Top Document: [sci.astro] ET Life (Astronomy Frequently Asked Questions) (6/9) Previous Document: F.05 Could we detect extraterrestrial life? Next Document: F.07 What's a Dyson spheres? See reader questions & answers on this topic!  Help others by sharing your knowledge David Woolley <david@djwhome.demon.co.uk> Representative results are presented in Tables 1 and 2. The short answer is (1) Detection of broadband signals from Earth such as AM radio, FM radio, and television picture and sound would be extremely difficult even at a fraction of a lightyear distant from the Sun. For example, a TV picture having 5 MHz of bandwidth and 5 MWatts of power could not be detected beyond the solar system even with a radio telescope with 100 times the sensitivity of the 305 meter diameter Arecibo telescope. (2) Detection of narrowband signals is more resonable out to thousands of lightyears distance from the Sun depending on the transmitter's transmitting power and the receiving antenna size. (3) Instruments such as the Arecibo radio telescope could detect narrowband signals originating thousands of lightyears from the Sun. (4) A welldesigned 12 ft diameter amateur radio telescope could detect narrowband signals from 1 to 100 lightyears distance assuming the transmitting power of the transmitter is in the terawatt range. What follows is a basic example for the estimation of radio and microwave detection ranges of interest to SETI. Minimum signal processing is assumed. For example an FFT can be used in the narrowband case and a bandpass filter in the broadband case (with center frequency at the right place of course). In addition it is assumed that the bandwidth of the receiver (Br) is constrained such that it is greater than or equal to the bandwidth of the transmitted signal (Bt) (that is, Br >= Bt). Assume a power Pt (watts) in bandwidth Bt (Hz) radiated isotropically. At a distance of R (meters), this power will be uniformly distributed (reduced) over a sphere of area: 4 * pi * R^2. The amount of this power received by an antenna of effective area Aer with bandwidth Br (Hz), where Br >= Bt, is therefore: Pr = Aer * (Pt / (4 * pi * R^2)) If the transmitting antenna is directive (that is, most of the available power is concentrated into a narrow beam) with power gain Gt in the desired direction then: Pr = Aer * ((Pt * Gt) / (4 * pi * R^2)) The antenna gain G (Gt for transmitting antenna) is given by the following expression. (The receiving antenna has a similar expression for its gain, but the receiving antenna's gain is not used explicitly in the range equation. Only the effective area, Aer, intercepting the radiated energy at range R is required.) Gt = Aet * (4 * pi / (w^2)), where Aet = effective area of the transmitting antenna (m^2), and w = wavelength (m) the antenna is tuned to. f = c / w, where f is the frequency and c is the speed of light. c = 2.99792458E+08 (m/sec) pi = 3.141592654... For an antenna (either transmiting or receiving) with circular apertures: Ae = <eta> * pi * d^2 / 4 <eta>r = efficiency of the antenna, d = diameter (m) of the antenna. The Nyquist noise, Pn, is given by: Pn = k * Tsys * Br, where k = Boltzmann's constant = 1.38054E23 (joule/kelvin) Tsys = is the system temperature (kelvins), and Br = the receiver bandwidth (hertz). The signaltonoise ratio, snr, is given by: snr = Pr / Pn. If we average the output for a time t, in order to reduce the variance of the noise, then one can improve the snr by a factor of sqrt(Br * t). Thus: snr = Pr * sqrt(Br * t) / Pn. The factor Br*t is called the "time bandwidth product," of the receive processing in this case, which we'll designate as: twp = Br * t. We'll designate the integration or averaging gain as: twc = sqrt(twp). Integration of the data (which means: twp = Br * t > 1, or t > (1 / Br) ) makes sense for unmodulated "CW" signals that are relatively stable over time in a relatively stationary (steady) noise field. On the other hand, integration of the data does not make sense for timevarying signals since this would distroy the information content of the signal. Thus for a modulated signal twp = Br * t = 1 is appropriate. In any case the snr can be rewritten as: snr = (Pt * Gt) * Aer * twc / (4 * pi * R^2 * Br * k * Tsys) Pt * Gt is called the Effective Isotropic Radiated Power (EIRP) in the transmitted signal of bandwidth Bt. So: EIRP = Pt * Gt, and snr = EIRP * Aer * twc / (4 * pi * R^2 * Br * k * Tsys) This is a basic equation that one can use to estimate SETI detection ranges. ####################################################################### # If Rl is the number of meters in a light year (9.46E+15 [m/LY]), # # then the detection range in light years is given by # # # # R = sqrt[ EIRP * Aer * twc / (4 * pi * snr * Br * k * Tsys) ] / Rl # # # # If we wanted the range in Astronomical Units then replace Rl # # with Ra = 1.496E+11 (m/AU). # ####################################################################### Note that for maximum detection range (R) one would want the transmit power (EIRP), the area of the receive antenna (Aer), and the time bandwidth product (twp) to be as big as possible. In addition one would want the snr, the receiver bandwidth (Br), and thus transmit signal bandwidth (Bt), and the receive system temperature (Tsys) to be as small as possible. (There is a minor technical complication here. Interstellar space contains a plasma. Its effects on a propagating radio wave including broadening