Real-Time SAR Processing
The ability to image ground surfaces with airborne and spaceborne sensors is very important in many applications, e.g. the detection of changes as a result of natural disasters. Sensors in space are very accurate and can provide continuous monitoring of large areas. Unfortunately, satellites offer only very limited space and electrical power for the payload, which sets the parameters for the development of a space-based sensor. A cost-effective solution for earth observation is the use of electro-optical (EO) sensors, which are lightweight and require little power, but can only be used in daylight and good weather conditions. The use of active sensors emitting electromagnetic waves with millimetre or centimetre wavelengths, combined with synthetic aperture radar (SAR), provides high-resolution images in all weather conditions, day and night. The MIRANDA-35 SAR sensor developed at Fraunhofer FHR is an advanced, state-of-the-art design that enables high-resolution, real-time imaging in many applications. The use of the Ka-band (35 GHz) makes the sensor sensitive to small structures compared to lower frequency ranges; road textures and obstacles on railway tracks can therefore be detected. An advanced frequency modulated continuous wave (FMCW) generator allows a radar bandwidth of more than 2 GHz, which corresponds to a distance-independent resolution of a few centimetres in both the range and cross-range directions. A very high image contrast is achieved by the high signal to noise ratio of the heterodyne receiver.
The quality of the radar measurements using an FMCW system depends directly on the frontend components used. The linearity and phase stability of the generated signal (chirp) at the carrier frequency play a decisive role, as it is used both for transmission and down-conversion. The core component of the chirp generator is a Direct Digital Synthesizer (DDS) that generates a signal in the MHz range. The DDS is controlled by an ultra-stable master clock that also drives other frontend components. This technique ensures the coherency within a single chirp as well as between sequenced chirps, enabling high-resolution SAR processing. The output spectrum of the chirp generation module is between 8.4 GHz and 9.1 GHz. This signal is then passed to the frequency multiplier-by-four and finally amplified to +33 dBm (2 W). A small portion of the transmitted signal is passed to the dual-channel
heterodyne receiver, where it is mixed with the signal from the received radar echo to obtain an intermediate frequency (IF) in the lower MHz region. The IF signal is then digitized, stored and processed in the high-performance backend, which was developed specifically for MIRANDA-35. It enables a quick-look capability by using an advanced real-time, on-board SAR-processor. In small aircraft, the processor must cope with unstable flight conditions, which requires a highly accurate inertial measurement unit (IMU) and advanced correction algorithms. This issue is probably not applicable in a satellite application. In this project the MIRANDA-35 system will be used as a flight demonstrator. The frontend and backend will be modified so that they can be installed in an aircraft yet to be determined and will meet the project requirements. For real-time data processing reasons, it is intended to limit the resolution to 50 cm x 50 cm, which requires a radar bandwidth of 300 MHz. The real-time SAR image stream (Level-1) including geodata is made available via USB interface for further processing steps. In the current state of development, the radar frontend has a volume of about 20 x 40 x 30 cm3 and a mass of about 5 kg. A power of 2 W is available at the output of the amplifier. The slotted wave guide antennas have an opening angle of 3.3° in azimuth and 15° in elevation resulting in an antenna gain of about 27 dBi.