Geography Reference
In-Depth Information
comprehensive review of the different code
elements. It is these codes that provide the
information necessary for a receiver to determine
position.
A GPS receiver can calculate its position using
either
pseudorange
or
carrier phase
observations.
Pseudoranges are used primarily for instantaneous
positioning in navigation (Ashkenazi and Dodson
1992). In pseudorange observations, distance
between satellite and receiver is calculated by
measuring the time delay between the
transmission and receipt of a unique segment of
the C/A code. However, because the receivers
have less accurate clocks than those on board the
satellites, there will be a degree of unknown error
in the distance calculated. This uncorrected
observed distance is known as pseudorange. In
pseudoranging, three satellites are used to
determine an uncorrected location for the
receiver, and a fourth satellite is used to remove
the errors associated with the receiver clock. This
ranging method is typically capable of positioning
to within 15-30 m. However, this accuracy is
degraded because of the problem of
selective
availability
(SA).
SA was introduced in March 1990 by the US
DoD to deliberately degrade the positional
accuracy possible with the civilian system (until
1990, 15-metre accuracy was possible). SA can
take two forms, dither, which affects the satellite
clock, and epsilon, which refers to the errors
introduced into GPS satellite
orbital parameters
(Gilbert 1995). The effect of both errors is to
reduce positional accuracies to approximately 100
m horizontally and 150 m vertically (Ashkenazi
and Dodson 1992) (Figure 43.1). Technological
developments, including new differential solutions
enabled improvements in the positional accuracy
available to civilian users. These solutions are
discussed below. SA can also be countered by the
use of carrier phase observations.
Carrier phase observations improve upon the
accuracy of the pseudoranging method by
working with the carrier signals L1 and L2 rather
than the C/A or P codes. In carrier phase
observations, the nature of the signal carrier is
compared rather than the coded message.
HOW GLOBAL POSITIONING WORKS
GPS works by using a variation on trilateration, a
technique used by surveyors for many years.
Kennedy (1996) and Barnard (1992) provide a
comprehensive explanation of the trilateration
method. In summary, the location of an
unknown
position is calculated by measuring the distance
between the unknown position and that of several
fixed,
known positions. In the context of GPS, the
ground receiver represents the unknown position
and the constellation of GPS satellites the known
fixed positions.
In trilateration, the distances between a
minimum of three locations, or
ranges,
are
measured by monitoring a signal between the
satellites and ground-based receivers. Both the
GPS receiver and the satellites transmit similarly
coded radio signals, and the time delay between
receipt of the signals is used to determine distance
between the satellite and receiver. To measure time
delay, the GPS signal from the satellite must
contain information about the exact time that the
signal left the satellite. In addition, the signal from
the satellite must contain information about the
exact location of the satellite at the moment that
the signal was sent. Without this, the GPS receiver
would not know the location of the satellite, since
GPS satellites are continually orbiting the Earth.
Therefore, all GPS satellites continuously transmit
two signals: L1 at 1575.42MHz and L2 at
1227.60MHz. Both signals carry coded
information in binary format (0 and 1 strings).
Three basic codes are transmitted: the
navigation
code, the
coarse acquisition
(C/A) code and the
precise
(P) code (van Sickle 1996). The C/A code
is available to all civilian users and is used for the
standard positioning service (SPS), whereas the use
of the P code is restricted, and encrypted, by the
US DoD. There are several important elements to
the three codes, including satellite position or
ephemeris, atmospheric correction
(required to correct
for delays to the signal as it travels through the
ionosphere),
antispoofing
(intentional degrading of
the signal), satellite
almanac
(the location of all
other satellites) and satellite
health
(the quality of
the signal). Van Sickle
(ibid.)
provides a
