On large reflector antennas as radar target for low-frequency synthetic aperture radar
Earth remote sensing provides actual, often crucial and unique information. Global or regional applications can be addressed in the domain of meteorology, climatology, geology, seismology as well as effects caused by the people. The European Copernicus program is a flagship program and delivers data from the Sentinel-satellites developed by the European Space Agency (ESA) as well as data from some other satellites. ESA secures new directions in Earth observation within the ESA Explorer program. BIOMASS as an Earth explorer mission has been launched in 2025. It carries a unique synthetic aperture radar (SAR), which operates at 435 MHz. A single active transponder assists the mission for instrument characterisation and calibration, from an ESA-location near NewNorcia in Australia. Traditional corner-reflectors are less suitable at lower frequencies, if a large mono-static backscattering is desired, because of required large dimensions. Natural persistent targets are also less frequently available at lower frequencies. The latter targets provide comparable (coherent) responses in subsequent SAR observations. It has motivated to investigate other precision structures as a radar target, like a large reflector antenna. Knowledge of the back-scattered field is necessary into the direction back to the radar receiver. If at a sufficient level, the phase information can already be used. It is convenient for SAR interferometry observations. A start has been made to predict the scattering from large reflector antennas by means of analysis. SAR measurements have been inspected, but on another frequency than the BIOMASSfrequency (Sentinel-1, C-band, data-availability). Measured results at C-band do not inform about results for P-band. But C-band data are available, and it assists in methods for further investigations.A reflector antenna has nearly perfect electrical conducting (PEC) properties for the reflectors. There are deviations as discussed with quantitative values in a chapter on several reflector materials. There are also constraints. The scattering from the total antenna structure with its peculiar focal structure is often not known at the radar frequency. The radar signal is often in a band for which the reflector antenna has not been designed. The scattering depends strongly on the specific antenna structure and focal peculiarities. Analysis methods are outlined as well as a possibility to influence the level of the mono-static back-scattering.A repeated SAR-observation can assist to consolidate the obtained result. Large reflectorantennas are available worldwide for telecommunication, radio-astronomy or other applications. They are often equipped with a pointing capability. Pointing towards the SAR-satellite is desirable to provide a high mono-static back-scattering, irrespective of a precise knowledge of the antenna feeding structure. Knowledge of the latter assists to be able to make more accurate predictions.Analysis is needed, because there are many different reflector antenna configurations. But usually not all antenna configuration details are available. Also, the reflector antenna is usually not available to be pointed to the SAR satellite, because of its own functional programming. The analysis of a large reflector antenna can be carried out, using PEC assumptions, for the BIOMASS frequency. A Method of Moments (MoM) approach is suitable, but then one must know all antenna details. The Grasp antenna analysis program with its MoM extension is suitable and can inform about both radiation and scattering characteristics.It can be useful to know if one can consider adaptations, like the use of a controllable closing mechanism. Antennas in the Atacama Large sub-Millimeter Array (ALMA), in the Northern Extended Millimeter Array (NOEMA) and at Pico Veleta have such a closing mechanism for front-end protection. It provides a possibility to influence the level of mono-static backscattering.Mono-static back-scattering into the direction of the SAR receiver is known to vary veryslowly over an angular range for a corner reflector as target. Large reflector antennas provide such an effect for a much more limited angular range. But when there is a PEC plate or feed-array in the focal region (as in Westerbork antennas), the latter angular range is a bit wider. But the scattering from the feed-array is often not known outside or even inside its operational band. In analyses and predictions, the actual positioning of the PEC plate in the focal region must be known, but it might be used conveniently. It has been demonstrated for the 25 m antenna in Dwingeloo. It has been confirmed by SAR observations in C-band (Sentinel-1). Several antennas have been analysed and SAR observations in C-band have been carried out. A special case shows that the antenna radiation function is quite different from the antenna scattering function. An explanation is described for a unique radio-telescope with a spherical reflector, the ROT54 antenna, located in Armenia.Arrays of large antennas can be of interest. The Westerbork radio-telescope array is veryaccurate, both mechanical and electrical for its design frequency (L-band). It can providean array of radar targets in P-band and act as a sort of calibration scenario. Other arrays of antennas can be considered as well, but it needs further work.A start has been made with investigations of Sentinel-1 SAR data, when both the Westerbork antennas and the Dwingeloo antenna have been pointed towards the Sentinel-1 satellite. For such a purpose the Sentinel Application Platform (SNAP) software has been used, known in the Copernicus program. SNAP has as well some restrictions for such a subject and dedicated SAR analysis programs may provide better results. But it is also meant to indicate to the reader, that SNAP as an available tool is useful in general sense.A regulation of the International Telecommunication Union (ITU) does not allow BIOMASS to operate over Westerbork, as it is located within the zone for the Space Object Tracking Radar (SOTR). The SOTR has priority to use P-band (ITU ruling).With a worldwide availability of large antennas, it is a consideration that a large antenna can act as a radar target for BIOMASS, provided it can be pointed to the SAR. In such a case such a target can possibly complement the active SAR-transponder for interferometric observations. And it can be a better alternative than the corner-reflector especially at low frequencies.The analyses presented are in general at the BIOMASS frequency. Grasp is recommended, with options for different analysis methods, with the possibility to derive the complex fields, inclusive polarisation and level. A simple way is adopted to normalise the incoming plane-wave in Grasp calculations and so to determine the mono-static backscattering in dBmeter squared. P-band SAR observations are desirable to confirm such analyses. It invites to carry out further work. It has been decided to present quite some facts and related material. It can serve further investigations in related directions
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- Technische Universiteit Eindhoven
- Eindhoven, Noord-Brabant
- Netherlands 5612 AJ
- Building: De Zaale - Auditorium
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On large reflector antennas as radar target for low-frequency synthetic aperture radar
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