NOS RTK TEAM FINAL REPORT

August 31, 2000

Office of Coast Survey

Center for Operational Oceanographic Products and Services

Office of National Geodetic Survey

Cary Wong (N/OPS1)

Jack L. Riley (N/CS11)

Doug Martin (N/OPS1)

Lloyd C. Huff (N/CS1)

Lucy Hall (N/NGS22)

Stephen Gill (N/OPS)

Rick Foote (N/NGS22)

Edward Carlson (N/NGS2)
CONTENTS 

PURPOSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

The RTK Team . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

BACKGROUND ..............................................................1

Differential GPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Kinematic GPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Real-Time and Post-Processed Kinematic GPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

On-The-Fly Kinematic GPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

NOS HYDROGRAPHIC SURVEY SPECIFICATIONS ...............................3

Basic Minimum Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Horizontal and Vertical Accuracy Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Maximum Allowable Horizontal Error for a Sounding .....................4

Maximum Allowable Vertical Error for a Sounding .......................4

Kinematic GPS Horizontal and Vertical Accuracy Budget . . . . . . . . . . . . . . . . . . . . . . . . 5

Relevant Dynamic Parameters in Vertical Measurement . . . . . . . . . . . . . . . . . . . 5

Hydrographic Accuracy Requirements for Three-Dimensional GPS . . . . . . . . . . 6

Chart Datum Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

KINEMATIC GPS IN HYDROGRAPHY ..........................................7

Operational Requirements (RTK, OTF RTK, or Post-Processed techniques?) . . . . . . . . . 7

Survey Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Maximum Existing Water Level and GPS Control . . . . . . . . . . . . . . . . . . . . . . . . 8

Mid-Level Water Level Datum and GPS Control . . . . . . . . . . . . . . . . . . . . . . . . . 9

Mid-Level GPS and Inadequate Water Level Datum Control . . . . . . . . . . . . . . . . 9

Inadequate GPS and Mid-Level Water Level Datum Control . . . . . . . . . . . . . . . . 9

Inadequate GPS and Inadequate Water Level Datum Control . . . . . . . . . . . . . . . 10

Issues: Benefits & Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Inspiring Field Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

GPS Expedition to Tangier Island, Chesapeake Bay . . . . . . . . . . . . . . . . . . . . . . 11

RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

ATTACHMENT 1 – Memo Creating RTK Team . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

ATTACHMENT 2 – RTK Team Charter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

APPENDIX A – Glossary of GPS Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

APPENDIX B – Glossary of Tide Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1

PURPOSE

The RTK Team

Upon the recommendation of the National Ocean Service (NOS) Survey Team, a Real-Time

Kinematic (RTK) Requirements Team was created (see ATTACHMENT 1). The RTK

Requirements Team (“the RTK Team”) was task to document requirements for the application of

RTK global positioning system (GPS) technology in support of hydrography, tides, and related

NOS navigation missions and programs1. The NOS Survey team helped develop a charter for the

RTK Team (see ATTACHMENT 2). The charter specifies that the purpose of the RTK Team is

to “investigate and report on applications of RTK GPS to support [chart-quality hydrographic

surveying]”2. Here, chart-quality hydrographic surveying (“hydrography”) means meeting, or

exceeding, applicable NOS hydrographic survey specifications3 and International Hydrographic

Organization standards4

. This is the final report of the RTK Team investigations into the use of

kinematic GPS methods for NOS hydrography.

BACKGROUND

To avoid possible confusion that may result from colloquial terminology, a short discussion of

GPS concepts is presented. A glossary of general GPS terms is found in APPENDIX A. 

Subsequent terms appearing in quotes or italics are contained in the glossary in one form or

another. Some general knowledge of GPS theory and technology is assumed.

Differential GPS

The term “differential GPS” (DGPS) encompasses a family of GPS relative positioning

techniques. DGPS uses two or more GPS receiver-antenna units to position an unknown point or

set of points relative to a known point or set of points. DGPS improves upon the positioning

accuracy otherwise attainable in absolute point positioning methods, which use a single GPS

receiver-antenna unit.

For the purpose of our discussion, DGPS can be divided into two primary surveying categories: 

meter level and centimeter level. Meter-level DGPS surveys utilize GPS code phase

measurements and are primarily based on the C/A code modulated on frequency L1; for example,

a L1-only GPS receiver for the rover and (preferably) a L1/L2 base station. Centimeter-level

DGPS surveys utilize GPS carrier phase measurements and are based on the carrier signal

present on both L1 and L2. There are many variations employed in both meter- and centimeterlevel DGPS (e.g., meter-level DGPS augmented with carrier smoothing and carrier-aided

tracking, centimeter-level DGPS utilizing L1 only, etc.)5

.

Meter-level DGPS has been used as the primary source of horizontal control for all NOS

hydrographic surveying operations in support of nautical charting since 19926

. The United States

Coast Guard (USCG) meter-level DGPS reference station network has been in operation for

almost as long7

. Hydrographers and the general marine navigational community use the term
2

“DGPS” to refer to meter-level DGPS. Hereafter in our discussion, “DGPS” will refer to meterlevel differential positioning based primarily on code phase measurements.

There are several forms of centimeter-level DGPS surveying and specific terms have been

adopted by the GPS community-at-large to differentiate between those techniques.

Kinematic GPS

The term “kinematic GPS” refers to the form of centimeter-level differential positioning which

(primarily) uses carrier phase observables. Intuitively, “kinematic” GPS implies that differential

corrections are formulated in conjunction with a mobile GPS receiver (i.e., a rover in conjunction

with at least one static base station). Although many “static GPS” techniques also exist which

use both code- and carrier-phases and are used in hydrography, we will focus discussion on

mobile, kinematic GPS-based methods.

