RC4500 Antenna Controller Chapter 6 Support
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6.2.2 Timekeeping
There are several versions of time (system, sidereal, referenced and GPS) discussed within this manual.
System time is maintained by the RC4500’s real time clock. The real-time clock is backed up by battery
so that system time is available as soon as the RC4500 powers up. The system time is used to calculate
sidereal time for maintaining track tables. Since satellite’s do not experience time shifts (such as from
Standard Time to Daylight Savings Time or when moving from one time zone to another), it is
recommended that system time not be modified while active track tables are present. If system time is
changed, the information stored in track tables for inclined orbit satellites will no longer be valid.
The RC4500’s system time is set to approximately Universal Coordinated Time (UTC) at the factory. It
will vary from UTC due to the tolerance of the real-time clock.
If the optional GPS receiver is installed, the RC4500 parses UTC from the data sent by the GPS receiver.
This data is only available when the GPS receiver is sufficiently locked on to GPS satellites to determine
UTC. The RC4500 allows the user to synchronize system time to the UTC reported by the GPS receiver.
The period of a satellite’s motion is one sidereal day (approximately 23 hours 56 minutes 4 seconds).
Entries in the track table for an inclined orbit satellite are stored at intervals of 1/48th of a sidereal day.
The RC4500 determines at what point in a sidereal day (with respect to the RC4500 reference) it is by
calculating how many sidereal days have passed from January 1, 1992 until the present system time.
In several screens the RC4500 displays a reference time. The user may designate a three letter
timezone designation and an hourly offset from system time. This allows the user to display local time or
some other reference time without modifying system time. If system time is maintained close to UTC, the
reference time displayed may be of use to operators for coordinating events.
See
section 5.2.2.3 for details on time maintenance.
6.2.3 Drive System
The RC4500 implements several mechanisms for the driving and monitoring of the azimuth, elevation and
polarization axis.
Position Sensing
The RC4500 senses absolute axis position using feedback from various sensors (resolvers for azimuth
and elevation, potentiometer or resolver for polarization). The sensed voltage from the potentiometer is
scaled appropriately for the particular mount. The sensed positions are displayed in angular format.
The boresight of the antenna is displayed for the azimuth and elevation axis. In elevation, this angle is
with respect to the local horizontal. In azimuth, this angle is with respect to the centerline of azimuth
travel.
Jam and Runaway Sensing
The RC4500 continuously monitors the axis positions to detect incorrect movement of the mount. If an
axis has been commanded to move and the RC4500 does not detect movement within a prescribed time,
the controller will declare a “JAM” condition and not allow further movement in that axis until the condition
has been reset.
Similarly if the RC4500 senses movement in an axis when no movement should be occurring, the RC4500
will declare a “RUNAWAY” condition. Like JAM, the RUNAWAY condition must be reset before further
movement in the axis may occur.
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