NOTICE TO MARINERS
No.14/07 B8, C5, D2, E1, F2, G1
GLAs PUBLIC DGPS TRANSMISSIONS
The Differential Global Positioning System (DGPS) is provided as a marine aid to navigation, giving 24 hour a day, all year round service with overlapping signal coverage up to 50 nautical miles around the coasts of the United Kingdom and Republic of Ireland. The differential signals are transmitted using a terrestrial network of medium frequency marine radiobeacons in the 283.5 - 315 kHz band. For mariners equipped with a suitable receiver the signal provides both real time integrity monitoring of GPS derived positions and the capability of fixing their positions to better than 5 metres (95% probability) in moving applications. Greater accuracy can be achieved in stationary applications.
All mariners are advised:
1. DGPS relies inherently on GPS, the operation and characteristics of which is under the control of the US Department of Defense. Users are cautioned that the signal availability and accuracy may be subject to change without warning.
2. The DGPS service is provided primarily for use in monitoring the integrity of GPS to enhance the safety of marine navigation. The provision of greater accuracy for marine navigation is a secondary feature.
3. Signal reception may become unreliable, under certain extreme environmental conditions, towards the limits of the geographical coverage.
4. All radio navigation systems are susceptible to interference (including jamming) and environmental effects, which can adversely affect their availability. The GLAs strongly advise that no single aid to navigation
system should be used in isolation and that DGPS users should use all alternative means available to cross check the information received. Users should also ensure that they have a receiver which gives sufficient warning
of the complete loss of the DGPS signal and reversion to GPS.
5. Various DGPS receiver types are available, some of which may not
provide appropriate or timely warnings in respect of the system.
6. DGPS should not be confused with Space Based Augmentation Systems (SBAs) such as EGNOS (European Geostationary Navigation Overlay Service). EGNOS enabled receivers generally do not receive DGPS signals nor do DGPS receivers receive EGNOS signals.
7. Reference should also be made to Volume 2 of the Admiralty List of Radio Signals. In particular, Mariners' attention is drawn to the section dealing with
horizontal datums on charts and satellite derived position notes.
Captain D. Glass,
Director of Navigational Requirements.
London, EC3N 4DH.
30th March, 2007