Real-Time and Post-Processed Kinematic GPS

Operationally, the kinematic GPS solution can be determined either in real-time or after postprocessing. “Real time” in the term “real-time kinematic GPS” (RTK GPS) simply refers to the

fact that the carrier phase corrections are transmitted in real time via some (wireless) data link;

e.g., via very-high frequency (VHF), HF, or ultra-HF (UHF) radio transmission–c.f., USCG realtime DGPS via HF radiobeacon. All post-processing techniques require the kinematic GPS

carrier phase “observables” to be continuously recorded. Long, uninterrupted observation

sessions could pose a data management problem. Post-processed kinematic GPS methods might

not exist were it not for the complexity associated with real-time data links and the resolution of

the carrier phase ambiguity in real time. Use of the GPS carrier phase permits resolution at the

millimeter level, provided the ambiguity of the integer-number of carrier phase wavelengths can

be determined and maintained. Incorrect ambiguity resolution translates to a degradation of

positioning accuracy and, depending on user accuracy requirements, the percentage of system

availability.

On-The-Fly Kinematic GPS

Kinematic GPS technology is not new to NOS. Starting in 1983, NOS has developed centimeterlevel kinematic GPS techniques to support land, air, and marine applications8

. At first these

techniques relied on some form of static initialization procedure to resolve carrier phase

ambiguity. During static initialization, the kinematic GPS receivers are required to be stationary

for some period of time after loss of satellite signal lock (e.g., after receiver power-up or recovery

from antenna signal interruption). In 1989 NOS extended kinematic GPS technology to

circumvent the static initialization requirement. Subsequently, carrier phase ambiguity could be

resolved in the presence of relative motion between two GPS receiver-antenna units “on-the-fly”

(OTF)9

. Not to be confused with the operational term “real time”, OTF refers to the

mathematical technique that resolves GPS carrier phase integer ambiguities without requiring a

GPS receiver to be stationary at any time. Indeed, OTF can be applied in real time during RTK

GPS, or kinematic GPS data can be post-processed via OTF going either, or both, forward and

reverse on the time line8

. Because no GPS receiver can ever be completely immune to loss of
3

satellite signal lock the application of OTF is vital to the practical use of kinematic GPS in

hydrography and any other mobile application.

The utility of OTF is currently being applied in kinematic GPS for hydrographic calibration

activities within NOS Coast Survey (CS). CS has been employing post-processed kinematic

GPS methods using OTF since 1993 in support of hydrography8,10. Consequently, the term

“OTF” has become so commonplace in this regard that NOS hydrographers typically use it to

refer to the specific concept of post-processed OTF kinematic GPS. NOS hydrographers usually

speak of “OTF” in referring to all kinematic GPS endeavors not computed in real time.

Summary

In summary, the following GPS terminology is used in the NOS hydrographic community:

DGPS – Meter-level differential positioning based on code phase measurements

RTK GPS – Or simply “RTK”; centimeter-level (i.e., less-than-or-equal-to one

decimeter) differential positioning based on GPS carrier phase

measurements transmitted in real time; static initialization may be

necessary, but “OTF RTK” usually implied

OTF RTK – RTK GPS technique where static initialization is not necessary

Post-Processed OTF – Sometimes simply “OTF”; centimeter-level differential positioning

based on GPS carrier phase measurements computed off-line (i.e., not in

real time); and, static initialization is not necessary. The ability to postprocess forward and reverse on the time line provides a means of data

quality assurance.

NOS HYDROGRAPHIC SURVEY SPECIFICATIONS

Concepts related to water level observations are discussed in this section. A glossary of some

important tide terms is found in APPENDIX B. Subsequent water level terms appearing in

quotes or italics are tabulated in the glossary. Some general knowledge in the theory of tides is

assumed.

Basic Minimum Criteria

The Hydrographic Surveys Division of the Office of Coast Survey document NOS Hydrographic

Surveys Specifications and Deliverables3 contains the minimum criteria for shallow-water

hydrographic surveys. These requirements are used by NOAA field units and by organizations

under government contract to deliver data for application to the nautical charts produced by

NOAA. The specifications detailed are based, in part, on the International Hydrographic

Organization Standards for Hydrographic Surveys4. Additional details for specific project areas
4

Sounding Measurement Error Budgets

(at 95% confidence level)

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80 90 100

Depth [meters]

Error [meters]

Required Horizontal Accuracy Required Vertical Accuracy

Figure 4

are provided in the “Letter Instructions” for NOAA field units and in a “Statement of Work” for

contractors.

Horizontal and Vertical Accuracy Budget

NOAA hydrography references all horizontal positions to the North American Datum of 1983

(NAD83) which, by definition, uses the Geodetic Reference System 1980 (GRS80) ellipsoid. 

Bathymetry data and least depths on all wrecks, rocks, and obstructions (collectively,

“soundings” hereafter) are referenced to a chart datum–usually implemented to mean lower low

water (MLLW). All heights, such as bridges and overhead cables, are referenced to mean high

water (MHW).

MAXIMUM ALLOWABLE HORIZONTAL ERROR FOR A SOUNDING

Total error in the measured position of a sounding, at the 95 percent confidence level, must not

exceed:

5 meters + 5 percent of the depth

MAXIMUM ALLOWABLE VERTICAL ERROR FOR A SOUNDING

Total error in the measured depth of a sounding, at the 95 percent confidence level, must not

exceed:
5

The constant a = 0.5 meters and represents the sum of all remaining non depth-dependent

measurement errors. Parameter d is the measured depth of a sounding in meters and the term

(b @ d) represents the sum of all remaining depth-dependent measurement errors. A scale factor

of b = 0.013 is used (i.e., 1.3 percent).

