TAC 27 Index
It looks like a cellular telephone, but actually does much more interesting things than that. Here's how it works. Twenty-four American Navstar satellites transmit continuous, synchronised time signals, together with details of their positions in orbit. What this device does is to locate two satellites and measure the difference in timing of their signals due to their differing distances. From this range difference, and the known positions of the satellites, it calculates a locus of possible positions for itself: a hyperboloid of revolution, if you will. Two further pairs of satellites provide two further hyperboloids, and the intersection of these three calculated surfaces defines a point in space. The coordinates of this point are compared to the rotating geoid, and a latitude, longitude and altitude are derived. Once a second.
Since the thing knows and understands the mysteries of the Ordnance Survey grid, it can tell you your grid reference. It can also tell you your coordinates in the American, German, Swiss, Swedish or Irish grids, too, if that interests you. And because it updates once a second, it can (and does) calculate your heading and speed when you're on the move. (It has a little mental model of the Earth's magnetic field, so will give you bearings relative to magnetic, grid or true north, as you like.)
Now, it also has a memory for waypoints. Push a button, and it will remember the coordinates of the point you're standing on. Or type in the coordinates of somewhere you want to go. Later, you can call up the memory of a waypoint, and ask the GPS 45 to guide you to it. On the little screen, it'll show you a perspective view of a road: walk down the road, and you'll get to the preset point. Stray off course, and the road veers around the screen until you come back on line again. You can string a lot of waypoints together into a route, and the GPS 45 will lead you, lamb-like, from one point to the next, reassuring you all the while with figures for distance-made-good and estimated-time-en-route. Amazing. Spooky.
Designed for maritime use, it's robust and waterproof, and it has a back-up capacitor to keep all your menu settings intact while you're changing batteries (which you'll need to do after twenty switched-on hours). It's only right at this point to declare that the present reviewer is in love with this device, with that ghastly, gut-wrenching, when-will-we-meet-again love one usually forgets after the age of sixteen. But some problems must, in all fairness, be acknowledged. The astonishing display of a ten-figure grid reference (to the nearest metre!) is completely spurious. The best accuracy attainable is within ten or fifteen metres. And a very, very tiny footnote in the manual says "Subject to accuracy degradation to 100m ... under the US DOD-imposed Selective Availability Program." Which means that the US Department of Defense have jiggered the accuracy of civilian GPSs in case they fall into the hands of terrorists, who could use them to ... well, nothing springs to mind, actually. But they could probably do something really bad. No doubt about it.
So a whole new set of route-finding reflexes needs to be acquired to use this thing properly. Waypoints and lines of march should be chosen to lie well clear of dangerous ground. But remember that the positional error is not cumulative: the GPS 45 constantly draws you back on course for whatever fuzzy point you've set, in complete contrast to the horrible strayings that can occur when trying to walk a compass course in thick mist. The wee screen can be dangerously seductive. This reviewer has gone quite spectacularly on his neck while trying to follow the little road on the screen, instead of watching his feet. In fact, it's better to walk briskly and check the screen from time to time. Little detours around obstacles can throw the track information off for a while, and the temptation is to stop until things sort themselves out. Of course, the thing then loses all idea of your present ground track, and freezes whatever information it last had. A blank screen and a warning message would be much safer, but probably didn't seem relevant to a manufacturer aiming at the marine market. Oh yes. And it costs #299. Love hurts.
Ed. - Late news from The Guardian OnLine, 11/4/96: "Non-military [GPS] users will now be able to pinpoint things to within 50 feet instead of the previous 300 feet." This sounds like good news - although just how quickly it will happen remains unclear, as does how you obtain 15m accuracy on this type of grid reference.
TAC 27 Index