Research Concepts RC2500 - Manual (Page 34)

RC2500 Antenna Controller Chapter 4 Inclined Orbit Satellites 27
Research Concepts, Inc. • 5420 Martindale Road • Shawnee, Kansas • 66218-9680 • USA www.researchconcepts.com
The apparent motion is slightly greater and somewhat skewed when viewed from the surface of the earth.
The exact shape of the pattern varies with the longitudinal position of the satellite and the place on the
earth from which the satellite motion is viewed. To estimate the height and width of an inclined orbit
satellite's motion as viewed from the surface of the earth, a good approximation is to multiply the results
of the equations above by 1.2.
Examination of these relationships show that the figure eight pattern is much taller than it is wide. For
example, if the orbital plane of a satellite is inclined with respect to the earth's equatorial plane by 5
degrees, the apparent height and width of the figure eight pattern of the satellite's apparent motion as
viewed from the surface of the earth is:
Height (North to South): 1.2 * 2 * 5 = 12 degrees
Width (East to West): (1.2 * 5 * 5) / 115 = 0.26 degrees
This example shows that the apparent motion of the satellite is practically a straight line. This knowledge
of the satellite's apparent motion as viewed by the antenna is exploited by the RC2500B tracking antenna
controller.
The 23 hour, 56 minute, and 4 second period of the satellite's apparent motion is referred to as a sidereal
day. A sidereal day is the time that it takes the earth to complete exactly one revolution. The '24 hour'
solar day is the period of time that it takes the sun to reach the same point in the sky. The solar day is
longer than the sidereal day because the earth is in orbit around the sun and the earth must rotate more
than 360 degrees for the sun to reach the same point in the sky.
Sidereal time refers to the time reference used to record time during a sidereal day. A sidereal time clock
would progress up to 23 hours, 56 minutes, 3 seconds and then wrap around to 0 hours, 0 minutes, 0
seconds. All sidereal times used by the RC2500B controller and referenced in this manual are in
seconds.
The apparent motion of the satellite as seen by the antenna is at it greatest when the satellite is passing
through the earth’s equatorial plane. The apparent motion of the satellite is zero at the endpoints of the
figure eight pattern when the satellite appears to reverse direction.
4.2 RC2500B Tracking Algorithm
The tracking algorithm used on the RC2500B can be divided into 3 distinct parts, or submodes -
STEP_TRACK, PROGRAM_TRACK, and SEARCH. A TRACK_ERROR submode is also implemented.
These submodes are summarized here to provide an overview of the tracking algorithm. The following
sections will provide much greater detail. Notice that certain words are italicized in the following
subsections. The italicized text refers to parameters which are specified by the user either at the time of
system installation (via CONFIG mode), when a track is initiated (via SETUP mode), or once a track has
been established (via the TRACK mode menu). The idea is to get the user familiar with the parameters
which he or she must either specify or adjust. All of these parameters are described in more detail in
later sections.
The TRACK submodes will be described in chronological order as seen by a user initiating a track on an
inclined orbit satellite. A track is initiated from SETUP mode. The user peaks the antenna on the inclined
orbit satellite, selects the satellite name, longitude, inclination and frequency band, and then sets the
satellite's horizontal and vertical polarization positions. At this point the controller enters STEP_TRACK
mode. Note that not all versions of the software support polarization control.
In the STEP_TRACK submode the controller periodically performs peakups on the inclined orbit satellite.
It stores azimuth and elevation positions of these peaks in a track table in non-volatile memory. The track
table divides the sidereal day into 48 time segments. Whenever the current sidereal time equals the
starting time for one of the 48 time segments, a peakup occurs and the antenna azimuth and elevation
values are stored into the appropriate position within the track table. The track table stores a map of the
satellite's apparent motion as seen by the antenna.
During a step_track operation, the controller peaks the antenna by monitoring the received signal
strength. Signal strength information is available to the controller via the AGC (automatic gain control)
input. The AGC input accepts a DC signal generated by the AGC circuits of a satellite receiver, or by a
beacon receiver. The AGC setup and configuration procedure is described in section 4.4.1.