Global Positioning System Reference
In-Depth Information
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C
HAPTER 1
[Fi
[1]
I
NTRODUCTION
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A new and exciting era of positioning on land, on the sea, and in space began with
the launch of the first global positioning system (GPS) satellite on February 22, 1978.
The primary purpose of the satellite system was to meet the needs of the military and
national security, in regards to positioning and timing, on a 24-hour per day basis all
around the world and under all weather conditions. Very soon, however, the potential
benefits of GPS for civilian applications became apparent, with that number rapidly
increasing and no end in sight twenty plus years later.
The satellites transmit at frequencies L1 (1575.42 MHz) and L2 (1227.6 MHz)
modulated with two types of codes and the navigation message. The codes are the
civilian C/A-code and the encrypted military P(Y)-codes. At present the L1 carrier
is modulated with both types of codes, whereas L2 is modulated with a P-code only.
Modernized GPS will transmit a second civil code on L2 and a third civil code on a
new carrier L5 (1176.45 MHz).
There are two types of observables of interest to users. One of them is the pseu-
dorange, which equals the distance between the satellite and the receiver plus small
corrective terms due to receiver and satellite clock errors, the impact of the ionosphere
and troposphere on signal propagation, and multipath. Given the geometric positions
of the satellites as a function of time, i.e., satellite ephemeris, four pseudoranges are
in principle sufficient to compute the location of the receiver and its clock correction.
Pseudoranges are a measure of the travel time of the codes, C/A or P(Y). The sec-
ond observable, the carrier phase, is the difference between the received phase and
the phase of the receiver oscillator at the epoch of measurement. Receivers are pro-
grammed to make phase observations at the same equally spaced epochs. In addition,
receivers keep track of the number of complete cycles received since the beginning
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