WHAT IS A GPS AND DO I NEED ONE? Bernie O'Shea

Some of us have been wandering around the bush for a long time, sometimes with a map and/or compass, sometimes without. We've seen lots of good views, climbed up and down ridges, crossed rivers and camped in beautiful spots. Sometimes a member of the party had been there before, or had learned about it from a friend. Sometimes we just saw a name on a map and thought "That might be a nice place to see ..." Mostly we got where we wanted to go, or at least fairly close to it, and haven't come to any lasting harm. There is a long tradition in bushwalking that when we don't go or end up exactly where we intended, then that is alright, as long as we get home for dinner. Never lost - only a slight temporary geographic embarrassment! After all, it's easy to find the summit of a mountain - that's the place with the 360 view. The best place to cross the flooded river is not found on a map. It's the place with the big fallen tree across it. This is why aids to navigation like GPS will always be secondary to planning, common sense, experience and a good map. (But a GPS might be handy in locating the fallen tree bridge the next time!).

OK then, what is GPS? It is the Global Positioning System and it is a very clever and complicated navigation system designed and maintained by the American Department of Defense (DOD). It uses a number of satellites to provide instantaneous position, velocity and time information almost anywhere on the globe, at any time, and in any weather. It is used for personal navigation (bushwalking, boating, driving etc.), aircraft navigation, offshore survey and ship navigation, dredging, land surveying, mapping, and more esoteric applications.

There is a network of 24 satellites (plus a few spares) that orbit about 11,000 nautical miles up (roughly 25,000 kilometres from the earth's centre, or 20,000 kilometres above the earth's surface). There are six different orbital paths and each satellite makes two complete orbits around the earth in just under 24 hours. (That's a speed of about 4 km per second). Each satellite weighs about 1 tonne, has on board a number of extremely accurate atomic clocks (worth heaps of money) and is about 5 metres across with solar panels extended. The transmitter power is only 50 watts or less, so we need very sensitive radio receivers to pick up their signals from that distance. That such a receiver can be built into a handheld device the size of a mobile phone, is fairly amazing (at least to us old guys!)

The whole shebang is monitored by the DOD from five ground stations throughout the world (none in Australia). Satellites are monitored every 1.5 seconds, for precise orbit and clock performance, which information is then fed back to them at intervals by the Master Control Station at US Airforce Space Command at Colorado Springs, using three of the monitoring stations for uplink. This is necessary because the whole system depends on each satellite knowing precisely where it is at all times, and broadcasting this, together with the precise time, so that the ground units can work out their own position.

The actual information broadcast by a GPS satellite contains:

(1) A "pseudo-random code". This identifies which satellite is transmitting.

(2) Ephemeris data - which contains important information such a status of the satellite (healthy or unhealthy), current date and time, and

(3) Almanac data, which tells the GPS receiver where each GPS satellite should be at any time throughout the day.

Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system. So each satellite in effect is broadcasting a message which essentially says, "I'm satellite #X, my position is currently Y, and this message was sent at time Z." Of course this is a gross oversimplification, but that's the idea.

The bushwalker standing on the Bogong high plains in the middle of a blizzard and complete whiteout, and out of sight of any snowpoles, will be glad that information is available to her GPS receiver! To determine its (and her) position, it needs to compare the time the signal was transmitted with the time it was received. The signal travelled at the speed of light, so the machine can work out an accurate distance from that satellite. If it can add in the distances from a few more satellites, there is a way to mathematically work out exactly where it is on earth. And if the bushwalker can tell it the position of the nearest hut from his map, then it can work out a direction and distance to go. AND if the bushwalker starts moving, the GPS will notice its position is changing, and can work out a direction and speed - and from that even an estimate of when they will reach the hut. Ahhhhh!! Pity they haven't taught it how to put the billy on yet!!

So this is what a GPS receiver can be used for. It is all just manipulating information received from some satellites, and mathematical computation, so there is a similarity to computers. Most GPSs have some memory for the user to store positions (waypoints), sequences of waypoints (routes), and to record where it has been (tracks). Most also have provision for a cable connection, so that these items can be transferred to and from a home computer. Some GPSs have more memory to store a background map, which is useful for displaying position. Otherwise the position will have to be communicated by means of a latitude & longitude, or a map grid reference, which a human will have to relate to a paper map. Of course, you can connect the output to another computer in an aircraft, and let the pair of them drive the autopilot. Maybe one day to the computer in charge of your car? - Horrors! Some GPSs also have a magnetic compass and a pressure altimeter incorporated to improve accuracy and performance.

How accurate is it? Well, it varies (day to day and minute to minute, depending on the geometry of the satellites). Until May 2000, the US government was introducing deliberate random errors into the GPS signals, so that only the US military could get the best results. That has now stopped, although Uncle Sam still can access a slightly more accurate signal than those available to civilians. What used to be errors of approximately 100 metres have now improved to 10 - 20 metres - quite good enough for bushwalking! If the receiver has access to another signal from an extra, stationary, precisely located transmitter, so that errors can be cancelled out, then it can be accurate enough even for surveying work - that is, down to centimetres.

So - how much would you expect to pay? Without the complimentary steak knives, you can buy one of these toys (that is the pocket sized, waterproof bushwalk model) for about $300. Another, non-waterproof model with internal mapping facilities is about $600ish, while the models with everything including the magnetic compass and altimeter are about $900 or so. Of course, you may be able to find some bargains in the secondhand market, or by purchasing overseas.

Well, that is a rundown on the state of play in navigation. The system only became fully operational in 1995, so things are moving fast. Hardly a week goes by without somebody thinking of a new way to use this information. Proof of distances and heights flown by glider pilots in competition. Local councils cataloging their outdoor furniture and bike paths, and also recording positions of wrecked and dumped car bodies. And games for enthusiasts, like "Geocaching" which is essentially a treasure hunt for hidden goodies placed by other players - the positions being published on the Internet.

 

References:

http://www.gme.net.au/
http://www.ja-gps.com.au/
http://gpsinformation.net/
news:sci.geo.satellite-nav (Usenet news group)