The Global Positioning System: A Shared National Asset (1995)
Chapter: Findings
autopilot is preferred to manual control during long voyages because it saves fuel and reduces crew workload. This equipment requires stable velocity inputs, which are unavailable from the SPS with SA present. Methods exist to smooth or limit SA errors, such as the integration of inertial navigation systems with GPS, but vessel operators may be unwilling to bear this additional cost burden.
The Coast Guard's DGPS service itself is also affected by the presence of SA. In order to keep up with the high rate of clock dither present with SA, the system's radiobeacons must broadcast differential corrections at a high update rate. These corrections then require most of the bandwidth available on the 283 KHz to 325 KHz signal. A slower correction rate would allow the broadcast of other safety-related information that may be critical to mariners in the coastal and harbor regions.
Integrity
Under current operational procedures, the GPS master control station (MCS) does not monitor the integrity of the SPS. An improperly operating satellite can be detected by observing errors in the broadcast of the Y-code, but it is possible for errors to exist in the C/A-code regardless of the status of the Y-code. Because of this situation, the Coast Guard has stated that DGPS radiobeacons would still be required even in the absence of SA. Other integrity issues for maritime DGPS users result from the potential lack of accurate electronic nautical charts used in ECDIS's as was discussed above.
Availability and Radio Frequency Interference
RF interference to both GPS and DGPS radiobeacon's are significant issues for the commercial maritime user because interference has a direct impact on signal availability. In the marine radiobeacon band (283 KHz to 325 KHz), atmospheric interference from electrical storms will occasionally interfere with operations. Vessels operating with additional sources of navigation information can cope with lapses in availability, but users of only GPS and DGPS cannot.
Findings
GPS and DGPS are now in use in the maritime community for a number of navigation, positioning, and surveillance applications.
The full benefit of GPS and DGPS will not be realized by maritime users until systems such as ECDIS's eliminate errors produced by inaccurate charts and incorrect plotting. Up-to-date digital hydrographic data is required for the electronic charts utilized by these systems.
