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diff --git a/08.-Passive-Radar.md b/08.-Passive-Radar.md index 7eb5cb0..5ebb94d 100644 --- a/08.-Passive-Radar.md +++ b/08.-Passive-Radar.md @@ -138,4 +138,21 @@ Range resolution depends on the sampling bandwidth, which for the KrakenSDR and This means that we can differentiate between two different objects that are 125m apart. Or in other words, two objects closer than 125m apart will be combined into a single range cell. -Another example, if we are using a DAB transmitter as the illuminator, then we only have 1.5 MHz of bandwidth. Therefore we achieve $\frac{c}{fs} = \frac{299792458}{1500000} = ~200m$ resolution per range cell.
\ No newline at end of file +Another example, if we are using a DAB transmitter as the illuminator, then we only have 1.5 MHz of bandwidth. Therefore we achieve $\frac{c}{fs} = \frac{299792458}{1500000} = ~200m$ resolution per range cell. + +# Other Advanced Passive Radar Notes + +## RD Display within RD Display DVB-T Phenomenon + +Sometimes you might see shrunken attenuated images of the entire RD display within the RD display itself. This is a phenomenon related to the way in which DVB-T signals are structured. + +In brief, the DVB-T signal is constructed in the following way: In the frequency domain we have a vector with 8192 frequency bins. Each of these bins are sub-carriers. Some of these carriers are data carriers, some of them are pilot carriers. The data carriers are used to transfer the useful data (MPEG-2 stream) while the pilot carriers contain fixed modulation data which is preliminary known by the receiver. The transmitter assembles this frequency domain vector (called symbol table) and transforms it to the time domain with iFFT. After transformation it sends out the symbol and assembles the next symbol table. (Of course, there are a lot of other processes besides this main principle but those are not important at the moment). + +The receiver isolates and synchronizes to the symbol in time domain, performs an FFT on the symbol and thus recovers the data carried on the data-carriers. + +Now, as the pilot signals are preliminary known by the receiver it is able to measure what kind of distortion happened with the pilot signals during the propagation and thus determine the response of the radio channel, which is finally used to compensate the data carriers. +From our perspective, the important point is that the pilot carriers are modulated the same way, transfer the same information and they are distributed all across the DVB-T signal spectrum at fixed and scattered positions. + +When we are calculating the range-Doppler map, we are seeking similarities in time-delay and frequency shift (as Doppler). Now, you can imagine that the pilot structure (as mask or frequency domain pattern ) in the received signal matches with its frequency shifted version as the pilots overlap each other. This matching results in dominant correlation peaks in the range-Doppler map. + +A possible solution to clean these out from the range-Doppler map is to construct the so-called missmatched filter. This filter is created by demodulating the received signal, scrambling the pilots and remodulating signal thus constructing a new reference channel.
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