Understanding

GPS Technology:

System History,

Architecture, and

Fundamentals

Edited by James A. Turner,

Geographic Sciences Laboratory

Manager, National Science 

and Technology Center

The information contained in this

Resource Note is excerpted from an

article in the June 28, 1999, edition

of the Oil & Gas Journal (Volume

97, Number 26). All of the material

contained herein is reprinted with

the permission of the publisher and

the authors:

Global Positioning Technologies

Provide Essential Exploration Tool

Simon Corbet, Martin Rayson

Quality Engineering & Survey

Technology Ltd., Newcastle, U.K.

Background

In the 1960s, the U.S. Air Force

and Navy began developing a

space-based positioning service, with

the potential for a worldwide, continuous, all weather tracking system.

On April 17, 1973, the Deputy

Secretary of Defense combined all

of the systems into a comprehensive Department of Defense system, named Navstar GPS.

The Navstar GPS Joint Program

Office (JPO) was established on

July 1, 1973, to manage the new

system. Now fully operational,

GPS provides suitably equipped

users with accurate 3D positions,

velocity, and time parameters. The

service allows for simultaneous,

unlimited numbers of users and is

provided globally, continuously, and

throughout all weather conditions.

Today, GPS allows for instantaneous

positioning to within an accuracy of

100 m (95%) in the horizontal and

140 m (95%) in the vertical for

most commercial activities.

GPS Constellation

GPS comprises three major segments: space, control, and users. The

role of the space segment is to provide a sufficient number of satellites

such that a receiver may observe four

satellites anywhere in the world at

any time. To obtain this coverage,

the GPS constellation consists of 24

operating satellites that are arranged

in six near circular orbital planes

with four satellites located in each

plane at an orbit height of almost

20,200 km (10,900 nautical miles).

This height is such that the satellites follow a ground track that

repeats every 23 hrs, 56 min.

The control segment’s main functions consist of tracking the satellites

for orbit and clock determination,

prediction modeling, time synchronization, and data uploading to

each satellite. 

The GPS user segment consists of a

number of military and civilian

receivers that can receive, understand, and process the radio 

frequencies transmitted by the GPS

satellites. They may be used for a

variety of applications such as navigation, positioning, time transfer, surveying, and attitude determination.

GPS Systems

Standalone GPS. At the low end of

the market, simple, small, and inexpensive (possibly hand-held) receivers

can deliver 100 m level absolute

positions instantaneously and

independently (only using signals

broadcast by the satellites). This

is often referred to as “standalone

GPS,” or pseudorange point 

positioning. Receivers are now

widely available and can cost as

little as $200.

Differential GPS. Digital GPS

involves the use of a stationary reference receiver located at a known

fixed point and an unknown

remote, possibly mobile, receiver.

To determine a set of corrections,

the pseudoranges observed at the

reference receiver are compared to

expected transmissions. These

corrections can then be broadcast

to the remote receiver, and if

applied to its observed pseudoranges, can improve the positional

fix. Typical distances from reference

station to mobile station are on the

order of 1,000 km.

Combined GPS. The use of a

combined reference station DGPS

position solution is now widely

used. This essentially utilizes differential corrections from a number of

reference stations, which are combined in either the position,

adjustment, or correction domain

to give an overall solution. The

precision of the position fix is not

necessarily improved from the

DGPS solution, however, multiple

reference station solutions provide a

more stable position fix, improved

quality control, and reliability

measures.

RTK GPS

If the carrier-phase data are used, the

precision of the position fix can be

dramatically improved. In this case, a

similar scenario to DGPS is

employed with a static reference

station at known coordinates and

an unknown remote, possibly

mobile, receiver. The reference

receiver broadcasts the phase data

to the remote receiver, which

enables it to compute its position

relative to the reference station

coordinates. The precision of the

RESOURCE NOTES

NO. 42 DATE 08/18/00

Range

GIS and GPS
position fix can be on the order of 1

cm; however, the reference station to

remote station distance must typically be 10 km to achieve such results.

Very Precise GPS. At the top end

of GPS-positioning technology,

data collected by sophisticated

receivers located at several points over

several days, or even continuously

and permanently, can deliver relative positions with millimeter

accuracy.

GPS Accuracy

One of the most interesting (yet

often misunderstood) features of

GPS is the enormous range in the

accuracy with which it can be utilized. It ranges from 100 m to 1

mm - a total of five orders of magnitude. No other surveying system

offers such possibilities, and the key

to the potential user is to identify

the positioning requirements in

order to obtain the most suitable

system. There are essentially two

methods of positioning with GPS

using either pseudoranges or carrier

phases. However, the differences

between these two are becoming

less distinguishable.

Differential GPS

Differential GPS typically provides

a horizontal positional precision of 1

5m (95%). Because there are difficulties over radio licensing and distances

between reference and mobile

receivers, a number of companies

have established Differential GPS

services. Such services comprise a

network of reference stations constantly transmitting encrypted DGPS

corrections via land or satellite-based

radio communication links.

Users of the system subscribe to the

service, allowing them to demodulate

and unscramble the incoming signal

and feed the corrections into a GPS

receiver, or third-party software.

System Service Providers

There are a number of DGPS service providers. For example, the

U.S. Coast Guard operates a free

maritime DGPS service, broadcasting differential corrections from a

number of reference receivers

around the U.S. Horizontal positioning is guaranteed to 10 m

(95%), and integrity alerts within

10 sec. The USCG DGPS system

was originally planned to cover only

coastal and Great Lake regions; however, this has now been expanded to

cover inland waterways.

In addition, Racal Survey provides a

satellite-based DGPS service, known

as Land Star. DGPS corrections are

generated at each of the reference

stations, combined, then uplinked

to a high-power, spot beam satellite.

Users are equipped with a receiver

and antenna to receive both the GPS

signals and the DGPS corrections

from the Land Star satellite. The

service costs about $500 per day.

Fugro also operates a similar satellite-based DGPS service to Racal

known as OmniStar.

Alternatives

Other alternative systems that may

soon become viable include the

Russian and European satellite systems. The Global Navigation

Sputnik System (Glosnass) is the

Russian Equivalent to GPS. Satellite

launches began October 12, 1982,

and more than 70 now orbit the

earth. As of May 1999, only 15 satellites were still in operation.

A Note of Caution

Although GPS offers many benefits

over traditional survey techniques,

it is also a system that should be

treated with a great deal of care. For

example, setting up a high-resolution

form of GPS, such as a differential

GPS (DGPS) network, can be

difficult and expensive, requiring

expert geodetic knowledge. However,

like all technologically advanced systems, gross and systematic errors can

be readily introduced from the

peripheral systems such as GPS

receivers and navigation software.

The U.S. Department of

Transportation is the primary

interface for all civil GPS matters,

creating the Civil GPS Service

Interface Committee to meet this

obligation. Relevant information,

such as a list of receiver manufacturers and points of contact can be

found on the USCG Navigation

Center’s web site:

(http://www.navcen.uscg.mil)

Contact

James A. Turner

Geographic Sciences Laboratory

Manager, National Science and

Technology Center

Denver Federal Center, Building 50

P.O. Box 25047

Denver, CO, 80225-0047

phone (303) 236-0840

fax (303) 236-3508

email jturner@blm.gov

Range

Resource Notes are intended to be early

announcements of technical and informational topics for

Bureau of Land Management personnel and some of their customers.

Information in this Resource Note is based on the opinion and

experience of the author and has not been peer-reviewed. Conclusions

and opinions expressed herein do not

necessarily represent those of BLM. Use of trade names does not

imply U.S. Government endorsement of commercial products.

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