A Guide for the Uninitiated Part 1
The Bad News about GPS.
GPS – just what is it?
It is short for Global Positioning System. The system is based on a number of satellites orbiting the Earth. The satellites consistently transmit their location and time signals. GPS receivers are receiving the signals and use them to calculate their present position. There are enough satellites to provide continuous coverage around the globe. Obviously, in real life, the term GPS refers to a receiver, not to the satellites.
The important thing to remember is that a receiver needs to see at least 3 satellites (preferably more) in order to calculate the position, and that due to the nature of the signal, the receiver can see only satellites that are above the horizon. Terrain obstructions, foliage, or even a human body can block or attenuate satellite signals as well. In most situations there are enough satellites visible to a receiver to calculate the position. If your receiver cannot lock on to three or more satellites, change your position. Simple turning around to face the opposite direction may do the trick. In the woods, under very heavy foliage, you might need to look for a clearing. If you are in a canyon or such, try moving closer to the middle of it or to one of the sides.
The screen dump above shows that there are 10 satellites above the horizon, the GPS is receiving signals from 7 of them, but is using only 6. The signal from satellite #13 is too week or distorted and is not used. Signals from satellites # 19, 22 and 23 are blocked completely.
Selective Availability (SA).
GPS satellites send a number of different signals. Some of them are very precise and available to the US military and some other selected users. You and I are not the selected ones, so we can only receive signals that are not so accurate... In fact, a purposeful random error is introduced into the signal. As an effect of it, the position indicated by a popular handheld GPS receiver, at any time, may be wrong by 100 meters or more. That’s the bad news. There is also not so bad news. In reality, most of the time, the SA error is smaller than 50 meters. “Most of the time” is the key.
The system is potentially VERY accurate. Used with some sophisticated equipment it can pinpoint a position with sub meter accuracy (a meter is just a tad more than a yard). Receivers that are sold in sporting goods stores are not so sophisticated and, theoretically, could be accurate to better than 20 meters, if it were not for Selective Availability.
The number of satellites being received and their respective positions influence the accuracy as well. In Illustration #1, the position is solved as a 3D Fix (three dimensional) with an Estimated Position Error (EPE) of 19 meters. Such numbers indicate fairly reliable working of the system. If the position is indicated as a 2D (two dimensional, no altitude solution), some considerable errors may be present. A 3D solution is what you really want, but do not get fooled by the 19 meters accuracy indication. It does not account for Selective Availability. Since SA error is usually smaller than 50 meters, a total error of 70 meters can be anticipated when the receiver indicates a 3D Fix and 20 meters EPE. At the same time, without using some additional equipment (DGPS), the error due to the SA can be larger than 100 meters! Further in the guide you will find ways of dealing with the SA. My experience shows that in most cases the position indicated by a handheld GPS is accurate to within 50 meters, but NEVER BET YOUR LIFE on it!
The GPS receiver – what it can and cannot do.
The basic function of a GPS receiver is to calculate it’s present position. It does the required calculations once every second. The receiver can also store in the memory a large number of the positions. Based on that information, a number of other calculations are also performed by the receiver. The most important of these are the distance and direction between the present position and another location (or between any two locations). In order to do that, the receiver has to know where the “other” location is. There are two basic ways of telling it to the receiver. One is by storing your present position as a “waypoint”. The other one is by inputting the coordinates of a waypoint. The coordinates for input can be also obtained from a map. A GPS can show all of these locations in a numerical format or as icons on a plotter screen. The numerical format is very useful when you want to find your location on the map. The plotter screen gives more of the feel for the present position in reference to other locations and route traveled.
A GPS is not meant to be a replacement for a map. It is meant to be used in conjunction with one. Even on top of the line receivers with mapping displays, the maps are not as detailed as an average topographical map. An old trusty paper map is an excellent source of information and can always give a better overall view. It can get wet or torn, but it doesn’t need batteries to operate and it cannot break down. Do yourself a favor and have one with you every time you go off the beaten tract. More importantly, a GPS is not a replacement for a compass. The compass rose displayed by a GPS can easily fool one into believing that a compass is a built-in feature. The “compass” displayed by a GPS is based on calculations done by the unit. Since the receiver “knows” where it was a moment ago and it also “knows” where it is right now, it can calculate the direction between the two and show it in the form of a compass rose.
Have a look at the screen shot above. It is NOT a compass. The display is meant to graphically show in which direction the unit is moving. Because of Selective Availability (mentioned earlier), the receiver is fooled into thinking that it moves in a direction of 44 degrees with a speed of 1.2 kilometers/hr. In reality it is sitting still on my desk and it’s facing directly west! Because of SA, when moving slowly, do not rely on the track and bearing indicated by a GPS unit. The faster you move, the more accurate the readings are. When you move at a fast steady pace, the readings become usable. GPS can tell you what course to take to get to the intended target, but it is the compass that will tell you in which direction it is. There are situations however where a compass may be less accurate than a GPS, so on occasion you might want to make a “run for it”, to get a reliable direction reading from the GPS. Also, due mostly to the presence of SA, bearings to locations in close proximity (less than 200 meters) may be very inaccurate, so keep that compass handy. It will allow you to continue on the previously determined heading when GPS becomes useless.
A GPS, being an electronic device, is subject to failure. Personally I have never experienced one, but it is possible. Even the best of equipment can break down, be mechanically damaged, lost, or simply run out of batteries. Most of the receivers on the market can survive a lot of abuse. Mine was dropped a number of times (accidentally) on the ground and into the mud and water, kicked and stepped on. It still works the same as when I took it out of the box. I have seen a number of reports where a receiver failed and lost its ability to lock onto the satellites or lost memory and all saved positions with it. One particular brand was notorious for this, but the manufacturer has since corrected the problem. The bottom line is; if you are using a GPS for real, always have a compass and a map with you.