Constraints on Communications Objects

Two types of constraints - Communications and Interference - are provided for transmitters and receivers. A third type of constraint - Noise - is provided for receivers and radar. A fourth type of constraint - RF Environment - is provided for all three objects.

Communications Constraints

The Comm page allows you to impose a variety of communications link constraints 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.

Communications constraints

Field	Description
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.
Rcvd Isotropic Power (RIP)	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 pre-receive gains and losses that may be defined for the receiver as well as propagation losses.
Flux Density	Limits the power flux density at the receiver. This value represents the flux density as seen by the receiver, including all pre-receive gains and losses that may be defined for the receiver as well as propagation losses.
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.
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.
C/No	Limits the carrier-to-noise density ratio measured at the antenna output. This value includes any pre-receive gains and losses that may be defined for the receiver as well as propagation losses.
Power at Receiver Input (receiver only)	The received power at the input of the receiver front end amplifier (i.e. after the antenna and the cables etc). This accounts for the antenna gain and the cable losses etc., and is adjusted for polarization mismatch loss.
C/N	Limits the true carrier-to-noise ratio. This value includes any pre-receive gains and losses that may be defined for the receiver as well as propagation losses.
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.
Link Margin	Allows for filtering of report results based on the Link Margin calculation.
Eb/No	Limits the bit energy to noise power density ratio. These values include all pre-demodulation gains and losses.
Polarization Relative Angle	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.
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.
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.
User Plugin (receiver only)	A constraint defined by a user-supplied plugin script. The plugin script is not automatically reloaded after you make changes to it. To reload the script, click Reload.

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 (e.g. frequency constraint invalid value is shown as -0.1 Hz).

Interference Constraints

The Interference page allows you to set constraints in terms of certain link performance indicators as affected by interference. For example, if you set BER+I Max to a given value, access is excluded for periods during which BER, taking into account the effects of interference, exceeds that value. This is in contrast to the BER constraint on the Comm page, which would exclude access for periods during which BER exceeds the given value, irrespective of interference effects.

Before you can use these constraints, you must first define an interference environment with a CommSystem object that identifies the participating transmitters and receivers and the interference sources. If the scenario contains more than one CommSystem, you can select the CommSystem to be used for interference constraints on the scenario's RF Environment page.

STK uses the first CommSystem in the scenario unless another has been selected.

You can also opt to exclude time intervals that meet a given constraint.

Interference constraints

Noise Constraints

This page enables you to impose constraints on radar or a receiver related to system noise temperature. You can also opt to exclude time intervals that satisfy a given constraint.

The following constraints are available:

RF Environment Constraints

Environmental factors can affect the performance of a communications link or a radar system. An object's RF Environment properties page enables you to apply the following models to your analyses. When enabled, the models will affect all RF phenomena in the scenario except where overridden or varied on the individual STK object.

• Environmental data
• Rain, clouds and fog
• Atmospheric Absorption
• Urban and terrestrial
• Tropospheric scintillation
• Custom models