If one or more Antenna objects are attached to the GPSReceiver or TrackingInstrument object, the name of each antenna appears in a drop-down list. Select the antenna that you wish to designate as the default antenna. The default antenna is always used for the simulation of measurements and for the processing of measurements when an antenna is not specified by the tracking data. Antenna objects are useful for establishing a reference point for observations that differ from the center of mass of the satellite and for restricting the direction from which measurements will be simulated or processed.