Note that the sounding measurement error budgets are independent of any scale-of-survey that

might be in use during the hydrographic project. A summary of the sounding error budgets is

shown in Figure 1.

The summation of all the error sources associated with a hydrographic sounding measurement

must fit into this three-dimensional error budget. The maximum allowable error in the horizontal

position of a sounding must be less than or equal to the combined positional errors arising from

inaccuracies in vessel heading, roll, and pitch, as well as the contribution from the GPS

positioning subsystem. The maximum allowable error in measured depth must be less than or

equal to the combined depth errors arising from inaccuracies in slant-range timing, vessel heave,

roll, pitch, and draft, water column sound velocity, and water level (tide), as well as any residual

biases in the equipment. Only a portion of the three-dimensional error budget total can be

allotted to kinematic GPS.

Kinematic GPS Horizontal and Vertical Accuracy Budget

The vertical accuracy requirement is nearly an order of magnitude smaller than the horizontal

accuracy requirement (e.g., 0.5 meters versus 5 meters). Unfortunately, height measurement

using GPS is inherently less accurate than the (corresponding) determination of horizontal

position. A detailed analysis of the relevant dynamic parameters which affect the measurement

of sounding depth is needed. Although such an analysis is beyond the scope of this document, a

first order approximation to the GPS component of the vertical error budget total can be

estimated. Some relevant parameters, along with their “practical” (c.f., NOS Hydrographic

Surveys Specifications and Deliverables3, Section 5.4.5, “Error Budget Analysis for Depths”)

expected contribution to depth error model are:

RELEVANT DYNAMIC PARAMETERS IN VERTICAL MEASUREMENT

(practical depth error model contributions shown at the 95% confidence level)

• Vessel heave, pitch, and roll: 0.30 meters

• Vessel dynamic draft: 0.15 meters

• Water column sound velocity: 0.25 meters

• Water level (tide): 0.35 meters

“Vessel dynamic draft” encompasses the notion of static draft as well as changes in settlement10

as a vessel’s hull moves through the water and as a vessel’s load changes due to, say, fuel

consumption. Change in vessel squat (or trim) is assumed to be measured by the pitch sensor. 

Some of these dynamic parameters can be replaced by high-accuracy height measurements

obtained from kinematic GPS, provided the frequency of motion is sufficiently low. The
6

measurable frequency is dictated by the available position update rate from the GPS receiverantenna unit. Modern kinematic GPS receivers are nominally capable of one to ten position

updates per second (1 to 10 Hz). In general, the potential exists to measure vessel draft (not

heave) and water level using kinematic GPS for hydrography. There are numerous international

technical papers which discuss the measurement of tides using kinematic GPS11, 12, 13.

A more detailed error budget analysis is warranted to determine exactly which parameters can be

effectively measured by GPS positioning, while assuring compliance with the total error budget

requirements. The list of practical depth error contributions above is somewhat casual in the

sense that the root-sum-square error is . 0.54 meters, slightly larger than the required minimum

vertical error of 0.50 meters. Nonetheless, order-of-magnitude requirements are presented

below.

HYDROGRAPHIC ACCURACY REQUIREMENTS FOR THREE-DIMENSIONAL GPS

(at 95% confidence level)

GPS Horizontal Error: # 2.5 meters (final solution), # 5.0 meters (available in real time)

Position update rate $ 1 per second

GPS Vertical Error: # 0.20 to 0.35 meters (depending on the vertical parameters measured)

Position update intervals:

– For dynamic draft measurement: 20 to 60 seconds

– For water level measurement: # 6 minutes temporal, and

# 1 kilometer spatial

The NOS goal for the availability of the three-dimensional GPS solution for hydrography is 97%

(or better), at the respective position update rates shown above. Here, “availability” refers to the

existence of a positioning solution which meets the stated accuracy requirements, without regard

to whether the calculations occur in real-time or offline (i.e., from RTK GPS or post-processed

kinematic GPS, respectively).

CHART DATUM ISSUES

Traditional hydrographic surveying scenarios use tide and water level gauges and discrete tidal

zoning to produce tide reducers to MLLW. The error budget is separated into: (i) component

parts of water level measurement error, (ii) error in tidal datum computation using less than a 19-

year tidal datum epoch, and (iii) error associated with extrapolation of the tide or water level

using time and range correctors (tidal zoning). 

For kinematic GPS surveying scenarios, the error associated with water level measurement

relative to a tide staff leveled to permanent bench marks is replaced by the error in measuring

water level height relative to the ellipsoid (NAD83/GRS80) using kinematic GPS. The error

associated with traditional tidal zoning is replaced by the errors associated with determining the

relationship between the ellipsoid and MLLW throughout the survey area.
7

The MLLW datum relative to GPS must be determined in all scenarios. Tide gauges must be

installed at “critical locations” wherein ellipsoidal height relationships are established by

occupying tidal benchmarks with GPS. This should be accomplished prior to actual

hydrographic surveying operations. The error associated with tidal datum determination using

short periods of observations (i.e., less than 19 years) will always exist as long as the chart datum

is based on a tidal datum.

In summary, the kinematic GPS water level error budget would need to have estimates of:

• Errors of knowing OTF elevations of the water surface relative to the ellipsoid at the time and

location of each sounding

• Errors of establishing or recovering MLLW/Ellipsoidal relationships at shore points using

appropriate combinations of new tide gauge measurements and datum determinations;

benchmark and tidal datum recoveries from historical locations; and occupations of tidal

bench marks with GPS to establish the relationships

• Errors in interpolating and extrapolating the MLLW/Ellipsoidal relationship using various

means such as interpolation, offshore GPS/tide gauge measurements, numerical estuarine

models, and continuous zoning approaches such as “Tidal Constituent And Residual

Interpolation” (TCARI)14.

