35. Can I improve the position I get from my handheld GPS unit?

This question generally refers to those handheld GPS units designed for outdoor activities, such as, cycling, orienteering, and hiking which are in the $500 or less price range.

When a position is determined using one GPS receiver without the benefit of corrections from a secondary source, it is known as point positioning; that is, the GPS receiver is working in autonomous mode. Due to various error sources, the largest WAS the Department of Defense (DOD) intentional signal degradation (i.e., selective availability - SA), the typical accuracy is (was) usually better than 100 meters of the true position. Also, by averaging several readings your accuracy may be improved.

To increase the accuracy of autonomous GPS, differential corrections can be applied to the GPS observations. Differential corrections eliminate or reduce errors associated with the measurements made to each satellite (pseudo-range) that was used in determining the point position. There are two common methods for applying these corrections; real-time differential GPS and post-processed differential GPS. Both methods require information from a secondary source (e.g., a second GPS receiver located at a known point).

Real-time differential GPS requires a radio link between the rover receiver (i.e., your handheld unit) and a base receiver (i.e., the GPS receiver located at a known point). Via this radio link corrections to the pseudo-ranges are made before the point position is computed. Once the position is computed all the pseudo-ranges and pseudo-range corrections are not usually saved. Even if this information were saved, it would be somewhat inappropriate to further correct the data. Depending on the equipment, distance from the base station, and a host of other variables your positional accuracy can be improved to the sub-meter level.

Post-processed differential GPS requires that GPS data be collected simultaneously at both the rover and base receivers. These data must include pseudo-range measurements to common satellites and precise time stamps for each measurement. After the measurements are made, the base and rover station data are combined via a computer with post-processing software and corrections are applied to the rover data. Depending on the equipment, the distance from the base station, and a host of other variables; this method can also improve your positional accuracy to the sub-meter level.

For both real-time and post-processed differential GPS, you are required to have additional hardware. Real-time GPS requires that your handheld GPS receiver includes an attachment capable of receiving radio signals containing differential corrections. Post-processed GPS requires that your receiver can output appropriate data in a proper format (e.g., RINEX format). Post-processing also requires software that will allow you to combine data from the rover and base stations, and a base station collecting appropriate data in a proper format.

A general rule of thumb is that if a receiver is real-time or post-processed capable, then the manufacture will make accessory equipment (e.g., real-time rover radio receiver) and software (e.g., post-processing software) available, although usually at an additional cost. The good news is that numerous base stations, supporting real-time and/or post-processed, differential GPS are available throughout the United States. The U.S. Coast Guard provides real-time differential correction free of charge to anyone within the range of their broadcast beacons. Also, the National Geodetic Survey provides GPS data for post-processing activities from about 400 continuously operating reference stations (CORS) throughout the United States. Users may freely download these CORS data via the Internet at www.ngs.noaa.gov/CORS