A Look at GPS


Exciting new advances in GPS technology benefit GIS users
by Kyle Bohnenstiehl


Removal of Selective Availability

By now almost everyone who has ever heard of GPS knows that on May 2, 2000 the Department of Defense turned off the signal that degrades GPS accuracy in the Department of Defense-run network of satellites. The capability to determine your location to within 5 meters anywhere on the planet or in earth orbit for as little as $49 is a remarkable event in human history. The benefits of increased autonomous positioning to recreational GPS users are apparent, but how will the removal of Selective Availiability (SA) help the GIS mapping and surveying community? Several realtime satellite and land based DGPS beacon receivers that I have seen tests from seem to indicate that there is no great jump in positioning accuracy in a CA code DGPS scenario. Will we see quicker position fixes on our survey grade units, better RTK initialization, and decimeter accuracy out of single frequency DGPS units? Probably not, but it is nice to know that the system is finally running at the level it was always capable of. The increase in commerce related to this event will be large, and this is one technology that is sure to find many new uses in the future if the applications are mature and well thought out.

One type of GIS application that will benefit are the new color LCD Palm Computers running some type of mapping software such as ArcPad by ESRI or Solo CE from Tripod Data Systems combined with a GPS. These small computers can display aerial or satellite imagery, soils information, vegetation types, hazards, detailed topography, utility lines and even the location of other parties in the area if connected via a wireless network. Before the removal of SA, the GPS position was subject to inaccuracies as great as 100m which would cause the position to jump all over the screen and render such technology essentially useless. Now the GPS position is accurate to around 5-10m and we are able to navigate from one location to another with some degree of confidence. We know that where the cross hairs are located is really where we are. With SA turned off, GPS users now have positioning that is more accurate than most USGS 1:24000 quadsheet maps so new sources of high accuracy basemaps will be in demand. The technology and data exist now for a mobile GIS at an amazingly affordable price of around $1200. Companies that introduce this technology to the broad user community in an easy-to-use format will certainly succeed. Land managers, farmers, ranchers, law enforcement agencies and scientists can all benefit from such technological advances and will be part of a large new market of users. This is all made possible because of real time GPS accuracy on a level that is acceptable to many people across the current and potential user community. With the planned implementation of the new L2 and later L5 GPS signal in a few years, we can expect an even better level of autonomous real time GPS positioning, perhaps on the order of one meter. It is a brave new world indeed, one that geographers and surveyors have awaited for centuries.

For more information, see an excellent summary of the benefits of SA removal and how specific applications will benefit, at the NOAA web site. For updated daily global assessments of GPS accuracy see the Air Force Space Command web site.

Real-time Differential GPS (DGPS)

The Coast Guard is placing a series of DGPS beacons along the nation's coastlines, waterways, and major rail routes to implement real-time differential GPS service as a navigational aid. Land-based users within range of a beacon can also benefit from this no-cost service. Some factors that may effect your adoption of this technology include:

Users must decide if it is worth the technological and financial investment to improve their GPS accuracy to around 1-2m, which may only provide slightly better utility than an autonomous GPS fix of about 5-10m. However, DGPS does provide a consistently more reliable accuracy than autonomous positioning which in some scenarios could degrade down to +/- 30m. In all cases, if you need to know how accurate your GPS position is, you must stand on top of a known point that you have a coordinate for and compare the differences.

There are two competing satellite based DGPS services which are reliable and can provide a position to within 0.4m under ideal conditions, but are costly at around $800 for a one year subscription. This is still the only DGPS system for some parts of the world, but it does not have global coverage yet. The same hardware issues are present here as with the Coast Guard beacon system. These DGPS beacon systems would be better utilized if beacon technology was simpler and more affordable with a seamless integration into the GPS unit or an OEM GPS card and antennae. (For more information on the Coast Guard DGPS infrastructure visit U.S. Coast Guard. Also see OmniSTAR and www.raycal.com for information on commercial DGPS beacons. For information on radio beacon hardware see Garmin, Inc. and also The GPS Store as starting places.)