Research Concepts RC2500 - Manual (Page 39)

32 RC2500 Antenna Controller Chapter 4 Inclined Orbit Satellites
Research Concepts, Inc. • 5420 Martindale Road • Shawnee, Kansas • 66218-9680 • USA www.researchconcepts.com
4.3.3 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 which 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 (for known as Greenwhich Mean Time - GMT). Note that the
real time clock used in the controller will properly account for leap years. The RC2500B tracking
algorithm is year 2000 compliant.
4.3.4 Receiver AGC Signal
To implement the tracking algorithms, the controller requires an input signal, which indicates the strength
of the received signal. Such a signal is generated within a satellite receiver or modem, 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, which 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 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 perform the controller setup 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 controller has the provision to process two channels of AGC information. This means that two
receivers may be connected to the controller (one receiver is required). The two channels are referred to
as AGC1 and AGC2. The controller chooses 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 or if the AGC input range
traverses zero volts. In these cases, an input of zero volts (ground potential) corresponds to a
possible satellite signal. To account for these cases, the installer must adjust the offset pot of the
unused channel such that it results in a measured signal strength near zero.
2. 2. AGC C Band Threshold - When the receiver is tuned to a C band satellite and the signal strength
is above this value, it is assumed that a satellite signal is present. The same threshold is applied to
both AGC channels.