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have befallen it on its trip down to the earth. However, the satellites are so far out in
space that the little distances between receivers on earth are insignificant. Thus, the
signals that reaches both of receivers will have travelled through virtually the same slice
of atmosphere, and so will have virtually the same errors. The reference receiver is on a
point that's been very accurately surveyed and is kept there. This reference station
receives the same GPS signals as the roving receiver but instead of working like a normal
GPS receiver (using timing signals to calculate its position), it uses its known position to
calculate timing. The difference between calculated and received timing signal is an
"error correction". The stationary receiver transmits this error information to the roving
receiver so it can use it to correct its position measurements. Briefly, differential GPS
(DGPS) is a technique that uses data from a receiver at a known location to correct the
data from a receiver at an unknown location. The DGPS corrected solution is much more
accurate than a normal GPS solution. DGPS involves the cooperation of two receivers,
one that's stationary and another that's roving around making position measurements.
DGPS can eliminate all errors that are common to both the reference receiver and the
roving receiver on the ship, Fig. 2. Most systematic errors can be eliminated using a
DGPS
,
but not errors due to environmental factors or receiver design. DGPS allows to
obtain high accuracies e.g., positioning of a few meters /1/.
GPS technology not only overcomes the limitation of climate, but also measures the
structure displacement in three-dimensional directions. The accuracy of a few milimeters
can be obtained by using a DGPS carrier-phase approach, and the sampling frequency of
10 Hz or even 20 Hz is now available, providing higher accuracy.
Each GPS satellite transmits on two frequencies, L1 (1.575 GHz) and L2 (1.227 GHz).
L1 is the primary signal used for most civilian applications, and L2 is used for computing
ionospheric corrections in some cases. The L1 signal can be divided into three compo-
nents: carrier wave, tracking codes, and navigation message. Information about the
satellite positions (or orbits) is contained in the navigation message. Each satellite
transmits its navigation message with at least two distinct spread spectrum codes: the
Coarse / Acquisition (C/A) code, which is freely available to the public, and the Precise
(P) code, which is usually encrypted and reserved for military applications. Both the C/A
and P(Y) codes impart the precise time-of-day to the user. The C/A code is a 1.023 with
1.023 million rectangular pulses (chips) per second so that it repeats every millisecond.
Each satellite has its own C/A code so that it can be uniquely identified and received
separately from the other satellites transmitting on the same frequency. Both the naviga-
tion message and tracking codes are modulated on the carrier wave, a continuous radio
signal at the L1 frequency. As with the tracking codes, the GPS receiver can correlate to
the carrier wave to gather ranging information to each satellite. Because the frequency of
the carrier wave is much higher than the frequency of the code chipping rate (2 MHz for
the C/A-code vs. 1.575 GHz for L1), its cycles are much shorter, and thus, it is possible to
make more accurate satellite ranging measurements using the carrier wave component of
the satellite signal. However, this method is significantly more complex.
GPS tracking codes are designed so that when the code is successfully correlated in
the receiver, an unambiguous range to the satellite is derived. The receiver knows exactly
how far through the long code sequence it currently is. For carrier wave tracking, it is not
so simple. Every wavelength looks exactly like every other one and each is about 20 cm
long only. The receiver can determine the phase angle of the current cycle - but not how
many cycles lie between it and the satellite. Because the receiver directly measures the