Synthetic Aperture Radar (SAR)

A Synthetic Aperture Radar (SAR) achieves high resolution in the cross-range dimension by taking advantage of the motion of the vehicle carrying the radar to synthesize the effect of a large antenna aperture.

The SAR type is available only for radars mounted on aircraft, launch vehicles, missiles, or satellites (or on sensors that, in turn, are mounted on these vehicles).

See the Technical Notes for a discussion of SAR constants and equations.

The availability of certain report and graph elements depend on whether you enable SAR mode.

The following parameters are available for SAR:

Pulse Definition
Modulator
Pulse Integration

Pulse Definition

Parameter	Description
PRF or Unambiguous Range	Select either Pulse Repetition Frequency (PRF) or Unambiguous and enter an appropriate value.
Range Resolution or Bandwidth	Select either Range Resolution or Bandwidth. Range Resolution is a user-defined parameter inversely related to the pulse-compressed RF bandwidth of the transmitted signal.
Pulse Compression Ratio, Pulse Width, Scene Depth, or FM Chirp Rate	Select one of the following complementary factors: Pulse Compression Ratio. The ratio of compressed to uncompressed RF bandwidth. Pulse Width. The width of the transmitted pulse (the inverse of uncompressed RF bandwidth). Scene Depth. The range swath along the radar line of sight to the target. FM Chirp Rate. The frequency modulation rate of the pulse compression (1/sec²).
Range Broadening Factor	An analysis weighting factor to compensate for signal processing losses in the range dimension. Enter the appropriate value. The default value is 1.2.
IF Bandwidth	IF Bandwidth is the analog-to-digital sampling rate of the SAR. This bandwidth assumes a quadrature sampling without oversampling; in the case of real-valued sampling, the IF Bandwidth must be twice the sampling rate. This rate determines the amount of data that can be collected in a given amount of time.
Number of Pulses	Number of pulses in the modulating pulse sequence.

Modulator

Type	Description
Use Signal PSD	If selected, STK computes the power spectral density based on the radar's operating mode, currently pulsed mode only for SAR. For a pulsed waveform, the computed PSD is ±n (n is user selectable, default value is 15) null points on the RF spectrum. The pulsed signal spectrum follows a sinc pattern. The first null point is at the 1/ pulse width (e.g., the default value for the pulse width is 1.0e^-7 seconds; the computed spectrum is ±150 MHz for the default value 15 for n. The spectrum sample rate is adaptive and based on the spectrum bandwidth used by the signal. This ensures that the sampling of the spectrum is at a sufficient rate for accuracy. The S/T continuous mode models the carrier to be a pure sinc wave. By enabling PSD analysis, you can change this to an impure carrier with a Gaussian power density distribution. The spectrum spreads to ±6 sigma over the bandwidth specified in the Power Spectral Density and RF Spectrum Filters group on the Radar Basic System properties page. PSD Analysis use enhances the fidelity of the radar performance analysis. STK computes the amount of RF power at the radar's transmitter and the transmitter filter due to the signal spectrum characteristics and the bandwidth. On the receive side of the radar, the received power computes based on the incoming signal spectrum, the receive side filter characteristics, and the bandwidth. PSD analysis also improves the radar performance analysis under jamming. The received jamming power computes on the basis of the jammer signal spectrum, bandwidth, the radar's receive side filter and the bandwidth. The jamming power represents the unwanted signal power as seen by the radar's receiver. The pulsed radar signal spectrum is a train of sinc-shaped spectrums. The envelope of the peak amplitudes of these sinc spectrums also follow a sinc characteristic curve. A SincEnvSinc filter is available as a filter type to do match filtering on the radar signals.
PSD Limit Multiplier	The PSD limit multiplier is used to extend the bandwidth of the PSD used in spectral overlap computations with the receive radar spectral filter. The bandwidth of the PSD can be computed by the equation: BW = 2 / Pulsewidth * n where n is the PSD limit multiplier.

Type

Description

Use Signal PSD

If selected, STK computes the power spectral density based on the radar's operating mode, currently pulsed mode only for SAR.

For a pulsed waveform, the computed PSD is ±n (n is user selectable, default value is 15) null points on the RF spectrum. The pulsed signal spectrum follows a sinc pattern. The first null point is at the 1/ pulse width (e.g., the default value for the pulse width is 1.0e^-7 seconds; the computed spectrum is ±150 MHz for the default value 15 for n. The spectrum sample rate is adaptive and based on the spectrum bandwidth used by the signal. This ensures that the sampling of the spectrum is at a sufficient rate for accuracy.

The S/T continuous mode models the carrier to be a pure sinc wave. By enabling PSD analysis, you can change this to an impure carrier with a Gaussian power density distribution. The spectrum spreads to ±6 sigma over the bandwidth specified in the Power Spectral Density and RF Spectrum Filters group on the Radar Basic System properties page.

PSD Analysis use enhances the fidelity of the radar performance analysis.

STK computes the amount of RF power at the radar's transmitter and the transmitter filter due to the signal spectrum characteristics and the bandwidth.

On the receive side of the radar, the received power computes based on the incoming signal spectrum, the receive side filter characteristics, and the bandwidth. PSD analysis also improves the radar performance analysis under jamming. The received jamming power computes on the basis of the jammer signal spectrum, bandwidth, the radar's receive side filter and the bandwidth. The jamming power represents the unwanted signal power as seen by the radar's receiver.

The pulsed radar signal spectrum is a train of sinc-shaped spectrums. The envelope of the peak amplitudes of these sinc spectrums also follow a sinc characteristic curve.

A SincEnvSinc filter is available as a filter type to do match filtering on the radar signals.

PSD Limit Multiplier

The PSD limit multiplier is used to extend the bandwidth of the PSD used in spectral overlap computations with the receive radar spectral filter. The bandwidth of the PSD can be computed by the equation:

BW = 2 / Pulsewidth * n

where n is the PSD limit multiplier.

Pulse Integration

The following options are presented for selecting and defining the Pulse Integration Mode of your SAR:

Parameter	Description
SAR Analysis Mode	A synthetic aperture radar may trade off integration time for azimuth resolution. The integration time-azimuth resolution product is determined by geometry and the parameters defining the SAR. The analysis mode option menu specifies a fixed value to apply to either integration time or azimuth resolution for analysis purposes. This selection affects report and graph elements and constraints downstream. Azimuth resolution is defined as the resolution characteristic of the azimuth dimension, usually applied to the image domain. Azimuth resolution is fundamentally limited by the Doppler bandwidth of the system. Select either Fixed Azimuth Resolution or Fixed Integration Time, and enter the appropriate value.
IF Bandwidth	IF Bandwidth is the analog-to-digital sampling rate of the SAR. This bandwidth assumes a quadrature sampling without oversampling; in the case of real-valued sampling, the IF Bandwidth must be twice the sampling rate. This rate determines the amount of data that can be collected in a given amount of time.
Azimuth Broadening Factor	An analysis weighting factor to compensate for signal processing losses in the azimuth dimension. Enter the appropriate value in the textbox. The default value is 1.2.
Multiplicative Noise Ratio	The contribution of ambiguous range and sidelobe returns to the image formation process. Enter the appropriate value in the textbox. The default value is -20 dB.