the bandwidth of the signal. This effect was first calculated by Drake & Helou and later by Cordes & Lazio. The magnitude of the effect is direction, distance, and frequency dependent, but for most lines of sight through the Milky Way a typical value might be 0.1 Hz at a frequency of 1000 MHz. Thus, bandwidths much below this value are unnecessary because there will be few, if any, signals with narrower bandwidths.) Now we are in a position to carry out some simple estimates of detection range. These are shown in Table 1 for a variety of radio transmitters. We'll assume the receiver is similar to Arecibo, with diameter dr = 305 m and an efficiency of 50% (<eta>r = 0.5). We'll assume snr = 25 is required for detection (The META project used a snr of 2733 and SETI@home uses 22; more refined signal processing might yield increased detection ranges by a factor of 2 over those shown in the Table 1.) We'll also assume that twp = Br * Tr = 1. An "educated" guess for some of the parameter values, Tsys in particular, was taken as indicated by the question marks in the table. As a reference note that Jupiter is 5.2 AU from the Sun and Pluto 39.4 AU, while the nearest star to the Sun is 4.3 LY away. Also any signal attenuation due to the Earth's atmosphere and ionosphere have been ignored; AM radio, for example, from Earth, is trapped within the ionosphere. The receive antenna area, Aer, is Aer = <eta>r * pi * dr^2 / 4 = 36.5E3 m^2. (Scientific notation is being used here; 1E1 = 10, 1E2 = 100, 1E3 = 1000, so 36.5E3 is 36.5 times 1000.) Hence the detection range (light years) becomes R = 3.07E04 * sqrt[ EIRP / (Br * Tsys) ]. Table 1 Detection ranges of various EM emissions from Earth and the Pioneer spacecraft assuming a 305 meter diameter circular aperture receive antenna, similar to the Arecibo radio telescope. Assuming snr = 25, twp = Br * Tr = 1, <eta>r = 0.5, and dr = 305 meters. ++++++ Source  Frequency  Bandwidth  Tsys  EIRP  Detection   Range  (Br) (Kelvin)  Range (R)  ++++++ AM Radio  5301605 kHz  10 kHz  68E6  100 KW  0.007 AU  ++++++ FM Radio  88108 MHz  150 kHz  430  5 MW  5.4 AU  ++++++ UHF TV  470806 MHz  6 MHz  50 ?  5 MW  2.5 AU  Picture       ++++++ UHF TV  470806 MHz  0.1 Hz  50 ?  5 MW  0.3 LY  Carrier       ++++++ WSR88D  2.8 GHz  0.63 MHz  40  32 GW  0.01 LY  Weather Radar      ++++++ Arecibo  2.380 GHz  0.1 Hz  40  22 TW  720 LY  SBand (CW)       ++++++ Arecibo  2.380 GHz  0.1 Hz  40  1 TW  150 LY  SBand (CW)       ++++++ Arecibo  2.380 GHz  0.1 Hz  40  1 GW  5 LY  SBand (CW)       ++++++ Pioneer 10  2.295 GHz  1.0 Hz  40  1.6 kW  120 AU  Carrier       ++++++ It should be apparent then from these results that the detection of AM radio, FM radio, or TV pictures much beyond the orbit of Pluto will be extremely difficult even for an Arecibolike 305 meter diameter radio telescope! Even a 3000 meter diameter radio telescope could not detect the "I Love Lucy" TV show (reruns) at a distance of 0.01 LightYears! It is only the narrowband high intensity emissions from Earth (narrowband radar generally) that will be detectable at significant ranges (greater than 1 LY). Perhaps they'll show up very much like the narrowband, short duration, and nonrepeating, signals observed by our SETI telescopes. Perhaps we should document all these "nonrepeating" detections very carefully to see if any long term spatial detection patterns show up. Another question to consider is what an Amateur SETI radio telescope might achieve in terms of detection ranges using narrowband FFT processing. Detection ranges (LY) are given in Table 2 assuming a 12 ft (3.7 m) dish antenna operating at 1.42 GHz, for various FFT binwidths (Br), Tsys, snr, time bandwidth products (twp = Br*t), and EIRP values. It appears from the table that effective amateur SETI explorations can be conducted out beyond approximately 30 light years provided the processing bandwidth is near the minimum (approximately 0.1 Hz), the system temperature is minimal (20 to 50 Degrees Kelvin), and the EIRP of the source (transmitter) is greater than approximately 25 terawatts. Table 2 Detection ranges (LY) for a 12 foot diameter amateur radio telescope SETI system, operating at 1.420 GHz. ++  EIRP  +++++  100TW  25TW  1TW  100GW  ++++++++ Br  Br*t  Tsys  snr  Detection Range  (Hz)   (kelvin)   (LY)  ++++++++ 0.1  2  50  25  28  17  3.4  1.1  ++++++++ 0.1  1  50  25  20  12  2.4  0.76  ++++++++ 0.5  2  50  25  12.7  6.4  1.3  0.4  ++++++++ 0.5  1  50  25  9  4.5  0.9  0.3  ++++++++ 0.1  20  50  25  90  54  11  3.4  ++++++++ 1.0  200  50  25  90  54  11  3.4  ++++++++ REFERENCES: Radio Astronomy, John D. Kraus, 2nd edition, CygnusQuasar Books, 1986, P.O. Box 85, Powell, Ohio, 43065. Radio Astronomy, J. L. Steinberg, J. Lequeux, McGrawHill Electronic Science Series, McGrawHill Book Company, Inc, 1963. Project Cyclops, ISBN 0965070700, Reprinted 1996, by the SETI League and SETI Institute. Extraterrestrial Civilizations, Problems of Interstellar Communication, S. A. Kaplan, editor, 1971, NASA TT F631 (TT 7050081), page 88. User Contributions:Top Document: [sci.astro] ET Life (Astronomy Frequently Asked Questions) (6/9) Previous Document: F.05 Could we detect extraterrestrial life? Next Document: F.07 What's a Dyson spheres? Part0  Part1  Part2  Part3  Part4  Part5  Part6  Part7  Part8  Single Page [ Usenet FAQs  Web FAQs  Documents  RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
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