KINEMATIC GPS IN HYDROGRAPHY

Operational Requirements (RTK, OTF RTK, or Post-Processed techniques?)

Strictly speaking a real-time, highly accurate and precise, 3-D position is not necessary for

hydrographic surveying operations. Real-time horizontal positions need only to be good enough

to maintain survey line navigation (DGPS 5-meter horizontal accuracy sufficient for real time).

Vertical positioning is inherently an off line procedure in hydrography. Soundings are formed by

reviewing and editing associated sensor data (i.e., vessel attitude and speed) as well as testing for

potentially anomalous depths, managing sound velocity profile data and performing speed and

refraction corrections, managing tide data and, finally, merging everything together into a vertical

position solution. Nominal post-processing time required to “clean & merge” soundings might

be, at best, 1:1 as compared with acquisition time (1 hour of time on-line data acquisition

requires 1 hour of off-line data processing). Hence, both real-time and post-processed kinematic

GPS scenarios are feasible.

Survey Scenarios

Kinematic GPS requires an accurate horizontal and vertical reference frame in order to determine

an accurate position on each sounding, relative to NAD83, and to determine an accurate depth of

each sounding, relative to MLLW or other appropriate (local) chart datum (tide or water level;
8

e.g., Great Lakes).

The issue of determining separation between kinematic differential GPS vertical datum and the

local chart datum is important to resolve for each survey area. These relationships may or may

not be well know. The complexity of the geodesy of the area, the tidal characteristics of the area

and the geographic complexity and size must be taken into account. Constant relationships may

be adequate in small areas; simple interpolation may be adequate in others; or complex spacial

interpolation may be required using numerical models and continuous zoning procedures (e.g.,

TCARI14 and other tidal datum transformation models15).

Moored vessels or GPS buoys have been used by NOS, USACE, and organizations in Canada in

several special projects for determining offshore tidal datums and their relationships to the

ellipsoid. Simultaneous tide measurements from offshore GPS platforms, with onshore control

tide stations, have been successfully used to determine tidal datums and separation between chart

datum and the ellipsoid at offshore positions. This technique is operationally feasible for future

survey operations16.

There are a number of possible operational survey scenarios, depending on the availability of

suitable horizontal and vertical control. The availability of control must be evaluated during

survey planning. Establishment of the appropriate control prior to survey operations (soundings)

must become an integral part of the survey process. Establishment of the proper control for some

survey areas may not be trivial and consequently may not be accomplished without considerable

effort and time.

The following is a list of some possible survey scenarios:

(1) MAXIMUM EXISTING WATER LEVEL AND GPS CONTROL

C The survey area contains NGS approved continuously-operating reference stations

(CORS). For example, USCG, United States Army Corps of Engineers (USACE),

Federal Aviation Administration (FAA), or International GPS Service for Geodynamics

(IGS) CORS might be within a useable distance of the survey site.

C The survey area has a combination of operational NOS NWLON water level stations with

known chart datum relationships to ellipsoidal heights and/or historical water level station

locations with a minimum of three bench marks with known up-to-date chart datum

elevations relative to the ellipsoid.

C The relationship between local chart datum and the ellipsoid is known throughout the

survey area with confidence based on interpolation techniques between the known shore

locations.

C Carrier phase corrections are broadcast or archived at a suitable epoch update for real-
9

time or post-processed kinematic GPS.

(2) MID-LEVEL WATER LEVEL DATUM AND GPS CONTROL

C The survey area has high accuracy reference network (HARN) marks, but there is no

suitable CORS station nearby. A GPS base station must be set up on one of the HARN

marks to record and/or broadcast the required carrier phase corrections to the survey

vessels.

C The survey area has a network of historical water level station locations with known upto-date chart datum elevations on the bench marks. Some of the water level station

benchmarks must be occupied temporarily with GPS to establish the chart-datum-toellipsoidal-height relationship.

C Some developmental (continuous or discrete zoning) work is required to determine the

chart-datum-to-ellipsoidal-height relationship in offshore areas. Offshore GPS buoys

may be deployed and TCARI may be used to establish these relationships

(3) MID-LEVEL GPS AND INADEQUATE WATER LEVEL DATUM CONTROL

C The survey area has HARN marks but no operational or historical water level stations

with up-to-date chart datum elevations. A GPS base station must be set up on one of the

HARN marks to record and/or broadcast the required carrier phase corrections to the

survey vessels.

C Prior to collection of soundings, water level stations need to be established, chart datum

elevation relative to the bench marks established and the bench marks occupied with GPS

to establish the chart-datum-to-ellipsoidal-height relationships. Significant development

work may have to be done to extend the chart datum/ellipsoid height relationship over the

entire survey area (depending on the particular tide regime and the geographic area).

(4) INADEQUATE GPS AND MID-LEVEL WATER LEVEL DATUM CONTROL

C The survey area has no CORS or HARN marks established but has an adequate network

of operating or historical water level stations with chart datum established. A GPS base

station will have to be established to record and/or broadcast carrier phase observables

during survey operations.

C The water level station bench marks need to be occupied with GPS to establish the chartdatum-to-ellipsoid-height relationships and significant developmental work may have to

be done to determine the relationship throughout the survey area.
10

(5) INADEQUATE GPS AND INADEQUATE WATER LEVEL DATUM CONTROL

C Significant pre-operational survey field work must be done to establish the horizontal and

vertical control necessary for the survey to be accomplished using kinematic GPS.

