GPS Landing System Battle Intensifies

ICAO recognises the need to put the future world system into space and envisages a Global Satellite Navigation System, but it is reticent for the reasons outlined below to accept GPS as the core of that system. However whilst maintaining this stance, ICAO is in a Catch 22 situation since GPS and Glonass, the Russian counterpart, are the only systems likely to be in existence for many years to come.

In order to learn more about the overall world situation in regard to GPS, and where we stand in Australia, I recently arranged to be briefed by Captain Ian Mallett, Chairman of the CAA Global Satellite Implementation Team. Set out below are the main aspects of that briefing plus additional industry sourced information.

The Background to GPS

Whilst there is no doubt that GPS represents one of the most significant advances in aerial navigation since man first started flying, it must be remembered that it was conceived as a military system and remains so.

The US DoD which owns the system, has allowed it to be used for other than military purposes but degrades the GPS signal for civil use to 100m accuracy through a process known as “selective availability” (SA). Using decoding devices, an accuracy of 10m can be achieved for military purposes which is sufficient for smart weapon accuracy. Selective availability is one of the practical problems limiting the adoption of GPS as a precision approach system, however there are ways around it as will be outlined later.

Although the US Government has offered the world the free use of the GPS system with a guarantee of six years notice of intention to decommission, there is an understandable reluc- tance on the part of the international aviation community to embrace GPS as the core of FANS since the US DoD has made it very clear that it intends retaining control of the system, even beyond Initial Operational Capability (IOC) which is the point at which the system is formallyreleased for civilian use.

The recent introduction of the 24th satellite into service completed the planned constellation of 21 satellites plus three spares, and guarantees 100m accuracy positioning, for 95% of the time, 24 hours a day, anywhere in the world.

US Overlay Programme

It goes without saying that an item of approach equipment must meet certain technical standards before a descent using that equipment may be made below the initial approach fix. These standards are laid down in technical service orders (TSOs).

Because GPS is a military system, with system integrity monitoring in ground equipment rather than in the aircraft, (which is normally unacceptable for civil use), ICAO has not produced any international standards for GPS receivers, yet paradoxically a decision on GPS versus MLS has been set down to be made at the end 1995! Because of the lack of international standards the US is proceeding with it’s own programme (codenamed Overlay) for approval of the system for civil use, for non precision approaches. The first phase of Overlay has commenced with approval being granted for GPS to be used on certain non precision approaches, providing VOR and NDB guidance is available as supplemental equipment. The existing approach charts are to be used in this interim phase.

Phase II will commence later this year when IOC is granted by the US DoD and the system is released for non military use. In anticipation of this next phase, TSO C129 was released late last year which called for, inter alia, Receiver Autonomous Integrity Monitoring (RAÍM) in which the receiver itself checks the validity of the satellite signals. Unfortunately this requires six satellites to be read simultaneously and as this can only be achieved at any one location on the earth’s surface for 65% of the time, it places operational limitations on the use of the equipment. In addition, unfavourable local conditions eg, mountains or even trees can severely limit satellite acquisition. Finally, any errors developing in the system can take up to two hours to rectify which in itself is a significant operational limitation.

Accordingly although GPS receivers meeting TSO C129 will be cleared for stand alone non precision approaches to flight planned destinations, alternates will require supplemental guidance as for phase one.

subject to satisfactory operation during the previous phases, Phase III is planned for late next year at which point, stand alone use of GPS for enroute and non precision approaches will be approved.

Finally, as an indication of the speed with which GPS is being introduced into the US aviation market, Trimble recently announced that it’s TNL 2000T and TNL 2100T GPS stand alone GPS equipment under TSO C129 A2 for enroute navigation receiver systems had received FAA approval as navigation.

Australia’s position vis a vis GPS

So where does Australia stand in relation to approval of GPS for other than as a supplemental VFR navigation aid? As outlined earlier, ICAO has yet to set international standards for GPS which is severely limiting countries such as Australia proceeding down the GPS path. Clearly it would be unwise and very expensive to attempt to develop a unique set of standards for local use. As Captain Mallet was at pains to stress, if international standards existed, there wouldn’t be a problem.

An added complication which is providing a strong imperative for Australia to seek to have the way ahead resolved is that DMEA (Distance Measuring Equipment Australia) is scheduled to be taken out of service at the end of 1995, again because of frequency problems. Of course in the worst case, RPT could continue to use the good old fashioned radio compass, which would still meet all legal requirements, but as DME arrivals are a feature of the present system in Australia we would suffer a significant reduction in operational capability. The most likely outcome is that GPS receivers meeting TSO requirements will be approved as an enroute aid for RPT operations in the interim pending resolution of the international situation. This equipment should be part of an integrated package but again in the interim will most likely be approved as an add on.

Hopefully the Overlay programme in the US will provide sufficient operational experience in the next year or so to make the way ahead much clearer than it stands at present.

The Fiji Trials

Fiji, with the assistance of the US FAA is trialling the use of GPS with airborne equipment made available by the US. With only 19 domestic RPT aircraft, the whole fleet could be equipped with GPS for the cost of one ground navigation aid. This should prove to be a worthwhile supplemental trial to the US Overlay programme.

