The Global Positioning System
(GPS) uses navigation satellite communications to find a receiver's position on the
Earth. While GPS was developed to guide ballistic missiles to their targets, by far,
civilian users of this "free" service predominate. This transition to peaceful use is
somewhat analogous to that of the liquid crystal display (LCD) research that led to
the Kent State University, Ohio, Viet Nam War riots in the '60s, where four students
were shot by the Ohio National Guard. Although the research laboratory was burned
by student agitators against the Viet Nam war, we consumers now have inexpensive
LCDs in watches, TVs, computers, telephones, etc.
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.
The Global Positioning System (GPS) is well described in some website links below.
The essence is that satellite navigation is practical for the hiker since the price
of many handheld units has fallen to less than $150.