The US Department of Defence created the Global Positioning System, a world wide all-weather positioning resource, in 1973, for both military and civilian use. It is based on a constellation of 24 satellites, in six distinct planes approximately 22,000km above mean sea level. The satellites act as reference points for ground receivers to calculate their 3D position, velocity and time.

Each satellite transmits a precise coded time message and orbit information. The receiver calculates its distance (range) from the satellite by the travel time. By knowing the position of three satellites and their respective ranges, a GPS receiver can theoretically determine a three dimensional position. However a fourth observation is required to adjust the receiver clock to the satellites clock. A GPS receiver calculates a 3-dimensional position by incorporating any four or more range measurements into its solution. These range measurements are called pseudo-ranges, as they are not a true range due to contributing errors that bias the measurement.

The first satellites were launched in 1978, with the full complement of 24 satellites achieved in the early 1990's, allowing position fixes at any time of the day. GPS surpassed all original expectations with positions being within 20 metres of true for a single receiver. The satellites stable orbit and the accuracy of the onboard atomic clock being better than first thought.

The largest error to positions obtained by a GPS receiver up until the 1 May, 2000 was Selective Availability or SA, a United States Government program of degrading the position accuracy of a GPS receiver. See White House Press Release

It has been found through testing GPS receivers after SA has been switched off that a modern recreation GPS receiver is appreciably more accurate than when previously tested prior to SA. Empirical tests are still being conducted but most GPS manufacturers claim horizontal accuracy of less than 15 metres. This is a vast improvement in performance to the days during Selective Availability. This improvement is due to improvements in GPS receiver technology and manufacturing techniques, and more precise satellite orbit information being broadcast.

Now instead of trying to down grade the GPS signal, the United States government through the Federal Aviation Administration (FAA) and the United States Coast Guard are trying to improve (or augment) the position obtained by a GPS receiver. These organisations have established a free to air correction signal that will increase the accuracy of a modern 12 channel recreational type GPS receiver to around 3 metres. A brief theory behind the corrections supplied to the GPS receiver are outlined below in Code phase Differential GPS. The various 'free to air' and commercial services available for Differential GPS follow the description of Code phase Differential GPS.

Code phase Differential GPS

To overcome the correctable errors that affect the GPS receiver a technique called Differential GPS (DGPS) was developed. It involves a GPS receiver (the Base Station) collecting GPS measurements over a known position (previously surveyed), during the same period of time that one or more GPS receivers (the Rovers) are operating. As the two GPS receivers are viewing the GPS satellites through the same part of the sky it is assumed that the errors are the same for the base and roving GPS receivers.

The base station receives the same GPS signals as the roving receiver but instead of using the timing signal from the satellites to obtain a position it uses its known position to calculate timing. It compares this theoretical time, with the time it actually took. Any difference is the error in the satellite's time signal, which equates to it pseudorange; this comparison is done for all the satellites in view.

The pseudorange corrections computed by the base GPS can be applied to the roving GPS in two ways. By processing the two sets of data in a computer or by establishing a communication link between the base and the rover GPS and applying the corrections in realtime.

For post processed differential GPS you need a computer and a suitable software package. Measurement data is downloaded from the base and the roving GPS receiver to the computer. Pseudo range corrections are computed for the Base GPS by using the satellite number and the time the measurements were taken, pseudorange corrections are then applied to the roving GPS receiver's data. This method is used mostly for mapping features on the ground.

In Realtime Differential GPS the roving GPS receiver is corrected for errors during operation. This correction is transmitted from the base GPS via a radio data link to the roving GPS. The transmitted data includes the satellite number, its timing difference and the rate of change of the error. This rate of change helps the roving receiver interpolate its position in between radio updates, or if the signal is lost for a short period of time. Realtime differential GPS is ideal for setting out points and accurate navigation to a point

Code phase differential is the most common technique to gain better accuracy when using a GPS receiver. The accuracy is approximately 3 metres with a Code Phase GPS receiver such as one used for recreation purposes and 1 metre or less with a GPS receiver with carrier phase smoothing. This position accuracy is fine for navigational purposes and indeed for some land surveys tasks such as GIS asset mapping. When position needs to be accurate to the centimetre level, as is required for most land surveys, a better solution is required. Differential GPS is still the method, but you need Carrier phase Differential GPS for accuracy of position to 1cm.

Before Carrier phase Differential GPS, which is used primarily by Surveyors and those industries that require precise navigation, I will list the various free to air and commercial services available for Differential GPS.

