Research Concepts RC2000C - Manual (Page 37)

RC2000C Az/El Tracking Antenna Controller Chapter 4 Inclined Orbit Satellites 29
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4.3.4 Real Time Clock
The controller contains a real time clock (powered by the same lithium battery that powers the
controller's non-volatile memory) which is used to calculate the sidereal time. The period of the
satellite's motion is one sidereal day. The controller maintains a track table that contains the satellite's
azimuth and elevation positions as a function of sidereal time. The controller uses the time and date
maintained by the real time clock to calculate the sidereal time. The user specifies the Time and Date
via CONFIG mode prompts.
It is important that the user not change the time value when the time changes from Standard Time to
Daylight Savings Time. The satellite does not experience the time shift. If the time is changed forward
or backward by one hour, the data in the program track table will no longer be correct. For this reason
the user should not change the time as the time standard changes. If this causes confusion the time
may be specified as Universal Coordinated Time (formerly known as Greenwich Mean Time - GMT).
Note that the real time clock used in the controller will properly account for leap years.
4.3.5 Receiver AGC Signal
To implement the tracking algorithms, the controller requires an input signal that indicates the strength
of the received signal. Such a signal is generated within a satellite receiver, and is referred to as an
AGC signal. (AGC is the abbreviation for Automatic Gain Control.) On satellite receivers, this signal
may also be referred to as a 'Signal Strength' or 'Tuning Meter' output. An AGC output typically varies
in proportion to the received power of the transponder that the receiver is currently tuned to.
The signal strength input is used to peak up the antenna while step tracking. The step tracking
operation positions the antenna so as to maximize the received signal strength. Signal strength is also
used to determine whether or not a satellite transponder is currently active. If the signal strength
reading falls below a threshold set by the user, the controller assumes that a satellite signal is not
present. If the satellite signal is lost while step tracking, the SEARCH sub-mode receives control.
When the PROGRAM_TRACK sub-mode is active, the presence of a satellite signal governs whether or
not track table update operations are performed. The procedures required to set up the controller for
various receiver AGC configurations are covered in detail in the next section.
4.4 Configuring the Tracking System
This section describes the procedures that the user must follow to configure the tracking system. The
procedures for adjusting the AGC gain and offset pots, determining the AGC threshold parameters, and
entering other CONFIG mode data pertaining to the tracking system, are all covered in this section.
4.4.1 AGC Adjustment and Configuration
The use of the AGC signal was described in the previous section. The controller has the provision to
process two channels of AGC information. This means that two receivers can be connected to the
controller (at least one receiver is required). The two channels are referred to as AGC1 and AGC2.
The controller uses the stronger of the two as its AGC input. Each channel has separate gain and
offset potentiometer adjustments. The installer uses the pots to transform the receiver's AGC signal
swing into a range of values which is easily measured by the controller.
In addition to the potentiometers, there are three CONFIG mode items that are related to the AGC
system. The three items are ...
1. AGC Polarity Flag - This flag indicates to the controller whether a stronger satellite signal
corresponds to a higher AGC voltage (positive polarity) or a lower AGC voltage (negative polarity).
Both AGC channels must have the same polarity. Each AGC channel contains an internal pull
down resistor. This means that if one of the AGC channels is unused, its input is pulled to ground.
This can cause problems if the AGC Polarity Flag specifies negative polarity, because an input of
zero volts (ground potential) corresponds to the strongest possible satellite signal. For this case the
installer must place a jumper from connector J1, pin 11, to the unused AGC input channel and
adjust that channel's potentiometers (after negative polarity has been specified) so that its signal
strength value is 0. J1 - pin 11 has a potential of 5.7 volts.