Communications Constraints

Communications constraints are communications link constraints you can impose on a transmitter or receiver. You can also opt to exclude time intervals that meet a given constraint.

Constraints on the receiver also apply to the jammers. When the receiver constraint does not close the link with a transmitter, it may exclude the jammer as well.

Field	Description
Bit Error Rate (BER)	Limits the bit error rate. Starting with STK 9, the minimum threshold setting for the BER was lowered to 1.0e-30. The BER values on the Comm Link reports and the Comm System Link Information reports may go down to 1.0e-30. The minimum default value for the BER constraint remains at 1.0e-25. However, the minimum allowed has been changed to 1.0e-28. This lowering of minimum BER values allows proper constraining of the Access time intervals based on the minimum BER.
C/N	Limits the true carrier-to-noise ratio. This value includes any prereceive gains and losses that may be defined for the receiver as well as propagation losses.
C/No	Limits the carrier-to-noise density ratio measured at the antenna output. This value includes any prereceive gains and losses that may be defined for the receiver as well as propagation losses.
Communications User Plugin	You need to provide a plugin script. The plugin script is not automatically reloaded after you make changes to it. To reload the script, click Reload. This constraint only applies to Receiver objects.
Doppler Shift	Limits the amount of frequency shift due to the relative velocities of the objects involved. The constraint values specify the frequency range relative to the transmitted frequency. For example, if the transmitted frequency is 5 GHz and the Min and Max Doppler Shift are 100 kHz, the allowed frequency range is 4.9999 to 5.0001 GHz.
Eb/No	Limits the bit-energy-to-noise-power-density ratio. These values include all pre-demodulation gains and losses.
Flux Density	Limits the power flux density at the receiver. This value represents the flux density as seen by the receiver, including all prereceive gains and losses that may be defined for the receiver as well as propagation losses.
Frequency	Limits the range of frequencies to those to which the receiver responds. To assist in making frequency choices, use the Band field to select a particular frequency band or choose Custom to specify a particular frequency range.
G/T	Constrains the dynamically computed ratio of the link budget receiver gain value (G) along the link vector and the total receiver noise temperature (T). The noise temperature includes the antenna's external (environmental) noise, antenna noise, antenna to LNA and LNA to receiver cables, LNA noise figure, etc.
Link EIRP	EIRP (in dB) computed in the direction of the link being established at the current time instant between a transmitter and a receiver. This is obtained by computing the gain (dB) in the direction of the link and then adding the final transmitter amplifier power (dB) to it.
Link Margin	Enables you to filter report results based on the Link Margin calculation.
Polarization Relative Angle	This is the relative angle between the transmitter's plane of polarization and the receiver's plane of polarization. It is computed dynamically at each time step from transmitter and receiver position and attitude information. Polarization mismatch between the receiver and the transmitter antennas transmits into a loss value in dB.
Power at Receiver Input (receiver only)	This is the received power at the input of the receiver front-end amplifier, meaning after the antenna, cables, etc. It accounts for the antenna gain, cable losses, etc., and is adjusted for polarization mismatch loss.
Rcvd Isotropic Power	Specifies the minimum and maximum desired received isotropic power level at the receiver antenna input. This range represents the received isotropic power as seen by the receiver and includes any prereceive gains and losses that may be defined for the receiver as well as propagation losses.
Spectral Flux Density	The Flux Density is computed across the receiver's bandwidth, as seen by the receiver's RF front end. The bandwidth is the receiver's total bandwidth.
System Noise Temperature	You have two main options to specify this constraint: Use Total System Temperature: This option is only available if you select the Compute option on the radar or receiver Definition page AND you select the Compute option with at least one Component in the System Temperature tab of the Basic Definition page AND if you specify a Min and/or Max System Noise Temperature. If you select this option, the specification of selected components is disabled and the total system noise temperature calculated by STK is used in constraint evaluation. Use Selected Components: This option is only available if you select the Compute option on the radar or receiver Definition page AND you select the Compute option with at least one Component in the System Temperature tab of the Basic Definition page AND if you specify a Min and/or Max System Noise Temperature. Select one or more of the following components: Earth, Sun, Atmosphere, Urban Terrestrial, Rain, Cosmic Background, Clouds & Fog, Tropo Scintillation, and External. Only those components that are selected to be calculated in the Radar System Temperature window or Receiver System Noise Temperature window will be available for selection here.

In the case where constraints are computed between invalid Communications pairs, STK displays messages in the Message Viewer that the link pair is not valid. An example of this is a link between a transmitter object and an antenna object, instead of a receiver object. The constraint data report displays invalid default values for the metric, such as frequency constraint invalid value is shown as -0.1 Hz.