This is a guest article about how a GPS works. It was written by RJ Stapell, of High Trail Expeditions, who I met at the Overland Expo 2012.
The GPS System
The GPS navigation system currently maintained by the U.S. Department of Defense consists of:
- 24 satellites (together with spares)
- The satellites maintain six orbital planes approximately 12 miles above the earth
- Each satellite orbits the earth twice every 24 hours
- The system is designed so that at least 4 satellite are visible at any time of the day – anywhere in the world
The GPS system is based on line of site transmission from the satellite to the GPS receiver. A GPS handheld or vehicle-mounted receiver requires a strong signal from a minimum of 4 satellites in order to accurately report the receiver’s position, speed and direction of travel. The GPS signal from the satellite can be adversely affected by:
- Canyon walls, tall buildings or other large objects or structures
- Dense foliage or tree canopies
- Antenna blockage by metal from cars or trucks
- Low batteries
The exact position of each satellite is known at all times, which is continuously transmitted. Knowing the exact position of each satellite and the actual distance from the GPS receiver, the current position of the GPS receiver can be determined. The distance from the satellite to the GPS receiver is “measured” by determining the time it takes for the GPS signal to reach the GPS receiver from the satellite.
GPS Unit Operation
Most typical handheld GPS units operate in a similar manner. Each manufacturer has its own design regarding menus and button-functions. Reading and understanding the specific manual for your particular unit is very important to get the most from your handheld receiver.
The first step is the initial setup of your unit, which covers display options, units of measurement, map datum and time format. The following categories are critical in order to effectively use the GPS receiver in conjunction with a map and/or compass:
- Coordinate Display Format: latitude-longitude/UTM/MGRS (for military users)
- Map Datum: the GPS receiver must be set to map’s datum
- NAD27/NAD27 CONUS for most USGS Topographical Maps
- WGS84 for newer and non-governmental map sources
- Headings: true north or magnetic north
- Time Format: 12 hour or 24 hour format/local time, specific time zone or GMT (ZULU) time (especially important in aviation applications)
- Units of Measure: feet or meters/miles or kilometers/mph or kph
Each GPS receiver will have a number of different types of displays to give your current position information. Most units will also allow you to customize the type of data displayed with the terrain map, 3-D map or digital compass. This allows the user to choose the type of information that will be most helpful based on the mode of travel and type of terrain.
An important display (that is often overlooked) is the Satellite Display Map. As noted above, it is critical for your GPS unit to receive a strong signal from 4 satellites in order to accurately report your position.
The Satellite Display map presents a map of the satellites that are “visible” to your GPS unit from your current position. In addition, most Satellite Display Maps also show the signal strength from these “visible” satellites and the location accuracy of the position fix. The accuracy of your position fix is also known as your “Estimated Position Error”. Most GPS units will give you this information on one of its displays. The EPE should be checked on a regular basis.
By using and understanding the information from the Satellite Display Map, you will be able to determine the accuracy of your unit in a given location and whether the unit can be used for navigation.
Your GPS receiver is an electronic “gizmo”. No matter how hard it tries to stay on, it will eventually run out of juice and unless you are prepared, you will become “lost”.
Consequently, the following is highly recommended:
- Become skilled in using the “forgotten” map and compass as a primary navigation tool with your GPS as a secondary navigation device
- Bring lots of spare batteries
- Mark your GPS unit with bright tape (most units are black and are easily missed or left behind
- GPS units do not do well when they are cold, wet, run over, dropped and, of course, forgotten
- Before you really need your GPS unit to get you back home, read your manual, set your unit up so that it will give you the information you need, and PRACTICE, PRACTICE, PRACTICE!
Suggested Route/Trip Planning Checklist:
Prior to Departure
Mark your trailhead and end of trail locations together with interim waypoints on the map. Mark landmarks and other easily identified features on the map even if not located on your trail or route.
Pull the waypoints from the map (latitude-longitude or UTM) and load the data into the GPS receiver.
CRITICAL—verify that your GPS receiver is loaded with same map datum as the map you are using and that your compass (by declination) and GP unit (at initial setup) are working on TRUE NORTH .
In The Field.
At trailhead obtain a position fix and verify coordinates to the stored trailhead waypoint.
Orient your map to the existing surroundings and compass.
CRITICAL—verify that your GPS receiver is operating in 3D Mode and NOT in 2D Mode.
Proceed on trail with either compass/map or with GPS using the “GOTO” feature to navigate to the next waypoint.
Do not use GPS receiver continuously, but only for occasional position fixes and to cross check on your map and compass work.
Use “Back Track” function on your GPS to return to your start for an “out and back” trip.
General Rule is to be aware of one’s approximate location relative to key reference points at all times.
Use a notebook to make notes starting from the trailhead, especially at trail junctions, landmarks and changes in direction.