Obviously, there are numerous combinations of these situation elements that may exist in a given

hydrographic survey project area. The important point to note is that the complete situation must

be identified early on in the project planning stages so that a ample time and resources can be

organized and mobilized to establish the vertical control necessary (prior to beginning survey

operations). As noted in the above scenarios, this may entail conducting static GPS observations

to establish a control point for a base station or installing a water level measurement system to

establish a chart datum.

Although real-time solutions might appear to be the preferred operational scenario, it has been

demonstrated that NOS kinematic GPS hydrographic operations should not be limited to RTK

alone. Scenarios based on post-processed kinematic GPS positioning may be the best approach

in many instances.

Issues: Benefits & Costs

When GPS control and chart datum are established prior to survey operations, the survey vessels

can accomplish their survey without the need to establish and monitor traditional tide and water

level gauges. The survey personnel may only need to construct a GPS base station if a CORS

station is not nearby. Pre-survey activities dealing with tide gauge and GPS control are

traditionally accomplished right at the beginning of surveys, but they could be scheduled and

accomplished much earlier in the project cycle. The potential benefit to be gained from

accomplishing this earlier in the project cycle stems from the reduction in the time it takes to

produce “final” sounding data for a nautical chart product. That time would be reduced because

the post-processing and application of tide data, tidal datums, and tidal zoning process would

become streamlined. Soundings would be automatically referenced to chart datum in the course

of field data acquisition and processed on-board or during shore-based processing operations. 

Both horizontal and vertical accuracy (vis-a-vis more accurate tide and dynamic draft

measurements) could be increased in hydrographic surveying. Increased accuracy means

hydrographic surveying is organized for change as future, more-demanding charting requirements

arise. Hydrographic data accessible via electronic navigation charts will be commensurate with

the best capabilities of GPS marine navigation.
11

As in any satellite-based differential GPS positioning technique, the availability of kinematic

GPS solutions (i.e. “fix” data) depends on four key issues17:

C Time to first fix

C Recovery of data after an interruption

C Proximity of reference station(s) for computational purposes

C Proximity of reference station(s) for purposes of radio data link coverage

The first three issues are intimately tied to the technical problem of acquiring and maintaining

phase-lock in the carrier tracking loop of the kinematic GPS solution.

In the case of post-processed OTF, where forward and reverse kinematic GPS computations can

be compared, “NOS has found that GPS vertical positions of suitable accuracy are typically

available only 85% of the time.”13. However, recent investigations by NOS have shown that

hydrographic requirements for kinematic GPS availability are within reach, using a combination

of “carrier triple differences”, “code double differences”, and Kalman filtering13.

Inspiring Field Work

GPS Expedition to Tangier Island, Chesapeake Bay

A week-long experiment was conducted in Chesapeake Bay in the vicinity of Tangier Island. 

The experiment utilized the BAY HYDROGRAPHER, on which had been installed two

additional dual frequency GPS receivers. The BAY HYDROGRAPHER was operated in close

proximity to several NOS tide gauges varying in location from Annapolis MD to Windmill Point

VA and the data from the tide gauges were used to determine the accuracy and precision of the

GPS-based estimate of the ellipsoidal height of the free water surface. This experiment was a

cooperative effort with The XYZ’s of GPS, Inc. Shore-based reference sites were setup and

operated at ranges from 40 to 140 Km to Tangier Island. The results indicated the ability of

kinematic GPS (in a post-processing mode) to estimate the water levels with accuracy on the

order of 10 cm at distances greater than 100 km between the BAY HYDROGRAPGER and the

reference site. The test successfully demonstrated the ability to utilize multiple reference sites in

the kinematic GPS solution. The tests also provided the first characterization of a newly

developed (by The XYZ’s of GPS, Inc.) double code difference, triple phase difference GPS

algorithm. This work was reported on at the ION 2000 Annual Meeting in Anaheim CA, January

2000.
12

RECOMMENDATIONS

This report represents preliminary efforts and findings based on considerable discussion and

research by the RTK team. This report represents only the first phase of NOS moving towards

conducting hydrographic surveys using kinematic GPS. The Team has developed several

specific recommendations based on what was learned in the process of preparing this report.

The Team recommends that:

1. A new crosscutting team of appropriate technical personnel should be established that

will continue this effort in a formal manner. The new NOS RTK team should also have a

"home" within a line office, possibly NGS, such that the line office is responsible for

keeping the momentum of the team going and takes ownership for its' success. Through

NGS, the team could gain access to GPS hardware and expertise, as well as have a home

base that would be capable of providing R&D funding.

2. Starting immediately, all new GPS equipment (receivers and antennas) purchased for

NOS hydrographic surveying should be dual frequency and either equipped for RTK

operations, or have an optional upgrade path to full RTK operations.

3. Greater use should be made of the BAY HYDROGRAPHER to field test operational

scenarios so that standard kinematic GPS operating procedures can be developed for

hydrographic surveying. 

4. Project instructions for NOS hydrographic surveys should be modified to include a

requirement for the acquisition of GPS phase observables at one-second intervals on the

survey platform and at one or more suitable shore sites. The observed data would be

provided to NGS or another designated group within NOS that would be tasked and

funded to process the data, to determine the time history of latitude, longitude, and Height

of the shipboard GPS antenna at one-second intervals, estimate the performance and

investigate any problems encountered in the data processing.

5. Future Hydrographic Survey Contracts should be modified to require the acquisition of

GPS phase observables at one-second intervals on the survey platform and at one or more

suitable shore sites. An initial strategy may be to specify that both traditional surveying

procedures and kinematic GPS procedures be employed as the contractor conducts patch

tests. The phase observables would be processed, by the contractor in a post-mission

mode, to determine the time history of Lat, Lon, and Height of the shipboard GPS

antenna at one-second intervals. The observed data and the processed results would

become deliverables under the contract. The observed data would be provided to NGS or

another designated group within NOS that would be tasked and funded to process the

data, to determine the time history of Lat, Lon, Height of the shipboard GPS antenna at

one-second intervals, estimate the performance and investigate any problems encountered
13

in the data processing.

