Viavi 87339, Operation Manual

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Principals of Operation
6 -6
6.1.3 SPS Standard Positioning Service
The Clear Acquisition Code, or C/A, is the principal civilian ranging signal and is always
broadcast in a clear or unencrypted form. The use of this signal is sometimes called the
Standard Positioning Service or SPS. This signal may be degraded intentionally but is
always available. The signal creates a short Pseudo Random Noise (PRN) code
broadcast a rate of 1.023 MHz. The satellite signal repeats itself every millisecond. The
C/A code is also used to acquire the P Code.
6.1.3.A PPS Precise Positioning Service
P Code is never transmitted in the clear and is encrypted with a W code. When
encrypted the signal is know as P(Y) code and is not available to civilian users. The C/A
PRN’s are unique for each satellite however, the P-code PRN is actually a small
segment of a master P-code approximately 2.35 × 1014 bits in length
(235,000,000,000,000 bits) and each satellite repeatedly transmits its assigned segment
of the master code.
6.1.4 Position Calculation
Position calculations consists of the following elements:
•
Deciding which satellites to acquire and track
•
Code and frequency correlation
•
Measure distance to satellites
•
Obtain satellite positions
•
Adjust local clock bias
•
Perform triangulation calculations (Trilateration)
•
Adjust for time delay errors
6.1.4.A SV’s to Acquire and Track
The L1 and L2 frequencies broadcast a GPS Navigation Message (Nav Data) as part of
their signal. This low frequency (50 bits per second) data stream provides the receiver
with a number of critical items required in determining a position. A data bit frame
consists of 1500 bits divided into five, six second 300-bit subframes. A data frame is
transmitted every thirty seconds. An entire set of twenty-five frames (125 subframes)
makes up the complete Navigation Message that is sent over a 12.5 minute period.
Fig. 6-7 GPS Navigation Data
Subframes