Portuguese DGPS Network

During the last decade, maritime DGPS stations have been installed by most of the developed countries, providing high-quality, free service to all users. Portugal has recently joined this service, with two DGPS stations at Carvoeiro and Sagres and a Control Station located at the headquarters of the Portuguese Lighthouse Authority. These have been operating since December 2002 and a one-year trial is currently underway with the aim of validating the stations so that they can be declared fully operational.

With Selective Availability (SA) turned off, stand-alone GPS accuracy has improved significantly from previous the level of 100m (95 per cent). The US authorities currently ensure a horizontal accuracy between 13m (95 per cent) and 36m (95 per cent). Although these figures do not consider three important GPS error sources (ionospheric and tropospheric delay plus receiver noise), they have proven to be pessimistic and users are, generally, getting accuracy in the order of 7 to 8m in the horizontal plane. One of the reasons for this above standard performance is the number of satellites in the current constellation. In the early years of this new millennium the number of GPS satellites has remained consistently above 28, despite performance standards requiring a constellation of only 24 operational satellites.
Nevertheless, accuracy in the order of 7 to 8m may be difficult to get in certain places at certain times, when the number of visible satellites falls significantly, due to:

• Periodic maintenance of the caesium clocks and repositioning manoeuvres of satellites
  
• Shading of the satellites by mountains, large structures and buildings
  
• Unscheduled outages of satellites
  
  Despite all these variables, GPS accuracy improved dramatically after the removal of SA, making it sufficient for many applications and groups of users. However, extra accuracy is still essential in some maritime applications, such as piloting and navigation (including harbour manoeuvres), dredging, buoy positioning and hydrographic surveying.
  The transmission of differential corrections to the GPS signals allows elimination of most of the errors of GPS, improving its accuracy to better than 2m, as long as the Reference Station is not too distant from the user (less than 200 NM).
  Aside from accuracy, the removal of SA does not overcome the main issue for mariners: integrity, which is the capacity of the system to broadcast timely warnings indicating when it should not be used for navigation. Real-time monitoring of satellite health is performed at the Master Control Station, which uses data collected by the six GPS Monitor Stations distributed around the globe.
  However, these six stations follow each satellite for only approximately 93 per cent of time. This means there are gaps in the monitoring network and the satellites are unmonitored for some periods each day. Besides, these ground stations have a slow reaction time and, as a consequence, navigators could be given incorrect positions for 2 to 6 hours without any warning.
  One of the best ways to provide an independent assessment of satellite health is the use of differential stations. DGPS stations permanently monitor the signals of visible satellites and if they detect any malfunction or failure in a healthy satellite then they eliminate it from the navigation solution and flag that satellite's number in the appropriate message. Detection of bad data from a healthy satellite and broadcast of the appropriate warning is done within approximately 10 seconds.
  
  What Has Changed?
  The removal of SA not only improved the accuracy of GPS but is also enabling better results with DGPS.
  SA errors were completely compensated by the differential technique but the corrections lost their validity after a short period of time because SA was a random, fast changing, error. Therefore, to obtain good DGPS results corrections had to be transmitted very frequently, thus placing a considerable strain on the communication channel used to broadcast corrections to users. With the removal of SA, this strain to broadcast very frequent corrections has been relieved because the remaining GPS errors have a slower changing rate: tropospheric/ionospheric delays and errors in the satellite ephemeris and clocks do not change as quickly as the random shifts generated by SA.
  Therefore, if DGPS corrections are broadcast every 10 seconds, as they were before discontinuing SA, then good results may still be obtained even if DGPS corrections are lost (due to interference or other reasons) for periods of up to 15 minutes. The result is increased robustness of the DGPS service and reduced susceptibility to interference.
  Another benefit for the DGPS service of SA being set to zero is the possibility - due to a reduction in the amount of data to be broadcast - of transmiting crucial safety of navigation information using a special text message. An example is Navigational Warnings and meteorological or hydrographic data. The same messaging may also be used to broadcast information on scheduled outages and maintenance actions at the DGPS stations.
  In a near future, the spare datalink capacity gained after SA was turned off may be used to broadcast messages containing phase corrections capable of giving sub-metre accuracy in real-time. Carrier phase corrections are similar to pseudo-range corrections and are computed using the carrierphase measurements made at the DGPS Reference Station.
  
