8.3.4 Steptrack Parameters

The behavior of the satellite step track is adjustable with a couple of parameters. This permits to tune the step track performance for special preconditions arising from the antenna and also the satellite. The first parameters listed below are setup parameters, they are set once for an ACU installation to adapt the ACU to the antenna and the beacon receiver.

• AZAntennadiameter ELAntennadiameter --- The diameter values are used by the ACU software to evaluate the antenna's beamwidth. There are separate values for both axes to handle offset antennas as well.
• BeaconRXtype --- With this parameter you specify which type of beacon receiver. For a sat-nms LBRX beacon receiver set it to 'SATNMS' and set the receiver's IP address accordingly. To make ACU and beacon receiver work together you should take care of the following:
  • ACU and beacon receiver must be connected to the same Ethernet segment.
  • Both devices must have assigned IP addresses in the same subnet.
  • The LBRX beacon receiver must be configured for the correct LO frequency. The displayed receive frequency must be the true RX frequency rather than the L-band frequency.
  • At the sat-nms LBRX the ACU's IP address must be set as the 'UDP destination address'.

The setting 'VOLTAGE' is used with any other type of beacon receiver or with a sat-nms LBRX beacon receiver which has no TCP/IP connection to the ACU.When operating in SATNMS mode, the ACU will automatically determine the beacon frequency from the sat-nms beacon receiver. Also the beacon receiver's background activities like frequency tracking and noise reference measurements get synchronized to the step track sequence in this mode. These features are not available in the VOLTAGE operating mode.

• BeaconRXIPaddress --- You need to enter the beacon receiver's IP address in 'dotted quad' notation here if the receiver tape is set to 'SATNMS'.
• BeaconRXvoltagescale BeaconRX0Vlevel --- These parameters define the slope and offset of the beacon level voltage. The values must be set to match settings of the beacon receiver. With the sat-nms LBRX beacon receiver you can set these parameters there as well, chapter '8.3.2 ACU And Beacon Receiver' explains how to find the best settings for this.

The parameters in the table below are to be set individually for each satellite. They are set at the 'Tracking' page and stored with each target memory.

• BeaconRXfrequency --- This parameter is only of interest if a third party beacon receiver is used. The beacon frequency (you must enter the frequency received by the antenna [MHz], not the IF frequency seen by the receiver.) is used by the ACU to calculate the antenna's beamwidth and an approximated beam pattern. With the sat-nms LBRX beacon receiver, the ACU automatically reads the frequency from the receiver.
• Trackingcycletime --- The cycle time specifies how often the ACU shall perform a step track cycle. The value is to be entered in seconds. In fact, the parameter does not specify a cycle time but the sleep time between two tracking cycles. This means, the true cycle time is the time the ACU needs to perform one step track cycle plus the time entered here. 300 seconds (5 minutes) is a good starting value for this parameter. Inclined orbit satellites probably will require a shorter cycle time, very stable satellites can be perfectly tracked with one step track cycle every 15 minutes (900 seconds).
• Trackingstepsize --- The tracking step size is a very important parameter for the performance of the tracking. It defines the size of every depointing step, the ACU makes in order to find out where the optimal antenna pointing is. Setting too high values will cause significant signal degradations during the step track cycle because the antenna moves a too large amount away from the satellite. Setting the value too small will let the beacon level jitter mask the level differences caused by the test steps, the antenna will not track the satellite properly.The step size is specified as a percentage of the antenna's half 3dB beamwidth. The ACU calculates the beamwidth from the antenna diameter and the beacon frequency. Expressing the step size in this relative way keeps the value in the same range, regardless of the type of antenna. The recommended value for this parameter is 15-20%. You may want to start with 20% and try to reduce down to 15% if the signal degradation during tracking becomes too high.The tracking step size is a common parameter for both axes. If both axes behave differently, you can tweak the antenna diameter settings in the setup. Specifying a larger diameter makes the ACU using a smaller step size for this axis.If the tracking step seems to be completely out of range, you should check if the beacon frequency is set properly. The frequency must be the true receive frequency at the antenna, entered in MHz, not an L-band frequency or other IF.
• Trackingmode --- The tracking mode parameter switches the steptrack on or off. With the operation modes 'STEP' and 'ADAPTIVE' the ACU performs steptrack.
• Levelaveraging --- When measuring the beacon level, the ACU takes a number of samples and averages them. The standard value of 5 samples normally should not be changed. Larger values will slow down the ACU execution cycle.
• Levelthreshold --- If the beacon level drops below this value, the ACU raises a fault signal. Steptrack is inhibited while the beacon level is too low, the antenna position freezes.
• Recoverydelay --- After the the ACU has done the tracking steps for the elevation axis, it waits some time before it starts tracking the azimuth axis. This is to let the beacon level settle after the final position has been found. A typical value for this parameter is 4000 msec.
• Measurementdelay --- During a steptrack cycle, the ACU positions the antenna to a certain offset and then measures the level. Between the moment when the antenna reached commanded position and the beacon level measurement the ACU waits some time to let the beacon level settle. The optimal delay value depends on the beacon receiver's averaging / post detector filter setting and is a quite critical for the steptrack performance.If the delay is too short, the beacon voltage does not reach its final value, the steptrack does not properly recognize if the signal gor better or worse after a test step. If the delay is too long, the impact of fluctuation to the measures level grows and may cover the small level difference caused by the test step. With the sat-nms LBRX beacon receiver, best results are achieved if the receiver is set to 0.5 Hz post detector filter bandwidth and a measurement delay of 1500 msec.
• Smoothinginterval --- This parameter controls the smoothing function. Setting it to zero disables smoothing. A detailed description of this function you find at chapter '8.3.3 Smoothing'