Global Positioning System Reference
In-Depth Information
begin, an up-to-date ephemeris is needed. It can take several minutes for all
relevant data to be received so the initial session can be lengthy (which is known
as a “cold start”). Receivers store this data in nonvolatile memory so that if a new
session is started again before that orbital information has significantly drifted, the
startup time can be very rapid (perhaps a few seconds).
It is also possible to relay the ephemeris data to the GPS receiver via a non-
GPS route, for example, by a cellular radio data signal. This arrangement is part of
a methodology known as assisted GPS (AGPS) and can accelerate greatly the time
taken for a cold start and is an essential part of the E911 emergency calling
system.
Matrices are used to solve sets of simultaneous equations containing the
timing information received from the available satellites and after processing, the
positional information can be estimated. It is expressed in various formats (e.g.,
latitude and longitude in degrees, minutes, seconds [or using decimals], height in
meters above mean sea level, and velocity). The geodetic standard WGS-84 is
used as the model for the approximate shape of the Earth. Many GPS receiver
modules have a navigational output signal (via an RS232 port), and will present
their data according to the standards defined by the National Maritime Electronics
Association (NMEA). This is a simple text-based protocol that generates standard
“sentences” containing fields of latitude, longitude, time, altitude, velocity,
satellite availability, and quality information. Proprietary formats are also
available. These modules greatly facilitate the embedding of GPS into consumer
appliances very cheaply.
The accuracy of the readings varies according to a large number of factors,
including system limitations such as clock quality and code parameters,
atmospheric propagation anomalies, and everything that affects local signal
strength in the local environment. A typical domestic receiver should be giving a
positional performance better than 3m most of the time. By increasing the
sensitivity of the receivers (with more sophisticated correlation techniques) it is
now possible to use the GPS devices, at least to some extent, indoors, especially
near windows. It is remarkable that GPS transmitters in orbit, more than 20,000-
km distant and transmitting only 27W of power (about half the power of a
conventional incandescent electric lamp) can still be providing useful positional
information indoors. Figure 6.9 shows how small a typical GPS receiver module
has now become.
6.2.5
Timing, Differential GPS (DGPS), GNSS Augmentation Services, and
Surveying
There are many specialist applications for GPS. The accurate local timing that is
effectively locked to a network of atomic clocks can be used as a substitute atomic
clock for other radio applications. Some of these, for example, may be positioning
systems based on terrestrial radio.
