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4
SYSTEM DEFINITION AND CONSTRAINTS
4.1 Astra 19.2°E Constellation
During our case studies, we have considered the Astra 19.2°E constellation, formed by a group of four satellites (1KR, 1L, 1M and 1N, ordered according to their launching date). They are co – located at the 19.2° East orbital position in the Clarke Belt, and are owned by SES S.A.. They provide for services downlinking in the 10.7 GHz – 12.75 GHz range of the 𝐾𝑢 band.
The satellites at the Astra 19.2°E position primarily provide digital TV, digital radio and multimedia services to Europe and North Africa, principally to Algeria, Austria, Belgium, France, Germany, Morocco, the Netherlands, Poland, Spain, Switzerland and Tunisia (Fig. 4.1).
The frequency plans of the four satellites are shown in (Fig. 4.2 – (a), (b), (c), (d) ):
Fig. 4.1: Coverage Area
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Fig. 4.2 – (b): Astra – 1L Frequency Plan
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Fig. 4.2 – (d): Astra – 1N Frequency Plan
4.2 System Definition
In (Fig. 4.3) is described an easy block diagram of the system used during the experiments, developed by the Fraunhofer FHR Institute:
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As we can see, the first part is a normal satellite receiver, with three LNA (Low Noise Amplifier). The entire spectrum is amplified by an LNA with 39.5 dBi of gain, and then split into two branches: the frequencies between 10.7 − 11.7 𝐺𝐻𝑧 are considered as “low band”, and are demodulated by an oscillator with a frequency of 9.75 𝐺𝐻𝑧. The frequencies between 11.7 − 12.75 𝐺𝐻𝑧 are considered as “high band”, and are demodulated by an oscillator with a frequency of 10.6 𝐺𝐻𝑧. Thus, the two bands are amplified by others LNAs (57 dB of Gain) with band 950 − 1250 𝑀𝐻𝑧 and 1100 − 2150 𝑀𝐻𝑧 respectively. After that, two local oscillators bring the signal near the base band, where a narrowband filter catch the parts between 0 − 65 𝑀𝐻𝑧. At the end, an ADC with 𝑓𝑠 = 130 𝑀𝐻𝑧 converts the analog
signal to digital with 16 bits of representation. The parabolic antenna is a Fuba sat – antenna with 85 cm. of diameter and a gain of 39.5 dBi.
The block diagram described above is equal for the vertical and horizontal polarization, and the entire system must be replicated for both surveillance and reference channels. Thus, we will have two antennas and eight outputs.
4.3 Operational Proof
The first operational proof done with the radar was the simulation of a punctual scatter using a 30 𝑚. ca. delaying cable LMR – 400 – UF (Fig. 4.10) between the receiver and the surveillance antenna.
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The antennas were pointed both towards the Astra 19.2°E satellites constellation for the signals recording (Fig. 4.11); an a posteriori cross – correlation processing has been done between the two signals. The support structure of the two antennas has an arm of 3 𝑚. length ca.
Fig. 4.11: Antennas Configuration (Courtesy of Fraunhofer FHR)
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As we can see in (Fig. 4.12), the peak of the cross – correlation is well visible and it has a high signal to noise ratio (SNR) of 25÷30 dB.
Moreover, as explained in (Section 3.3.6), no unwanted peaks are visible in the map, reinforcing the thesis that none direct signal reconstruction is necessary.
4.4 Close Satellites Interference
One possible source of interference is due by the co – satellites interference.
In general, specially for satellites that broadcast on the same region of interest with their footprints (e.g. Europe), an FDM (Frequency Division Multiplexing) technique is applied; but the enormous development of the satellite communications and services has required the exploit of the entire 10.7 − 12.75 𝐺𝐻𝑧 band.
Since the satellite broadcastings are not thought for radar applications, can not be excluded that currently and/or in the future can be more than one transmission on the same channels (thus, with the same carrier frequencies). Of course, these satellites would be placed in different angles over the Equator, allowing the interference cancellation by the receiver antenna (Fig. 4.4).
Fig. 4.4: Typical Parabolic Antenna Pattern
In first approximation, the HPBW (Half Power Beam Width) of a parabolic antenna can be written as:
𝜃3𝑑𝐵 = 1.22
𝜆
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where 𝜆 is the carrier wavelength of the transmission, and 𝐷 is the diameter of the parabolic antenna. In our case, with 𝐷 = 85 𝑐𝑚. and considering a carrier frequency of 𝑓0 = 11.7 𝐺𝐻𝑧, we obtain 𝜃3𝑑𝐵 ≃ 2°.
Thus, if we point the antenna with sufficient angular (azimuth and elevation) accuracy towards the reference satellite, it is possible attenuating the signals from the others undesirable satellites, due to its small angular aperture (spatial multiplexing).
The same argument cannot be done on the surveillance channel of the radar (in this thesis work we do not consider the direct path interference due to the main satellite on the surveillance channel at zero doppler). On the target act all the signals produced by the geostationary satellites along the LOS (Line of Sight) of the target. The reflected signals by the target are not spatially separable by the surveillance antenna (Fig. 4.5), thus, if their respective satellites are broadcasting on the same channels, we will obtain a strong interference. At the limit, if the satellites broadcast using different carrier frequencies, we obtain a general increasing of the noise level.
Fig. 4.5: Close Satellites Interference
Considering an ideal punctual scatterer, two different interference satellites in the same reference band (in addition to the reference satellite), and considering the same average power level for each
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transponders, the RD map in (Fig. 4.6 – (b)) is obtained. The RD map in (Fig. 4.6 – (a)) shows the case without interferents along the surveillance channel for a comparison.
Fig. 4.6 – (a): RD map without interferents on the surveillance channel
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In the previous image we can appreciate a general increase of the noise level, but the SNR is still really high around 50 dB.
More interesting is the situation where some others satellites broadcast with a higher power respect to the reference satellite (this is not uncommon, because being situated in different azimuth angles over the Equator, the weather condition over their footprints could be really different, with the need of more transmission power for some transponders).
In (Fig. 4.7 – (a), (b)) the RD maps for two different cases are shown: in the first image, one interferent has the same power of the reference satellite, instead the other one has a power of 1 dB higher with respect to the reference satellite. In the second one, both the interferents satellites have a power of 1 dB higher with respect to the reference signal.
In the worst case (Fig. 4.7 – (b)) the peak is still well visible, but we can notice an increase of the noise level. However, since the SNR is still around 30 dB, the degradation is not as much as expected, declaring that the close satellite interference does not create problems during the detection procedure.
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Fig. 4.7 – (b): RD map where both the interferents have a power of 1dB higher with respect to the reference signal
More unlucky situation, for example more interferent satellites or a higher power for the interferent transponders, haven’t been taken under consideration because they are not realistic situations.
