Atmospheric Adaptive Arrays - Pilot-Tone
Schematic 9010-009 shows the possible use of a PIN photodetector array in ground-based optical SETI receivers. This implements space diversity heterodyne reception in the presence of considerable atmospheric turbulence. The definition of the word "space" in this context refers to spatial and not outer-space. Space-diversity reception might be an alternative to laser guide-star adaptive telescopes techniques, or a complementary technique to further improve the signal-to-noise ratio. The technique could also be used with incoherent optical receivers if both the pilot-tone and main channel were intensity or polarization modulated onto the optical carrier.
The pilot-tone, whose frequency may be above or below the main signal channel, may itself contain a low-bandwidth message, perhaps a Rosetta stone for decoding the main wideband signal channel. It would probably be intensity or polarization modulated so that incoherent optical systems could detect the simple message. This could be the "signpost", but placed adjacent to the main signal, not in the microwave bands. The pilot-tone approach also has the advantage that any residual Doppler drift (chirp) will be removed when the signal is mixed with the pilot-tone and the difference frequency extracted as the 2nd I.F., since this is a common-mode effect.
The theory behind the pilot-tone method is as follows, and makes no specific assumption about modulation techniques employed by ETIs, i.e., whether intensity, polarization, frequency or phase modulation, analog or digital:
Let the pilot-tone carrier (part of the received optical signal) at fp be given by:
Ep(t).sin[wpt + dø]
Let the modulated signal (part of the received optical signal) at fs be given by:
Es(t).sin[wst + ø(t) + dø ]
The phase disturbances dø, are essentially common to both the signal and the pilot-tone, since the latter are almost identical in optical frequency and travel the same optical path.
If we heterodyne a local-oscillator laser operating at frequency fo with both these signals, we obtain the difference frequency signals or first I.F. from the photodetector proportional to:
Ep(t).Eo.sin[(wp - wo)t + dø]
Es(t).Eo.sin[(ws - wo)t + ø(t) + dø]
The pilot-tone signal may be passed through a narrow bandpass filter (BPF) and amplifier, to produce what is effectively a strong electrical (second) local oscillator (L.O.) signal for the electrical mixer. It may be also be used to lock a narrow-band Phase Locked Loop (PLL) whose Voltage Controlled Oscillator (VCO) is employed as the strong, amplitude-stable and clean 2nd L.O.. The information signal may be passed through a wideband BPF and applied to the other port of the electrical mixer. The difference frequency output of the electrical mixer at the second I.F. is proportional to:
Ep(t).Es(t).Eo2(t).cos[(ws - wp)t + dø(t)]
The second I.F. produced after the low-pass filter (LPF) has all the atmospheric-induced phase noise eliminated. The frequencies and bandwidths given in brackets are arbitrary, and used to help clarify the technique. Each array pixel has one of these circuits whose in-phase outputs are simply added and taken to the Multi-Channel Spectrum Analyzer (MCSA). Those pixels that produce weak signals have the least effect on the output signal-to-noise ratio, since the noise at the 2nd I.F. is the result of noise beating with a weak signal.
The amount of dispersion (group-delay) through the atmosphere is sub-nanosecond and is insignificant. Modulation bandwidths and frequency separations into the GHz range would not impair the ability of the pre-detection combining receiver system to sum the signal modulation envelopes in phase.
This pilot-tone technique which has previously been applied to radio-frequency and fiber-optic communication systems, should find future application in deep-space laser communications, particularly after we become space-faring. It is such a logical approach, that we should not be too surprised if it is eventually found to be employed by ETIs. It certainly would make our life a lot easier by substantially increasing the detectability of their signals.