Quoted from here:

http://www.coseti.org/lemarch1.htm

DOMESTIC RADIO SIGNALS

Sullivan et al. (1978) and Sullivan (1981) considered the possibility of eavesdropping on radio emissions inadvertently "leaking" from other technical civilizations. To better understand the information which might be derived from radio leakage, the case of our planet Earth was analyzed. As an example, they showed that the United States Naval Space Surveillance

TABLE 3: Characteristics of the Electromagnetic Spectrum

Spectrum Frequency Wavelength Minimum Energy

Region Region [Hz] Region [m] per photon [eV]

_____________________________________________________________

Radio 3x10(6-10) 100-0.01 10(-8) to 10(-6)

Millimeter 3x10(10-12) 0.01-10(-4) 10(-6) to 10(-4)

Infrared 3x10(12-14) 10(-4)-10(-6) 10(-4) to 10(-2)

Optical* 3x10(14-15) 10(-6)-3x10(-7) 10(-2) to 5

Ultraviolet 10(15)-3x10(16) 3x10(-7)-10(-8) 5 to 10(2)

X-rays 3x10(16-19) 10(-8)-10(-11) 10(2-5)

Gamma rays > 3x10(19) <10(-11)> 10(5)

* Visible - SAK definition

System (Breetz, 1968) has an effective radiated power of 1.4x10 to the 10 power Watts into a bandwidth of only 0.1 Hz. Its beam is such that any eavesdropper in the declination range of zero to 33 degrees (28 percent of the sky) will be illuminated daily for a period of roughly seven seconds. This radar has a detectability range of leaking terrestrial signals to sixty light years for an Arecibo-type (305-meter) antenna at the receiving end, or six hundred light years for a full-up Cyclops array (one thousand dishes of 100-meter size each).

Recently, Billingham and Tarter (1992) estimated the maximum range at which radar signals from Earth could be detected by a search similar to the NASA High Resolution Microwave Survey (HRMS) (now the SETI Institute's "Project Phoenix") assumed to be operating somewhere in the Milky Way galaxy. They examined the transmission of the planetary radar of Arecibo and the ballistic missile early warning systems (BMEWS). For the calculation of maximum range R, the standard range equation is:

R=(EIRP/(4 pi phi min))1/2

Where phi(min) is the sensitivity of the search system in [W m-2]. For the former NASA HRMS Target Search from Arecibo, phi(min) = 10 to the -27 power and the NASA HRMS Sky Survey phi(min) = ~10 to the -23 power (f)1/2, where f is the frequency in GHz. Table 4 shows the distances where the Arecibo and BMEWS transmissions could be detected by a similar NASA HRMS spectrometer.

All these calculations assumed that the transmitting civilization is at the same level of technological evolution as ours on Earth. Von Hoerner (1961) classified the possible nature of the ETC signals into three general possibilities: Local communication on the other planet, interstellar communication with certain distinct partners, and a desire to attract the attention of unknown future partners. Thus he named them as local broadcast, long-distance calls, and contacting signals (beacons). In most of the past sixty SETI radio projects, the strategy was planned with the hypothesis that there are several civilizations transmitting omnidirectional beacon signals. Unfortunately, no one has been able to show any positive evidence of this kind of beacon signal.

Another possibility is the radio detection of interstellar communications between an ETC planet and possible space vehicles. Vallee and Simard-Normandin (1985) carried out a search for these kind of signals near the galactic center. Because one of the characteristics of artificial transmitters (television, radar, etc.) is the highly polarized signal (Sullivan et al., 1978), these researchers made seven observing runs of roughly three days each in a program to scan for strongly polarized radio signals at the wavelength of lambda = 2.82 centimeters.

RADAR WARNING SIGNALS

Assuming that there is a certain number N of civilizations in the galaxy at or beyond our own level of technical facility, and considering that each civilization is on or near a planet of a Main Sequence star where the planetoid and comet impact hazards are considered as serious as here, Lemarchand and Sagan (1993) considered the possibility for detecting some of the "intelligent activities" developed to warn of these potentially dangerous impacts.

Because line-of-sight radar astrometric measurements have much finer intrinsic fractional precision than their optical counterparts, they are potentially valuable for refining the knowledge of planetoid and comet orbits. Radar is an essential astrometric tool, yielding both a direct range to a nearby object and the radial velocity (with respect to the observer) from the Doppler shifted echo (Yeomans et al., 1987, Ostro et al., 1991, Yeomans et al., 1992, and Ostro, 1993).

Since in our solar system, most of Earth's nearby planetoids are discovered as a result of their rapid motion across the sky, radar observations are therefore often immediately possible and appropriate. A single radar detection yields astronomy with a fractional precision that is several hundred times better than that of optical astrometry. The inclusion of radar with the optical data in the orbit solution can quickly and dramatically reduce future ephemeris uncertainty. It provides both impact parameter and impact ellipse estimates. This kind of radar research gives a clearer picture of the object to be intercepted and the orientation of asymmetric bodies prior to interception. This is particularly important for eccentric or multiple objects.

Radar is also the unique tool capable for making a survey of such small objects at all angles with respect to the central star. It can also measure reflectivity and polarization to obtain physical characteristics and composition.

For this case, we can assume that each of the extraterrestrial civilizations in the galaxy maintains as good a radar planetoid and/or comet detection and analysis facility as is needed, either on the surface of their planet, in orbit, or on one of their possible moons.

The threshold for the Equivalent Isotropic Radiated Power (EIRP) of the radar signal could be roughly estimated by the size of the object (D) that they want to detect (according to the impact hazard) and the distance to the inhabited planet (R), in order to have enough time to avoid the collision.

One of the most important issues for the success of SETI observations on Earth is the ability of an observer to detect an ETC signal. This factor is proportional to the received spectral flux density of the radiation. That is, the power per unit area per unit frequency interval. The flux density will be proportional to the EIRP divided by the spectral bandwidth of the transmitting radar signals B (expressed in units of Hertz).

The EIRP is defined as the product of the transmitted power and directive antenna gain in the direction of the receiver as EIRP = P(T).G, where P(T) is the transmitting power and G the antenna gain. This quantity has units of [W].

According to the kind of object that the ETC wants to detect (nearby planetoids, comets, spacecraft, etc.), the distance from the radar system and the selected wavelength, a galactic civilization that wants to finish a full-sky survey in only one year will arise from a modest "Type 0" (flux density ~10 to the 13 power W/Hz, R ~0.4 A.U., D ~5000 m, and lambda ~1 m) to the transition from "Type I" to "Type II" (flux density ~2x10 to the 24 power W/Hz, R ~0.4 A.U., D ~10 m, lambda ~1 mm).

Lemarchand and Sagan (1993) also presented a detailed description of the expected signal characteristics, as well as the most favorable positions in the sky to find one of these signals. They also have compared the capability of detection of these transmissions by each present and near future SETI projects.