congressionally mandated, joint DOD

and DOT effort to reduce the proliferation and overlap of federally funded radionavigation systems.

The FRP is designed to delineate policies and plans

for U.S. government-provided radionavigation services, and is issued biennially, or more frequently if

20
necessary. Nonetheless, although civilian access to

GPS signals is clearly established in the FRP, signal

availability and accuracy remains subject to change

without advance warning, at the discretion of DOD

(U.S. DOT/DOD, 1984).

The true impact of S/A on the day-to-day

operational effectiveness of civilian GPS activities

remains rather unclear. A complete review of S/A

effects has not been performed by the authors, nor

has such a review been published as yet.

E.

S/A

Future Of GPS Technology

Barring significant new complications due to

from DOD, GPS as an industry will likely

continue to develop in the civilian community. There

are currently more than 50 manufacturers of GPS

receivers, with the trend continuing to be toward

smaller, less expensive, and more easily operated

devices. While highly accurate, portable (handheld) receivers are already available, current speculation envisions inexpensive and equally accurate

“wristwatch locators,” and navigational guidance

systems for automobiles. However, there is one

future trend that will be very relevant to EPA’s use of

this technology. Community base stations and regional receiver networks are GPS management and

technological innovations that will make GPS surveying easier and more accurate.

Community Base Stations

Regional Networks of Receiver Stations

The development of networks of geographically

separated GPS receiver stations for differential

positioning across large areas is a concept that is

currently under consideration by some of the EPA

Regional Offices. These receivers will be strategically

based to provide for optimal differential positioning

across their entire jurisdictional area. This would

make GPS surveying one step easier in that a second

receiver station would not need to be positioned at a

fried location for each survey event. A survey team

would simply need to identify the receiver station

best used for differential positioning in that area, and

either record in real time the reference station error

correcting data or acquire the logged base station

data for use in post processing.

The development of a national network of

community base stations is still a research topic at

this time. Concerns include how far from a base

station a receiver may be before degradation in

accuracy occurs. This will ultimately determine the

required density of base stations. consequently,

additional research is required before large investments can take place.

Real-Time Differential Correction

Real-time differential correction is required for

certain applications such as precise navigation and

may be a practical long term solution for all GPS data

collection. Differential correction requires an “anchor” point of known coordinates. By using existing

or other communications satellites data can be collected at these sites, processed, immediately unlinked,

and then broadcast via another channel into a receiver. Several commercial firms are currently exploring the marketing of such data via leased mobile

satellite services (MSS) with user subscriptions.

The satellite industry is moving to set aside part

of the L-band radio frequency to accommodate GPS

differential correction data streams. If accepted by

the FCC, this would enable differential correction

data to be a standard broadcast just like the GPS

locational data. A simple upgrade to existing GPS

receivers would enable real-time correction and provide true coordinates on the receiver display. This

technological development would certainly impact

the need for widespread networks of community

base stations.

Similarly, low cost field units are now appearing that provide digital, voice and video links to

Earth-orbiting satellites with user-transparent relay

links to anywhere in the world. Satellite access costs

are also falling below the $10/minute threshold. One

could envision that these units could be deployed to

emergency response sites or set up as nodes in areas

where intensive data collection will occur.

21

Use

A. GIS Applications

Section 4

Of GPS For Environmental Applications

GPS has been assessed as both a data collection

and quality assurance tool for GIS applications.

Positional data can be collected quickly and accurately and recorded within a common geographic

reference system. Point locational information can

be obtained in a fraction of the time required for

conventional surveys, permitting a field crew, for

example, to quickly and accurately gather positional

data for a large number of wellheads or sample

collection points. Line or polygonal features, such as

a street network or boundary of an industrial facility,

can be digitally mapped equally as easily. The GPS

receiver is simply transported to a convenient starting point, placed in logging mode, and the street

network is followed or the facility boundary circumnavigated until the feature or features of interest are

digitized. The data could then be extracted from the

receiver’s data logger, loaded to the appropriate GIS

platform, and the positional data converted to the

desired GIS data import format.

With the data logging capabilities of most GPS

receivers, the coordinates, time, and other attribute

information may be collected and then exported to a

GIS data base with no manual digitizing operation.

Since GPS provides a common reference system,

data from GPS sources and from sources rectified

with GPS register with each other and with the

multisource data within the GIS data base. Moreover, the spatial quality of GIS data can be established by comparing the representative positional

attributes of the data with their “true” GPS-derived

locations.

The following represent a few of the enhancements which GPS lends to EPA GIS applications:

1. Provides a bridge to collection of state plane

coordinates for GIS projects and to establishment

of legal boundaries and absolute site locations for

waste sites;

2. Gathering of precise locational data for specific

site features such as well heads, underground

tanks or drums;

3. Obtain accurate positional information without

4.

the need for line of site between points necessary

to do a traditional survey, thereby rendering insignificant buildings and other obstructions in and

around large urban areas. This capability also

eliminates the need to obtain access to hostile

properties and sensitive areas such as wildlife

reserves and wilderness areas; and

Provide a high quality locational accuracy assessment tool for implementation and enforcement of

the EPA’s Locational Data Policy. Accuracy

assessments can be conducted within a very short

period of time compared to conventional survey

techniques. GPS-derived data sets can be used for

comparison to data collected through other methods and representing the same features.

B. Field Sampling

1.

2.

3.

Provide a means to navigate to any point (e.g.

sample location) in the air, on the sea, or on the

land. A single C/A receiver will deliver navigational accuracy of approximately 20 meters. Postprocessing can reduce this error to about 3 meters

or less;

Ensure positional accuracy for data gathered during times when relative controls are not possible

due to heightened levels of urgency (e.g. oil spill

events). In these situations, positional data can

later be referenced to a state coordinate system

with the knowledge that accuracy will not be lost;

and

Allow for the collection of samples at known,

well-defined locations within a site in a much

shorter time than locating collection points by

23
conventional survey methods. The reduced time,

which can be as short as a few seconds to record

a GPS position, greatly reduces the exposure time

of personnel doing the sample collecting.

C. Remote Sensing

1. Provide photogrammetric control by establishing

photo-control points of horizontal and vertical

positions. An accurate position can be obtained

without the expense and time required for conventional surveys. Since GPS utilize a worldwide

standard reference system, limits of a common

grid reference system due to independent benchmarks are avoided; and

2. Provide sensor navigation and positioning for

remote sensing instruments such as cameras, line

scanners, and other active and passive measurement devices. These instruments can be accurately positioned during data collect and the position data base can be stored for post processing

and greater accuracy. This technique applies to

aircraft-based systems as well as space satellites.

D. Real-time Attribute Coding Software

An innovative software product recently made

available is GeoLink software, from GeoResearch of

Billings, Montana. GeoLink allows automatic realtime input of GPS satellite data into an ARC/INFO

GIS format, using a mobile or base station PC.

ARC/INFO is the GIS software currently in use by

the EPA. With this capability digital map products

can be created in the field. Preceded geographic

feature attributes can be simultaneously attached to

the collected GPS data or added as a feature location

is acquired. Attributes and positions then can be

displayed graphically on the PC as they are acquired;

this capability allows easy updating of old data and

visual verification of newly acquired data. It has

advantages over photo interpretation with the GPS

user being able to see under trees and through identification of very small objects. The use of GPS for

windshield surveys is the simplest and most cost

efficient method of spatial attribute updating

(Puterski, 1990).

24
Section 5

Performing a GPS Survey

A GPS survey of the type used in the Old

Southington, Connecticut and San Gabriel, California studies (see Appendix D) can be divided into five

major segments (Slonecker and Carter, 1990);

A.

B.

C.

D.

E.

Planning

Reconnaissance

Survey Execution

Data Reduction and Processing

Integration of GPS into a Data Base

Each of these segments is described below.

Particular attention is drawn to problems experienced in the field. Appendix F, Field Charts, contains a number of checklists and forms which complement the discussion. These prototypes were

developed for experimental projects using the hardware and software available. While obviously oriented toward a particular receiver and software,

these forms are useful models in performing a survey

with any equipment. The other appendices provide

descriptions on additional sources of information

and ancillary data vital to the success of a GPS

survey.

A. Survey Planning

Define Objectives of Survey

It is clearly important to initially establish the

ultimate objectives of a GPS survey. Recognition of

these objectives early in the project planning process

will help to focus the rest of the planning phase. The

accuracy requirements for the positional data needs

to be defined, paying particular attention to the EPA

Locational Data Policy (Appendix C). From the

discussion above, some distinct survey objectives

might include:

• registration of remotely sensed photography or

imagery;

●

●

evaluation of locational data quality of existing

data; and

sample data collection following precise coordinates in a monitoring plan.

Define Project Area

This step is designed for establishing the overall project area and defining the limits of the survey.

Maps and/or aerial photos should be utilized extensively to familiarize the crew with the area prior to

the actual field work. For identifying the study area

and surrounding environment, 7.5 minute topographic

maps are ideal. For locating particular sites by

address a local street map will be required. A

complete understanding of the project area transportation network will also enable the field crew to

maximize the effectiveness of their field time.

Determine Observation Window and

Schedule Operations

This involves determining the precise window

of satellite availability and scheduling accordingly.

The incomplete status of the satellite constellation

dictates that surveys are currently restricted to several hours per day. Optimization of the schedule is

dependent upon the size of the crew, the level of

accuracy desired, the logistics of setup, and the travel

between control points.

A current satellite visibility almanac is invaluable in planning a survey mission. Provided by

several vendors, these almanacs provide information

on the availability of satellite coverage. Since satellite orbits are periodically adjusted, these almanacs

require updating every several months. Most vendors provide updates using either electronic bulletin

boards or regular mail. For final verification of

availability, contact the U.S. Coast Guard GPS Users

Service for information on the entire system or any

25
individual satellites that may be deactivated during

your scheduled field work. Information is also

broadcast by WWV/WWV# at 15 minutes past each

hour.