6. NOS should establish and/or nurture existing operational partnerships with the private

GPS industry. One possible means to accomplish this would be to continue to work

cooperatively with the several developers and suppliers of GPS processing software to

assess the reliability/ availability of solutions at different levels of accuracy and precision. 

The sets of GPS phase observables from shipboard antennas and relevant shore based

antennas, are available from the BAY HYDROGRAPHER, NOS hydrographic survey

platforms, and Hydrographic Survey Contracts, could be used for algorithm testing.

7. The Joint Hydrographic Center partnership of NOAA with UNH should be encouraged to

explore GPS technology through applied research and operational testing for purposes of

implementing kinematic GPS for NOS hydrographic surveying. 

8. NOS should move quickly to determine ellipsoidal heights of MLLW for the US coastal

waters and to update MLLW to the current epoch. NOS should also continue to improve

the accuracy and spatial resolution of the geoid in the coastal waters of the US because

uncertainties in the local geoid height and the ellipsoidal height of MLLW translate

directly into uncertainties in elevations and depths relative to local MLLW.

9. NOS should plan for and provide adequate R&D funds to rapidly address new issues of

RTK-GPS as they are brought to light by the increased exposure associated with

additional field data and the attempts to establish practical field operating procedures.

10. The future RTK team or others in NOS should further investigate the issues of

post-mission processing of RTK data as compared to true real-time operations. Issues to

be investigated include: (i) cost and complexity of real-time communication of GPS

phase observables from the reference station to the moving platform, and (ii) impact of

post-mission vs. real-time knowledge of the Lat, Lon, Height of a shipboard antenna on

the time required for the hydrographic field unit to process, review and forward a

hydrographic survey data set. 

11. NOS should anticipate that upon the decision to convert NOS hydrographic surveying to

RTK-GPS, it may be necessary to purchase of a significant amount of RTG-GPS

equipment in addition to that which was acquired by the ongoing slow purchase of such

equipment, as stated in recommendation (2), above.
14

ATTACHMENT 1
15

ATTACHMENT 2
16

APPENDIX A

A. Glossary of GPS terms (adapted from http://www.navtechgps.com/glossary.asp

and http://www.analog.com/techsupt/prod_briefs/glossary.html)

1. absolute 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).

2. ambiguity - The initial bias in a carrier-phase observation of an arbitrary

number of cycles. The initial phase measurement made when a GPS

receiver first locks onto a satellite signal is ambiguous by an integer

number of cycles since the receiver has no way of knowing when the

carrier wave left the satellite. This ambiguity remains constant as long as

the receiver remains locked onto the satellite signal and is resolved when

the carrier-phase data are processed.

3. availability - The percentage of time that the services of a navigation

system can be used within a particular coverage area. Signal availability is

the percentage of time that navigational signals transmitted from external

sources are available for use. Availability is a function of both the physical

characteristics of the operational environment and the technical

capabilities of the transmitter facilities.

4. baseline - A baseline consists of a pair of stations for which simultaneous

GPS data has been collected.

5. base Station - Also called a reference station. A receiver that is set up on a

known location specifically to collect data for differentially correcting

rover files. The base station calculates the error for each satellite and,

through differential correction, improves the accuracy of GPS positions

collected at unknown locations by a roving GPS receiver.

6. C/A code - The Coarse/Acquisition or Clear/Acquisition code modulated

into the GPS L1 signal. This pseudo random noise (PRN) code is a

sequence of 1023 pseudo random binary biphase modulations on the GPS

carrier at a chipping rate of 1.023 MHz, thus having a code repetition

period of 1 millisecond. The code was selected to provide good acquisition

properties. Also known as the "civilian code." C/A codes are transmitted

only on the L1 frequency.

7. 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.

8. carrier-aided tracking - A signal processing strategy that uses the GPS

carrier signal to achieve an exact lock on the pseudo random code.

9. carrier phase measurements - GPS measurements based on the L1 or L2

carrier signal.

10. code phase GPS - GPS measurements based on the C/A code.
17

11. control point - Also called a control station. A monumented point to

which coordinates have been, or are being assigned by the use of surveying

observations. The National Geodetic Survey maintains a nation-wide set of

control points.

12. cycle slip - A discontinuity of an interger 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.

13. differential positioning - Precise measurement of the relative positions of

two or more receivers tracking the same GPS signals.

14. differential GPS (DGPS) - A technique used to improve positioning or

navigation accuracy by determining the positioning error at a known

location and subsequently incorporating a corrective factor (by real-time

transmission of correction or by post-processing) into the position

calculations of another receiver operating in the same area and

simultaneously tracking the same satellites.

15. dilution of precision (DOP) - an indicator of satellite geometry for a

unique constellation of satellites used to determine a position. Positions

tagged with a higher DOP value generally constitute poorer measurement

results than those tagged with lower DOP.

16. distance root mean square (DRMS) - It is the root-mean-square value of

the horizontal distance error. The root-mean-square value of the distances

from the true location point of the position fixes in a collection of

measurements. As typically used in GPS positioning, 2 drms is twice the

root mean square of error ellipse. This implies that the probability of

finding the true horizontal position is within 95%.

17. elevation mask angle - That angle below which satellites should not be

tracked to avoid interference problems caused by buildings and trees and

multipath errors.

