Noise environment at ATA

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In unpopulated bands (those not in active use by humans), Gaussian white noise (GWN) is the rule, only occasionally broken in nature. The noise environment at the ATA is a complex mix of generated artifacts.

However, the telescope by its very nature introduces correlations (or redundancy) into our signals. For example, the light-travel time across our array is ~1 microsecond and across 1 dish is ~0.01 microsecond. Therefore, we will hesitate to believe "discoveries" of signals with redundancy on timescales between 0.01 and 1 microsecond, since there are known array artifacts in this range.

Then there are the Walsh function frequencies which range from 100 Hz to 12.8 kHz. Converting this frequency range to microseconds, we can expect artifacts over time durations of 50 microseconds to 10,000 microseconds.

Then there is self-generated RFI. In a very broad sense of RFI, we can consider receiver noise to be RFI. Essentially, we are making measurements of the electric field arriving from the sky using instruments that are "hot." Our receivers are white-hot at 1.4 GHz. The pitifully weak signals we receive from the sky are completely overwhelmed by the self-generated noise of our receivers. To a pretty good approximation, receiver noise is GWN.

There is another kind of self-generated RFI, which is in our signal processing room. Here the signals from the antennas are digitized at very high rates and processed with 2-3 GHz CPUs. In order to run this computing equipment we need lots of clocks ticking along with periods anywhere from 1 second (10^6 microseconds or 1 Hz) down to 10^-4 microseconds (10 GHz).

Finally, there is RFI generated by sources other than the ATA. This stuff is all over the place. It looks like sine waves everywhere thanks to the human love of clocks. It looks like dispersed pulsars because radar transmitters often use chirped pulses. It can even look like GWN in constrained frequency ranges. But it isn't noise from the galaxy or cosmic microwave background or even from our antenna receivers. It is satellite signals.

The only reason we have hope of overcoming all these sources of noise is that we can either identify them and throw them away, or search in sky-frequency ranges where they don't appear.

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