The Agency has arranged for EPA users to

acquire one such almanac, the Satellite Visibility and

Geometry Analysis Software (SATVIZ) from

Trimble Navigation. SATVIZ is an easy-to-use PC

based software program which provides information

critical to the various components of planning a GPS

survey: satellite availability, elevations, azimuths,

and GDOP calculations. Appendix A provides information on how to contact Trimble Navigation.

Several rules of thumb exist regarding available windows and angles above the horizon. Most

almanac software will yield good results for windows within one half degree of the survey station

(approximately 30 miles). If your survey mission

will span greater distances, then you may wish to

iterate calculations for several planned survey locations. For angles above the horizon, optimal results

are achieved with satellites 25 degrees above the

horizon. However, as little as 10 degrees works fine

in areas of minimal obstruction.

Accuracy is heavily dependent upon the amount

of observation time and number of observations

taken at each point. It is generally agreed that

observation time can be reduced by increasing the

quality of observation time, i.e. observing a maximum

number of satellites during 3-D viewing periods.

Establish Control Configuration

In this step, known control points and/or benchmarks are located for both horizontal and vertical

control. This is usually accomplished by researching

the records of various Federal, State and local agencies such as the National Geodetic Surveyor the state

geodetic survey. It is advisable to have, where

possible, at least two control points each for both

vertical and horizontal positions so that there is a

double check for all control locations. The reference

for NGS benchmark information is provided in Appendix A.

It is of paramount importance that the reference

datum within which the monument is located be

defined. The discussion provided later in this section

explains the reasons in detail. For horizontal coordinates, the North American Datum of 1927 (NAD 27)

or the newer Datum of 1983 (NAD 83) will

be specified. For vertical control coordinates, the

National Geodetic Vertical Datum of 1929 (NGVD

29) or the new North American Vertical Datum of

1988 (NAVD 88) will be referenced. If the NGS has

redefined the benchmark coordinates to correspond

to the newer datums, coordinates will be available

for both datums.

If the monument is located in a controlledaccess setting, the appropriate individuals should be

contacted to obtain admittance. The station recovery

section of monumentation data sheets provided by

NGS describe in detail how to locate a particular

point and whom to contact for access.

Sensitivity to factors contributing to multipath

are particularly important for positioning a receiver

station and antenna. In particular, control points/

areas should not be near power lines, substations or

large metal objects which can cause multipath interference and corrupted data. Since observation of

these proximate features may not be possible until

the survey reconnaissance is performed, having

backup sites ready will save time.

Choosing control points for use as base stations

may require a physical inspection of the site. Ideal

locations will have a near 100 percent clear view of

the sky and be easily accessible. They should also be

located in area of low vehicular and pedestrian traffic.

Select Survey Locations

Obtain a list of the facilities or routes targeted

for data collection. A good suggestion is to organize

the site lists alphabetically by city and alphabetically

by street name within each city. This will facilitate

initial route planning to visit each and serve as a

master list. If possible, plot the general location on

afield map and highlight a local street map to serve

as a general navigation aid. Similarly, plot potential

base stations to serve as control points on a 7.5

minute topographic map and local street map.

The survey points/areas must be accessible

during the satellite window of availability, which

currently may occur during unusual hours (e.g. 1 am

to 5 am). If the survey point to be obtained is located

on private property, care should be taken to pursue

appropriate notification and access protocol. This

includes preparation of a letter of introduction and

formal contact with the property owner/manager. A

sample letter is included in Appendix F, Field Chain.

Access to points “on-site” within private property

such as business facilities are typically not appropriate for night time surveys.

26
The points/areas should have continuous and

direct line-of-site to the path of the satellites in the

sky. Based on the view provided by satellite window

planning software such as SATVIS, and the survey

team’s knowledge of the natural and man-made

topographic features, it may be possible to predict

masking.

As with control points, obstructions and other

factors can cause interference and corrupted data

during the survey. It is advisable to note any adverse

conditions on a form when collecting data. This will

be helpful in the postprocessing phase.

If point data being collected is to be used as

control for photogrammetric operations, then the

point locations must be photoidentifiable on the

imagery to be used for photo registration. If the

registration is to be used with historic imagery, the

locations should be landmarks present and identifiable for the entire period of history to be reviewed.

Such landmarks might be corners of the street network that have remained constant, street/railroad

intersections, hydrants or other public works.

Equipment Logistics

Survey planning action items in this area include: determination of equipment availability (laptop

PC, GPS units, transport vehicle, monumentation

equipment), and checking equipment for necessary

repair and maintenance (batteries charged in PC and

GPS unit, PC disk loaded with necessary software

and has available disk space).

This is the time to collect and pack field survey

equipment. In addition to the above items, experienced crews carry everything from a compass and

tape measure to manuals and almanac printouts. A

suggested checklist is provide in Appendix F.

B. Reconnaissance

The Reconnaissance phase is an important part

of a successful GPS operation and is usually performed by an individual or crew at some point prior

to the arrival of the full field team. The purpose of

this phase is to:

Locate and Verify Control Point Locations

This is critical to the success of the overall

survey. Often, monuments have been damaged,

stolen, buried or vandalized. If a control point cannot

be recovered, a replacement must be located. This

can drastically change the schedule and logistics of

the field survey. Also, it may be found that a

monument’s location has shifted somewhat. There

are currently state and federal initiatives to improve

coordinate networks as part of the effort to upgrade

to NAD 83 (see discussion of datums below).

Plan to visit each of the control points at least

twice during the survey. Collecting redundant data

is useful in determining the quality and accuracy of

the overall survey. The duration of each fix should

be approximately 3-5 minutes.

Preview Instrument Locations

Obtain permissions and verify accessibility. It

will often be necessary to coordinate activities with

property owners, local law enforcement, and/or land

management officials in order to ensure safe and

authorized access to the instrument locations. Field

verify that there are not any visible multipath-contributing or masking features. Identify any natural or

man-made obstacles to direct access to survey point.

Physically Establish Point Locations

This is accomplished by using a standard surveying marker such as an iron pipe, a hub and tack,

or a brass nail. All points should be documented with

detailed descriptions in a log book (refer to Appendix

F for a sample form). If nearby multipath or masking

features are unavoidable, note their presence. It may

be necessary to physically offset the GPS control

point from an obstructed benchmark. This can

effectively be accomplished using a compass and

tape measure.

C. Survey Execution

The actual GPS survey consists of:

Establishing a Schedule of Operations

This involves determining the window of satellite configuration availability and scheduling the

GPS sessions. This is dependent on the size of the

crew, the level of accuracy desired, and the logistics

of setup and travel between control points. Maximum data quality and collection efficiency can be

27
obtained by arranging travel time to coincide with

periods of 2-D or no satellite coverage and actual

data collection periods to coincide with periods of 3-

D satellite visibility.

Presurvey: The Day Before

Plan on arriving the day before. Charge all

batteries. Many GPS collection systems utilize a

battery system which requires either 8-hour or overnight charging. For example, the Polycorder used

with the Trimble Pathfinder system utilizes a Ni-Cad

battery that should not be charged over 8 hours.

Review the travel routes to survey sites and base

stations, if required, and coordinate with local personnel. Review use of unfamiliar equipment and

understanding of procedures.

Predata Collection: Establishing a Base

Station

The type of survey will dictate if a base control

station in the field is required. If required and the

location is not secure or if the data collection period

is particularly long, some of the survey crew may be

required to remain at this site. Logistical considerations will need to be scheduled, i.e. shut down

periods for downloading files, changing battery packs,

and when to terminate collection. Once setup at a

base station begins, the GPS units will need to be

initialized. Depending upon the location and familiarity with equipment, this activity can take anywhere from a few minutes to a couple of hours.

Data Collection: Performing the GPS Survey

The crew must warm-up, check and program

the receiver for proper operation. Most vendors

currently recommend collecting fixes for discrete

point data for a period of 3-5 minutes, at a one or two

second interval. Many software packages require

approximately 35-40 readings per point to perform

statistical analyses such as t-tests.

Depending on the unit being utilized, sufficient

battery power must be available and the receiving

antenna must be leveled on a tripod and centered

exactly over the control point location. Log sheets

containing critical information on position, weather,

timing, height of instrument, and local coordinates

must be maintained. Once the session is completed,

the receiving equipment must be disassembled, stored,

and log and tape files documented.

If another session is scheduled, this process

must be conducted quickly and efficiently so that the

crew can beat the next location and be set up in time

for the scheduled window of satellite availability.

If the survey to be performed will span over

numerous days, it is likely that the data will be

transferred from the GPS to a lap top PC with some

regularity. Data from the base station as well as the

roving unit will need to be collected with equal

frequency.

Leveling

If a correlation to a known vertical datum is

required, a leveling survey must be performed from

a known benchmark to at least one of the GPS control

points and closed back. It is desireable to level to

more than one of the GPS points and to close on a

second vertical control point.

Locating Facilities

A recommended strategy for locating facilities

involves conflation of the street address and facility name. Often an address is located but confirmation of the name is not possible. Reasons may vary

from vacancy to a change in ownership. Data should

still be collected and any discrepancies well documented. Many problems can resolved at a later time.

Other problems, such as poor signage in rural areas,

can be overcome by asking for information from

local postal workers and delivery persons.

Returning From The Field

This is the time to perform other postsurvey

tasks. Before leaving the site, document any unique

problems. After returning from the field, complete

other housekeeping chores such as recharging the

system and cleaning the equipment. Use checklists

to make sure equipment is in working order and any

consumable supplies are reordered.

