process GPS satellite signals.
More than 100 different receiver
models are already in use. The
typical handheld receiver is
about the size of a cellular telephone, and the newer models are
even smaller. The handheld units
distributed to US armed forces
personnel during the Persian Gulf
War weighed only 50 grams.
The core of GPS receivers
comes in two major variations:
sequential single-channel and
parallel multi-channel. Single-channel GPS units have only one
radio receiver unit, and they must
step sequentially through all possible satellites. This takes time and
degrades their accuracy, since they
may lose a "lock" each time they
switch channels. Parallel units have
from between 4 and 12 receivers,
each dedicated to one particular
satellite signal, so strong locks can
be maintained on all the satellites.
There are some two-channel units
out there, but in practice these are
only slightly better than single-channel units.
Today's GPS receivers are
extremely accurate, thanks to their
parallel multi-channel design.
GARMIN's 12 parallel channel
receivers are quick to lock onto
satellites when first turned on and
Just So You Know ...
the latitude, longitude, and altitude (or some similar measurement) of its current position. To
make the navigation more user-friendly, most receivers plug this
raw data into map files stored in
We can use maps stored in
the receiver's memory, connect
the receiver to a computer that
can hold more detailed maps in
its memory, or simply buy a
detailed map of any area and
find the way we need using the
receiver's latitude and longitude
readouts. Some receivers let us
download detailed maps into memory or supply detailed maps with plug-in map cartridges.
A standard GPS receiver will not
only place us on a map at any particular location, but will also trace a
path across a map as we move. If we
leave our receiver on, it can stay in
constant communication with GPS
satellites to see how location is changing. With this information and its built-in clock, the receiver can give several
pieces of valuable information:
Figure 2. GARMIN GPS receiver.
they maintain strong locks, even in
dense foliage or urban settings with
tall buildings. Certain atmospheric
factors and other sources of error
can affect the accuracy of GPS
receivers. GARMIN GPS receivers
are accurate to within 15 meters on
average. Newer GARMIN GPS
receivers with WAAS capability can
improve accuracy to less than three
meters on average.
The most essential function of a
GPS receiver is to pick up the transmissions of at least four satellites
and combine the information in
those transmissions with information in an electronic almanac, all in
order to figure out the receiver's position on Earth. Once the receiver
makes this calculation, it can tell you
•Odometer — distance you have
•How long you've been traveling.
•Speedometer — current speed of
•Average speed of your vehicle.
•A "bread crumb" trail showing your
exactly where you have traveled on
•The estimated time of arrival at the
destination if you maintain your current speed.
NUTS & VOLTS
Reader Wayne Lauritzen wished to expand on the PRN encoding used in GPS,
so he submitted the following. Thanks, Wayne! — Editor Dan
For the C/A code, the Gold codes are a set of 32 orthogonal sequences of ones and
zeros, each of which has a length of 1023 elements, or chips. The chip rate is 1.023 Mbps, so
the code repeats every millisecond. The codes are easily generated by tapped feedback shift
registers. In the actual application, the zeros are replaced by negative ones. These sequences
are used as the pseudorandom codes which are assigned to the indivdual GPS satellites. The
satellites are often refered to by their PRN number, where PRN stands for Pseudo-Random-Noise. In addition to modulating its ranging message onto the carrier, each satellite modulates
its unique PRN onto the L1 carrier wave with binary biphase modulation (BPSK). Effectively,
this means that the L1 carrier wave, first modulated by the ranging message, is next multiplied
by the PRN. Each time the PRN changes state, the signal gets a 180 degree phase shift. This
modulation significantly widens the spectrum, hence the "spread spectrum" terminology. The
spread spectrum is very resistant to jamming and, in its uncorrelated state, looks like low
power noise. The GPS receiver generates its own local PRN sequence for each of the satellites which it expects to see, and cross correlates that generated Gold code with the received
signal. Since the sequences are orthogonal, a good correlation will be found only for the satellite which generated exactly the same code. The correlation effectively extracts a signal from
each satellite which can be further demodulated to produce the ranging message.
The Gold codes were named for their discoverer, Robert Gold. Refer to his paper
"Optimal Binary Sequences for Spread Spectrum Multiplexing" in IEEE Transactions on Info
Theory, Oct 1967, pages 619-621, for more information.
To obtain this last piece of information, we would have to have given
the receiver the coordinates of destination, which brings us to another
GPS receiver capability: inputting
Most receivers have a certain
amount of memory available for storing navigation data. This greatly
expands the functionality of the
receiver, because it essentially makes
a record of specific points on Earth.
The basic unit of user input is the waypoint. A waypoint is simply the coordinates for a particular location. You
can save this in the receiver's memo-