GPS Q&A
By Creighton Smith

Q. I have noticed that some GPS units can be purchased with optional real-time differential or a beacon receiver. What are these, and do I need one?

A. Real-time Differential GPS (DGPS) and beacon receivers are two methods used to gain a more accurate fix on a position by projecting real-time corrections to counter errors in GPS data. DGPS and beacon receivers work similarly to post-processed differential, but instead of correcting the rover data after you've gathered it, a radio signal is broadcast to carry the corrections in real time. The two methods, which are both considered real-time differential, allow for more accurate navigation and relocation of assets. The major advantage to both is that you get instant results in the field instead of gathering data and calculating results at a later time.
      A real-time system consists of a radio reception device that is tuned to a broadcast frequency and connected to the GPS receiver. The GPS receiver calculates accurate position fixes as it receives corrections. With DGPS you build your own reference station, whereas a beacon receiver uses correction signals that are broadcast free of charge by the U.S. and Canadian Coast Guards, as well as by many other countries.
      There are many methods available to send correction data from the reference station to the rover receiver. Digital communication systems, as well as cellular telephony, can communicate real-time corrections, or you can use a licensed set of VHF or UHF radio modems. There are also companies that offer accuracy improvement services. Typically these companies work through FM sub-carrier frequencies and use "pager" technology to receive the corrections. The fees for these services are relatively inexpensive, typically around $100 per month. Some companies offer correction data using a satellite link.
      In deciding which type of system to use, you may wish to visit the U.S. Coast Guard web page at www.navcen.uscg.mil/dgps/default.htm. This site shows the location and status of each of its broadcast reference stations and lists the broadcast range of each station. If one or more of these stations are within 200 miles of your location, a beacon receiver may be your best choice. Otherwise, you may need to set up your own base station to broadcast corrections to the rover receiver.
      In that case, you would place a GPS receiver and a data link over a known position and enter the true coordinates for this position into the base GPS receiver. This receiver then begins broadcasting the correction messages. The rover receiver and data link are set up to receive corrections on the same frequency that the base is broadcasting. It is important to note that the accuracy of the real-time system will depend upon the quality of the receiver and the strength of the correction signal. Sub-meter is typically possible with one to three meters.
      The drawback to DGPS is that the type of radio broadcast system and the system's power level limit the control station's broadcast range. Without radio contact, there is no real-time correction. You should easily be able to detect any problems because most GPS receivers capable of receiving corrections will also display quality indicators. Setup parameters will also prevent you from working with uncorrected data. If radio contact is lost, most people rely on post-processed differential.
      There is no way to predict the exact range of a real-time system. However, a good rule of thumb for portable transmission stations is 25/25/25. This refers to 25 watts of power broadcast from 25 feet above ground level, producing an effective range of 25 miles over relatively even topography. If your survey area is hilly or wooded, the effective range of your radio system can be significantly limited. The signal will travel farther through damp forest conditions than it will over dry land. A beacon system has a maximum receiving range of approximately 250 miles, while the UHF maximum is approximately 25 miles.
      There are several applications that can benefit from using real-time corrections, such as navigating quickly to items of interest like survey monuments, or to hard assets such as power poles. If you've entered the point name and coordinates as a destination waypoint, you can use the navigation functions of the GPS unit to reach the desired point. Without real-time DGPS the 100-meter error introduced by selective availability can make navigation to known points difficult. Anyone needing accurate values for infield decision making will benefit as well. Your vendor can assist you with finding the best solution to fit your needs.

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