D. Data Reduction and Processing

Data reduction and processing consist of:

28
Data Transfer

There are currently two common methods for

collecting data in the field, using an intermediate

device such as a Polycorder or directly into a

laptop/notebook personal computer. With the latter

method some users subsequently perform all processing directly on the same device. More commonly, data is transferred to another machine. This

consists of reading the raw data from the GPS cassette tapes or other media into a structured database

for processing. As with any computer data backup

copies should be made immediately.

Initial Processing

The electronic GPS data stream may not be

immediately useable. It normally consists of satellite

navigation messages, phase measurements, user input field data and other information that must be

transferred to various files for processing before

computations can be accomplished. Depending upon

the hardware and software vendor, many of these

operations are transparent to the user. There are five

components to the initial processing phase performed

by GPS “firmware,” software that comes with the

GPS units:

1. Orbit Determination: using satellite navigation

messages, one unambiguous orbit for each satellite is computed;

2. Single-Point Positioning: clock corrections and

parameters for each receiver are computed;

3. Baseline Definition: general locations of receiv4.

5.

ing stations are established, computing the best

pairs of sites for baseline definition;

Single Difference File Creation: the differences

between simultaneous phase measurements to the

same satellite from two sites. This is the basic data

from which network and coordinate data will be

derived; and

Data Screening and Editing: automatic and manual

screening of the single difference files and editing

of data obviously affected by breaks, cycle slips,

or multipath.

In some instances, depending on the type of

maintenance and upgrades that are going on to the

NAVSTAR constellation at the time of the survey,

utilization of the actual ephemeris rather than the

ephemeris projected prior to the survey date may

improve solution accuracy. Actual ephemerides are

available 2 weeks after a given survey date.

In the data screening and editing step above,

there are at least three considerations that might be

taken in editing. Outlier position data can be removed from a data file. This editing should be

guided by establishing an absolute deviation threshold, using the mean coordinate as a reference. The

threshold criteria might be varied to determine the

sensitivity of the solutions to this editing. Data

points collected immediately after a break in the data

stream, such as in the event of masking, should be

edited out because these positions will be less reliable. Finally with S/A operational, removal of twodimensional positions (e.g., positions obtained when

the satellite configuration was not strong enough for

obtaining three-dimensional positions) may be advisable because S/A seems to have such a large effect

on the altitude; although altitude is not specifically

solved in a 2-D fix, the altitude of the position

impacts the solution.

Computation

This component uses the pre-processed data to

compute the network of sites and give a full solution

showing geographical coordinates (latitude, longitude and ellipsoidal height), distances of the vectors

between each pair of sites in the network, and several

assessments of accuracy of the various transformations and residuals of critical computations. If the

standard deviation for a differential mode position is

greater than 5 meters, removal of outlier coordinates

and recalculation of a position mean is advised

(Lange, 1990).

A number of software programs exist to assist

with this phase of processing. In particular, the EPA

Environmental Monitoring Systems Laboratory, Las

Vegas, Nevada (EMSL-LV), is currently developing

a suite of programs that will be available in the future

to all regional offices. These programs will include

PLOTSSF and CLEANSSF. Designed to work with

the Trimble SSF file, these modules can be used to

display statistics about each point and editing of

outliers. Further information is provided in Appendix E.

E. Integration of GPS Data into a GIS

Data Base

Rectification of Datum

In recent years, NGS has redefined the shape of

the earth, called a reference datum. Analogous to

adding a leap second to the official clock, the new

29
datums differentially correct the shape and therefore

positions on the earth’s surface. Unlike the clock, the

adjustment is not uniform over the earth’s surface.

Consequently, it will require many years to convert

old maps and digital data bases.

If the GPS data are to be introduced into an

existing map data base, it may be necessary to

convert either the existing data or the GPS data in

order to match the reference datums. The North

American Datum of 1927 (NAD 27) has been replaced in recent years as the official horizontal datum

by the North American Datum of 1983 (NAD 83).

Although NAD 83 is much more consistent and

accurate than its predecessor, many surveying and

mapping coordinates in the United States are still

referenced to NAD 27. Consequently, datum inconsistencies confront the land surveying and mapping

communities.

The principle differences between NAD 27 and

NAD 83 are functions of the reference ellipsoids or

spheroids (i.e., simple geometric approximations for

the shape of the earth) chosen for each datum. NAD

27 depends upon an early approximation, known as

the Clarke Spheroid of 1866, while NAD 83 relies on

the more precise Geodetic Reference System of 1980

(GRS 80). The Clarke Spheroid of 1866 was designed to fit only the shape of the conterminous

United States, utilizing a specific Earth surface coordinate pair as its center of reference. On the other

hand, GRS 80, and the essentially equivalent

WorldGeodetic System of 1984 (WGS 84), is a

geocentric ellipsoid, utilizing the Earth’s center of

mass as its center of reference. This fact facilitates

computing correct geometric relationships on a global and continental scale.

As NAD 27 has been in use for over 50 years,

there is certainly a potential for datum inconsistency

in the development of a digital cartographic or geographic data base. The impact of the datum change

must be viewed in light of 1) the mapping scales of

interest within the cartographic or geographic product or data base, and 2) the magnitude of the shifts

between datums. The differences between NAD 27

and NAD 83 vary with respect to location, from as

little as zero to in excess of 100 meters.

If the average datum shift within a region of

geographic interest is less than the stated accuracy

standard necessary to properly represent the area of

interest, then a “correction” or transformation to

NAD 83 is not necessary. A transformation between

datums will likely be necessary only when the difference between datums exceeds the reqisite accuracy

of the geographic product (Dewhurst, 1990). As of

1986, the National Geodetic Survey had completed

adjustment of 270,000 geodetic control stations to

NAD 83 with 1 part in 100,000 accuracy. Many

states desire higher accuracy in their networks now

that it is obtainable using high-grade GPS equipment. Many are in the process of or are considering

upgrading Order A and B networks to 1 ppm accuracy. Those committed to this upgrade as of June,

1990, include: Washington, Oregon, New Mexico,

Wisconsin, Tennessee, and Florida. NGS has issued

an implementation strategy for such upgrades. The

coordinate recordation will include a reference in the

following format, “NAD 83 (Adjustment of 199X)”

(Bodnar, 1990).

Several different methods of transforming coordinate data are well accepted in the geodetic and

surveying communities. A rapid and accurate transformation methodology, known as NADCON, has

been developed by the National Geodetic Survey

(NGS). NADCON software has been adopted by the

Federal Geodetic Control Committee (FGCC) as the

standard for coordinate conversions between NAD

27 and NAD 83, Results indicate that NADCON is

accurate to approximately 15 centimeters for the

conterminous United States, where good geodetic

control exists. Remote areas where geodetic control

is sparse or nonexistent may experience somewhat

less accurate results, but seldom in excess of 1.0

meter (Dewhurst, 1990). This conversion is recommended for map data that is smaller than 1:200 scale.

Copies of NADCON are available from the

National Geodetic Survey. For further information

contact:

The National Geodetic Survey

National Geodetic Information Branch

N/CG 174

NOAA

Rockville, MD 20852

Phone: (301) 443-8631

For Agency users of Environmental System

Research Institute’s ARC/INFO GIS software,

NADCON is being released with Revision 6.0. For

users who will not be installing that version in the

immediate future, a program called CDATUM is

currently available from EMSL-LV. This program,

written in FORTRAN 77 for the VAX, will convert

datums for any coverage.

30
Copies of CDATUM are available from EMSLLV. For further information contact:

Mason J. Hewitt

GIS Program Manager

EMSL-LV

P.O. BOX 93478

Las Vegas, NV 89108

Phone: (702) 798-2377 FTS 545-2377

Users of vertical measurements from GPS receivers should also understand the heights provided

by GPS are not elevations above sea level. In

conventional surveying horizontal measurements are

referenced to a theoretical ellipsoid, a mathematically defined regular surface, and vertical measurements are referenced to a geoid, an irregular surface

along which the gravity potential is equal at every

location. The geoid is equivalent to the surface to

which oceans would conform over the entire earth if

free to adjust based on mass and rotational forces.

Because the earth’s mass is unevenly distributed the

geoidal surface is irregular. Three-dimensional coordinates of GPS data refer to heights above the

ellipsoid, not the geoid.

Simple software techniques are typically provided by vendors to assure discrepancies in the

undulations between the geoid and ellipsoid with

magnitudes no greater than 30 meters. Where precise measurements referenced to mean sea level are

required, most more elaborate computer programs

are available from NGS to obtain accuracies of the

order of 5 to 10 millimeters. Some of these require

additional field work with respect to sampling data at

numerous known benchmarks. The professional

surveying community and GPS vendors are currently working closely to provide more efficient

solutions to the derivation of vertical measurements.

NGS is currently readjusting vertical geodetic

data to produce the North American Vertical Datum

of 1988 (NAVD 88). The NAVD 83 readjustment

will remove distortions from the continent-wide vertical geodetic (height) reference system. This project

is planned for completion in 1991. The vertical

adjustment combines a large-scale resurvey of the

U.S. vertical control network, replacement of missing or destroyed benchmarks, and a readjustment of

the entire vertical control network. The result will be

a greatly improved, up-to-date height reference system for all of North America. The conversion of the

vertical datum will result in up to a 2-meter shift.

This may not be a concern for most spatial data as the

amount of shift falls within the range of uncertainty

of vertical accuracy for differential positioning (National Geodetic Survey, 1990).

31

References

Ackroyd, Neil, and Robert Lorimer. Global Navigation: A GPS User’s Guide. Lloyd’s of London Press,

Ltd, 1990.

