Introduction
A very important point to remember is that any single GPS receiver, used autonomously, is subjected to many sources of error. (Autonomously means: a single receiver collecting data with no contact or input from other receivers collecting data at the same time.) These errors include satellite orbit and clock errors, signal propagation errors, and even errors introduced intentionally by the U.S. Department of Defense. All GPS receivers will be affected by these combined errors; resulting in errors that range from dozens to hundreds of meters. This is true for all receivers, regardless of manufacturer or receiver type.
      By obtaining data from two receivers (one stationary base receiver at a known location, and the other receiver roving freely) and using a process known as differential correction, most of these errors can be removed. When the data from two receivers is used for a differential correction most of these errors are eliminated. Whether for centimeter, sub-meter, or even 2 to 5 meter accuracy; you will require at least two receivers, and you will have to perform a differential correction. Again, no manufacturer is immune to this principle.
      There are two ways that differential correction is typically performed; either in real-time via special telemetry links between the two GPS receivers, or as a post-processing procedure on a computer. In general, both techniques are equally accurate.
      Real-time differential GPS requires a wireless link between the base receiver and the rover receiver(s). The wireless link is used for the transmission of real-time differential GPS (DGPS) correction data between base and rover GPS receivers. In some vehicle tracking applications, the rover transmits data to the base station so a correction can be performed and the accurate rover position can be displayed or reported to a dispatcher. However, in GIS data collection applications, the opposite is true. Typically the corrections are transmitted from the base receiver to rover so that the corrections can be applied to the rover data and the roving user can view their accurate location on the screen for real-time navigational purposes. The advantage of real-time differential GPS (DGPS) is accurate navigation in the field.
      Post-processed differential GPS (DGPS) is performed in a computer after both the base and rover GPS receivers have been transferred to the PC. The advantage of post-processed data collection is there is no communication requirement between the base and rover GPS receivers. The base data is stored for later use, and then special software applies corrections to the rover data.

Which is Appropriate?
The benefit of using real-time DGPS is that the user can navigate accurately. The cost of real-time DGPS is the expense and/or maintenance of the telemetry link between base and rovers. As a rule, users must decide case-by-case whether the need for accurate real-time navigation is sufficient to justify the required data link.
      In the past, most users established their own telemetry system and this often constituted a significant expense. For most applications, the cost and hassle of acquiring, licensing, and maintaining a radio system outweighed the benefits of collecting real-time DGPS positions. These days, however, there are a variety of options for acquiring real-time DGPS correction data very easily and inexpensively.
      The Radio Technical Commission for Maritime Services has defined an industry standard format for the transmission of real-time correction data. This format is known as RTCM-SC-104. There are now several commercial and governmental sources of real-time DGPS data transmission. These RTCM DGPS transmissions vary widely in quality and range. There are many ways that the DGPS corrections can be transmitted from the GPS base station to the roving user. For example, today these corrections are being sent via cellular telephones, UHF and VHF radios, via pager services and other FM sub-carrier broadcasts, and even via satellite for extremely wide area coverage.

Sources of Real-Time
DGPS The following section summarizes three sources of RTCM data. This is not intended to be a complete list, nor is it intended as an endorsement of any particular real-time differential data source. The goal here is simply to mention a few common sources of RTCM data and to briefly describe how each one works. The major commercial and government sources of real-time correction data on the market today are:
• MSK beacon transmitters
• FM sub-carrier
• Wide area DGPS via satellite

