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TECHNICAL DESCRIPTION INTRAC-305
Page 54 INTRAC-305 MANUAL - Issue 3.2 © 2011 Advantech Wireless
the raw satellite position estimate and the orbital model is
filtered with another tracking filter (known as the "relationship
algorithm") capable of tracking and correcting transients. This
is then combined with the basic model to form a reliable
predictor that tracks mean windage, refraction and
stationkeeping manoeuvres without error.
The INTRAC tracking filters are designed in such a way as to
enable the model to provide the required accurate pointing
prediction at all times. Even when not verified by
measurements, as for example occurs with loss of beacon, the
tracking filters are capable of accurately predicting the satellite
orbit for many days. Under INTRAC control, pointing is always
controlled from the internal satellite orbit model. When a
measurement cycle is performed it is always done as a
perturbation with respect to current pointing. Thus, unlike
conventional steptrack, INTRAC is always on track when a
measurement cycle is performed. INTRAC never uses the
measurement cycle for the purpose of directly bringing the
beam on track. INTRAC simply performs one measurement
cycle in each axis every 10 minutes in order to up-date the
parameters used in the orbital model and for the rest of the
time keeps the beam correctly pointed.
As a result of the combination of thermal noise, fade,
scintillation, random windage-induced platform-reference
motion and other noise sources the beacon signal will, during a
measurement cycle, contain noise additional to that directly
attributable to the measurement cycle itself. Careful algorithm
design ensures that this noise has zero mean value and has a
value of standard deviation such that it is equivalent to thermal
noise of a certain effective value of C/No. By special design of
the measurement cycle the INTRAC system minimises this
effective value of C/No in a way that is not possible with
conventional steptrack methods. Furthermore the INTRAC
measurement cycle design discriminates so effectively against
the slow component of received beacon signal power
fluctuation, caused for example by rain fades, that it almost
completely suppresses errors caused by linear beacon ramps
of all practicable slopes.
The INTRAC algorithm also incorporates adaptive
compensation for imperfections in the antenna drives. As a
result its performance is largely unaffected by servo backlash,
AC track motor drive rate and transportation rate (motor to axis
rate) and coast because of the specific choice of perturbation
pattern and the use of high resolution position transducers. The
INTRAC servo algorithm dynamically calibrates the mechanical
coast of the antenna and automatically compensates for it if it
is within reasonable limits (less than 1/20 beamwidth).
Wind affects tracking in two ways. The antenna structure is
distorted by the wind load and this distortion shifts the beam
pointing relative to the angle transducer reading. This