Bodnar, Captain A. Nicholas. National Geodetic

Reference System Statewide Upgrade Policy. National Geodetic Survey, June 1990.

Corn, David. Surveying By Satellite. Catalyst,

February 1990, pp. 43-44.

Denaro, R. P. and R. M. Kalafus. Advances and Test

Results in Differential GPS Navigation. The Journal

of Navigation, vol. 43:1, pp. 32-40, 1990.

Dewhurst, W. T. Shift-On-A-Disk. Converting

Coordinate Data from NAD 27 to NAD 83. ACSM

Bulletin, No. 126, 1990, pp. 29-33.

Hartl, P. H. Remote Sensing and Satellite Navigation: Complementary Tools of Space Technology.

Photogrammetric Record, vol. 13:74, pp. 263-275,

1989.

Hum, J. GPS, A Guide to the Next Utility. Trimble

Navigation Ltd. Sunnyvale, California, 1989, 76 pp.

Kruczynski, L. R. and A. F. Lange. Geographic

Information Systems and the GPS Pathfinder System: Differential Accuracy of Point Location Data.

Trimble Navigation, Limited study report, 1990.

Kruczynski, L. R. Differential GPS: A Review of

the Concept and How To Make It Work. Trimble

Navigation, Limited study report, 1990.

Kruczynski, L. R., W. W. Porter, D. G. Abby, and E.

T. Weston. Global Positioning System Differential

Navigation Test at the Yuma Proving Ground. The

Institute of Navigation, 1986.

Lange, A. F. and L. R. Kruczynski. Global Positioning System Applications to Geographic Information

Systems. Proceedings from the Ninth Annual ESRI

ARC/INFO User Conference, Palm Springs,

Calfornia, 1989. 4 pp.

Langely, R. B. Innovation Column: Why is the GPS

Signal So Complex? GPS World, Vol. 1:3, pp. 56-

59, 1990.

Locational Data Policy Implementation Guidance

(LDPIG). U.S. EPA Office of Information Management Resources, Washington, D. C., 1991.

McDonald, K. D. GPS Progress and Issues. GPS

World, Vol. 1:1, p. 16, 1990.

McDonald, K. D., E. Burkholder, B. Parkinson, J.

Sennott. GPS for the Environmental Protection

Agency, Course 355. Navtech Seminars, Inc., November 1991.

National Geodetic Survey. National Geodetic Reference System Statewide Upgrade Policy. 1990.

Puterski, R. GPS Applications in Urban Areas.

Proceedings from the Tenth Annual ESRI ARC/

INFO User Conference, Palm Springs, California,

vol. 1, 1990, 12 pp.

Slonecker, E. T. and J. A. Carter. GIS Applications

of Global Positioning System Technology. GPS

World, Vol. 1:3, pp. 50-55, 1990.

U.S. Department of Defense/Department of Transportation, 1984. Federal Radionavigation Plan. Final Report: March 1982-December 1984. U.S.

Department of Defense Document #DOD-4650.4

and U.S. Department of Transportation document

#DOT-TSC-RSPA-84-8.

Wells, D., N. Beck, D. Delikaraoglou, A. Kleusberg,

E. J. Krakiwsky, G. Lachapelle, R. B. Langley, M.

Nakiboglu, K-P Schwarz, J. M. Tranquilly, and P.

Vanicek. Guide to GPS Positioning. Canadian GPS

Associates. Fredericton, New Brunswick, Canada.

1988.

33

Appendix A

Sources Of Additional Information On GPS

Information Sources Within the EPA

The Environmental Monitoring Systems Laboratory - Las Vegas (EMSL-LV) is EPA’s Center for

Excellence in remote sensing, GIS and geoprocessing.

EMSL-LV has extensive experience with various

geopositioning techniques including GPS technology. It is strongly recommended that EMSL-LV be

contacted prior to action involving geoprocessing

for advice on appropriate geopositioning methods or

for technical assistance. For more infonnation please

contact:

Mason Hewitt

(702) 798-2377

(FTS) 545-2377, or

E. Terrence Slonecker

Information Sources Outside the EPA

GPS Satellite Clock Behavior and Related

GPS Information

The U.S. Naval Observatory provides dial-up,

on-line access to GPS information computer fries.

Access requires full duplex, seven data bits, even

parity, upper case.

U.S. Naval Observatory

Washington, D.C. 20392-5100

300 or 1200 baud (202) 653-1079

2400 baud (202) 653-1783

Precise GPS Orbit Information

Government: Precise orbital positions and

velocities based on post computations of tracking

data collected from stations of the Cooperative International GPS Tracking Network (CIGNET) are available from the National Geodetic Survey (NGS).

Satellite orbital data are scheduled to be available 2

weeks after the tracking data are collected. For a

description of formats, fee schedule, or to order data,

contact:

National Geodetic information Center,

N/CG17

National Geodetic Survey

National Ocean Service, NOAA

Rockville, MD 20852

(303) 443-8775

Commercial: Precise orbital data is available

from the Aero Service Division, Western Atlas International, using data obtained from its tracking network stations. For a description of format, fee

schedule, or to order data, contact:

Mr. Jim Cain

Manager, GPS Service Division

Western Atlas International

3600 Briarpark Drive

P.O. Box 1939

Houston, TX 77251-1939

(713) 784-5800

GPS Satellite Status and Health

NAVSTAR/GPS Operational Control System

Falcon Air Force Base

Colorado Springs, CO 80914-5000

Recorded daily operations report

(719) 550-2115

Live Mission Operations Controller

(719) 550-2363

35
Performance of GPS Satellite Survey

Systems

GPS Test Coordinator

Instrument Subcommittee

Federal Geodetic Control Committee

c/o National Geodetic Survey, NOAA

N/CG14, Rockwall 306

Rockville, MD 20352

(301) 443-8171

The Civil GPS Service (CGS)

The CGS is evolving under the guidance of a

steering committee representing the civil user community and the Department of Transportation. CGS

will serve as a source of information and point of

contact for civil users, and GPS status and information will be disseminated by the Civil GPS Information Center. For information, contact:

Chairman, CGS Steering Committee

U.S. Department of Transportation

Research and Special Programs Administration

400 7th Street NW, Room 8405

Washington, DC 20590

(202) 366-4355

The U.S. Coast Guard GPS Information

Center (GPSIC)

The U.S. Coast Guard GPS Information Center

(GPSIC) is now (June 1990) providing GPS operational advisory broadcasts (OAB) on a “test and

evaluation” basis. Users have access to the service

24 hours a day, although live operation of the center

initially will be limited to 8:00 a.m. to 4 p.m. eastern

time, Monday through Friday, except federal holidays. Information available through the center consists of current constellation status, future scheduled

outages, and an almanac suitable for making GPS

coverage and satellite visibility predictions.

A voice recording that presents a brief summary of the constellation status can be reached at

(703) 866-3826. More detailed information is available through a computer bulletin board system:

300, 1200, or 2400 bps (703) 866-3890

4800 or 9600 bps (703) 366-3894

For additional information on the center or the bulletin board write to:

Commanding Officer

U.S. Coast Guard

Omega Navigation System Center

7323 Telegraph Road

Alexandria, VA 22310-3998

(703) 866-3806

A brief GPS update is also provided hourly by

GPSIC and radio stations W and WWVH, operated by the National Institute of Standards and Technology on frequencies of 2.5, 5, 10, and 20 MHz.

WWV, at Fort Collins, CO. broadcasts this message

at 15 minutes past the hour. WWVH, in Hawaii,

repeats the same message at 15 minutes before the

hour.

GPS Bulletin

The GPS Bulletin is prepared and disseminated

bimonthly by the National Geodetic Survey (NGS),

Charting & Geodetic Services, National Ocean Service (NOS), National Oceanic and Atmospheric

Administration (NOAA) under the auspices of the

Global Positioning System Subcommission, Commission VIII, of the International Coordination of

Space Techniques for Geodesy and Geophysics

(IUGG). Although the Bulletin is intended to serve

as a means to disseminate GPS information primarily to those involved in the practice of high-precision geodesy and geodynamics, there is information

of general interest for the GPS community.

For more information, contact:

Miranda Chin

N/CGl14, Rockwall 419

National Geodetic Survey, NOAA

Rockville, MD 20852

(301) 443-2520

Available GPS Training

American Congress of Surveying and

Mapping (ACSM)

ACSM offers a comprehensive GPS training

program that involves a series of l-to 3-day courses

36
at locations nationwide. Courses available include: P.O.B. Publishing Co.

Basic GPS for Surveyors, Planning and Executing

GPS Surveys, Processing GPS Data, and Data Adjustment and Transformation. For more informa- P.O.B. offers a series of seminars entitled “Praction, contact: tical Surveying with GPS.” For more information,

contact:

American Congress of Surveying and Mapping

210 Little Falls Street P.O.B. Publishing Company

Falls Church, VA 22046-4392 5820 Lilley Road, Suite 5

(703) 241-2446 Canton, MI 48187

(313) 981-4600

Canadian GPS Associates University of Montana

Offers courses taught by experts that are tailored to specialized audiences at locations world- The University of Montana School of Forestry

wide. Courses cover all aspects of GPS satellite and Center for Continuing Education, in cooperation

navigation and surveying technology. For additional with the USDA Forest Service Missoula Technology

Information, contact: &Development Center, offers a course on Introduction to Satellite Navigation in Resource ManageDr. David Wells ment. For more information, contact:

Canadian GPS Associates

P.O. BOX 3184 Center for Continuing Education

Postal Station B University of Montana

Fredericton, New Brunswick, E3A 5G9 Missoula, MT 59812

(406) 243-4623 or 243-2900

Canada

(506) 453-5147 California State University

Navigation Technology Seminars, Inc. Seminars, workshops, and short courses on

(NAVTECH) GPS satellite surveying are offered through the Department of Civil and Surveying Engineering, School

NAVTECH offers a curriculum of courses that of Engineering, California State University, Concover about 12 different aspects of GPS satellite tinuing Engineering Program. One- to 3-day courses

navigation and positioning technology. The curricu- are offered either at the Fresno campus or off campus

lum includes a course on Basics of the GPS and one at a hosted facility. For additional information,

on Basics of Surveying with GPS. For additional contact:

information, contact: Department of Civil and Surveying Engineering

Carolyn P. McDonald, President

Navigation Technology Seminars, Inc.

