
Chapter 10 259
GPS Modulation
GPS Concepts
The TLM word, which is 30-bits long, is structured using the preamble for the first 8-bits, 16
reserve bits (9 through 24, all set to zero), and the 6 parity bits for bit locations 25 through 30.
The HOW also contains 30-bits where the first 17 represent the most significant bits (MSBs)
for an incrementing time-of-week (TOW) count. Additionally, the HOW message contains the
two bits (bits 23 and 24) used for parity computation along with the 6 parity bits.
During a GPS signal transmission, a pulse signal is generated every 6 seconds at the
EVENT 1 rear panel connector. This pulse coincides with the beginning of each subframe
starting with the second subframe. The pulse is synchronized to the RF output, which
compensates for any internal signal delays.
Chip Clock Reference
The GPS reference clock (chip clock) has the advanced feature of being adjustable with a
factory set value of 10.23 Mcps (the GPS standard). You also have the convenience of using the
internal chip clock or having an external source provide the reference. When an external
reference is used, it is applied to the DATA CLOCK input connector.
NOTE Whenever an external source is providing the chip clock reference, it is
imperative that its frequency matches the value set with the
GPS Ref (f0)
softkey.
The P and C/A code chip rates are dependent on the value set using the
GPS Ref (f0) softkey
whether you are using the internal or an external chip clock reference. The P code chip rate
matches the reference clock rate and the C/A code chip rate is one-tenth of the reference
value (.1 x reference clock rate). Refer to Figure 10-1 on page 257 for a block diagram showing
how the GPS signal is generated within the ESG.
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