Research Concepts RC2500 - Manual (Page 35)

28 RC2500 Antenna Controller Chapter 4 Inclined Orbit Satellites
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The PROGRAM_TRACK submode is active whenever a satellite's track table contains valid satellite
position data for the current time. When PROGRAM_TRACK is active the antenna smoothly tracks the
satellite by interpolating between track table azimuth and elevation position entries. Once a complete
track table has been established for a satellite, the controller will remain in the PROGRAM_TRACK
submode.
The SEARCH submode is active when the satellite signal is lost and track table data is not available. In
this submode the controller performs a search for the satellite in the region where it calculates the
satellite will be found, based on its knowledge of the satellite's apparent motion. If the satellite is found
while the controller is performing a search, the STEP_TRACK submode will receive control. If the
satellite is not found while performing the search, the controller will wait Search Retry Interval minutes
and perform another search. If while waiting for the Search Retry Interval to expire, the sidereal time
advances to a time for which valid track table data is available, the PROGRAM_TRACK mode will
become active.
The automatic search may be disabled via the Search Enable CONFIG mode item. If the automatic
search is disabled, the user is simply prompted to position the antenna on the satellite and hit the ENTER
key.
The TRACK_ERROR submode becomes active whenever an error occurs. Some of the errors which can
occur are: antenna jammed, antenna limit reached while tracking, antenna runaway, track table data
corrupted, system CONFIG data corrupted, or a peak limit error has occurred (while peaking the antenna
the controller had to move too far - possibly peaking on an adjacent satellite). Errors are described in
Chapter 7.
4.2.1 STEP_TRACK
In the STEP_TRACK submode, the controller will periodically jog the antenna in elevation and then
azimuth to peak up the antenna's received signal strength. This step_track operation will be referred to
as a peakup. The STEP_TRACK submode is active whenever there is no track table information
available for the current sidereal time and a satellite signal is present.
When STEP_TRACK mode is active, two events can trigger a step track operation. The first is when the
current sidereal time reaches a value that corresponds to one of the 48 track table entries. The second is
when the controller calculates that the antenna pointing error exceeds the value specified by the user.
The Max Track Error CONFIG mode item allows the user to specify the maximum tracking error in tenths
of a dB.
The controller constantly calculates the antenna pointing error based on the following data: the apparent
motion of the satellite (determined by the Inclination of the satellite’s orbital plane), the antenna's
beamwidth (determined by the Antenna Size and the satellite frequency Band), and the length of time
since the last peakup. To determine the peakup interval, the controller assumes that the satellite is
always moving at its maximum velocity as seen by the antenna. The maximum velocity occurs when the
satellite is passing through the earth’s equatorial plane. When the controller determines that in length of
time since the last peakup occurred the satellite could have moved so far as to allow the antenna pointing
error to exceed the Max Track Error specified by the user, a peakup will be initiated.
The size of the azimuth and elevation movements (or step sizes) for the step_track operation is also
determined by the Max Track Error value. The step size corresponds to the angular movement of the
antenna which would change the received signal strength by Max Track Error tenths of a dB. It is
calculated based on the antenna's beamwidth and radiation pattern.
Reducing the Max Track Error value results in more frequent step_track operations with smaller step
sizes. There are limits to how small the max track error may be. This is discussed in section 4.4.5 -
Tracking Problems. Step_track operations will occur more frequently for satellites with larger orbital
Inclination values, or with larger antenna sizes. For a given Antenna Size and satellite Inclination, step
track operations will occur more frequently for Ku band transponders than for C band transponders. (The
user is prompted to enter satellite Band data during setup.) For hybrid satellites (both C and Ku band),
the TRACK menu system allows the user to switch bands.
• The STEP_TRACK submode will pass control to these TRACK mode submodes on the
following conditions:
• PROGRAM_TRACK..receives control when the current sidereal time equals a time for
which track table data is available.