Transponders

With a satellite, facility, or GPS Satellite highlighted in the Object Browser, click the button or select Transponder from the Object Catalog to attach a transponder to the satellite. The following properties can be specified for a transponder:

Transponder Properties
Property Description
Type Defines the type of ranging signals received and retransmitted by the transponder. Choices are:
  • SatToGround - for use in processing ground-based two-way ranging.
  • SatToSat - for use in processing space-based two-way ranging.
  • GroundToSat - for use in processing 4L BRTS (Bilateral Relay Transponder System, part of the TDRS tracking system) ranging. This type of transponder is attached to a facility.
  • Relay - for use in processing 4L ranging, normal two way TDRS ranging, One-Way Bistatic Range, 2L CA Pseudo-range, TDOA, FDOA, SD TDOA and SD FDOA. This type of transponder is attached to the relay satellite and its delay is added in both the forward and return link directions of the 4L range measurement. The delay is added on the forward link only for the One-Way Bistatic Range.
  • SatToRelay - for use in processing 4L ranging, normal two way TDRS ranging. This type of transponder is attached to the user satellite.

A satellite can have multiple transponders to model the use of multiple ranging paths used in the tracking of the satellite. Each transponder can have its own constant bias and state estimate. In most cases, transponders are limited to one of each type. However multiple Relay transponders are allowed to support TDRS, and multiple SatToGround transponders are allowed to support Range and Doppler.
TransponderID Enter an integer representing a unique identification number for this transponder. See the special note below related to transponder ids on TDRS spacecraft.
EstimateBias A correction to the constant transponder bias will be estimated by the filter if set to true and two-way range measurements are processed. Also controls whether transponder bias can be varied by the Simulator.
BiasData

Select TimeUnits or DistanceUnits and then set the BiasModel.
BiasModel

Choices for the BiasModel include:

  • GaussMarkov = Bias will be modeled as a scalar exponential Gauss Markov sequence
  • RandomWalk = Bias will be modeled using a Wiener (Brownian motion) sequence
  • Vasicek = Bias will be modeled using a Vasicek stochastic sequence. This is a two-parameter model that solves for both a short-term and long-term bias.

    • Reference the Stochastic Model section for the description and inputs associated with each model.
RFInfo Information related to relay transponder selection and use. Currently only used during the processing of TDOA and FDOA measurements:
  • CenterFrequency - center of transponder frequency band.
  • Bandwidth - the frequency span allocated to the transponder is from the center frequency minus one half the bandwidth to the center frequency plus one half the bandwidth.
  • AddFreqTranslation - The additive change in the frequency of the signal as it passes through the transponder. A typical value of -2300 MHz might be used with an uplink in the 14 GHz range and a downlink in the 12 GHz range.
DefaultAntenna If the transponder is attached to a satellite, to which one or more antennas are also attached, the name of each of those antennas will appear in a dropdown list. Select the antenna that you wish to designate as the default antenna for the transponder. 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. If no default antenna is specified and no antenna is specified in the tracking data then no antenna processing will be applied in either the simulation or processing of measurements.
DefaultAntenna2 Available only if Type (above) = Relay. In the case of relay transponders, the first antenna is designated to be the ground link antenna while the second antenna is the space link antenna. Selection works the same way as for DefaultAntenna.

Specific transponder paths are identified in UTDF formatted tracking data for TDRS spacecraft. In order to model the commonly used signal paths properly during the processing of 4L Range and 4L BRTS Range observations, each TDRS satellite should be configured with eight transponders with the following transponder ids:

SA1 - SA1 = 11
SA2 - SA2 = 22
MA - MA - 1 = 331
MA - MA - 2 = 332
MA - MA - 3 = 333
MA - MA - 4 = 334
MA - MA - 5 = 335
MA - MA - 6 = 336

where the transponder ids are constructed from a combination of the forward path and the return path where: 1 corresponds to SA1, 2 corresponds to SA2 and 3 corresponds to MA. The final digit in the MA ids above reflects the fact that there are 6 separate transponder paths using the MA service. In all cases shown here, the forward and return paths are the same.