MAIN HEAD Trusting GPS DECK HEAD We have all heard reports about unreliable GPS systems, but what can seafarers do about it? We review the options: alternative position satellites, DPGS and expensive GPS systems which know how accurate they are BODY All navigators are taught that reliance on a single source of navigational information is foolhardy. When using satellite derived positional data from a single source, such as GPS, the navigator should be regularly checking the validity of the displayed position with reference to other inputs such as radar, depth soundings and visual data. Part of the 'problem' with a modern GPS installation is that it is reliable and accurate, diminishing the user's experience of having to cope with fault conditions. "If it's always right, do I really need to keep checking so regularly?" can become the thought process. Well publicised incidents involving GPS malfunctions have increased navigators' awareness of the system not being perfect but at the same time it has led to better equipment and system design, which in turn is diminishing the likelihood of a navigation fault occurring without an appropriate automatic alarm being given. SUBHEAD Alternative position satellites Ideally the marine world needs an additional global navigation satellite system (GNSS), entirely independent from GPS. Position fixes from the two systems could then be compared easily by the navigator (and automatically by the navigation system) to more positively flag positional inaccuracy problems. It was originally expected that the Russian GLONASS system would fill this role and IMO agreed a resolution 'Performance standards for shipborne GLONASS receiver equipment' in 1996. Unfortunately the international community has not embraced the use of GLONASS, mainly because of questions about the continued reliability of its positioning service. For many years the European Union has been investigating whether it should provide its own 'Galileo' global navigation satellite system. This has now reached the stage of a well thought out system that ensures good stand-alone accuracy and integrity but is also engineered to properly complement GPS to enable the highest possible integrity from affordable dual-service receiving equipment. The current Galileo brochure talks about an effective operational service from 2008 but some commentators doubt whether that date can be achieved; in the worst case the whole project could perhaps still be cancelled. This means that for quite a while the world has to rely on GPS. So, how does the mariner increase the likelihood of the GPS fix being reliable? In many coastal areas the answer lies in the use of differential GPS (DGPS). SUBHEAD Differential GPS (DGPS) Over 30 countries now provide maritime users with local differential GPS stations. With an appropriate onboard receiver these enhance the basic accuracy of position fixing in most circumstances to better than 5 metres. (13 metres is the normally quoted figure for 'standalone' GPS). This accuracy advantage was particularly important when the Selective Availability (SA) function was utilised as a security measure on the GPS system, which limited standalone accuracy to about 100 metres. Now that the US has 'set SA to zero' the improved accuracy of DGPS is not so necessary for most maritime operations. However, use of DGPS offers integrity enhancements over that of standalone GPS, greatly enhancing safety. This is because special software running at the DGPS stations effectively uses the accurately surveyed site geographical position to check whether the pseudoranges from each GPS satellite are correct. If a faulty pseudorange is detected then it disables the broadcast of corrections applicable to that particular satellite and alerts user equipment that positions should not be calculated using that satellite. This stops unhealthy satellites degrading the positional accuracy or, when there are multiple failures or a small number of satellites in view, the user equipment can flag that the positional solution is degraded. Typically, user systems will be alerted within 10 seconds by DGPS if a satellite becomes unhealthy, as opposed to the tens of minutes that it can take the GPS authorities to assess and then broadcast that a satellite is unhealthy. SUBHEAD GPS self-assessment Even without a DGPS input GPS receivers themselves can be designed to make certain calculations (under most conditions) to judge the likelihood of the accuracy of its position fix. For example determine that there is a 95% probability the displayed position is within 15 metres of the actual position. It can do this, providing there is a 'reasonable' constellation of 5 or more satellites, by comparing the navigation solution from different selections of 4 satellite constellations. This procedure is known as receiver autonomous integrity monitoring (RAIM). Such a receiver is now called for in the new IEC maritime GPS receiver standard, known as IEC61108-1 (Second Edition), released in July 2003. It incorporates updates made in the year 2000 to IMO's requirements for GPS. The increased integrity requirement has been introduced to properly address the IMO words, "The equipment shall provide an indication if the position calculated is likely to be outside of the requirements of these performance standards". In the previous IEC standard this was restricted to calculations on HDOP (errors introduced into the position because of a poor satellite constellation), low position update frequency, loss of position and DGPS failures. Availability of the relatively new RAIM techniques together with affordable processing solutions, have allowed IEC to introduce this additional requirement into this new edition to better meet the IMO performance standard. SUBHEAD Safe, caution and unsafe The integrity of the displayed position has to be expressed in three states: safe, caution and unsafe. These integrity states are also required to be provided to other equipment across the digital interface. The manufacturer can use colours to indicate the integrity status, and not surprisingly green (safe), yellow (caution) and red (unsafe) have been chosen. The equipment must react within 10 seconds to display 'negative' changes in integrity. 'Safe' is displayed if the integrity calculation can be performed at a confidence level above 95% and that the calculated accuracy falls within the selected level. 'Caution' is displayed if the calculation of integrity is likely to be at less than a confidence level of 95%. This can happen if there are too few satellites available or the geometry of the available satellites limits RAIM calculation accuracy. The 'caution' label applies to the estimation of accuracy. The actual positional accuracy could be anything between excellent and poor! 'Unsafe' is applied when a good reliability integrity calculation can be made and the calculated error exceeds the selected accuracy. The IEC standard requires that the integrity indications must at least be selectable for 10 metre (95%) and 100 metres (95%) user accuracy requirements. Switching for additional accuracy levels may also be provided by the manufacturer. In fact the 100 metres requirement is historic because of SA issues and perhaps of little use today. Maybe 95% levels of 5, 10, 13, 15, 20, 25, 50 and 100 metres would be a useful (and compliant) set. RAIM is not as good an integrity solution as DGPS but in just about all circumstances it will alert the user when the positional accuracy is suspect because of satellite errors. On the occasions that it cannot make this judgement the user is alerted to this fact. There is always some time-lag before revised IEC standards become embedded in national maritime administration requirements for equipment type approval. The revision will certainly be adopted in Europe at the next revision of the Marine Equipment Directive. The date of this is difficult to predict but it will probably be late 2004 or early 2005.