What:
- A personal or mobile navigation device that uses satellites to obtain its position.
- Tells you where you are.
- Can guide you to a spot, not merely in a straight line (direct path) but optionally via waypoints that take you around obstacles.
- Records a track of where you have been.
Why:
Hikers, hunters, fishermen, boaters, even shoppers trying to find an automobile in a huge parking lot! Simply mark your position before heading out. You can later return to this mark, either directly via shortest path or via the route that it has been recording every five minutes or so.
They have already found a camp in the wilderness and identified its location with GPS. Then, using your CB or Amateur radio, you learn their location and enter a landmark or waypoint. Then you click on "Go To" that landmark or waypoint, and a compass appears telling you the direction and distance to your companions. See warning below about trees and cliffs.
- Find and/or mark a spot that has few or no landmarks.
Numerous applications; good fishing spots, grave markers, rural road bridges, the oversize deer you just shot and now you must go get help to pack it out, your boat sank but you still have your GPS, you dropped crab pots into the water and need to return to the buoys. Once a position is obtained, you can then use the Census Bureau's Tiger Map Server to produce a detailed map of the area. Very cool!
Fun! I have tested my Magellan 2000 on a commercial aircraft at over 500 miles per hour, it told me where I was, how fast I was flying and in what direction. Ever look out the window and wonder where you are? Getting a satellite lock through those tiny windows was very difficult; took ten minutes to get it in the right position, but at least it gives satellite quality indicators to help with this.
Just put one of these on your dashboard and use it as a very precise speedometer.
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.
Most GPS receivers have many slick features intended to not merely tell you where you are, but help you get somewhere.
The unit can give you a heading (course over ground) but only if the unit is moving. It would be nice to have a GPS with a fluxgate compass built in (able to produce a compass rose when standing still OR moving), but so far I have not seen one.
If you have four satellites (or more) and suitable configuration, it will give you an altitude above sea level. It exhibits short term unstability of several hundred feet, but you never need to calibrate it for sea level air pressure! Newer units perform sample averaging which stabilizes the reading and improves accuracy.
The GPS receiver can navigate you AROUND obstacles, either by backtracking a stored route, or by having entered the waypoints while sitting at the marina, or in a tent. Then you can put your maps and charts in a safe place and proceed on your way, checking the waterproof GPS receiver from time to time to make sure you are still on course, and if not, it will even guide to back onto "the beam". Magellan GPS 2000 and GPS 2000 XL provide a numerical reference (as well as a track display) for "cross track error", the Garmin III Plus provides a road display and a track display but I have not found a numerical cross-track error. Precision navigation through shoals might be better served with a numeric track error AND a graphical track display. At the cheap prices of GPS, get them both, one for numbers, the other for chart.
Most have a track plotter; ranging from the snapshot plot of the Magellan 2000, to moving plots that automatically shift as you travel (Magellan 2000 XL and above, and I think all Garmin units), to ones that have built-in maps (upper level Magellan but especially Garmin units) to ones that connect to a computer (Delorme, Garmin III Plus) and use a detailed cartographic database on CD-ROM.
A few units display actual cartographic maps (highways, rivers and lakes), a special aviation model has built-in airport control zone maps, very good for light aircraft. Larger, fixed-mount models have more sophisticated map display capabilities, some with special plug-in detailed maps.
- TWELVE CHANNEL RECEIVERS:
Not all have this; a one or two channel receiver can still track multiple satellites, but must multiplex (skip from satellite to satellite); and they take longer to acquire the satellite and are more likely to be disrupted while moving in a car or boat, especially near trees or cliffs.
- COMPUTER UPLOAD/DOWNLOAD:
Many units can interface to a computer, or to other navigation devices. The benefit is uploading a long list of waypoints, or, returning from a trek, downloading the fixes so that they can become waypoints for your return to that particular adventure or even evidence that you have traveled at certain times to certain places. Software is available to line up the downloaded fixes onto a map so that you have a detailed visual representation of where you were. You can enter points that represent boundaries of public or private land, depending on where you wish to go or avoid.
- EXTERNAL ANTENNA CAPABILITY:
Used to be an important consideration, but the Magellan GPS 2000 XL and the Garmin III Plus work just fine in every situation I have encountered.
Barometric altimeter (with auto-calibrate to satellite fix, and able to report absolute pressure in inches mercury and millibars for calibrating other barometric altimeters), thermometer (one Magellan model has a thermometer in it already), and it would be nice if it would wash clothing, cook supper, and teleport me to my favoriate camping spot...
Magellan has a GPS receiver with built-in email uplink to the Inmarsat communications satellite. It's expensive (a kilobuck) and the subscription to Inmarsat is not cheap, but just think of it, no matter how remote, no matter how deep the canyon you are in, you can send and receive email; and by hitting a button, include your present latitude and longitude. Cool, no?
GPS does not always work as expected! Here are some things that cause them to not work:
You may find yourself under a wet forest. The short wavelength is very sensitive to foliage, especially wet foliage. The Magellan GPS 2000 refused to work most of the time under the thick wet forest cover of the Pacific Northwest, but the later 12 channel units work fine in those conditions. Using an auxiliary, active (amplified) antenna will fix that problem.
Your batteries may die suddenly. Display might still be on, but you lose the satellites mysteriously.
The satellites may line up in a bad configuration (you cannot triangulate unless the satellites form a triangle!). Not all satellites work properly, either. You need at least three good ones in a triangle overhead. Most GPS give you a map, based on their stored ephemeris, of where they expect the satellites to be; and in difficult circumstances this helps you aim the antenna for better performance (or to get a fix at all).
- EMP (ElectroMagnetic Pulse)
that is to say, World War 3. The pulse will zap all satellites within a certain distance (and your receiver, too) making it all rather pointless for the survivors of armageddon. Learn to use a compass, altimeter, and map. Until then, GPS is excellent!
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.
