Australia's differential global positioning system
Your Global Positioning System (GPS) receiver will be unaffected by this action.
AMSA is discontinuing this service, as it is no longer required to support safe navigation in Australian waters.
DGPS is a ground-based GPS augmentation system, operating in the 285 to 325 kHz frequency range. It was introduced in the 1990s, to improve the accuracy and integrity of GPS information for ships navigating off the coast of Australia.
A network of 16 DGPS stations (see image below), each with a reference unit that generates corrections and provides integrity monitoring for GPS signals, makes up AMSA’s DGPS service. Their location was chosen based on the volume of traffic and degree of risk.
After the United States (US) Department of Defense made GPS available for civilian use globally in 1993, they introduced a deliberate position inaccuracy in the GPS signal. This was known as Selective Availability (SA). SA meant that GPS information could have inaccuracies of up to 200 metres. To mitigate this, some national aids to navigation authorities established marine radiobeacon DGPS stations.
These stations compute the difference (between the stations’ accurately-known position and GPS-provided position) and provide users within range of the DGPS radiobeacon, the ‘differential’ amount, thereby allowing shipboard receivers to improve positional accuracy to better than 10m. However, the nature of the radio signal means that its range (typically up to 150 nautical miles or 280 kilometres) is limited. It does not cover all of our coastal waters or indeed our EEZ.
DGPS also offers a means to monitor the integrity of the GPS constellation and positional information.
Why we are now discontinuing DGPS
Today, the accuracy of GPS exceeds the accuracy provided by AMSA’s DGPS service. Modern GPS receivers also incorporate automated integrity monitoring functionality, to warn the user when GPS information is degraded. Additionally:
- In the year 2000, US disabled SA. This improved the accuracy of GPS to better than 10 metres. US has stated it has no intention to ever use SA again.
- An analysis of the GPS Standard Positioning Service (SPS) Performance for 2018 indicates an average horizontal accuracy of better than 9 metres. The daily average accuracy of the SPS was measured to be 1.3 metres.
- The international (IMO) requirement for worldwide radionavigation systems in ‘harbour entrances, harbour approaches and coastal waters’ is that the system should provide positional information with an error not greater than 10 metres (95 % probability).
- In 2003, IMO’s GPS receiver performance standards introduced ‘Receiver Autonomous Integrity Monitoring (RAIM)’ technology. It alerts a user to any inaccuracies with particular satellites or their signals. This in turn reduces the relevance of our DGPS service for integrity monitoring purposes.
- The differential correction provided by AMSA’s DGPS service only applies to GPS. Today, there are other Global Navigation Satellite Systems (GNSS) such as GLONASS, GALLILEO and BeiDou. Anecdotal evidence suggests some 65% of GNSS receivers track GPS and at least one other GNSS.
- Multi-constellation GNSS receivers provide increasingly accurate positioning.
- In the next 2-3 years, an Australian Satellite-Based Augmentation System project, led by Geoscience Australia (GA), will offer higher accuracy positioning for GNSS users. To benefit from this service, though, you will need an SBAS capable GNSS receiver. More information on the Australian SBAS project is available at the Geoscience Australia website.
- The majority of equipment manufacturers no longer support the hardware and firmware for DGPS reference stations.
- Nations such as US, Japan and South Africa started to discontinue their services as early as 2015.
How the discontinuation of DGPS will affect you
For the vast majority of maritime users, discontinuation of DGPS should not impact the accuracy of satellite positioning or the safety of navigation.
There will be no impact on Stand-alone GNSS. However, GNSS receivers that have an integrated DGPS/DGNSS receiver, will no longer receive AMSA’s DGPS corrections and may alert or alarm. This will be no different to what happens today as the receiver moves out of range of a DGPS stations.
The effects can be summarised as follows:
- For GPS and DGPS capable receivers older than or fitted before 2003:
- If your receiver is DGPS enabled, the receiver will no longer receive AMSA’s DGPS signal. It will continue to receive GPS information. This information is likely to be accurate to better than +/- 10 metres.
- Receivers that are older than or fitted before 2003 may not include an integrity monitoring capability. This means that they are unlikely to alert or alarm if integrity of the satellite signal is degraded.
- If your GPS receiver is not DGPS enabled, then its performance will not change. For pre 2003 receivers, you should be aware that there is no integrity monitoring function built into your equipment.
- For GPS, GNSS or DGPS receivers newer than and fitted after 2003:
- If your receiver is DGPS enabled, the receiver will no longer receive AMSA’s DGPS signal but will continue to receive GPS information. This information is likely to be accurate to better than +/- 10 metres.
- Receivers newer than and fitted after 2003, will monitor the integrity of GPS information as received from GPS satellites. The receiver will alert or alarm if integrity of positional information is degraded.
What you should do if your GPS receiver is older than 2003
If your GPS receiver is older than or fitted before 2003, it might be time to upgrade your receiver. It is unlikely that pre-2003 receivers will monitor the integrity of the information they receive.
If integrity monitoring is important to you, then you may wish to replace your satellite navigation receiver with one that includes Receiver Autonomous Integrity Monitoring (RAIM).
For higher accuracy, a multi-constellation GNSS receiver may provide better accuracy than a GPS-only receiver.
The Australian Satellite-Based Augmentation System project, led by Geoscience Australia, will deliver high accuracy positioning for GNSS users. To benefit from this service in the future, you will need an ‘SBAS’ capable GNSS receiver. More information on the Australian SBAS project is available on the Geoscience Australia website.
More information about the GPS Standard Positioning Service is available at:
- GPS.gov: Official U.S. government information about the Global Positioning System (GPS) and related topics
- An analysis of Global Positioning System (GPS) Standard Positioning Service (SPS) performance for 2017, March 20, 2018, Space and Geophysics Laboratory, Applied Research Laboratories, The University of Texas at Austin.
- Discontinuance of the Nationwide Differential Global Positioning System (NDGPS); A notice by the Coast Guard on 03/21/2018, on the National Archives Federal Register.
- NDGPS General information; Navigation Centre, U.S. Department of Homeland Security.
- Trial of accurate positioning, Geoscience Australia.
How to have your say
If you have substantiated concerns that the safety of navigation could be adversely affected because of the discontinuation of our DGPS service, we would like to hear from you.