1900 N. Beauregard Street

Suite 106

School of Engineering - -

California State University, Fresno

Fresno, CA 93740-0091

(209) 294-2889

Alexandria, VA 22311

(703) 931-0500

37

Appendix B

Glossary of GPS Terms

Absolute positioning - positioning mode in which a

position is identified with respect to a well-defined

coordinate system, commonly a geocentric system

(i.e., a system whose point of origin coincides with

the center of mass of the earth).

Anywhere fix - The ability of a receiver to start

position calculations without being given an approximate location and time.

Baseline - A baseline consists of a pair of stations for

which simultaneous GPS data has been collected.

C/A code - The standard (Clear/Acquisition) GPS

code; also known as the “civilian code” or “S-code”.

Cadastral survey - survey performed to establish

legal and political boundaries, typically for land

ownership and taxation purposes.

Carrier - A radio wave having at least one characteristic (e.g. frequency, amplitude, phase) that can be

varied from a known reference value by modulation.

Carrier beat phase - the phase of the signal which

remains when the incoming Doppler-shifted satellite

carrier signal is beat (the difference frequency signal

is generated) with the nominally-constant reference

frequency generated by the receiver.

Carrier frequency - the frequency of the unmodulated

fundamental output of a radio transmitter.

Channel-A channel of a GPS receiver consists of the

radio frequency, circuitry, and software necessary to

tune the signal from a signal GPS satellite.

Chip - In the GPS world, a chip is the transition time

for individual bits in the pseudo-random sequence.

Clock bias - the difference the clock’s indicated time

and true universal time.

Control segment - a world-wide network of GPS

monitoring and control stations that ensure the accuracy of satellite positions and their clocks.

Cycle slip - a discontinuity of an integer number of

cycles in the measured carrier beat phase resulting

from a temporary loss-of-lock in the carrier tracking

loop of a GPS receiver.

Data message - a 1500 bit message included in the

GPS signal which reports the satellite’s location,

clock corrections, and health.

Differential positioning - precise measurement of

the relative positions of two receivers tracking the

same GPS signals.

Dilution of Precision - the multiplicative factor that

modifies range error. It is caused solely by the

geometry between the user and his or her set of

satellites; known as DOP or GDOP.

Doppler-aiding - a signal processing strategy that

uses a measured doppler shift to help the receiver

smoothly track the GPS signal. This allows more

precise velocity and position measurement.

Doppler shift - the apparent change in the frequency

of a signal caused by the relative motion of the

transmitter and receiver.

Dynamic positioning - See Kinematic positioning.

Ephemeris - the predictions of current satellite position that are transmitted to the user in the data

message.

Fast-switching channel - a single channel which

rapidly samples a number of satellite ranges. “Fast”

implies that the switching time is sufficiently short (2

to 5 milliseconds) to recover the data message.

39
Federal Radionavigation Plan (FRP) - Congressionally mandated, joint DoD and Department of Transportation (DOT) effort to reduce the proliferation

and overlap of federally funded radionavigation systems. The FRP is designed to delineate policies and

plans for U.S. government-provided radionavigation

services.

Frequency band - a particular range of frequencies.

Frequency spectrum - the distribution of signal amplitudes as a function of frequency.

Geodetic surveys - global surveys done to establish

control networks (comprised of reference or control

points) as a basis for accurate land mapping.

Geometric Dilution of Precision (GDOP) - see Dilution of Precision.

Handover word - the word in the GPS message that

contains synchronization information for the transfer of tracking from the C/A to P-code.

Ionosphere - the band of charged particles 80 to 120

miles above the earth’s surface.

Ionospheric refraction - the change in the propagation speed of a signal as it passes through the ionosphere.

Kinematic positioning - Kinematic positioning refers to applications in which the position of a nonstationary object (automobile, ship, bicycle) is determined.

L-band - the group of radio frequencies extending

from 390 MHz to 1550 MHz. The GPS carrier

frequencies (1227.6 MHz and 1575.42 MHz) are in

the L-band.

Mask Angle - the minimum acceptable satellite elevation above the horizon to avoid blockage of lineof-sight.

Multipath error-errors caused by the interference of

a signal that has reached the receiver antenna by two

or more different paths. This is usually caused by one

path being bounced or reflected.

Multiplexing channel - a channel of a GPS receiver

that can be sequenced through a number of satellite

signals.

North American Datum of 1927 (NAD 27) - older

and obsolete horizontal datum of North America.

NAD 27 depends upon an early approximation of the

shape of the earth, known as the Clarke Spheroid of

1866, designed to fit only the shape of the conterminous United States, and utilizing a specific Earth

surface coordinate pair as its center of reference.

North American Datum of 1983 (NAD 83) - official

horizontal datum of North America. NAD 83 relies

on the more precise Geodetic Reference System of

1980 (GRS 80), employs a geocentric ellipsoid model,

utilizing the Earth’s center of mass as its center of

reference.

North American Vertical Datum of 1988 (NAVD

88) - effort underway by NGS to readjust the North

American Vertical datum. The NAVD 88 readjustment will remove distortions from the continentwide vertical geodetic (height) reference system.

P-code - the Precise or Protected code. A very long

sequence of pseudo-random binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz,

which repeats about every 267 days. Each l-week

segment of this code is unique to one GPS satellite

and is reset each week.

Point positioning - See Absolute positioning.

Positional Dilution of Precision (PDOP) - measure

of the geometrical strength of the GPS satellite

configuration.

Precise Positioning Service (PPS) - the most accurate dynamic positioning possible with GPS, based

on the dual frequency P-code.

Proportional error - one means of expressing positional accuracy, expressed as the position error divided by the distance to the origin of the coordinate

system used, stated in parts per million (ppm).

Pseudo-lite - a ground-based differential GPS receiver which transmits a signal like that of an actual

GPS satellite, and can be used for ranging.

Multi-channel receiver - a GPS receiver that can

simultaneously track more than one satellite signal.

40
Pseudo-random noise (PRN) code - a signal with

random noise-like properties. It is a very complicated but repeated pattern of 1’s and 0’s.

Pseudo-range - a distance measurement based on the

correlation of a satellite transmitted code and the

local receiver’s reference code, that has not been

corrected for errors in synchronization between the

transmitter’s clock and the receiver’s clock.

Relative positioning - The determination of relative

positions between two or more receivers which are

simultaneously tracking the same GPS signals.

S-code - See C/A-code

Satellite configuration - The state of the satellite

constellation at a specific time, relative to a specific

user or set of users.

Satellite constellation - the arrangement in space of

a set of satellites.

Selective availability (S/A) - intentional degradation

of the performance capabilities of the NAVSTAR

satellite system for civilian users by the U.S. military, accomplished by artificially creating a significant clock error in the satellites.

Slow switching channel - a sequencing GPS receiver

channel that switches too slowly to allow the continuous recovery of the data message.

Space segment - the space-based component of the

GPS system (i.e. the satellites).

Standard positioning service (SPS) - the normal

civilian positioning accuracy obtained by using the

single frequency C/A code.

Static positioning - location determination when the

receiver’s antenna is presumed to be stationary in the

earth. This allows the use of various averaging

techniques that improve accuracy by factors of over

1000.

User segment - the component of the GPS system

that includes the receivers.

Y-code - classified PRN code, similar to the P-code,

though restricted to use by the military.

41

Appendix C

EPA Locational Data Policy

Locational Data Policy Implementation

Guidance (LDPIG). 1991. U.S. EPA Office

of Information Management Resources,

Washington, D.C.

1. Purpose. This policy establishes the principles for

2.

3.

collecting and documenting latitude/longitude coordinates for facilities, sites and monitoring and

observation points regulated or tracked under

Federal environmental programs within the jurisdiction of the Environmental Protection Agency

(EPA). The intent of this policy is to extend

environmental analyses and allow data to be integrated based upon location, thereby promoting

the enhanced use of EPA’s extensive data resources for cross-media environmental analyses

and management decisions. This policy underscores EPA’s commitment to establishing the data

infrastructure necessary to enable data sharing

and secondary data use.

Scope and Applicability. This policy applies to all

EPA organizations and personnel of agents (including contractors and grantees) of EPA who

design, develop, compile, operate or maintain

EPA information collections developed for environmental program support. Certain requirements

of this policy apply to existing as well as new data

collections.

Background.

a. Fulfillment of EPA’s mission to protect and

improve the environment depends upon improvements in crossprogrammatic, multi-media data analyses. A need for available and

reliable location identification information is a

commonality which all regulatory tracking programs share.

b. Standard location identification data will proc.

d.

e.

f.

vide a return yet unrealized on EPA’s sizable

investment in environmental data collection by

improving the utility of these data for a variety

of value-added secondary applications often

unanticipated by the original data collectors.

EPA is committed to implementing its locational

policy in accordance with the requirements

specified by the Federal Interagency Coordinating Committee for Digital Cartography

(FICCDC). The FICCDC has identified the

collection of latitude/longitude as the most

preferred coordinate system for identifying location. Latitude and longitude are coordinate

representations that show locations on the surface of the earth using the earth’s equator and

the prime meridian (Greenwich, England) as

the respective latitude and longitude origins.

