shiv wrote:
Could someone please explain to me what the hell a "low probability intercept radar " is?
Most of us are familiar how radio receivers work. AM/FM types of modulation and electronics that is used to demodulate the signal. Interestingly FM allows Bandwidth to be traded to achieve better Signal to Noise Ratio (SNR) I.e. even though the RF signal strength is low(compared to AM) the detector is able to resolve the signal from the wideband FM modulated signal.
Now switch to modern communication: like the digital cellphone one uses today (compared to those bulky analog cell phones 12 years ago) or the WiFi network modem (802.11B/G) in new laptops . These devices do not use analogue detectors at all, but use techniques like Direct Sequence Spread Spectrum (DSSS) techniques that spread the digital data stream using a Direct Sequence Spreading code. The Spreading code of logic "1" and "0" is orthogonal psudo-random sequence. Thus for a 1Megbit datastream applying 128bit psudo-random code (also called chips) on DSSS the bandwidth now becomes 128Mbit (chips). Using a digital signal processing one can reconstruct the Digital 1 from 0 even in extremely noisy situation because a digital receiver that knows the psudo-random code sequence used to spread the signal in the first place can detect if the data was 1 or 0 even if it received say 30 error bits in the 128 bit frame. because there is enough distinction between the "1" psudo-random possible sequence versus the "0" psudo-random code. This is called Processing-Gain or Spreading Gain. I.e. The receiver can do digital processing to obtain additional gain over and above what the analogue radio detector can provide (typically by using Analog to Digital Converter-ADC).
Thus DSSS technique allows the data signal to be radiated by flattening the signal over wide spectrum. Bigger the spreading code sequence the large is spread, so much so that it can be flattened to be close to ambient noise floor. Only the receiver that knows the spreading code can re-construct the signal back. The additional processing gain comes at the cost of digital processing required to synchronize with spreading frame and eliminating the noisy bits. This allows lower power transmission. That is the reasons why modern digital phones battery lasts so long (compared to old analog cell phones) and the radiated power is only few milliwatts even though the base station is many kilometers away.
Western military were first to use DSSS, for it offered security and since RF bandwidth is cheap (at least for military) it allowed very low transmitted power and it was so widely spread that all that enemy sees is noise floor that sometimes rises. Thus a very Low Probability of Intercept. (LPI)
The same concept when applied to Radar gives wideband LPI. In the case of BEL radar the datasheet says its processing gain (Rx Gain) is 72dB. That is fantastic and tells me that its spreading code is ~15 megabit long. As you can see it mentions various Bandwidth (56Mhz to 1.6MHz) (This is product of data-rate and number of bits in the Spreading-code. In Radar application DataRate = PRF).
BTW the GPS signal that gives us such fantastic timing resolution that we know our earth co-ordinates accurately, the signal on earth is so low that it is well below the RF noise floor. The GPS C/A uses 1023 chips for its spreading code (transmitted every 1 millisecond). Only the magic of DSSS processing gain that the digital processor is able to accurately extract the timing edge from GPS broadcast signal from 24,000Km above. For Military accuracy the GPS spreading code is many Gigabytes thus its security is virtually impossible to break.
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