Viavi 87339, Operation Manual

Page: 129 / 198

Principals of Operation
6 -12
To measure the travel time:
•
Receiver generates the same codes as the Satellite (PRN codes)
•
Measure delay between incoming codes and self generated codes
•
D = Speed of light * measured delay (Pseudo Range)
The first word of every single frame, the Telemetry word (TLM), contains a preamble
sequence 8 bits in length (10001011) used for synchronization purposes, followed by 16
bits reserved for authorized users. As with all words, the final 6 bits of the telemetry word
are parity bits.
The handover-word (HOW) immediately follows the telemetry word in each subframe. The
handover-word is 17 bits in length (a range of values from 0 to 131071 can be
represented using 17 bits) and contains within its structure the start time for the next
subframe, which is transmitted as time of the week (TOW).
Fig. 6-15 GPS Subframe Hand-Over Word
The transmission time in the first bits of the preamble are provided in the Navigation
Message in the TOW Message of the previous frame. This time is given in Satellite Time.
Information in the Navigation Message allows translation into Receiver Time. If the
preamble is validated, the arrival time of the first bits in the preamble is measured. This
time is given in Receiver Time.