GPS Q&A
By John Bohlke

Q.Do I need two or more GPS receivers in order to post-process the data? What are my options and considerations with obtaining base station data?
O.R. Eden Prairie, Minn.

A.Two or more GPS receivers are required to post-process GPS data. One or more of the receivers should be used as a base station. In a mapping-grade GPS receiver (decimeter to meter-level accuracy), usually only one base is necessary, whereas a survey-grade system (centimeter-level) can process data from multiple base locations. Many base station sources are available throughout the U.S. that would allow a person to post-process GPS data using only one other receiver.
     Some universities, the Coast Guard, the Corps of Engineers and various other private and government agencies are institutions that have established permanent base stations. Quite often this data can be accessed free of charge over the Internet or through a computer bulletin-board system. Information regarding the Continuous Operating Reference Stations (CORS) is found on the Internet at the National Geodetic Survey (NGS) FTP site, ftp://cors.ngs.noaa.gov. The FTP site contains general information about the service, base station data from many sites throughout the U.S. and various utility programs for using the data. The data can be retrieved in hourly segments or as a 24-hour file. Utility programs exist for unzipping the data and combining multiple file segments. We recently found that the file joining program, JOIN24PC.EXE, is not joining the files seamlessly. Therefore it may be better to join files with post-processing software.
     Several variations of GPS data can be obtained through base station sources. It is important to make sure that the data content, recording interval and file format match the requirements of the project, the roving receiver and the processing software.
     The required data content is dependent on the GPS application. Mapping-grade systems require the L1 frequency data and often times only the code phase portion of the L1 signal. Survey-grade receivers will require the L1 frequency data or both the L1 and L2 frequency data. The user should make sure that the base station meets the minimum data requirement.The processing software will ignore any superfluous GPS data from the base station.
     The base station recording interval is also relevant. Base stations typically record data at an interval that ranges from 1 to 30 seconds. The GPS operator should set the roving receiver to the same recording rate (or an even multiple of the recording rate) as the base station. GPS measurements will only process when the base and rover recording interval match. The base station site should offer the files in a RINEX format. The RINEX format is a standard in the GPS industry and works with most types of processing software. If the base station receiver is from the same manufacturer as the roving receiver, the RINEX file format may not be necessary.
     Another consideration of choosing a base station site is the baseline distance between the base and the rover. A mapping-grade GPS receiver does not have many restrictions on the distance from the base station; it is possible for a high-quality mapping-grade receiver to achieve sub-meter accuracy when it is 200 miles from the base station. A dual frequency receiver is capable of achieving survey-grade accuracy at long baselines, but a single frequency survey-grade receiver is most effective at baseline distances less than 15 miles. The accuracy of a GPS usually includes a nominal accuracy, plus a value in parts-per-million that is dependent on the distance from the base station. The primary concern is that the base and rover(s) are tracking a minimum of four satellites in common. Since the satellites are so far from the Earth, a baseline distance of 200 miles will still allow a user to track common satellites in a fairly open environment.
     After finding a local base station, test it on known control points before relying on it for actual projects.

Q.What type of equipment do I need to obtain real-time results in my mapping-grade GPS receiver?
L.D. Edison, N.J.

A. Several options exist for obtaining real-time Differential GPS (real-time DGPS) results. Most GPS rover receivers process the standardized real-time DGPS message, namely the RTCM message, to provide results while in the field. The accuracy of the real-time DGPS solutions ranges from sub-meter to several meters, depending primarily on the quality of the GPS receiver. A number of different sources broadcast the RTCM message.
     One real-time option involves the use of an FM-based receiver. Several companies broadcast the RTCM message on an FM radio frequency. A small FM receiver connects directly to the GPS rover receiver in order to achieve real-time results. The receiver hardware is typically inexpensive, but a subscription fee is required to use the service. The cost of the subscription may vary depending on the required accuracy. Like FM radio, the RTCM signal only reaches a limited area. The FM DGPS companies have good coverage in many large metro areas, but do not cover a lot of rural areas.
     The Coast Guard has constructed many GPS beacon sites around the country. The beacon sites consist of a GPS base station and an RTCM broadcast. Although the beacons were intended for navigation along waterways, the broadcast can also be used for land-based applications. We have found that many of the beacons broadcast over a 200-mile radius or more. The real-time beacon solution requires the purchase of a beacon receiver, but no subscription fee.
     A radio-modem system between a GPS base and rover receiver is another option that is available for real-time input/output compatible receivers. The radio-modem system allows the user to broadcast their own RTCM signal. This option provides a lot of flexibility because the base receiver and transmitter are movable. The base can be placed near the project area to work within range of the radio-modem system (typically 5-6 miles).
     With a land-based RTCM transmission, the user must not only have a view to the sky for the GPS but also must be in an area where the RTCM signal is being broadcast. In order to extend the range of real-time DGPS signals, several vendors broadcast the RTCM message from a satellite. Like the FM-based systems, the user must purchase the receiver hardware and a subscription service. Although the costs are more than the FM service, the range of the satellite transmission reaches much further. Since GPS requires a view to the sky, the satellite-based system presents only a very minor range restriction. Only one geo-synchronous satellite broadcasts the signal. Therefore, the RTCM signal will reach the real-time antenna unless the view to the sky is obstructed in the direction of that one satellite.

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