To specify a list of (A) satellites to be used in a simulation or filtering process, (B) trackers to be used in a filtering process, or (C) tracking strands to be used in a simulation process:
TIP: When selecting items in the Add Item window, you can use standard multi-select techniques using the Control or Shift key. If you later return to the Add Item window, you will find items that you have selected marked with an asterisk.
The AddItem window for the SatelliteList will list all satellites in the scenario. For the TrackerList it will list all trackers, i.e. it will include any facility or satellite that has at least one entry in its MeasurementStatistics list. For the TrackingStrandList it will contain a list of all possible generic tracking strands, where a generic tracking strand consists of a tracker, a primary object of interest designated by a * and possibly other scenario objects. The available list of tracking strands is constructed based on the satellites and trackers in the scenario and the list of measurement models associated with each tracker. Complete tracking strands are constructed during the simulation process by replacing the * with specific satellite names. The one exception to this process is the simulation of GNSS measurements, where the tracker and primary object are the same. In this case, the tracking strand is built as the GNSS receiver to the GNSS constellation, designated by the string "GNSS". Control over which GNSS satellites are used in the simulation is provided in the GNSS constellation object.
The tracking strand concept was developed to support more complicated tracking systems such as TDRSS, where relay satellites or multiple ground stations are involved in the computation of a single observable.
NOTE: Facilities which are not trackers, such as BRTS stations, will be added to the state definition for the simulator or filter if they have a transponder with an estimated bias and are involved in a tracking strand which may be used for simulation or the processing of measurements.
NOTE: If you do not specify a SatelliteList, the simulation or filter will generate data for all satellites in the scenario. If you do not specify a TrackerList, all eligible facilities and satellites in the scenario will be used in the filtering process. If you do not specify a TrackingStrandList, all possible tracking strands will be used in the simulation process.
The term 'tracker' is used here to refer to a data collector used in orbit determination. A tracker can reside on any platform, whether it is a facility or a satellite.
A closed-loop tracker identifies a sequence of data that makes up a 'track', and predicts where to look to collect the next point. A radar site is an example of a closed-loop tracker.
An example of an open-loop tracker is a Satellite Laser Ranging (SLR) site. An SLR site uses the predicted ephemerides of the target whose data it is collecting, points in an open loop, fires the laser and collects the returns. It does not process the returns to modify the pointing, and if it misses the target it might not know it until the data is reported to the central facility for OD.
A tracking strand is a sequence of objects in ODTK, specified in an order dictated by the measurement models, required for the modeling of an observation. Note that ODTK uses two types of tracking strands: full strands and generic strands where the primary object of interest is replaced with an *.