GPS Navigation Accuracy Tested

Enter GPS. The Global Positioning System (GPS), comprising a constellation of 24 satellites, was devised by the United States Defense Depar tment to give pinpoint accuracy 24 hours a day and engineered to be impervious to electronic jamming and interference. GPS is accessible for civilian use by aviators, surveyors, transport companies, mariners and a host of others.

Actually there are two systems: GPS and Glonass. Glonass is a Russian system also using 24 satellites (21 operational and 3 standby). Combining both the GPS and the Glonass systems will allow greatly increased accuracy. These systems will evolve to become the sole means of navigation when they eventually replace the existing navigation aids for general aviation.

The Fans (Future Air Navigation Systems) Committee of ICAO, after several years of consideration, concluded that the exploitation of satellite technology is the only viable solution that will overcome the limitations of the present system and address the needs of GA in the future. Until studies on performance of the system are concluded, joint agreements between the USA and USSR are reached and standards of equipment established, a GPSreceivercan only be used In conjunction with other approved navigation equipment.

There are approximately 14 GPS satellites (with additional ones being launched at the rate of one every 60-90 days) in orbit at this stage giving a minimum of 17 hours coverage with 24 hour coverage of all of Australia by 1992. Ultimately GPS-Glonass will provide highly accurate and reliable navigation for flights in areas without suitable ground based navigation systems. System integrity will be such that it will eventually enable future development of zero visibility landings.

GPS will be the primary system of the not-too-distant future, however the existing system of VOR’s etc will co-exist with GPS well into the next couple of decades. The NAV portion of a NAV/ COMM however most likely to be capable of being replaced by GPS. Affordable technological advances in navigation equipment for pilots/owners of aircraft have been few and far between

ATranspak GPS (por table) madein the USA by Trimble Navigation and distributed by Peter Patroni of IEI (Australia) in Melbourne was l:;>orrowed for evaluation on a recent flight over four days. The route was Canberra/Bourke/lnnamincka/Birdsville/Lake Eyre/Maree/Birdsville Track/Arkaroola/Leigh Ck/Wilpena Pound/ Broken Hill/Menindee/Griffith/Canberra. Almost the entire trip was flown at 500-1,500ft AGL, VFR, full SAR, with the intention of photographing floodwater, ruins and other places of interest to my friends and I. Unfortunately circumstances prevented the arrival of the Trimble GPS until one hour before departure so this precluded study of the operator’s manual until the overnight stop at lnnamincka.

The latitude and longitude of each of the abovementioned places together with several points that were not named on our WACs were entered in the memory and each given a number. Abbreviations or full names could have been used, however for speed of programming I chose to use numbers. After about 11/2 hours studying the Trimble operator’s manual I was able to enter and cross check some 30 fixes. Lack of time prevented detailed study and therefore the full potential of the GPS was not realised, however it was quite remarkable what this unit could do.

‘Kanowana’ Ruins, some 59nm west of lnnamincka, was to be the first trial for the GPS,especially as the landscape had changed dramatically due to recent floodwaters down the Cooper’s Creek. This leg, and the next leg to Birdsville, was flown at 1,000ft QNH and below making dead reckoning navigation very difficult. After leaving lnnamincka for the Kanowana Ruins and becoming established in cruise, the Trimble GPS was turned on to ‘NAV’ position. The unit instantly displayed the first waypoint (Kanowana) with bearing, distance, track groundspeed, ETE (estimated time enroute). Also on the screen was a simulated VOAtype display graduated in 1nm increments. The COi (Course Deviation Indicator) showed a blinking vertical bar which indicated we were 1 mile left of track. The unit is so sensitive that it will update to the second ETA and ETEs changed by winds, aircraft pitch variations and diversions from track.

That one display had me ‘sold’ as there is no single unit which gives so much information (for its size and relatively low cost) when compared to NAV/COM, ADF and DME with their limitations of scalloping, errors and/or range especially at low level.