Differential GPS (DGPS)

Whilst current GPS systems are capable (but not yet fully approved) of providing non precision approach facilities, the latest GPS development, DGPS, provides a precision approach facility which is capable of matching ILS/MLS precision approach accuracies. In fact one company, Trimble, maintains that its DGPS is capable of meeting both horizontal and vertical accuracy requirements for CAT-1 landing.

The Differential GPS landing system utilises a differential ground station, the position of which has been precisely established. The ground station, knowing its position, reads the satellites and determines the current error. This is trans- mitted to all aircraft capable of receiving the information and corrects their satellite reception. Of course a datalink is necessary between aircraft and the ground station.

Only one differential ground station is required per geographical area regardless of the number of airports or runways. The single station is capable of providing enroute navigation and traffic control, feeder navigation and traffic control, precision approach guidance, area surveillance, and surface operations and control.

All this can be provided without any radar input, but of course requires that all vehicles both air and surface borne, be fitted with the appropriate equipment. The economic benefits are considerable since the equipment is relatively inexpensive to purchase, install and maintain. There is no requirement for equipment on the airport surface, no long stretches of power cables and the ground station can be located anywhere.

At the same time as Trimble is proceeding with its developments, Honeywell has received US FAA approval for the development of an integrated GPS/flight management system (GPS/FMS) in conjunction with Continental Airlines, to provide a non precision approach capability at two Rocky Mountain ski resorts.

The equipment combines Honeywell’s FMZ-900 FMS with a 12 channel GPS receiver and will be installed in Continental Express ATR 42s and Dash 7s.

The equipment will be trialled in two phases, the second phase will include fitting the GPS/FMS in a Continental MD-80, and the development of prototype DGPS stations for test of a precision approach capability at the two ski resorts and a third station at Houston in Texas.

Finally, the US AOPA in its push for GPS to be adopted as the replacement for ILS has been trialling DGPS for precision instrument approaches using the Association’s Bonanza fitted with a Trimble six channel TNL-2000A panel mounted GPS receiver. In addition to conferring greater flexibility, AOPA has found that GPS guidance is easier to follow than ILS as it does not suffer from increasing sensitivity in the final phases of the approach. AOPA is using a Trimble Navigation TNL-2800 Landing and Tracking system as the ground station to transmit the DGPS signal.

GPS and the Private Flyer

As part of my research into GPS (and a very good excuse to justify the expense!) I took the opportunity to purchase a hand held Magellan Nav 5000A for my own use in VFR flight. My immediate impression was that here indeed was a piece of equipment that I could have done with a few times in the past! As anyone who has flown with a GPS will tell you, it has taken all the fun out of wondering just where you are on those long stretches where pin points are few and far between.

Used as a VFR assistance navaid, which is all it is cleared for at present in Australia, it is an excellent piece of equipment and well worth the money. However there are traps for the unwary or incautious which need to be addressed.

The CAA is concerned that a tendency might develop for VFR reliance on GPS alone for getting from A to B without any other form of flight planning. The rationale envisaged is that some will see it in the same light as the pocket calculator which has to some extent removed the need to understand basic mathematics. Who needs to be able to flightplan and map read when a little hand held gizmo will get you to where you want to go? There are essentially three problems. One is obvious and that is equipment failure such as the batteries suddenly going flat leaving you with a blank screen. Trying to change them in flight is not an exercise I would recommend, and in any case, if the problem is not identified soon enough, all data can be lost. Always remember that batteries provide their power whilst under load, which means that usually a battery that static test as two thirds active may have insufficient energy to operate the device in question when ‘on’ has been selected. Thus a battery that appears to be fresh may well fail after two or three applications, not a good thing whilst halfway to you destination, over mountains and in turbulent conditions etc etc. . . The second problem is the pilot who dials in B as his destination, gets airborne, punches the navigation button and leans back to enjoy the view, occasionally consulting the magic box. There is every chance that he will fly through control zones and restricted areas with never a thought. In addition, in flying a straight line course to his destination he will most likely deny himself good ground features in areas where prudence dictates a less direct route.

The third problem is incorrect entry of data, ie waypoints in the wrong position; here checking track angle and distance on the map will highlight errors and save subsequent embarrassment in flight.

My short experience with GPS confirms that it is a quantum leap forward in the art of finding your way around the country, but I strongly recommend that conventional flight planning and map reading remain the basis of VFR navigation, and that GPS be used as a supplemental aid. I use a CompuNav navigation computer to prepare a flightplan, checking against the GPS tracks and distances. Intelligent use of the facility to dial in waypoints can assist in keeping clear of those areas you need to avoid.

One final point to bear in mind is that aircraft tracking over a common waypoint using GPS are now at greater risk of collision because of the accuracy of the equipment. With the advent of silent VFR flights this might manifest as a problem as more and more pilots equip themselves with GPS.

The future of GPS

Clearly the time is rapidly approaching where the US Department of Defense can no longer claim to have exclusive control of GPS. To attempt to continue with the concept of S/A (selective ability) will surely fail against inevitable strong commercial pressures for the full capability of GPS to be available to all.

Finally, with little likelihood of a full Global Navigation System, independent of GPS coming into service for decades, and certainly not within the time scale of the demise of ILS, it would appear that ICAO will have to soon recognise GPS and establish international standards.

However, as stated at the beginning, there still remains the question of whether GPS or MLS will be the precision approach system of the future. This debate, to be fought out over next few years, stands to put the Olympics 2000 bid into the minor league of international competition.