  Portuguese DGPS Network
  Since December 2002 the two Portuguese DGPS stations have been operating on a pre-operational basis. A series of trials have been conducted which will be enable the validation of the service before it enters full operation. Two more Broadcast Stations are planned for the archipelagos of the Azores and Madeira in the Atlantic Ocean, expected to become operational in the year 2004.
  The purpose of the Control Station is to provide timely monitoring and control of the equipment installed in each Broadcast Station via the
  exchange of appropriate messages (RSIM protocol) over a communications network (fixed line telephone network). The Control Station is immediately notified of the occurrence of any alarm situation in the Broadcast Stations and also receives periodic reports from each. In addition to these reports the system operator is able to contact any of the Broadcast Stations and view the site's operation, on-line at any time.
  The Portuguese DGPS stations were designed to meet availability and continuity requirements established by the International Maritime Organization and therefore all essential components have stand-by units. This is because redundancy of major functions is fundamental to ensure that the operation of the station will continue uninterrupted in case of a hardware failure. The essential and most critical components are the Reference Station (the heart of a DGPS Station), the transmitter (the indoor component most likely to fail) and the transmitting antenna (which is very exposed to weather and to corrosive salty environments). The duplication of these critical components ensures not only high availability, but also high continuity of service.
  
  Preliminary Trial Results
  As already mentioned the Portuguese DGPS stations are being tested for a period of twelve months, during which several trials have been and will be conducted. One of these trials had the purpose of comparing the accuracy of GPS, DGPS and the EGNOS System Test Bed (ESTB).
  EGNOS stands for European Geostationary Navigation Overlay Service, which is the satellite based augmentation being implemented in Europe. Although EGNOS is not scheduled to be fully operational until 2004, the ESTB, a simplified version of EGNOS available within Europe, began transmitting signals via an INMARSAT satellite in February 2000.
  The following table summarises the results obtained on this trial and helps in understanding the performance of the three systems during this period.
  This experiment confirmed that stand-alone GPS is performing very well in terms of accuracy. However, the sort of accuracy being obtained is by no means assured and therefore to consistently get accuracy in the order of 1 to 3m differential services are still required.
  With regard to the maritime DGPS beacons, the accuracy advantage they provide is still significant, as they are able to ensure accuracy within 1 to 2m with almost 100 per cent certainty provided that the user is not too far distant from the DGPS station.
  To estimate the amount of spatial decorrelation of the DGPS corrections, for a period spanning 11th to 14th April the DGPS receiver was tuned to Carvoeiro Broadcast Station (which is approximately 40 NM distant from Lisbon, where the trial was conducted). From 14th to 17th April it was tuned to Sagres Broadcast Station (which is approximately 100 NM away from Lisbon). As expected, the results were slightly improved when the nearest Broadcast Station was employed: the increase of 60 NM in the separation from the nearest DGPS station resulted in a degradation of the accuracy in the order of 30 centimetres.
  Concerning the ESTB, it performed less well, at least in Portuguese territory, with the system proving unable to improve on stand-alone GPS accuracy. The fully operational EGNOS system, which will have a highly redundant configuration with many more ground stations, will surely perform better. Nevertheless, until that occurs, users must use the ESTB with caution, as the European Space Agency warns with the disclaimer on its web-page.
  
  Conclusion
  In terms of accuracy, it is clear that DGPS no longer offers its former dramatic benefits before SA was discontinued. Nevertheless, and despite the significant improvement in its accuracy, stand-alone GPS is not yet sufficient to comply with some accuracy requirements. However, the main benefit of DGPS is the integrity, which gives users an extra-assurance that the position is correct.
  With the termination of SA, DGPS broadcasts have also improved their services by reducing the susceptibility to interference and transmitting safety of navigation and meteorological messages. In the future, the addition of new messages containing phase corrections will allow sub-metre accuracy.
  Nevertheless, users, and in particular mariners, are now able to get a freely available high accuracy service adequate for the more demanding applications, including hydrographic surveying.