The State/EPA Data Management Program is a

successful multi-year initiative linking State

environmental regulatory agencies and EPA in

cooperative action. The Program’s goals include improvements in data quality and data

integration based on location identification.

Readily available, reliable, and consistent location identification data are critical to support

the Agencywide development of environmental risk management strategies, methodologies,

and assessments.

OIRM is committed to working with EPA

Programs, Regions and Laboratories to apply

spatially related tools (e.g., geographic information systems (GIS), remote sensing, automated mapping) and to ensure these tools are

supported by adequate and accurate location

identification data. Effective use of spatial

43
tools depends on the appropriate collection and

use of location identifiers, and on the accompanying data and attributes to be analyzed.

g. OIRM’s commitment to effective use of spatial

data is also reflected in the Agency’s comprehensive GIS Program and OIRM’s coordination of the Agency’s National Mapping Requirement Program (NMRP) to identify and

provide for EPA’s current and future spatial

data requirements.

4. Authorities.

a. 15 CFR, Part 6 Subtitle A, Standardization of

Data Elements and Representations.

b. Geological Survey Circular 878-B, A U.S.

Geological Survey Data Standard, Specifications for Representation of Geographic Point

Locations for Information Interchange.

c. Federal Interagency Coordinating Committee

on Digital Cartography (FICCDC)/U.S. Office

of Management and Budget, Digital Cartographic Data Standards: In Interim Proposed

Standard.

d. EPA Regulations 40 CFR 30.503 and 40 CPR

3 1.45, Quality Assurance Practices under EPA’s

General Grant Regulations.

5. Policy.

a. It is EPA policy that latitude/longitude

(“lat/long”) coordinates be collected and

documented with environmental and related

data. This is in addition to, and not precluding,

other critical location identification data that

may be needed to satisfy individual program or

project needs, such as depth, street address,

elevation or altitude.

b. This policy serves as a framework for collecting and documenting location identification

data. It includes a goal that a 25-meter level of

accuracy be achieved; managers of individual

data collection efforts determine the exact levels of precision and accuracy necessary to support their mission within the context of this

goal. The use of global positioning systems

(GPS) is recommended to obtain lat/longs of

the highest possible accuracy.

c. To implement this policy, program data managers must collect and document the following

information:

(1) Latitude/longitude coordinates in accordance with Federal Interagency Coordinating Committee for Digital Cartography

(FICCDC) recommendations. The coordinates may be present singly or multiple

times, to define a point, line, or area, according to the most appropriate data type

for the entity being represented.

The format for representing this information is:

t/-DD MM SS.SSSS (latitude)

t/-DDD MM SS.SSSS (longitude)

where:

• Latitude is always presented before longitude

• DD represents degrees of latitude; a

two-digit decimal number ranging from

00 through 90

• DDD represents degrees of longitude; a

three-digit decimal number ranging from

000 through 180

• MM represents minutes of latitude or

longitude; a two-digit decimal number

ranging from 00 through 60

• SS.SSSS represents seconds of latitude

or longitude, with a format allowing

possible precision to the ten-thousandths

of seconds

• + specifies latitudes north of the equator

and longitudes east of the prime meridian

• - specifies latitudes south of the equator

and longitudes west of the prime

meridian

(2) Specific method used to determine the

lat/long coordinates (e.g., remote sensing

techniques, map interpolation, cadastral

survey)

44
(3) Textual description of the entity to which

the latitude/longitude coordinates refer

(e.g., north-east corner of site, entrance to

facility, point of discharge, drainage ditch)

(4) Estimate of accuracy in terms of the most

precise units of measurement used (e.g., if

the coordinates are given to tenths-ofseconds precision, the accuracy estimate

should be expressed in terms of the range

of tenths-of seconds within which the true

value should fall, such as “+/- 0.5 seconds”)

d. Recommended labelling of the above information is as follows:

• “Latitude”

• “Longitude”

• “Method”

• “Description”

• “Accuracy.”

e. This policy does not preclude or rescind more

stringent regional or program-specific policy

and guidance. Such guidance may require, for

example, additional elevation measurements

to fully characterize the location of environmental observations.

f. Formats, standards, coding conventions or other

specifications for the method, description and

accuracy information are forthcoming.

6. Responsibilities.

a. The Office of Information Resources Management (OIRM) shall:

(2) Provide guidance and technical assistance

where feasible and appropriate in implementing and improving the requirements

of this policy

b. Assistant Administrators, Associate Administrators, Regional Administrators, Laboratory

Directors and the General Counsel shall establish procedures within their respective organizations to ensure that information collection

and reporting systems under their direction are

in compliance with this policy.

While the value of obtaining locational coordinates will vary according to individual program requirements, the method, description

and accuracy of the coordinates must always be

documented. Such documentation will permit

other users to evaluate whether those coordinates can support secondary uses, thus addressing EPA data sharing and integration objectives.

7. Waivers. Requests for waivers from specified

provisions of the policy may be submitted for

review to the Director of the Office of Information

Resources Management. Waiver requests must

be based clearly on data quality objectives and

must be signed by the relevant Senior IRM Official prior to submission to the Director, OIRM.

8. Procedures and Guidelines. The Findings and

Recommendations of the Locational Accuracy

Task Force supplement this policy. More detailed

procedures and guidelines for implementing the

policy are issued under separate cover as the

Locational Data Policy Implementation

Guidelines.

(1) Be responsible for implementing and supporting this policy

45

Appendix D

EPA Case Studies

I. Puget Sound Near-Shore Habitat

Inventory

EMSL-LV, EPA Region 10, the Washington

Department of Natural Resources, and the Puget

Sound Authority have been participating in an

interagency project the last few years to inventory

near shore habitats of Puget Sound. Aircraft

multispectral scanner (MSS) imagery and field

verification data of representative habitat types were

used to conduct the inventory. Both of phases of the

project, data collection and verification were able to

effectively use GPS to provide valuable information.

During MSS data acquisition, GPS provided accurate

positional information at all times, so that image data

could be subsequently rectified to earth coordinates.

Collection of ground verification data is an

ideal use of GPS technology. Near-shore habitats of

Puget Sound do not have natural or cultural features

which can be used as ground control to gee-reference

MSS imagery. Field data assists the image analyst

to identify surface features visible on the MSS imagery. Field experts in marine and estuarine near-shore

habitats visit representative habitat types and characterize each site by substrate type, vegetation, and

orientation to Puget Sound. This information is

recorded on field sheets. GPS data is collected at

each field site to ensure that image analysts can

accurately correlate field data to image data.

The field data is used to assess the accuracy of

thematic maps produced from analysis of MSS imagery. Therefore, exact location of data collected in

the field is necessary. Use of GPS technology during

field reconnaissance ensures gee-positional accuracy of reference data for image analysis and verification data for assessing absolute accuracy of the

MSS classifications.

Logistics planning for field data collection,

using GPS, in near-shore habitats is complicated by

the limited GPS satellite visibility during periods of

low tide. Logistics planning for field data collection

involves coordinating low tide with GPS satellite

visibility windows. All field activities not directly

involved with data collection such as travel between

sites and data reduction were arranged around low

tide and satellite visibility.

Procedures to characterize sites within nearshore habitats of Puget Sound utilizing GPS were

tested prior to the MSS data acquisition mission. A

Trimble Pathfinder portable position recording systems instrument was operated in a remote mode. A

Trimble 4000st instrument was operated in the reference or base station mode. Spatial data as well as site

characterization data were collected at representative near-shore habitats in the Puget Sound area.

Data forms, pre-survey and post-survey forms proved

invaluable. The forms assist field crews to properly

operate and maintain GPS equipment and data files.

The experience of this project indicates field

work is best organized as a three step process:

• Mission planning

• Data collection

• Post Processing data

Mission Planning

The first step in mission planning is to define

the job requirements, project area, and number of

points of data collection. The second step in mission

planning is to develop a work plan. The plan must fit

job requirements and be scheduled efficiently. Since

the satellite constellation is presently incomplete, the

geometry of the constellation is also important. Most

suitable geometry has a single satellite directly overhead and the remaining three equally distributed in

the hemisphere 10 degrees above the horizon.

47
Mission planning involves scheduling travel

between data collection points. Point locations are

selected where GPS receivers have clear view of the

satellites. Schedules for sites with obstructions (i.e.

cliffs, buildings) must be arranged when satellite

visibility clears obstructions. Logistical optimization is achieved if travel occurs during periods without satellite visibility.

Before travel is initiated, the location of horizontal and vertical control monuments and benchmarks must be determined. These will be used in the

post-processing phase to provide differential correction. The last factor in mission planning requires the

review of data collection points in light of permits

and permission necessary to gain access to the site.

Data Collection

Lock on at least 4 satellites must be established

and lock maintained on the 4 satellites between all

measurement epochs. A recommended rule of thumb

is to record data at each collection point for approximately 3 minutes with a recording rate of 1 fix per

second.

Processing Data

Differential correction requires that a base station log data concurrent with logging at remote sites.

The correction algorithms require accurate knowledge of the position of the base. Raw measurements

are needed for “measurement space” differential

corrections. The rule of thumb used recommends

logging intervals of 5 seconds for fixes, and 10

seconds for raw measurements.

Low standard deviations should not be taken as

a measure of accuracy, but rather of repeatability.

S/A as well as other systematic errors and biases may

well introduce significant errors which can only be

removed by methods such as differential correction.

In many instances, standard deviations of corrected data sets are higher than those of the original

data. This is explained by the result of random noise

contributed independently by two receivers. However, the noise components can be expected to have

zero mean, so simple averaging of the corrected data

should effectively compensate.

