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CONCLUSIONS
This thesis work has been focused on the evaluation of the possibility to obtain high quality passive ISAR images using the DVB – S and DVB – S2 satellites as illuminators of opportunity.
Passive solutions are becoming really important for radar application, since a passive radar does not have an own transmitter, that leads to a decrease of the general cost of the system. Moreover, a bistatic solution can bypass the problem of a small RCS (Radar Cross Section) of the stealth targets: targets that are shaped and/or treated to minimize their monostatic RCS may nevertheless have higher bistatic RCS. In order to obtain the best images, a turntable geometry has been taken under consideration: this geometry guarantees the biggest aspect angle (i.e. the angle formed between the longitudinal axis of a target in flight and the axis of the radar beam) variation. The Astra 19.2°E satellites constellation has been taken under consideration as IOOs and a van (Mercedes – Benz 416 CDI) of the institute has been used as target.
These types of illuminators of opportunity have the distinction of a big antenna’s footprint, with which are able to cover a wide area (until an entire continent). Unfortunately, since the satellites have an altitude of around 36000 𝐾𝑚. over the Equator, the transmitted signal has a low power when arrives at the Earth surface.
The main interference that can act on the received signal (of the surveillance channel) is the close
satellites interference: since they are not thought for radar application, different satellites on different
azimuth angles over the Equator can broadcast on the same frequencies and with different powers (depending on the weather conditions under them). The reference channel is able to do a spatial multiplexing of the satellite signals, due to its thin antenna pattern; the surveillance channel instead cannot do it because the antenna is pointed toward the target, so it catches all the signals scattered by it, with a high risk of interference. The simulations done during this thesis work shown that the reference signal is strong against possible interferents: with a high probability this is due on the MPEG coding, that creates a noise – like signal, so during the cross – correlation process only the equivalent signal scattered by the target is able to create a peak on the Range – Doppler map. The interferents on the same frequency of the reference signal contribute only on a general noise level increasing.
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Moreover, despite the signal broadcasted by the satellites has some deterministic components (e.g. part of the PLHEADER), with their periodicities, the simulations show that no unwanted peaks appear on the RD maps. This is due on the long repetition time of these deterministic components: since the radar has to catch the signal scattered by the target, the total power link budget is low, with no possibilities to “see” beyond few kilometers from the surveillance antenna. For this reason, every signal folding is uncorrelated with the previous ones, preventing the unwanted peaks formation.
About the ISAR images, the work has been developed to obtain a countercheck: two different image formation algorithms (i.e. Range – Doppler and Back – Projection respectively) has been used to demonstrate the validity of these IOOs. Both the algorithms show that an image with a high SNR level is achievable, and the dimensions of the two spots are comparable with the target dimensions.
Unfortunately, since the radar developed by the institute has a total bandwidth of 65 𝑀𝐻𝑧 (in case of a real sampling), or about 130 𝑀𝐻𝑧 (in case of a complex sampling), a high resolution is not achievable: in cross – range direction the spot and target dimensions match quite well; instead, in range direction, the resolution is poorer then in cross – range. Nevertheless, within the larger boundaries given by the range resolution, the dimensions are compatible with reality.
At the state of the art, the radar developed by the Fraunhofer FHR Institute is able to obtain passive ISAR images where a dimensions evaluation of a target could be done. Given the current boundaries, an accurate target classification is impossible, since the bandwidth is not enough wide to obtain a high range resolution.
The DVB – S/S2 transmissions occupy a large part of the 𝐾𝑢 band (10.7 − 12.75 𝐺𝐻𝑧), so it is reasonable trying to get as much band as possible to improve the range resolution. One possible solution, easy but not cheap, is to use more receivers. The employment of more antennas for an array configuration can also lead to benefits for the cross – range resolution and/or the detection process.
For the future will be interesting to apply new image formation techniques, that use both the low and high band of the radar for the improvement of the range resolution. Moreover, since the satellites broadcast in both the polarization, polarimetric ISAR imaging algorithms could be apply.