18. ellipsoid height - The measure of vertical distance above the ellipsoid.

Not the same as elevation above sea level (or MLLW). A roving DGPS

receiver output position fix height in the same datum as the base DGPS

receiver.

19. epoch - Measurement interval or data frequency, as in making

observations every one second (1 Hz).

20. geoid - The particular equipotential surface that coincides with mean sea

level and that may be imagined to extend through the continents. This

surface is everywhere perpendicular to the force of gravity.

21. geoid height - The height above the geoid, often called elevation above

mean sea level.

22. ionosphere - The band of charged particles 80 to 120 miles above the
18

earth's surface, which represents a nonhomogeneous and dispersive

medium for radio signals.

23. ionospheric delay - A wave propagating through the ionosphere

experiences delay. Phase delay depends on electron content and affects

carrier signals. Group delay depends on dispersion in the ionosphere as

well, and affects signal modulation (codes). The phase and group delay are

of the same magnitude but opposite sign.

24. ionospheric refraction - The change in the propagation speed of a signal

as it passes through the ionosphere.

25. L-band - The group of radio frequencies extending from 390 MHz to

1550 MHz. The GPS carrier frequencies L1 (15735 MHz) and L2 (1227.6

MHz) are in the L-band.

26. local area DGPS (LADGPS) - A form of DGPS in which the user's GPS

system receives real-time pseudorange and, possibly, carrier-phase

corrections from a reference receiver located within line of sight

27. multipath - Interference caused by reflected GPS signals arriving at the

receiver, typically as a result of nearby structures or other reflective

surfaces. Signals travelling longer paths produce higher (erroneous)

pseudorange estimates and, consequently, positioning errors.

28. on-the-fly (OTF) - The term used to identify a technique that resolves

differential carrier phase integer ambiguities without requiring a GPS

receiver to be stationary at any time.

29. P-code - The precise or precision code of the GPS signal, typically used

alone by US and allied military receivers. A very long sequence of pseudorandom binary biphase modulations on the GPS carrier at a chip rate of

10.23 MHz which repeats about every 267 days. Each one-week segment

of this code is unique to one GPS satellite and is reset each week.

30. positional dilution of precision (PDOP) - Measure of the geometrical

strength of the GPS satellite configuration and translates into position

error. For example, a 3 m error in the pseudoranges will translate into a 6

m position error if the PDOP is 2.

31. phase lock - The technique whereby the phase of an oscillator signal is

made to follow exactly the phase of a reference signal. The receiver first

compares the phases of the two signals, then uses the resulting phase

difference signal to adjust the reference oscillator frequency. This

eliminates phase difference when the two signals are next compared.

32. post-processed differential GPS - In post-processed differential GPS the

base and roving recievers have no active data link between them. Instead,

each records the satellite observations that will allow differential

correction at a later time. Differential correction software is used to
19

combine and process the data collected from these receivers.

33. 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.

34. range rate - The rate of change between the satellite and receiver. The

range to a satellite changes due to satellite and observer motions. Range

rate is determined by measuring the Doppler shift of the satellite beacon

carrier.

35. real time kinematic (RTK) - The DGPS procedure whereby carrier phase

corrections are transmitted in real time from a reference station to the

user's roving receiver.

36. real-time differential GPS - A base station which computes, formats, and

transmits corrections usually through some sort of data link (e.g. VHF

radio or cellular telephone) with each new GPS observation. The roving

unit requires some sort of data link receiving equipment to receive the

transmitted GPS corrections and get them into the GPS receiver so they

can be applied to its current observations.

37. relative positioning - The process of determining the relative difference in

position between two locations; in the case of GPS, by placing a receiver

over each site and making simultaneous measurements observing the same

set of satellites at the same time. This technique allows the receiver to

cancel errors that are common to both receivers, such as satellite clock and

ephemeris errors, propagation delays and so forth.

38. receiver independent exchange format (RINEX) - A set of standard

definitions and formats that permits interchangeable use of GPS data from

dissimilar GPS receiver models or post processing software. The format

includes definitions for time, phase and range.

39. RTCM SC-104 - The special committee of the Radio Technical

Commission for Maritime Services that develops recommended standards

for DGPS.

40. rover - Any mobile GPS receiver collecting data during a field session.

The receiver's position can be computed relative to another, stationary

GPS receiver.
20

APPENDIX B

B. Glossary of tide terms (adapted from the NOS Tide and Current Glossary on the

CO-OPS web site: http://www.co-ops.nos.noaa.gov/tideglos.html)

1. accepted values - Tidal datums and Greenwich high and low water

intervals obtained through primary determination or simultaneous

observational comparisons made with a primary control tide station in

order to derive the equivalent of a 19-year value.

2. chart datum - The datum to which soundings on a chart are referred. It is

usually taken to correspond to a low-water elevation, and its depression

below mean sea level is represented by the symbol Z . Since 1989, chart

datum has been implemented to mean lower low water for all marine

waters of the United States, its territories, Commonwealth of Puerto Rico,

and Trust Territory of the Pacific Islands. See datum and National Tidal

Datum Convention of 1980.

3. datum (vertical) - For marine applications, a base elevation used as a

reference from which to reckon heights or depths. It is called a tidal datum

when defined in terms of a certain phase of the tide. Tidal datums are local

datums and should not be extended into areas which have differing

hydrographic characteristics without substantiating measurements. In order

that they may be recovered when needed, such datums are referenced to

fixed points known as bench marks. See chart datum.

4. mean high water (MHW) - A tidal datum. The average of all the high

water heights observed over the National Tidal Datum Epoch. For stations

with shorter series, simultaneous observational comparisons are made with

a control tide station in order to derive the equivalent datum of the

National Tidal Datum Epoch.

