The GPS satellites transmit two position information messages. The most precise, the "P" code, is encrypted and is normally available only to the military and other specially authorized users. Civilian GPS sets use the "C" code information. Until the spring of 2000, the Department of Defense, the operator of the GPS system, intentionally degraded the accuracy of the "C" code information. This degradation process was called Selective Availability (SA) and was intended to deny precise position data to unfriendly users. The DOD has discontinued the use of SA, to the delight of millions of GPS users. Today, the typical user will find that the position of a fixed terrestrial reference point will vary by not more than about 50 feet 95% of the time. Depending on atmospheric conditions, the observed position variation may be less than a few feet. This level of accuracy is sufficient for most, but not all navigation purposes.

When in use, SA created serious navigation accuracy problems for vessels navigating in near shore waters, especially near and in major port areas. Safety of navigation required a means to eliminate the intentional inaccuracy created by the DOD with SA. In response to this need, the Coast Guard established a coastwise network of radio transmitters from which they broadcast satellite error correction messages. The Coast Guard was able to avoid the cost and time delay of building new transmission stations by converting existing low frequency radio beacon stations for the accuracy improvement project. The system is called Differential GPS (DGPS). Receivers at each DGPS site monitor all GPS satellites in view. By comparing its precisely known position with the information received from each satellite the DGPS equipment can compute the corrections necessary to eliminate virtually all of the error inherent in the signal received from each individual satellite. The error corrections for each satellite visible from the DGPS station are assembled into a correction message that is transmitted on a low frequency (200-500 kHz) to all DGPS receivers within range, usually about 50 - 100 miles.

DGPS navigators contain two separate receivers, one for the signals from the satellites, the other for the low frequency correction signal. Although DGPS transmitters are normally some hundreds of miles apart it is necessary to ensure that they do not interfere with one another. For this reason, the DGPS transmitters operate on different frequencies within the low frequency beacon band. Most of the DGPS receivers can automatically search for and tune to the strongest signal, eliminating the need for the operator to make this selection. However, many DGPS systems allow the user to manually select the desired frequency, a capability of value in atmospheric conditions that effect signal propagation on the low frequency bands.

Reception of the low frequency DGPS signals requires a separate antenna from that used to receive the gigahertz GPS signal. Two types of low frequency antennas are used, an "E" field antenna resembling a short whip or an "H" field antenna that is usually built into the dome-like GPS antenna. The base of the "E" field antenna must be connected to the vessel's ground system. The "H" field antenna is usually a bit more expensive, however it does not require a separate ground wire, making it somewhat easier to install. The "H" field antenna may be somewhat more immune to the static electricity interference caused by nearby lightning storms.

The elimination of SA may make it unnecessary for some mariners to install differential capable GPS equipment. However, DGPS provides a means for informing GPS receivers of an "unhealthy" satellite that is transmitting erroneous information and has not yet been corrected or turned off by a ground based control station. If you routinely navigate in areas where precise position location is important DGPS is well worth the added cost. DGPS provides superior accuracy, with occasional maximum errors of less than ± 15 feet 95% of the time and typical errors less than ±8 feet. The use of DGPS also provides noticeably improved stability in both the speed over ground and course data displays.

GPS use is not limited to marine navigation. The aviation community is a major user of GPS and will, according to current planning eventually become almost totally dependent on the GPS system. The need for a high degree of air navigation precision and reliability has led to the development of an aviation version of DGPS, the Wide Area Augmentation System (WAAS). In the U.S., WAAS will consist of 22 ground reference stations similar in function to the Coast Guard's DGPS stations. Since aircraft quite distant from each monitoring station need the correction data, the WAAS correction messages will be transmitted to geostationary satellites for retransmission to aircraft. The error correction transmissions from the satellites are at the same frequency as the GPS satellite transmissions, eliminating the need for a separate receiving antenna for the differential correction signals. Aviation DGPS systems will be programmed to accept these data and apply the corrections to the satellite signals they are using for navigation. The end result will be closely similar to what is achieved with marine DGPS. A few marine GPS units are now able to utilize WAAS differential correction data. It seems likely that WAAS capability will become part of the basic GPS system. A further aviation spurred refinement of DGPS, Local Area Surveillance System (LASS) will provide correction signals for airborne GPS used to make precision instrument landings. LASS accuracy is on the order of a foot or two, and includes instant warning of any system malfunction. Unless you are planning on running your boat up the centerline of a flooded runway this system appears to have no marine application.