Transponder

Transponder — Defines the electronics hardware, typically attached to a spacecraft, that receives and automatically re-transmits an incoming signal.

Description

The spacecraft Transponder model is required for modeling DSN two way range and Doppler data types. The Transponder object includes modeling of a retransmission delay due to the spacecraft transponder electronics. You can also specify a turn around ratio which is a multiplicative ratio describing how the frequency of the retransmitted signal differs from the received frequency. The incoming and outgoing frequencies are designed to be different so as to avoid RF interference between the signal transmitted by the ground station to the spacecraft and the return signal from the spacecraft to the ground station.

Fields

Field

Description

HardwareDelay

Transponder electronics delay between receiving time and transmitting time at the transponder. It is applied for both simulation and estimation, with or without ramp table use.

Data Type	Real
Allowed Values	Real >= 0
Access	set
Default Value	0
Units	seconds
Interfaces	script

PrimaryAntenna

Antenna resource used by the Transponder resource

Data Type	Antenna Object
Allowed Values	Any valid Antenna object
Access	set
Default Value	None
Units	N/A
Interfaces	script

TurnAroundRatio

Transponder turn around ratio which is used in both simulation and estimation. For the DSN Doppler data type where an input ramp table is not used, changing the transponder turn around ratio appreciably changes the measurement. For all DSN data types, changing the turn around ratio affects the media correction calculations which will typically result in a small change in the measurement. See the RunSimulator and RunEstimator help for additional details.

Data Type	STRING_TYPE
Allowed Values	A string in form of 'a/b' where a and b are real numbers
Access	set
Default Value	'240/221'
Units	N/A
Interfaces	script

Remarks

Turn around ratio affects media correction calculations

Suppose you are given a signal with multiple ‘n’ legs. In order to calculate the media corrections for a given leg, we need to know the associated frequency for that leg. The turn-around ratio is used to calculate the frequency for legs 2 through n. If media corrections are modeled, then, for both DSN range and Doppler measurements, the value of the turn-around ratio, as set in the Transponder resource, will have an effect on the measurements and thus both simulation and estimation processes will be affected.

Independent of media corrections, how does the turn around ratio, as set in the Transponder resource, affect DSN measurements?

Assume that media corrections are turned off so that we can ignore any, typically small, changes to the DSN measurements caused by media corrections. We make the following observations.

The value of Transponder.TurnAroundRatio has no effect on DSN range measurements.
If a ramp table is provided, then the value of Transponder.TurnAroundRatio has no effect on DSN Doppler measurements. In this case, the multiplicative turn around ratio used to calculate the computed measurement is based upon the Uplink Band given in the ramp table. (240/221 for S-band and 880/749 for X band)
If a ramp table is not provided, then the value of Transponder.TurnAroundRatio has a proportional effect on DSN Doppler measurements. For example, if the turn around ratio is doubled, then so is the DSN Doppler measurement in Hz.

For additional discussion on how the Transponder.TurnAroundRatio field affects the DSN measurements, see the RunSimulator and RunEstimator help.

Custom turn-around ratios for DSN Doppler data

As mentioned above, the DSN Doppler (TRK-2-34 Type 17) data type observation value depends upon the transponder turn-around ratio. As shown in the tables in the RunSimulator and RunEstimator help, for ramped Doppler data, GMAT only allows for the use of the standard S-band (240/221) and X-band (880/749) turn-around ratios. For Doppler data where a ramp table is not used, setting the Transponder turn-around ratio will correctly model the Doppler data. GMAT cannot currently accommodate custom turn-around ratios for ramped Doppler data.

Examples

% Create and configure a Transponder object

Create Spacecraft Sat1;
Create Antenna HGA;
Create Transponder Transponder1;

Transponder1.PrimaryAntenna  = HGA;
Transponder1.HardwareDelay   = 0.0;
Transponder1.TurnAroundRatio = '240/221';

Sat1.AddHardware = {HGA, Transponder1};
BeginMissionSequence;