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Acutime 2000 Synchronization Kit User Guide A-65
A Trimble Standard Interface Protocol
Signal level
The Signal Level (Byte 5) is a linear approximation of C/N0 which is
stated in antenna amplitude measurement units (AMUs), a Trimble
devised unit.
Note – SNR
(±3)
= 20log((signal counts/noise counts)*(BW/2)) where:
signal counts = 64 * AMU; noise counts = 90, and BW = 1000Hz.
The C/N0 is affected by five basic parameters:
• signal strength from the GPS satellite
• receiver/antenna gain
• pre-amplifier noise figure
• receiver noise bandwidth
• accumulator sample rate and statistics
The approximation is very accurate from 0 to 25 AMUs.
Codephase
The codephase (Byte 9) value is the average delay over the sample
interval of the received C/A code and is measured with respect to the
receiver's millisecond timing reference. Thus, it includes all receiver,
satellite, and propagation biases and errors. It is expressed in 1/16th
of a C/A code chip.
Doppler
The Doppler (Byte 13) value is apparent carrier frequency offset
averaged over the sample interval. It is measured with respect to the
nominal GPS L1 frequency of 1575.42 MHz, referenced to the
receiver's internal oscillator. Thus, it includes all receiver and satellite
clock frequency errors. It is expressed in Hertz at the L1 carrier.