Quality information
As mentioned above, the amplitude of the satellite's movement is used as a measure of the step
track quality. This is because the step track measurement uncertainty is an constant angle which
primarily depends on the antenna size.
Beside the amplitude, the ACU evaluates for each axis a figure called jitter. The jitter value
describes standard deviation of the measured peak positions with respect to the positions
calculated from the model. The figure is also expressed as a percentage of the antenna's
beamwidth, low values indicate, that the model ideally describes the antenna's path. High values
indicate that's something wrong. The step track results may be to noisy at low amplitudes or the
model does not fit at all. This may be the case if a satellite gets repositioned in the orbit.
You may set a threshold value for the jitter. The ACU raises a fault if at least one axis exceeds the
threshold value. If this happens three consecutive times, the models gets reset, all data in the
tracking memory gets marked invalid.
In the tracking memory the ACU records all steptrack peak positions. The tracking memory is
used as a data source for the adaptive tracking and for the smoothing function as well. Each
record contains a time stamp, the azimuth / elevation values and the beacon level measured after
the peaking. The tracking memory has a fixed size (1000 entries). Once 1000 records are stored,
the memory works in a first in - first out manner. How many days of tracking data fits into the
memory depends on the tracking interval. The diagram below shows the memory depth vs. the
tracking interval.
The tracking memory is volatile, it is erased when the ACU gets reset or switched off. Saving the
tracking memory in regular intervals to the ACU's flash memory would damage the flash memory
quite quickly.
(C) 2022, SatService GmbH
www.satnms.com
ACU-ODM-UM-2209 Page 69/73