Trimble Transpak comes complete with carrycase, cigarette lighter plug and battery pack (Nicads or 8 AA cells). If the aircraft electrical system goes u/s and you are concerned about draining the 4 hour life of the battery pack, the GPS can be turned off and only turned on when the necessary information is required. Ninety-nine waypoints can be stored or it can be upgraded to 999 at additional cost. ‘WPT’ (waypoint), one of the eight switching positions on the unit, allows storing of fixes by label or numbers which enable alphabetic or numeric searching or scanning of the waypoint library.

in the past but with the potential, abilities and price of GPS that is rapidly changing. The Trimble GPS, compared to other navigation systems, is certainly affordable at the 1990’s price of close to $5,000 (depending on $A-$US).

The GPS can calculate track and distance between any two waypoints in the library without relying on satellite tracking. Switching to ‘SETUP’ enables the selection of land, sea or air mode and metric, imperial or nautical measurements. The ‘TIME’ function displays the date and time (24 hours, UTC or local) ETA, time of last fix and ETE. Asterisks in the ETA and ETE locations indicate that the GPS is either not performing fixes or that you are moving away from the intended fix.

‘DIST’ function. When the GPS is switched to this position it calculates the range and bearing between any two waypoints using a great circle route. (This is faster than the normal map method and the only way if you lose your protractor!) Diversions are ‘a piece of cake’ as the GPS gives you your position (‘POS’) at any time. You only need to enter a new fix (or select a waypoint from the library) to find distance, bearing and ETEinstantly.

Satellite ‘window’ time is not 24 hours yet but it was more than adequate for this trip. Some signal loss was experienced due to shielding by the airframe but a remote antenna, as distinct from the inbuilt type, would have solved the problem. The hours of satellite window availability are obtained in ‘STS’ (satellite status) function which gives the number of satellites currently being tracked and whether the Trimble GPS is making position fixes. The remaining time of the present window and the start time for the next window is also displayed. If you are not in a ‘window’ it shows the start time of the next one and its duration. Fantastic!

More time would have allowed a better assessment of this remarkable unit and its other functions. The encouraging aspect of purchasing a multi-channel GPS such as the Trimble Transpak is that it will not become redundant during your lifetime as some systems have in the past.

If aircraft owners were contemplating additional navigation equipment or replacement of existing equipment I would recommend giving serious consideration to buying a portable or panel mount GPS. Some aircraft have limited panel space which could be solved by placing a GPS on top of the instrument panel (no compass interference was observed). Open cockpit aircraft pilots would also find navigation easier without the worry of paperwork blowing away.

Trimble GPS users such as aviators, fishermen/women sailors and four wheeldriving adventurers will appreciate the versatility of this unit. I would suggest that flying schools, clubs or aircraft hire companies wanting to maximise aircraft usage should consider making a GPS available (for a small daily fee) to aircraft hirers.

Back to Kanowana Ruins – the GPS was so precise that we arrived directly overhead and, as only the foundation stones were visible, I had to orbit at the GPS fix so that my passengers and I could locate the ruins. This situation was repeated in several other places. The GPS was so accurate that the most difficult part was for the people on board to distinguish the barely recognisible features against the landscape. It was a relief when flying at such a low level over fairly desolate terrain to be able to enjoy the scenery and fly without spending a lot of time with my ‘head in the cockpit’ working on dead reckoning, measuring new tracks and calculating revised estimates, etc.

Some claims as to the accuracy of GPS have been overstated or as a consequence of initial system testing which allowed smaller errors. However, that testing was for military purposes but civilian receivers use a degraded signal giving accuracies to approximately 20 metres vertically and 30 metres horizontally. Land surveyors using ‘Differential GPS’ with reference and field receivers plus appropriate computer software can achieve accuracies of one centimetre or better.

As I said at the beginning of this article, there aren’t enough superlatives to describe the Trimble GPS. I am so impressed with it that my Glasair’s avionics shopping list will definitely include one. Peter Patroni at IEI Australia, Phone (03) 544 8411, Fax (03) 544 8648, distributes the Trimble GPS and will welcome enquiries.