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16.25.5. The satellites transmit two code signals, the precision (P) code on 1227.6 MHz and the coarse
acquisition (C/A) code on 1575.42 MHz (Figure 16.15). Both codes carry the same types of information.
The C/A code will be transmitted with intentional errors to deny the highly accurate position from
unauthorized users. The P code, like its relative, the encrypted Y code, does not include these intentional
errors. To circumvent the Y code encryption, differential GPS (DGPS) receivers are being designed that
will receive general GPS signals as well as a fifth signal from a ground-based transmitter. These
differential transmitters can easily determine the intentional error by comparing their GPS position to the
surveyed coordinates of the transmitter. The difference is the intentional error. The ground transmitters
will compute and relay the amount of intentional bias in the C/A code so receivers can remove the
position error without use of the P or Y codes.
Figure 16.15. Signal Bandwidth.
16.26. Clock Error and Pseudo Range. We assumed in the previous discussion that both the satellite
and the UE set were generating identical pseudo codes at exactly the same time. Practically speaking,
this is not the case. Each satellite carries an atomic clock accurate to 10-9 seconds. Achieving maximum
accuracy in synchronizing the codes would require all users to carry atomic clocks with comparable
AFPAM11-216 1 MARCH 2001 345
accuracies, significantly increasing both the size and cost of each receiver set. As a compromise, each
UE set is equipped with a quartz crystal clock.
16.26.1. Since the accuracy of a quartz crystal clock cannot approach that of an atomic clock, there is a
difference between satellite GPS system time and UE set time. As a result, the generation of the two
pseudo codes is not perfectly synchronized and a ranging error will be induced. Instead of determining
actual range, we will measure the apparent or pseudo range to the satellite. This particular problem area
is known as clock bias.
16.26.2. Clock bias affects all range measurements equally. The problem is determining the amount of
bias error. Using three satellites allows us to determine our position in three dimensions. By using a
fourth satellite and comparing pseudo codes, the UE set internally determines the amount of adjustment
necessary to make all of the measurements agree.
16.27. Satellite Clock Error. It might be safe to assume that since each satellite carries an atomic clock,
it would keep extremely accurate time. Since the compact dimensions of an orbiting satellite limit the
clock size, its accuracy does not approach that of ground based atomic clocks. As a consequence, there
will be some error in each satellite's clock when compared with "master" GPS system time. The
satellite's generation of the pseudo code will be slightly out of synch and some ranging error will be
induced. This problem is known as satellite clock error. To compensate for this type of error, the GPS
control segment comes into play. Monitor stations evaluate the accuracy of the satellite's clock and its
pseudo code generation. This information is then relayed to the master control station where the
necessary corrections to the satellite's transmissions are computed. Updated information is then
uploaded to the satellite via the ground antennas.
16.28. Ephemeris Error. Ephemeris is the ability to determine the location of a celestial body (in this
case a satellite) at regular time intervals. Ephemeris error then is caused by the satellite not being exactly
where we thought it was. By using estimation theory techniques, the computers at the master control
station predict what the satellite's position should be at a specific time. This predicted position is then
compared with the actual position as determined by the monitor stations. Updated information on the
satellite's future position is then uploaded to each satellite on a regular basis via the ground antennas.
Each satellite then continuously transmits these corrections to all users. In this way, ranging error caused
by uncertainty as to the satellite's exact position is minimized.
16.29. Atmospheric Propagation Error. We assumed that the satellite's RF signal traveled at the speed
of light, as it does in a vacuum like space. But just as light is refracted through a prism, the RF signal is
bent and slowed down as it enters the ionosphere. The degree to which the signal is affected depends on
the atmospheric conditions between the satellite and receiver and on the signal's angle as it passes
through the ionosphere. Atmospheric propagation error can cause position uncertainties up to 40 meters.
By noting the time delay between the two L-Band signals, much of the effect caused by atmospheric
　
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