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Tuesday, April 26, 2011

GPS dependancy

So we all know what GPS is. The appearance, in recent years, of consumer level devices that give your position graphically, that is, by displaying a map with a you are here marker on it, and which also give directions, provides much potential for humour.

First, a history lesson. As most of us know, GPS was developed by the DOD in the 70's and 80's, for military navigation. After the Korean airliner KAL 007 was shot down by the Soviets after straying into their airspace, President Reagan directed that GPS be made available for civilian use once it was implemented. The system was partially operational during the Gulf War, in which it was widely used, and was declared fully operational in 1995. Originally, the only location data a GPS receiver would supply was latitude and longitude, requiring one to keep a stack of maps nearby. Also, the system originally contained a feature known as Selective Availability. This feature was intended to degrade the usefulness of the system to unauthorized persons, by adding errors to the signal. These errors were psudorandom, and depended on the time of day. A key was produced daily, which when used in a specialized GPS receiver, would make corrections to the signal, giving a precise output. A typical error would be about 100 meters horizontally, and 50 meters vertically. In 1996, President Clinton issued an executive order declaring GPS a dual use system, and mandating that Selective Availability be shut off by 2006. After systems were developed that could deny the use of GPS to hostile forces by other means, Selective Availability was shut off in 2000. Now we have the inexpensive units with maps and routing algorithms and all manner of other bells and whistles that serious navigators and surveyors would just find annoying.

Now for the humour. I find the extent to which people become dependant on these devices to be somewhat pathetic. I was at a family reunion last august, and a relative was going on about how hard a time they had navigating, because they had accidentally reset their GPS, thus destroying all their stored maps. So I said: "But you can still get a latitude and longitude out of the thing, right?" to which they said "I don't think so." Doesn't anyone use paper maps anymore?  This pathetic dependence gets even more funny when people start running the consumer devices up against their limits. I strongly suspect that most people aren't aware that a GPS receiver doesn't tell you exactly where you are, it just tells you to within a certain range, which, for consumer grade units, is typically 3-5 meters. This, coupled with processing delays in the routing algorithms, can make life quite interesting when you're driving at speed on older freeways, with closely spaced ramps. I recall riding through Minneapolis and St. Paul with my sister and her fiance, a yooper who is overdependent on his GPS.  We missed one or two exits, and were directed onto another right at the last possible moment. Those routing algorithms are a piece of work too. They will send you through some of the weirdest routes possible. In the trip mentioned above, I drew on my experience of having been a commercial driver in the twin cities. The GPS gave conflicting directions. It apparently didn't know that Highway 62 is a godawful bottleneck that I prefer to avoid.

All this leads me to believe that it would be absolutely hilarious to turn Selective Availability back on. The outputs of the routing algorithms would be quite interesting if the receiver thinks that its 100 meters to the left of where it actually is. However, there is a work around for Selective Availability. It's called Differential GPS. In this system, two GPS receivers are used. One is set up over a point of known latitude and longitude. Readings are taken from this receiver, and the known values of latitude and longitude are subtracted from the reading. This provides a linear transformation that can be used to gain accurate readings from the second receiver, which is used normally. It would be quite humorous to see a group of ditzy teenage girls attempting this.

There is another method of boosting the precision of GPS devices, called RTK, or, Real Time Kinematic. The workings of this system are complex, so feel free to skip the following explanation:

GPS works by receiving multiple identical signals from satellites, comparing the signals to an internally generated copy of the signal, and using the difference in timing between these signals to calculate the distance to each satellite. Using these distances, the receiver can then calculate its location. The signal is a stream of pseudorandom data. An RTK system does the same thing, using the carrier signal, instead of the pseudo random data. The increase in accuracy comes from the increased speed with which the signal is received. The pseudorandom signal is a digital signal, transmitting 1020408.163 bits per second. The carrier signal is an analog signal, with a frequency of 1575.42 MHz. With a good A-D converter, considerably more than 1020408.163 bits per second are available.

In a nutshell, it also requires a base station, and if made available as on option on a car, would add 10k-15k to the price tag. Because of this, its used mainly in surveying and precision agriculture.

One could always use a sextant, but for driving, that would be more complicated than reading a map. However, it would be quite humorous to see a group of ditzy teenage girls attempting this.

Moral of the story: Your GPS won't save you, at least not for what you paid for it.

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