Research Concepts RC2000C - Manual (Page 67)

RC2000C Az/El Tracking Antenna Controller Chapter 5 Modes Function Description 59
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The Azim and Elev Constants relate changes in the antenna azimuth and elevation pointing angles to
changes in the antenna azimuth and elevation position count values. The Azim Constant and Elev
Constant are specified in position counts per radian. One radian equals 57.295 degrees. See section
4.4.2 for a complete description of these CONFIG mode items.
These quantities are referred to as the azimuth and elevation scale factors in chapter four, which
describes the tracking algorithms. A value must always be entered for the Elev Constant. Entry of the
Azim constant is optional. If the Azim Constant is initialized to a value of zero, when a track is initiated
on an inclined obit satellite, the controller will attempt to determine the azimuth scale factor based on
the positions of geostationary satellites in the vicinity of the inclined orbit satellite. If the user chooses to
let the controller determine the azimuth scale factor, it is very important to specify correct longitude
values for geostationary satellites (via SETUP mode).
5.10.11 Track Mode Parameters
The following parameters are used by the track system. These parameters are described in more detail
in chapter 4. These items may only be accessed when the Expert Access flag is set.
MAX TRACK ERROR
MAX TRACK ERROR(1-30):5 CONFIG
IN TENTHS OF A dB ENT,BKSP,SCRLL ^v
The Max Track Error CONFIG mode item specifies the maximum antenna tracking error in tenths of a
dB. This parameter has more influence over the operation of the tracking system than any other. The
significance of this variable is described in detail in sections 4.2.1 and 4.2.2 of this manual, which cover
the operation of the controller in the STEP TRACK and PROGRAM TRACK sub-modes.
In the STEP TRACK sub-mode, the value of this parameter determines how often peaking operations
occur. The controller will peak the antenna often enough so that in between peakups the signal
strength variation due to antenna pointing error will not exceed Max Track Error tenths of a dB.
In the PROGRAM TRACK sub-mode, the controller tracks the satellite based on the current sidereal
time, and data in the track table. The controller will reposition the antenna often enough so that the
error between the antenna's azimuth and elevation position and the azimuth and elevation positions
derived from interpolation of track table data is less than Max Track Error tenths of a dB.
The Max Track Error parameter specifies the azimuth and elevation step sizes during peaking
operations that occur during the STEP TRACK and PROGRAM TRACK sub-modes. Some users
erroneously conclude that the smallest value of Max Track Error leads to the tightest track. When the
value of this parameter is reduced to a point where the peakup azimuth or elevation step sizes
approach the value of the mechanical hysteresis (slop) of the antenna mount, the controller cannot
peakup properly. This can lead to PEAK LIMIT errors, or the antenna peaking itself off of the antenna.
Here is the mechanism that can lead to this undesirable result:
1. The Max Track Error parameter is set to a 'low' value that results in an elevation peakup step size of
just one position count.
2. When a peakup occurs, the controller measures the signal strength at the current antenna position,
and then moves the antenna up or down in elevation in an attempt to find the strongest satellite
signal. After recording the signal strength at the starting position, the controller moves the antenna
up in elevation by one position count.
3. Due to mechanical hysteresis, the antenna's pointing angle does not change even though the
antenna's elevation actuator has moved one position count.
4. Thermal noise in the receiver's AGC circuit or changing atmospheric conditions result in the
controller measuring a stronger AGC input at the 'new' antenna position.