Description of Global Positioning Satellite (GPS) Receivers

What:

Why:

How:

A "constellation" of satellites (different orbits) hopefully ensures that at any time of day or night, at least three (preferably four) navigation satellites are overhead any spot on earth -- up to a certain latitude, that is; GPS does not work well in arctic latitudes though I'm told by my good friend from Iceland that they work all the way to the arctic circle.

The receiver has no way of knowing how far distant a satellite is. But, it can compare the the arrival times of the beginning of each satellite's signature pattern, which is repeated over and over again and talked about more in a few paragraphs to come. In this manner it can determine the geometry of itself relative to the satellites, and the satellites know where they are by uploads from ground stations (the "ephemeris"), and from all that the GPS can compute its position on the earth.

Each satellite transmits, at the end of each signature, a portion of the ephemeris. The data rate is slow, about 50 bits per second, and it takes about 12 minutes or so for a GPS to obtain a complete ephemeris. Until then, it relies upon a stored, estimated geometry called an "almanac" or an older ephemeris from the last time it was used. This is why GPS will sometimes "wander" in position, particularly for the first 12 minutes of operation, if it has been turned off for some weeks. As it learns that the satellites are not where it thought they were, it recomputes where it thinks the receiver is located.

The computer in the GPS then performs spherical trigonometric calculations and displays where you are. In fact, if four or more satellites are properly distributed, it can even give you an altitude reading that is immune from barometric variation -- the starfish reading puts me at 35 feet below sea level, not bad considering my barometric altimeter might put me hundreds of feet below sea level.

The technology turned out to be so much more accurate and precise than expected, it created a problem for military security. Consequently, a disturbance is introduced called "Selective Availability" and the satellites are deliberately misinformed about their true position. The effect of this is an unpredictable error of less than 100 meters or so. Techniques exist for obtaining accuracy anyway. Averaging is easy and common, giving an estimated position accuracy of 30 feet or so (S.A. not included, which can add 60-300 feet of error). Differential GPS uses a receiver at a known position, it listens to the satellites, determines the position error, calculates a correction, and broadcasts on a radio signal the correcting information. Your Differential receiver listens to the satellites, then applies the correction, and your actual position is then known to a few feet. This is commonly done at harbors and rivers where position accuracy is essential and military threat not so great. Another one, Carrier Phase differential GPS is used by surveyors and is good for centimeters!

Selective Availability is supposed to be turned off eventually by act of Congress. When that happens, 30-foot accuracy should be repeatable and common. For this reason, my new unit, the Garmin III Plus, displays distances to waypoints all the way down to feet (or meters). At the moment, it is misleading accuracy, but if S.A. is turned off, then those feet will be useful.

All satellites are "talking" on the same frequency. It is an endless babble. Each satellite says something slightly different, however; and a template for each exists in the receiver. The template is a binary stream of 1's and 0's. An operation called "exclusive OR" is utilized to determine bit-for-bit matching. Older units simply converted all of the resulting success bits into a voltage, and when the template matched, the voltage would be highest. But false matches were common and the receiver would sometimes decide it had the satellites wrong, cancel and start over. Newer units appear to be entirely digital, with digital pattern matching that can lock onto multiple satellite patterns simultaneously. The single channel receiver cannot be moved while it is acquiring satellites and it doesn't much like to move even after that. The multiple channel receivers are not bothered by movement during or after satellite acquisition. Accuracy: I have no way right now to measure absolute accuracy, although some web sites have conducted exhaustive experiments on the subject. Right now, Selective Availability (deliberate error) is already greater than the accuracy of the GPS receivers, which is to say, they are very precise. My newer GPS each provide an "estimated position error" which is computed from the satellite geometry, signal strength and so on. EPE's of 160 feet down to 35 feet are common. If the satellites line up in a row, they cannot provide a triangulation, because they are not forming a triangle.
 
 

Slick Features:

WARNINGS:

GPS does not always work as expected!
Here are some things that cause them to not work:

When:

Right Now! GPS has been working for many years, but only in the past two years have GPS receivers reached a price level, size, battery efficiency, and sensitivity to make them applicable to such a wide variety of uses.

Where:

This is not an exhaustive list! but it will get you going quickly. Most camping, boating, hiking outfitters will have GPS. Major department stores (Shopko, Wal-Mart) have GPS so you can find them almost anywhere. NOTE: GPS come in "flavors" according to the lingo of the buyer and intended purpose: hiking, driving, flying, boating, mapping. These flavors influence the type and appearance of the navigation screens (display options) and the abbreviations you find. One will say HDG (heading) and another will say COG (Course Over Ground). Advertisements seldom indicate which way it is going to be, but the GPS Store has little icons to help you choose the application (icons indicating boating, aviation, hiking, bicycling, etc).

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