Low frequency (UHF) synthetic aperture radar (SAR) can be used to observe objects obscured by foliage. Such systems could be operated from space, but in this case the radar signals are susceptible to distortion by the ionosphere. This presents a considerable design challenge and the ionosphere is often the dominant degrading factor in these sorts of systems. The ionosphere controls the orbit choice, the selection of the transmitted waveforms and integration times, together with signal and image post-processing.
SERENE is designing the wideband ionospheric sounder CubeSat experiment (WISCER). WISCER will comprise a wideband (~100 MHz) beacon on a low cost CubeSat in order to measure and evaluate the ionospheric channel in anticipation of any future development of operational SAR systems.
The Rocket EXperiments for University Students (REXUS) programme allows students from universities and higher education colleges across Europe to carry out scientific and technological experiments on sounding rockets. The basic idea behind REXUS is to provide an experimental space platform for students in the field of aerospace technology. The REXUS programme is realised under a bilateral Agency Agreement between the German Aerospace Center (DLR) and the Swedish National Space Board (SNSB).
SERENE are involved in the Prototype Inflatable Antenna REXUS Deployment (PICARD) experiment. PICARD has been designed with the aim of increasing the TRL of inflatable antennas for space applications.
Since the advent of CubeSat spacecraft, Universities and private entities have been successfully designing and launching satellites at a fraction of the traditional cost, whilst still accommodating useful scientific payloads. Another recently established satellite format is the Pocketqube (PQ) – one eight the size of a CubeSat. This brings with it the challenge of working with substantially smaller power, mass and volume budgets.
For a PQ, SERENE is developing a radio frequency impedance probe (ImP) payload. The principle of operation is to measure the ionospheric plasma’s electric impedance at different excitation frequencies to determine the upper hybrid resonant frequency of the plasma, and hence determine the electron density.
Analytic Descriptions of the Ionospheric Impact on SAR
The ionosphere is the dominant degrading factor in low-frequency (UHF) space-based synthetic aperture radar (SAR). The ionosphere controls the orbit choice, the selection of the transmitted waveforms and integration times together with signal and image post-processing. We are quantifying the effects of the ionosphere (total electron content and irregularities) on SAR to offer mitigation strategies. The ionospheric effects considered are those that affect SAR resolution, interferometry, polarimetry, radiometric calibration and image quality and focus.
We are developing analytical (as opposed to numerical) techniques to quantify the various ionospheric impacts on SAR images. Using these analytic expressions, the dependency of the SAR performance on the ionosphere will be established. This will enable the SAR design space to be searched, the performance optimized for a given task, and mitigation strategies developed. These analytic expressions will be validated against numerical simulation and against experimental L-band PALSAR imaging of calibrated targets located in the equatorial region. At the same time we will measure the background quiescent and disturbed ionosphere using GNSS equipment.