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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. A 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.
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.
Because of Selective Availability (mentioned below), the receiver is fooled into thinking that it moves in a direction of aproximately 300 degrees with a speed of 23.5 In reality it is could be sitting still on my desk facing NNW Because of this 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. There are situations however where a compass may be less accurate than a GPS, (near to big metallic objects or at known rock that contains iron ore [like in Skye])
Remember you must have a compass and the ability to use it whenever you use your GPS. A 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. |
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Receiving signals The important thing to remember is that a receiver needs to see at least 3 satellites (preferably four) 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, 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 valley or gorge try moving closer to the middle of it or to one of the sides. If this does not work the only way is to get out of the valley or use traditional map and compass
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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. Remembering and adapting to the “Most of the time” is the key.
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GPS accuracy The system is potentially VERY accurate. Used with some sophisticated equipment it can pinpoint a position with sub meter accuracy. Receivers that are sold in mountaineering 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. 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, the error due to the SA can be larger than 100 meters! In most cases the position indicated by a handheld GPS is accurate to within 50 meters, but NEVER BET YOUR LIFE on it!
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More on Accuracy The basic purpose of a GPS is to indicate the position. The best news about it is, that even with the effects of SA in place, it does a very good job. For an mountaineer (unless near a large drop) it is usually sufficient to know the position within 50 meters. Your GPS should usually be sufficient to give you enough accuracy to put an X on the map. Remember that an X on a map covers most of the 50 meters anyway. The ability to use the information that this X shows from a map or to plot it back onto one is the key to GPS usefulness. For an outdoor enthusiast, it can never be overstressed.
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Finding the same place again (when you are actually at that place) In order to re-find your place with the GPS, check the surroundings for a very distinct and visible feature. It may be a rock, it may be a tree, Create a waypoint at this feature and then add it to your waypoint list with a name you can remember. When you want to return you switch to the units "go to" mode and then follow the directions (using a compass as mentioned earlier the unit is not a compass) When you get to the position of the feature you can set the unit to tell you this.
Warning - due to the error you may be near to but
not actually at the original position so use your knowledge gained earlier As you will no doubt be aware DO NOT USE THIS TECHNIQUE when near a large drop as the error upon return could put you in a compromising position. |
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Reliability You must ALWAYS remember 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.
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New Toy If you are
normal, the last thing you do is... read the users
manual. Put the batteries in and let’s push some buttons. If you have the unit turned on for a while, especially outdoors, a message “POSITION ACQUIRED” (or similar) will appear. Most new units will automatically lock onto satellites and acquire a position within 5 minutes. If it takes longer than that, you might have to enter the initial position manually. The booklet that came with the GPS receiver explains how to do this. If you have not lost it already.
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Basic setup – the things you have to do if you plan to use a GPS with maps. There are only two options you have to set, in order for the receiver to work properly with a printed map. The first option is DATUM. that if the receiver is set to display a position in a different datum than the one indicated by your map, the indicated position might be wrong by hundreds of meters. You need to set your GPS to OSG (Great
Britain Ordinance Survey) Like wise if you go on holiday to America and want to use your GPS, you need to look at a publication and find the code for North American maps and then enter this (when you go For topographic maps of North America, the most commonly used datum is NAD27. Some newer maps use WGS84)
The second option is the POSITION FORMAT. It determines the way in which the position is displayed by the GPS receiver. By far, the easiest to use is the UTM. Again, all current topographical maps for North America show the UTM grid. If you use a map, which does not have the UTM grid, you might have to set the unit to Degrees. Minutes or Degrees.Minutes.Seconds. In some countries, the map grid is different than the two mentioned. If that is the case, make sure that the chosen receiver, in fact, can use that particular format, before you spend the money
Basic setup – the convenience options. There are a number of options, which affect the way GPS works. They are mainly a matter of personal preference, or they may be simply dictated by a particular situation. To make following the guide easier, you might want to set up your receiver the way I do. It certainly is not the only way to do so, but I found it to work really well for bushwhacking. In no particular order there are: - in the system setup, under “UNITS” I use Bearing to “MAG” (magnetic). This will cause the unit to display course / bearing information in relation to the Magnetic North and match magnetic compass readings. It is not the same as True North! (as described elsewhere on this web site) You have to be careful with this setting. There are a number of areas where the earth's magnetic field is locally disturbed, sometimes very significantly. Your GPS may not account for it. - again, under “UNITS” I set “METRIC”. As Ordanance Survey uses meteric units If you must use feet and inches then the option is “STATUTE” - POWER SAVE – I leave it OFF. Setting it to ON may conserve batteries, but there is a tradeoff in the sensitivity and possibly in accuracy, especially under a tree canopy. If you use your receiver in wide open terrain, you can set that option to ON without adverse effects. - ALARMS – I have the ARRIVAL alarm set to ON, with the distance set to 200 meters), if navigating a route or to a waypoint. CDI (course deviation) alarm set ON to 500 meters. The CDI alarm alerts me if I stray too far to the side of the intended route. All other alarms I have usually set to OFF. - SOUND – usually I have it set to OFF. Since I hardly ever use alarms when bushwhacking, there is no need to scare the wildlife with beeping and buzzing noises.
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Before you go As with all navigation, before entering any unfamiliar
territory, always study an actual map (rather than the GPS screen
map) |
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Batteries Always carry spare batteries for your unit. In the wild, don’t throw out the old ones. They not only are a toxic waste, they might be able to provide enough power for one more position fix, when your back-up battery set dies unexpectedly.
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Words of caution and advice The techniques described above do work. I do not guarantee that they will always keep you out of trouble. In fact, if applied indiscriminately, they can put you into trouble. Some situations may require completely different methods, some will allow you to adapt the described ones.
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