CG-1280 Rev. N 09/13 - General Dynamics SATCOM Technologies

TheoryNOMINAL LONGITUDENORTH LATITUINCLINATIONSEARCHDIRECTION0SOUTH LATITUFigure 3-1Orbit Scan Theory3.2.2 Orbit Scan OperationTo use Orbit scan it is necessary to have correct site information loaded in the siteparameters. When an OPT target is built, the user must provide the nominallongitude (degrees east) and an estimated inclination. Then the Orbit scanparameter must be enabled. (Note: The longitude range is NOT used by Orbit scanbut is used to set up Box limits for the target's operation.)If no solutions are available when the target is selected for tracking the system willfollow this sequence. If Box limits are enabled, then OPT will check to see if thesystem is currently inside the box. If not, the system will go to box center. Then itwill check for a signal. If no signal is received, and Orbit scan is enabled, OPT willcalculate a satellite trajectory which places the ascending node at the nominallongitude and provides an inclination equal to the estimated inclination provided.The trajectory is assumed to be a circular orbit with a sidereal period. Thistrajectory is then followed in steps equal to a 5 dB signal change based on the 3dB beamwidth of the antenna. The signal level is monitored throughout theoperation. The satellite is assumed to be acquired if the level rises 0.5 dB abovethe Low signal level set for the target. If no satellite acquisition occurs after onepass, the system will stop at box center and issue an OPT cannot track alarm.It should be noted that the Orbit scan is only used when no OPT solutions exist fora given target. Once a solution exists, no matter how old it may be, the scan willNOT be used, even if there is no signal at the solution's position.At acquisition, the system enters steptrack to peak the antenna and then beginsstandard OPT operation.3-4