Synthetic Aperture Radars for Small Aircrafts

Ku-band and the X-band airborne SAR systems, RIAN-SAR-Ku and RIAN-SAR-X, have been developed and produced at the Institute of Radio Astronomy. The systems are designed to be operated from small aircrafts.

RIAN-SAR-Ku onboard AN-2 Fig. 1. RIAN-SAR-Ku system onboard Antonov AN-2 aircraft. RIAN-SAR-Ku onboard Y-12 Fig. 2. RIAN-SAR-Ku system onboard Y-12 aircraft.

Features of RIAN-SAR-Ku System

The Ku-band SAR system (RIAN-SAR-Ku) operates in a strip-map mode producing single-look SAR images of 3-meter resolution in real time. The radar can perform measurements at two linear polarizations. The system is also capable of detection and indication of moving targets.

  1. The transmitter is based on a traveling-wave tube power amplifier (TWT PA).
  2. The binary phase coding technique (M-sequences) is used for pulse compression.
  3. High pulse repetition frequency (PRF) of 20 kHz is used for detection of moving targets.
  4. The PRF is adjusted continuously to keep the ratio of the aircraft velocity to the PRF constant.
  5. A slotted-waveguide dual-polarization antenna with 1-degree beam and an alternative horn antenna with 7-degree beam are used.
  6. The antenna orientation angles are estimated with a high accuracy directly from Doppler frequencies of backscattered radar signals.
  7. Real-time SAR processing is performed with a time-domain convolution-based algorithm.
  8. The radar system is able to record range-compressed radar data, as well as pre-filtered data, navigation data, and SAR images.

Features of RIAN-SAR-X System

The recently developed X-band SAR system is capable of producing high-quality multi-look SAR images with 2-meter resolution in real time. The system is designed to be operated from light-weight aircraft platforms in side-looking or squinted strip-map modes.

  1. The transmitter is based on a modern solid-state power amplifier (SSPA).
  2. The radar transmits long pulses with a linear frequency modulation.
  3. A direct digital synthesizer (DDS) provides frequency sweeping.
  4. A digital receiver technique has been implemented.
  5. The radar uses a compact slotted-waveguide antenna with 10-degree beam.
  6. The antenna orientation angles are estimated with a high accuracy directly from Doppler frequencies of backscattered radar signals.
  7. SAR processing is performed by using a frame-based range-Doppler algorithm with a real-time motion compensation.
  8. Real-time multi-look processing has been implemented in order to reduce speckle noise.
  9. The SAR system is capable of recording original uncompressed radar data, as well as pre-filtered data, navigation data, and SAR images.

Table 1. Characteristics of the SAR Hardware Systems

Transmitter type TWT PA SSPA
Operating frequency Ku-band X-band
Transmitted peak power 100 W 120 W
Pulse repetition frequency (PRF) 5 - 20 kHz 3 - 5 kHz
Pulse repetition rate < 200 Hz / (m/s) Not used
Pulse compression technique Binary phase coding (M-sequences) Linear frequency modulation
Pulse bandwidth 50 MHz 100 MHz
Pulse duration 5.12 µs 5 - 16 µs
Receiver type Analog Digital
Receiver bandwidth 100 MHz 100 MHz
Receiver noise figure 2.5 dB 2.0 dB
System losses 4.0 dB 1.5 dB
ADC sampling frequency 100 MHz 200 MHz
ADC capacity 12 bit 14 bit
Antenna type Slotted-waveguide / Horn Slotted-waveguide
Antenna beam width in azimuth 1° / 7° 10°
Antenna beam width in elevation 40° / 40° 40°
Antenna gain 30 dB / 21 dB 20 dB
Polarization HH or VV / VV VV
Aircraft flight velocity 30 - 80 m/s 30 - 80 m/s
Aircraft flight altitude 1000 - 5000 m 1000 - 5000 m
Aircrafts used AN-2, Y-12 AN-2

Table 2. Characteristics of the SAR Data Processing Systems

Range resolution 3.0 m 2.0 m
Range sampling interval 1.5 m 1.5 m
Number of range gates 1024 2048(processed) / 4096(raw)
Range swath width 1536 m 3072 m
SAR processing algorithm Time-domain convolution Range-Doppler algorithm
Real-time motion error compensation (based on trajectory measurements) No Yes, 1st- and 2nd-order MOCO
Clutter-lock* Line-by-line Frame-by-frame
Pre-filtering Yes Yes
Azimuth resolution 3.0 m 2.0 m
Number of looks (in real time) 1 1 - 15
Ground mapping of SAR images Post-processing In real time
Raw data recording Range-compressed, 7-times decimated Uncompressed, no decimation
Recorded raw data rate 90 Mbps 625 Mbps
Recording pre-filtered data, navigation data, SAR images, etc. Yes Yes
Detection and indication of moving targets Yes No

*The antenna beam orientation angles estimated from the backscattered radar data are used to adjust SAR processing algorithms (the pre-filter, migration curves, reference functions, etc.).

Table 3. Detection and Indication of Moving Targets with RIAN-SAR-Ku System

Unambiguous velocity -100 m/s ... +100 m/s
Velocity estimation accuracy 0.2 m/s
Target position accuracy 1.5 m in range, 6.0 m in azimuth
Ground clutter rejection > 30 dB

Examples of SAR Images

A screenshot of the radar control display Fig. 3. Screenshot of the RIAN-SAR-Ku radar control display. All motion parameters are indicated, including the position of the antenna footprint on the ground with respect to the aircraft. Indication of moving targets Fig. 4. Real-time SAR image with detected and indicated moving targets obtained by the RIAN-SAR-Ku system.

Other examples: Fig. 4a, Fig. 4b, Fig. 4c, Fig. 4d, Fig. 4e, Fig. 4f, Fig. 4g. These SAR image are built without geometric correction.

45-look SAR image (RIAN-SAR-Ku) Fig. 5. 45-look SAR image of 3-meter resolution formed by using the algorithm with the built-in geometric correction (at the post-processing stage). The raw data were recorded by the RIAN-SAR-Ku system with the 7-degree horn antenna. The SAR image is imposed on a Google Map image of the scene in order to illustrate the achieved geometric accuracy. 7-look SAR image (RIAN-SAR-X system) Fig. 6. 7-look SAR image of 2-meter resolution obtained with the RIAN-SAR-X system.

Several SAR image frames were stitched to form this ground strip image. Open the full-resolution image (7.5 Mb). View the illustration of the frame stitching procedure (animated gif, 6.8 Mb).