Conclusions from this project include:

When 2-D fixes must be acquired, care must be

taken to supply the receiver with as good an estimate

of elevation as possible. Otherwise, very

large positional errors will result. Wherever possible, 3-D fixes should be acquired, even if this

requires re-visiting a site or waiting until satellite

availability y is favorable.

Reviewing data as soon after acquisition as

possible is highly desirable. All data should be

critically examined to search for clearly erroneous or

suspect points. The process of discarding outlier

points appears to be a good strategy and is

recommended.

Il. Old Southington Landfill Superfund Site

Southington, Connecticut

In 1988, EPA and the U.S. Geological Survey

conducted a pilot project designed to demonstrate

the use of GIS technology in the Comprehensive

Environmental Response, Compensation, and Liability Act (CERCLA), often referred to as Superfund,

Remedial Investigation (RI) process. The site chosen for this study was Old Southington Landfill, in

Southington, Comecticut. The database developed

for this project contained data layers derived from

both large-and small-scale sources, though there

were typically little or no large-scale, site-specific

data available in digital form that could be directly

input into an ARC/INFO GIS data base. Consequently, the existence of data of different source

scales and resulting variable resolution presented

obvious spatial relationship problems when the data

layers were combined.

This problem was solved by way of analytical

photogrammetry and digital cartography. These

technologies permitted the rectification and integration of aerial photography and digital data. Current

and historical aerial photography were obtained from

various sources and used to make large-scale digital

maps of roads, cultural features, hydrography, hypsography, historical site land-fill activity and other

thematic overlays. This data was combined with

property parcel maps, monitoring well locations and

sampling data to create a number of application

scenarios designed to show how GIS and remote

sensing technologies could be utilized to meet the

48
information needs of the CERCLA remedial investigation process.

During the development of this project, the

issue of spatial accuracy came to the forefront when

the large- and small-scale data layers were overlaid

at a common scale and significant spatial variations

were identified. In order to assess these variations, a

map accuracy test had to be conducted. The first step

in the accuracy testing scenario was to establish

engineering quality control around the site area. This

was done via a GPS survey in December, 1988.

Using well-defined aerial photo points and three

Magnavox WM101 GPS receivers, X, Y, and Z

coordinates were established for nine ground control

points over 2 days of static observation. Postprocessing results indicated accuracy of 2 centimeters in closure. These control points were then used

to photogrammetrically recompile the base maps

and conduct accuracy tests on all digital and analog

maps used in the project.

Ill. San Gabriel Basin Superfund Project,

California

In 1986, EPA conducted its first GIS demonstration project, using the San Gabriel Basin in the

eastern portion of Los Angeles, California. Basin

ground water had been determined to be heavily

contaminated with organic solvents, and the entire

basin had been placed on the National Priorities List

as a Superfund site. Furthermore, numerous Resource Conservation and Recovery Act (RCRA)

sites were identified in the basin, further complicating an already complex issue of contaminant source

identification. The overall objective of the San

Gabriel Basin Demonstration GIS project was to

illustrate some of the GIS capabilities useful in

support of implementing legislation and regulations

enforced by the EPA’s Office of Emergency and

Remedial Response, Office of Solid Waste, and

Office of Groundwater Protection.

The San Gabriel Basin Demonstration GIS

Project has since evolved into an operational GIS

mode, in support of EPA Region 9’s CERCLA

remedial investigation process. An extensive network

of monitoring and water supply wells, in conjunction

with sophisticated ground water flow modelling, is

being used to delineate the extent of contaminant

plumes and project their migration. The ultimate

goal of this effort will be to assess possible sources

and identify parties responsible for the contamination

of ground water, forming the basis for eventual

litigation for cost recovery. The EPA is committed

to quantifying the spatial accuracy of the GIS data

base, particularly with respect to well locations and

boundaries of potential source facilities.

The San Gabriel Basin GPS survey was conducted in late January and early February, 1989,

using three portable Trimble Navigation Pathfinder

GPS receiving units. The issues being addressed in

the San Gabriel survey were similar to those in the

Old Southington project, specifically the maintenance of spatial accuracy and quality control when

source data of varying scales and resolutions are

combined for display and analysis in a GIS data base.

Additionally, a third major objective was defined:

the test and evaluation of a Trimble Navigation

Pathfinder GPS receiver, its portability, and its ability to output spatial data directly into an ARC/INFO

GIS format.

The first step in the spatial accuracy testing of

the data base was to evaluate GPS as a means of

assessing the quality of the digital transportation

network coverage already existing in the GIS data

base. Streets and street intersections frequently

define facility boundaries, while proximity to them

commonly was the original basis for plotting well

locations. Assuming its spatial accuracy could be

verified, the transportation data layer or coverage

was generally regarded as a likely template or baseline

coverage for spatial quality assessment of the GIS

data base.

The GPS survey of the street network was

accomplished by simply driving the streets of the San

Gabriel Basin, in relative semikinematic mode, with

one Pathfinder receiver and data logger inside and

the antenna mounted atop the survey vehicle. Data

was collected at 1-second intervals, effectively digitizing the route followed. A second Pathfinder

(reference receiver) was placed atop an established

benchmark in a secure location, programmed to

collect data at 3-second intervals. The data collected

was downloaded in the field from the data recorder of

the roving unit to a portable personal computer,

converted to a binary format, and differentially corrected using the data file from the reference receiver.

In order to test and evaluate the convertibility

of GPS data to an ARC/INFO format, normal

postprocessing of the GPS data stream was modified

slightly. The downloaded binary file was converted

by means of a relatively simple program to a userdefined text file format containing X, Y, Z

coordinate values for each point of data collection.

The format of the text file allowed direct input of the

coordinate values, in units of decimal degrees, to

49
ARC/INFO in an ARC: GENERATE format. The

data files were then GENERATED into an

ARC/INFO coverage where they could be digitally

compared to the transportation coverage in the San

Gabriel data base.

Although specific methodology has yet to be

developed for quantitative comparison of digital line

features, the results of the survey have provided a

means of visual qualitative verification of the accuracy of the ARC/INFO transportation network coverage. Once these methodologies are established and

rendered operational, the transportation layer can

then be utilized as a baseline coverage in further

spatial QA/QC efforts.

The other primary objective of the San Gabriel

GPS survey involved selection of a sampling of

wellheads, based on their importance to the ground

water modeling effort. These wellheads were GPS

surveyed and the results used to rectify the well

locations as represented in the GIS data base. Collecting data in static differential mode, field surveyors collected and recorded data continuously for

5 minutes at each sample wellhead. Additionally,

the GPS receiver provided the surveyors with the

ability to record descriptive information related to

the wells and the data collection effort. Although

time constraints limited the survey sample size, the

technique was proven effective in both positional

data entry and accuracy assessment. Spatial accuracy an expanded GPS survey would permit rectification of the entire well locations coverage, and a

significantly increase the level of confidence with

which the EPA and its contractors could conduct

ground water modelling efforts.

Conversion of the downloaded well location

data files to ARC/INFO followed the same process

described for the conversion of the GPS-generated

transportation network, with a couple of slight variations. The only difference of consequence is that

records in the data files contain the mean position of

each wellhead, as averaged over 5 minutes of data

collection.

50
Appendix E

PLOTSSF and CLEANSFF Processing Software

Two software utility programs have been developed by EMSL-LV for visualizing and cleaning

GPS data files. These utilities are specific to Trimble

Navigation’s SSF format. The two utilities, called

PLOTSSF and CLEANSFF, operate in the PC DOS

environment and require a 286 CPU with coprocessor

and VGA graphics as a minimum.

CLEANSSF is an outlier rejector that iteratively calculates means and standard deviations of

positions and rejects any that lie beyond a fixed

number of standard deviations. The user is presented

with a display that boxes the data and allows the

viewing of outliers. Based on the needs of the user,

several iterations of the process may be used to

eliminate all outliers. This program has proven

useful on data sets after differential correction and to

get the tightest fit possible for GPS point data.

PLOTSSF is a visualization tool that displays

two concurrent plots, one X-Y and one elevation.

The two plots evolve, second by second, as the file is

processed. The user has the option to capture portions of the GPS data stream into separate files to

view tabular displays of position, elevation, and

time. In addition, ARC/INFO UNGENERATE files

may be used as background during the session to

provide a spatial reference for the GPS data. The

PLOTSSF utility is especially useful for reviewing a

GPS field session, analyzing distinct segments of a

transect, performing terrain analysis, or analyzing

travel routing.

The utility software is available to all Agency

GPS users. It includes some demonstration files that

allow the user to get a feel of how the two utilities

work. For further information, contact:

Mason Hewitt

GIS Program Manager

EMSL-LV

P.O. BOX 93478

Las Vegas, NV 89193

Phone: (702) 798-2377 FTS 545-2377

51

Appendix F

Field Charts and Forms

The following are a series of sample checklists

and other forms used in various GPS prototype

applications. These include the following:

• GPS Survey Steps

• Presurvey Checklist

• Field Equipment

• Last Minute Checks

• In the Field Checks

• Postsurvey Checklist

●

●

●

●

●

●

●

●

●

Sample Letter of Introduction

GPS Station Recovery Form

Trimble Pathfinder Pre-survey Checklist

Field Notes - GPS Data Acquisition

Planning and Data Sheet

Trimble Pathfinder Instrument Initialization

Field Notes - GPS Data Acquisition

GPS Field Sheet

GPS Data Reduction Lab Sheet

53
GPS Survey Steps

Define Objectives

Establish Study Area

Determine Observation Window

Schedule Operations

Establish Control

Select Survey Locations

Arrange Equipment Logistics

Perform Reconnaissance

Establish Base Stations

Conduct Survey

Transfer Data

Process Data
Presurvey Checklist

Obtain List of Facilities

Obtain Current Almanac

Call Coast Guard to Verify Satellite Availability

Obtain Control Points from NGS or Local Source

Obtain 7.5 min Topographic Maps

Obtain Local Street Maps

Prepare Letter of Introduction

Collect and Pack Field Equipment

55
Field Equipment

GPS Equipment

Laptop or Other Field Computer

7.5 minute maps

aerial photo if available

Camera

Film

Compass

Tape Measure

Binoculars

Field Forms

Clip Board

Calculator

GPS Hardware/Software Manuals

Mini Tape Recorder

Hard Copy of Almanac

Rain Gear

Two-way radio communication (i.e., CB, cellular phone, etc.)

