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5.1.2
ISMDETOBS Detrended phase and amplitude measurements
This log contains carrier phase and amplitude data that has been passed through a detrending filter.
The raw phase measurements include satellite motion, the rate of change of the ionosphere and
satellite and receiver oscillator drift. Thus, observing scintillation in the measurements without
detrending is not possible. A high pass Butterworth filter is used for detrending phase measurements.
There are 2 different techniques available for detrending amplitude measurements:
• Butterworth low-pass filter
• 60 second averaging filter
See the description of the ISMBANDWIDTH command for further details on the detrending filter and
how to modify the bandwidth of the filters.
The receiver collects phase and amplitude measurements at a 50 Hz rate (i.e., 50 sets of measurements
per second) and outputs them in this log every second. Each data block contains a base ADR and a
base power measurement representing the measurement at the time of week (TOW) specified in the
log header. This is followed by 49 sets of compressed data reporting the measurements taken at each
0.02 second epoch following the TOW. For example the first compressed value reports the
measurement at (TOW + 0.02); the second set at (TOW + 0.04) seconds, and so on.
The following section describes how to recover the full ADR and power measurements from the
compressed data.
ADR
The values stored in the compressed ADR data represent a delta value from the previous epoch's
measurement. So to compute the ADR measurement for a particular epoch, start with the base ADR
measurement and add in all previous delta values up to and including the delta value for the epoch in
question.
For example the ADR value for TOW + 0.04 would be:
(First ADR) + (delta ADR @ TOW + 0.02) + (delta ADR @ TOW + 0.04).
Power
The compressed power factor data fields give power measurements as a value relative to the base
power. The 12 bits of the power factor field should be treated as 2 parts:
• an 11-bit unsigned power factor in the lower 11 bits
• 1 "scaling" bit in the most significant bit representing the power scaling formula to be used.
One formula will calculate a power increase (scalar >= 1.0) from the base power and the other will
calculate a decrease (scalar < 1.0). The 2 formulas to be used are described below:
Power increase case (scaling bit = 0):
This_Power = Base_Power * 2048 / (power_factor + 1)
Power decrease case (scaling bit = 1):
This_Power = Base_Power * (power_factor + 1) / 2048
These formulas yield a scale factor that can be anywhere from 1/2048 to 2048. So this gives a
relative power range of ±33dB from the base power measurement.
Structure:
59
Message ID: 1395
Log Type: Asynch
GPStation-6 User Manual Rev 2
Logs
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