GPS & Hiking

The USAF space segment of GPS has 24 satellites that orbit the Earth about every 12 hours. Approximately half are in the hemisphere around the receiver postion, but those below 5 to 10 degrees above the horizon are not used due to blockage by hills, buildings or trees. Typically, eight satellites are electrically "visible" and are received in open territory.

Initial estimates of the receiver latitude (north-south angle), longitude (east-west angle), height, and time are entered or recalled from the receiver memory. The incoming signals are all on the same frequency, but are coded to allow separation and identification. Since there are four measurements to be solved for, at least four good satellite signals must be received. The range error to each satellite is used to solve for corrections to the initial latitude, longitude, height, and time. These corrections can then be added to the initial values to get new and better values for display and use. This process is repeated until the solution is said to converge (not change much).

GPS Position Errors

The Department of Defense (DoD) varies the satellite signals to insert errors and to degrade their accuracy. This reduces the risk of an enemy using GPS to steer missiles aimed at the U.S. However, the Department of Transportation wants the highest accuracy for flying and landing airplanes, and for the navigation of ships. The DoD restriction, known as Selective Availability (SA), results in horizontal position errors of less than 100 meters 95% of the time. The DoT, through the US Coast Guard, radiates measured GPS errors from radionavigation beacons at lighthouses so that users (except for enemies, of course) can remove them. A differential GPS receiver (DGPS) can receive the errors from these radiobeacons and subtract them from the raw GPS receiver positions to improve the accuracy to 1 to 10 meters. The premise seems to be that American boaters can afford DGPS receivers, but enemies can not afford to put them into ICBMs. This Government dicotomy will be resolved in 2004, when SA errors will be turned off.

For a hiker in forested areas, tree leaves can weaken the GPS signals. The greater the number of receiver channels, the higher the probability that four satellites can be received through gaps in the trees. If the hiker moves a few feet or simply continues walking, the signal strengths may improve. In clear areas, reception of eight satellites is relatively easy.

Map Coordinates

Hikers frequently use topographic survey maps known as quadrangles that are 7.5 minutes of position angle high in latitude and wide in longitude. These geodetic positions are not extensively marked on the maps, but metric grids are marked every 1000 meters. These metric grids are known as UTM (Universal Transverse Mercator), and are so convenient that GPS receivers can be set to solve for position in these coordinates. The UTM grids are usually offset slightly in angle from the map geodetic grid. Due to convergence of the meridians of longitude, the map is narrower at the top than at the bottom. The UTM measurements are called "eastings" and "northings", which are distances from the UTM reference lines. As an example, Mt. Black (3184.8 m or 10449 ft), WY is at 42-50-18.05706 N and 109-02-24.38794 W (in degrees, minutes, and seconds). This is equivalent to an easting of 0660 168 meters and a northing of 4744 727 meters (the space makes it easier to read the last three significant figures).

Reading the Topo Map

The coordinates are best read off using a sheet of viewgraph plastic that has squares of 1000 meters on a side. The left and bottom of a square are matched with the map grid, and the coordinates to 1000's of meters are read from the edge of the map. The location on the trail are then estimated with the minor 100 meter squares on the plastic. The edge reading and the estimated reading are then combined to get the full UTM easting and northing. Each set of coordinates should be taken where the trail changes direction, at a landmark, or perhaps at a stream crossing. Each location should be given a unique name of 6-8 letters and/or numbers. Use a naming method so that the most significant part comes first, and perhaps ends with a number. This allows the features to group when the names are sorted.

Loading the GPS Receiver

Receivers allow perhaps 20 t o30 points to be entered as a route. Each location on a route is called a waypoint. For each waypoint, enter the location name or abbreviation, the easting, and the northing. The route for the trip is then set up to proceed from one waypoint to the next. Current receivers allow the route to be traced backwards as well. This avoids entering a new route for the way back.

At the trailhead or car, enter the local position after taking a GPS fix. Once the route is selected, the direction and distance to the next waypoint will be displayed. Since the initial values were entered from the map, the location will vary from the GPS fix at that location. (Selective Availability will also contaminate the position fix.) GPS fixes should be logged into a notebook for all important locations. Later, the trail coordinate set can be updated for another trip.

As you travel along the route, the receiver will indicate the direction to the next waypoint. Since the waypoint location may be subject to error, careful interpretation is needed to avoid blunders. If time permits, a new GPS fix should be taken at each waypoint to update the coordinates.

In spite of the convenience of GPS, a compass and map should always be taken in case the GPS receiver or its batteries fail.