56
Last Minute Checks

Charge Batteries

Verify Almanac

Target Travel Route

In The Field Checks

Find Base Stations

Initialize Equipment

Begin Collecting Data

57
Postsurvey Checklist

Name:

Date:

Unit ID& Number:

1 System Battery Recharged?

2 All Data Files Downloaded?

3 All Data Files Erased?

4 Polycorder Battery Recharged?

58
Sample Letter of Introduction

Note: All letters requesting access to property should be on official stationary, include both a day and evening

phone number, and any other appropriate information. The example below has been used by an Agency

contractor.

July 4, 1991

To Whom it May Concern,

The below named individuals are employees of the Bionetics Corporation and under contract to the

U.S. Environmental Protection Agency (contract Number 68-03-3532). These individuals will be collecting field data in the area of Chattanooga, Tennessee, during the month of November, 1990.

Mary Brown

Bill Johnson

John Smith

Their efforts are in support of official U.S. Environmental Protection Agency research. Please

extend to them all possible courtesy and consideration.

Additional information may be obtained by calling 703-349-8970.

Sincerely,

E. Terrence Slonecker

Environmental Scientist

U.S. Environmental Protection Agency

59
GPS Station Recovery Form

Station Name

Project

ID

Date

Observer

Location

To find the station

Special requirements for GPS use (offsets, obstructions)

Access needs (owner, where to get keys to locked gates, etc.)

60
Trimble Pathfinder Pre-survey Checklist

Name:

Date:

Unit ID& Number:

Use of Unit: Base or Remote

Location of GPS Use: State

Site Name:

Directions: Check

respective option.

Menu Option

the blank to indicate the item was checked or fill the blank with the setting used for the

Enter a check or the setting used

5. Data Logging

1. Set Intervals . . . . . . . . . . . . . . . . . . . . . . . . .Position:

Raw:

2. Memory Check . . . . . . . . . . . . . . . . . . . . . . .Bytes:

Files:

Should be approx. 390,000 bytes and 49 files for a cleared memory

6. GPS Status. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Good Stats

2. Receiver Stats . . . . . . . . . . . . . . . . . . . . . . . . .Sys Batt: >10.0

CDU/DR Batt: OK

Ant volts: <5.0

Connect antenna, polycorder, and system battery to receiver to obtain accurate stats.

8.

7.

8.

System Options

0. Init. Altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Pos. Fix Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Altitude Ref. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Units of Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CLK/Time Zone

1. Set Time Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

GPS Parameters Recommended Settings

Dynamics Code:

El Angle Mask 10

Sig Lvl Mask: 6

PDOP Mask: 12

PDOP Switch: 10

*Note: GPS Parameters should be set the same for both the BASE and REMOTE units.

61
Field Notes - GPS Data Acquisition

Date: Tree/Begin:

Site Name:

Measurement Location

Sketch Location

GPS Survey Crew Members:

GPS Operator:

File Information:

Remote File Name:

Base File Name:

File Downloaded to PC

Differential Corrections:

Measurement Space File Name:

Solution Space File Name:

Time/End:

Field Data Recorder:

Position Fix Mode: 2D / 3D

Total # Fix/Corr. #Fix

/

/

62
Planning and Data Sheet

Adapted from “Practical Surveying with GPS,” Geotronics AB, 1989.

63
Trimble Pathfinder Instrument lnitialization

The following is a guide to set up a Trimble Pathfinder

Menu Option

6 GPS Status

*0 - Satellite selection

lists satellites being used for the GPS solution

GPS tracking status (Doing Posn fixes; No GPS Time yet; Only 2 Usable SV)

Solution mode employed i.e. 3-D displayed

PDOP, VDOP, and HDOP values displayed

*1 - Signal Levels (Useful if > 5)

2 - Receiver Status Good Stats

Sys Batt >10.0 <10.0 = must recharge polycorder

CDU/DR Batt OK ext = connected polycorder to external system

Ant volts <5.0 (check ant if not 3.5-4.2)

5.0 if not connected

3 - Local and GMT time - polycorder time (not satellite time)

* 7 SV Status

PolyTANS uses satellite vehicle (SV) orbit almanac data stored in the TANS receiver to compute the current

satellite visibility.

Scroll (up/down arrows) through satellite list to see current:

Satellite status

azimuth and elevation angles in degrees

User Range Accuracy (URA) in meters

Enabled field

Left/right arrows toggles between SV and enable fields. Once the Enable field is selected, the up/down arrows

change Enabled to Disabled. Do not disable a satellite you intend to use. This “is useful to disable a satellite

that has Selective Availability activated. An indication that SA has been activated is if the User Range

Accuracy (URA) field is >64. (URA is expressed in meters and cannot be changed in menu option 8, submenu

4).

Disabled satellites are not listed.

8 System Options

The user sets several operational parameters for both the TANS receiver and the POLYCORDER

0 Init. Altitude (very imp if taking 2D fixes)

1 Init. Position (initial frees are calculated quicker if a significant

distance between old fix and new fix occurs i.e.

between Nevada and Washington)

2 Posn Fix Mode use 0 Auto 2-D/3-D

64
3

4

*5

*6

*7

8

9

Altitude Ref

Units of Measure

North Reference

Magnetic Declination

Clock/Time Zone

See users manual

This is a very Important Setting

GPS Parameters

1 Dynamic Code

Then these parameters appear

El Angle Mask

Sig Lvl Mask

PDOP Mask

PDOP Switch

Beeper on/off

use 0 WGS-84 / NAD -83

use 0 KM: KPH: m

use 0 True North

use Manual:

Set GMT first w/ -1- PDT= -7

use 1 = land

use 10

use 6

use 12

use 10

GPS parameters should be set the same for both base and remote units

* Optional parameters to check. Check these settings if problems arise.

65
Field Notes - GPS Data Acquisition

Date: Time/Begin: Time/End:

Site Name:

Measurement Location

Sketch Location

Gps Survey Crew Members:

Gps Operator:

File Information:

Remote File Name:

base File Name:

Field Data Recorder:

Position Fix Mode: 2D 3D

File Downloaded to PC

Differential Corrections:

Total # Fix/Corr. #fix

Measurement Space File Name: /

Solution Space File Name: /

66
67
68
TECHNICAL REPORT DATA

(Please read instructions on the reverse before completing)

1. REPORT NO. 2. 3. RECIPIENT’S ACCESSION NO.

EPA 600/R-92/036 PB92-169358 4. TITLE AND SUBTITLE 5. REPORT DATE

GIS TECHNICAL MEMORANDUM 3: Global. Positioning February 1992

Systems Technology and Its Application in Environmental 6. PERFORMING ORGANIZATION CODE

Programs

7. A U T H O R ( S ) 8. PERFORMING ORGANIZATION REPORT NO.

Robert Puterski

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT NO.

Lockheed Engineering & Sciences Company

1050 E. Flamingo Road, Suite 126 11. CONTRACT/GRANT NO.

Las Vegas, NV 89119

12. SPONSORING AGENCY NAME AND ADDRESS

—

13. TYPE OF REPORT AND PERIOD COVERED

U.S. Environmental Protection Agency

Environmental Monitoring Systems Laboratory-Las Vegas 14. SPONSORING AGENCY CODE

P.O. BOX 93478

Las Vegas, NV 89193-3478 EPA 600/07

15. SUPPLEMENTARY NOTES

16. ABSTRACT Global Positioning Systems (GPS) are a location determination technology that offers significant opportunties for obtaining highly accurate locational data at low

cost . In order for the technology to perform up to its capabilities in Agency applica- tions, Environmental Protection Agency (EPA) staff will need to develop a greater understandingEPA has been collecting expertise in the use of this technology over the last of the technology itself, coordinate systems, surveying, and basic

geodesy.

3 years via pilot use of GPS systems to enhance locational control. in Agency projects.

In order to operationalize the use of this technology within EPA, there also exists a

need to develop concise standard operational procedures and methodologies for its use.

This document is a beginning toward fulfillment of these needs. It is intended to be

an introductory reference that describes the technology and how it could be employed in

EPA work. It provides an overview of survey methods from initial planning to data reduction and postprocessing. Ancillary but important issues such as reference datums and use with geographic information systems are covered in order to provide the reader

additional. context regarding the use of this spatial information in a project environment. Case studies performed by the Environmental Monitoring Systems Laboratory, Las

Vegas, are also included in this document as auxiliary background that may provide helpful techniques.

17. KEY WORDS AND DOCUMENT ANALYSIS

a. DESCRIPTORS b. IDENTIFIERS/OPEN ENDED TERMS c. COSATI Field/Group

18. DISTRIBUTION STATEMENT 19. SECURITY CLASS (This Report) 21. NO. OF PAGES

70 20. SECURITY CLASS (This page) 22. PRICE

EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE

“U.S. Government Printing Office 1992 — 648-003/41836

69