Free to Air DGPS services

Wide Area Augmentation System (WAAS) has been implemented this year, 2001, and covers the entire United States and a portion of Canada and Mexico. WAAS will also be established by the European Community and will cover all of Europe and parts of Northern Asia. WAAS can improve the accuracy of a single 12 channel, navigation type, GPS receiver to around 3 metres from true. The latest model GPS receivers can receive the WAAS signal. As both the WAAS and GPS signals are in the microwave band of the Radio spectrum it is relatively easy to intergrate the additional frequency from the WAAS broad cast to the GPS receiver.

WAAS improves (augments) the accuracy of GPS by calculating the errors in the GPS signal at a network of monitoring stations spread around the signal area and then transmitting the error corrections (pseudorange corrections) to WAAS satellites which in turn re-transmitts these corrections to WAAS capable GPS receivers in the signal coverage area. WAAS satellites are geostationary (staying in the same spot over the equator). The two satellites that transmitt WAAS signal to North America are over the Pacific and Atlantic Oceans. The two satellites that transmitt to Europe are over Africa and India. Depending where you are situated the WAAS satellites maybe near the horizon ie. at an angle of 40 degrees to the horizon, this makes it possible for trees and building in the signal path to block the signal.

Australia will not get WAAS in the forseeable future as the system is too expensive. The system was put in place primarily for the aviation industry, so Australia will use its existing VHF radio frequencies to broadcast the differential GPS corrections. This system will generally only be available to aircraft as it is on the restricted airband frequency. This type of system is called LAAS.

A link to the FAA's WAAS web page.

Local Area Augmentation System (LAAS) is being implemented and tested at commercial airports through out the world including Australia. LAAS should be in operational by 2004. To receive this DGPS correction message you need an 'airband' VHF radio and be in the coverage area. If you are in an aircraft you could receive this signal up to 200 kilometres away depending on your flying height. This system is designed to provide higher order accuracy when close to the airport. GPS receivers with carrier phase smoothing can achieve accuracy of position to 1 metre. This system will be only of benifit to the aviation industry.

More information on this system can be found on the FAA's LAAS web page.

Marine Beacons these beacons are some times described on brochures as USCG for United States Coast Guard. Marine beacons in Australia have been established by the Australian Marine Safety Authority (AMSA).

The signal from the marine beacon is Medium Frequency which is ideal for transmitting the pseudo range corrections long distances. It is high enough frequency to transmit data at 200bps and low enough to cover a large area of around 500 kilometres from the marine beacon. The radio transmission has three components: direct line of sight wave, a ground wave and a sky wave. The later two waves are what transmit the signal such large distances. The ground wave hugs and propogates along the surface of the earth gradually losing strength with distance. The sky wave reflects off the ionosphere and back to Earth transmitting the signal the greatest distance. Unlike the microwave frequency the medium wave signal penetrates through heavy foliage and buildings.

This system was established for the maritime industry but can be used by any one in the coverave area provided they have a beacon receiver.

More information on AMSA and its list of marine beacons can be found on Beacon Status

Commercial (you have to pay) DGPS providers

L-Band DGPS providers. There are only two Commercial suppliers of GPS pseudo range corrections in Australia and World Wide that use satellites to broadcast the corrections. They are Racal Landstar and Fugro Omnistar. These two companies have a network of GPS base stations through out the world that uplink pseudo range corrections to geostationary communications satellites. These satellites then beam down the correction signal to designated areas. This is still the only way to get pseudorange corrections throughout all of Australia and South East Asia.

The accuracy using this system combined with a carrier phase GPS receiver in horizontal is approx. 1 metre and when connected to a recreation receiver it is 3 metres. Both companies can provide a Wide Area Solution, as used with WAAS, to lessen errors associated with distance between the GPS user and the GPS base station. The typical signal hire for these systems is $2,500 per year.

FM radio DGPS providers - FM radio is the only other method in popular use for transmitting pseudorange DGPS corrections. This is done by putting the information on the sub carrier wave of the FM signal. The same signal used by commercial radio stations. In Australia it is on the JJJ radio frequency. If you can receive JJJ in stereo you will be able to receive the DGPS correction signal. The company that provides this service in Australia is AUSNAV.

Carrier phase Differential GPS

Carrier Phase refers to the radio signal by which the GPS information is sent. Code phase refers to the GPS information that is sent. As with code phase DGPS, carrier phase DGPS can be carried out in the realtime or later on with post processing. The reasons are as before, if you need to set out points or navigate to a point you will need accurate positions instantaneously, this method is also known as RTK or Real-Time Kinematic surveying. Mapping features on the ground or establishing survey control is often best done by post processing the GPS data.