5. mean lower low water (MLLW) - A tidal datum. The average of the

lower low water height of each tidal day observed over the National Tidal

Datum Epoch. For stations with shorter series, simultaneous observational

comparisons are made with a control tide station in order to derive the

equivalent datum of the National Tidal Datum Epoch.

6. mean sea level (MSL) - A tidal datum. The arithmetic mean of hourly

heights observed over the National Tidal Datum Epoch. Shorter series are

specified in the name; e.g., monthly mean sea level and yearly mean sea

level.

7. national tidal datum epoch - The specific l9-year period adopted by the

National Ocean Service as the official time segment over which tide

observations are taken and reduced to obtain mean values (e.g., mean

lower low water, etc.) for tidal datums. It is necessary for standardization
21

because of periodic and apparent secular trends in sea level. The present

National Tidal Datum Epoch is 1960 through 1978. It is reviewed annually

for possible revision and must be actively considered for revision every 25

years.

8. national water level observation network (NWLON) - The network of

tide and water level stations operated by the National Ocean Service along

the marine and Great Lakes coasts and islands of the United States. The

NWLON is composed of the primary and secondary control tide stations

of the National Ocean Service. Distributed along the coasts of the United

States, this Network provides the basic tidal datums for coastal and marine

boundaries and for chart datum of the United States. Tide observations at a

secondary control tide station or tertiary tide station are reduced to

equivalent l9-year tidal datums through the comparison of simultaneous

observations with a primary control tide station. In addition to

hydrography and nautical charting, and to coastal and marine boundaries,

the Network is used for coastal processes and tectonic studies, tsunami and

storm surge warnings, and climate monitoring. The National Water Level

Observation Network also includes stations operated throughout the Great

Lakes Basin. The primary network is composed of 54 sites with 139

seasonal gauge sites selectively operated 4 months annually for the

maintenance of IGLD. The network supports regulation, navigation and

charting, river and harbor improvement, power generation, various

scientific activities, and the adjustment for vertical movement of the

Earth's crust in the Great Lakes Basin.

9. primary control tide station - A tide station at which continuous

observations have been made over a minimum of 19 years. Its purpose is

to provide data for computing accepted values of the harmonic and non

harmonic constants essential to tide predictions and to the determination of

tidal datums for charting and for coastal and marine boundaries. The data

series from this station serves as a primary control for the reduction of

relatively short series from subordinate tide stations through the method of

comparison of simultaneous observations and for monitoring long-period

sea level trends and variations. See tide station, secondary control tide

station, tertiary tide station, and subordinate tide station.

10. tertiary tide station - A tide station at which continuous observations

have been made over a minimum period of 30 days but less than 1 year.

The series is reduced by comparison with simultaneous observations from

a secondary control tide station. This station provides for a 29-day

harmonic analysis. See tide station, primary control tide station, secondary

control tide station, and subordinate tide station.

11. tidal zoning - The practice of dividing a hydrographic survey area into

discrete zones or sections, each one possessing similar tidal characteristics.
22

One set of tide reducers is assigned to each zone. Tide reducers are used to

adjust the soundings in that zone to chart datum (MLLW). Tidal zoning is

necessary in order to correct for differing water level heights occurring

throughout the survey area at any given time. Each zone of the survey area

is geographically delineated such that the differences in time and range do

not exceed certain limits, generally 0.2 hours and 0.2 feet respectively;

however, these limits are subject to change depending upon type of survey,

location, and tidal characteristics. The tide reducers are derived from the

water levels recorded at an appropriate tide station, usually nearby. Tide

reducers are used to correct the soundings throughout the hydrographic

survey area to a common, uniform, uninterrupted chart datum. See tide

reducers.

12. tide - The periodic rise and fall of the water resulting from gravitational

interactions between Sun, Moon, and Earth. The vertical component of the

particulate motion of a tidal wave. Although the accompanying horizontal

movement of the water is part of the same phenomenon, it is preferable to

designate this motion as tidal current. See tidal wave.

13. tide (water level) gauge - An instrument for measuring the rise and fall of

the tide (water level). See ADR gauge, automatic tide gauge, Next

Generation Water Level Measurement System, gas purged pressure gauge,

electric tape gauge, pressure gauge, and tide staff.

14. tide reducers - Height corrections for reducing soundings to chart datum

(MLLW). A tide reducer represents the height of the water level at a given

place and time relative to chart datum. Tide reducers are obtained from

one or more tide stations within or nearby the survey area. Often, due to

differing tidal characteristics over the survey area, the tide reducers

obtained directly from a tide station must be corrected to adjust for time

and range of tide differences in the various zones of the hydrographic

survey area. See tidal zoning.

15. tide (water level) station - The geographic location at which tidal

observations are conducted. Also, the facilities used to make tidal

observations. These may include a tide house, tide gauge, tide staff, and

tidal bench marks. See primary control tide station, secondary control tide

station, tertiary tide station, and subordinate tide station.

16. type of tide - A classification based on characteristic forms of a tide curve.

Qualitatively, when the two high waters and two low waters of each tidal

day are approximately equal in height, the tide is said to be semidiurnal;

when there is a relatively large diurnal inequality in the high or low waters

or both, it is said to be mixed; and when there is only one high water and

one low water in each tidal day, it is said to be diurnal. Quantitatively

(after Dietrich), where the ratio of K1 + O1 to M2 + S2 is less than 0.25,

the tide is classified as semidiurnal; where the ratio is from 0.25 to 1.5, the
23

tide is mixed, mainly semidiurnal; where the ratio is from 1.5 to 3.0, the

tide is mixed, mainly diurnal; and where greater than 3.0, diurnal.
24

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