MSK Beacon Transmitters
The MSK beacon programs are a very exciting source of real-time DGPS data. Various governmental agencies around the world are providing real-time DGPS transmissions. These beacons transmit between 283.5 and 325 kilohertz, with typical geographic ranges from 160 to 500 kilometers (100-500 miles). Of course, the range of any individual transmitter is a function of power output, antenna height and efficiency, as well as the characteristics of the surrounding land.
      In general, there is no charge to receive and use these transmissions. The best advice regarding the purchase of a beacon receiver is to obtain both a beacon and GPS receiver prior to purchase to ensure that the beacon signal can be received in your work area. Lower quality receivers may have difficulty at the fringe of transmission range. Other common field problems include RF interference when used in vehicles and potential antenna grounding problems with whip style antennas.
      Ultimately, every country with any significant navigable coastline will probably have access to beacon transmissions. In most countries that are installing their own MSK beacon transmitters, a national, governmental department administers the program. In North America, both the U.S. and Canadian Coast Guards and the U.S. Army Corps of Engineers are installing dozens of transmission sites to provide coverage to all harbors and major inland waterways. Complete coverage is expected by 1996. Several Latin American countries are also in the process of establishing beacon transmission sites throughout South America.
      Internationally, the International Association of Lighthouse Authorities (IALA) coordinates and administers the installation of beacon transmitters in many countries. Most of northern Europe, the Baltic Sea and North Sea all have very extensive coverage based upon beacon transmitters already installed in Denmark, England, Finland, Germany, Holland, Norway, and Sweden. Southern Europe and northern Africa are also considering the best way to provide real-time DGPS to the Mediterranean Sea with a beacon network. In Asia and Australia, more than six beacon sites have already been installed. There are currently plans in place for several more.

FM Sub-Carrier
Both DCI and ACCQPOINT are companies that offer real-time DGPS services on a subscription basis. Such suppliers will establish GPS base stations, then transmit the DGPS correction data on standard FM broadcast frequencies (87 - 108 megahertz). It is common practice in the FM broadcast industry to transmit digital data on the FM sub-carrier, in addition to the normal programming. This technology is used by many personal paging systems, as well as for traffic broadcasts, differential GPS, and for MUZAK. The typical sub-carrier transmission has a range of about 100 kilometers (60 miles) or less.
      In general, the user would purchase a pager unit (or other small receiver) from the DGPS supplier. The pager will have been designed to receive and decode DGPS transmissions. Users can then subscribe to the differential services on a monthly or yearly basis. Typically, the suppliers will offer several levels of service, based upon the level of accuracy desired by the user in the field.
      As one might expect, DGPS transmissions via FM sub-carrier are available primarily in urban areas. It does not appear likely that FM sub-carrier DGPS coverage will be available in most remote, rural regions in the immediate future. However, this service is offered in hundreds of cities all over the world, and more cities are being added continuously.

Wide Area DGPS via Satellite
Another type of subscription DGPS service is wide area DGPS. A supplier offering wide area DGPS correction data usually transmits the correction data via satellite or high frequency radio. This form of telemetry offers a long range so that the supplier can offer DGPS to a specific, strategic regions.
      In addition to a large coverage area, there is another nice benefit to a wide area DGPS. Normally, the accuracy of differential GPS degrades as the distance between base and rover increases. This is primarily due to atmospheric and tropospheric differences at the base and rover locations. The wide area suppliers solve this problem by installing a network of several base stations that span or surround the coverage area. Ultimately, data from most or all of the base stations is analyzed to produce a set of customized differential corrections specific to the location of each individual rover in the field. This technique enhances the differential accuracy such that it is not uncommon for sub-meter differential accuracy to be achievable throughout the entire coverage region.
      In general, the user would purchase a receiver/processor package to obtain access to the wide area signal. Users can then subscribe to the differential services on a monthly or yearly basis. Typically, this type of service is more expensive than other subscriber services, However, it is by far the most dependable, robust, and accurate on the market. In many marine environments, this is the only differential option available. This type of service has long been popular in the offshore oil and gas industry where the nearest GPS base station might be hundreds of kilometers away.

About the Author:
Chuck Gilbert has over a decade of experience as a GPS user. He has been employed as an applications engineer for Trimble Navigation since 1989. If you have a suggestion or request for a future article, please drop a line to Chuck care of Earth Observation Magazine.

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