Carrier phase differential is explained very well in the following two papers, and are viewed in Adobe Acrobat (download):

GPS+GLONASS
Paper discussing Carrier phase Differential from an International GPS conference. GLONASS is the Russian equivalent of GPS

Carrier phase Differential GPS explained

GPS information from the University of Texas

More information can be obtained from other links on our About Us & Links page.

Selective Availability (although it has been switched off the capability has not been removed)

As the system is passive, the US Department of Defence could not monitor who was using the system. Worried that with this level of position accuracy hostile forces could turn their own system against them or their allies, they devised a policy called Selective Availability (SA). The U.S. Department of Defence in 1991, imposed SA by artificially creating an error, to downgrade position accuracy. Since 1991 and the 1 May, 2000 SA has only been switched off for a specific purpose.

This intentional error was random and affected the broadcast GPS time, it also had the provision to degrade the satellite orbit data that is transmitted. The net effect of this artificial error combined with 'natural’ errors, atmospheric and satellite, was to make the horizontal position accuracy of a single GPS receiver within 100 metres, 95% of the time, or as often stated 100 metres 2DRMS. The rate of change in position due to the error was also random. Most manufacturers claimed that a position gained from a GPS receiver will be typically 40 metres from true with Selective Availability on, although a position could just as easily be out by 120 metres.

Now that SA has been switched off a single GPS receiver operating autonomously is still subjected to several sources of error. This is regardless of manufacturer or receiver type. If positions gained over a short time period are to be within 1 metre or even 5 metres, two or more GPS receivers operating concurrently are needed; the technique is called Differential GPS

___White House Statement on Selective Availability - SA

For Immediate Release May 1, 2000

STATEMENT BY THE PRESIDENT REGARDING

THE UNITED STATES? DECISION TO STOP DEGRADING

GLOBAL POSITIONING SYSTEM ACCURACY

Today, I am pleased to announce that the United States will stop the intentional degradation of the Global Positioning System (GPS) signals available to the public beginning at midnight tonight. We call this degradation feature Selective Availability (SA). This will mean that civilian users of GPS will be able to pinpoint locations up to ten times more accurately than they do now.

GPS is a dual-use, satellite-based system that provides accurate location and timing data to users worldwide. My March 1996 Presidential Decision Directive included in the goals for GPS to: encourage acceptance and integration of GPS into peaceful civil, commercial and scientific applications worldwide; and to encourage private sector investment in and use of U.S. GPS technologies and services. To meet these goals, I committed the U.S. to discontinuing the use of SA by 2006 with an annual assessment of its continued use beginning this year.

The decision to discontinue SA is the latest measure in an on-going effort to make GPS more responsive to civil and commercial users worldwide. Last year, Vice President Gore announced our plans to modernize GPS by adding two new civilian signals to enhance the civil and commercial service. This initiative is on-track and the budget further advances modernization by incorporating some of the new features on up to 18 additional satellites that are already awaiting launch or are in production. We will continue to provide all of these capabilities to worldwide users free of charge.

My decision to discontinue SA was based upon a recommendation by the Secretary of Defense in coordination with the Departments of State, Transportation, Commerce, the Director of Central Intelligence, and other Executive Branch Departments and Agencies. They realized that worldwide transportation safety, scientific, and commercial interests could best be served by discontinuation of SA. Along with our commitment to enhance GPS for peaceful applications, my administration is committed to preserving fully the military utility of GPS. The decision to discontinue SA is coupled with our continuing efforts to upgrade the military utility of our systems that use GPS, and is supported by threat assessments which conclude that setting SA to zero at this time would have minimal impact on national security. Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened. This regional approach to denying navigation services is consistent with the 1996 plan to discontinue the degradation of civil and commercial GPS service globally through the SA technique.

Originally developed by the Department of Defense as a military system, GPS has become a global utility. It benefits users around the world in many different applications, including air, road, marine, and rail navigation, telecommunications, emergency response, oil exploration, mining, and many more. Civilian users will realize a dramatic improvement in GPS accuracy with the discontinuation of SA. For example, emergency teams responding to a cry for help can now determine what side of the highway they must respond to, thereby saving precious minutes. This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world.

May 1, 2000

President Clinton: Improving the Civilian Global Positioning System (GPS)

THE WHITE HOUSE

Office of the Press Secretary

Additional information about GPS and the Selective Availability is available online at the Interagency GPS Executive Board web site: http://www.igeb.gov

to the top