NOTE: VIEW WITH MONOSPACED FONT Aug 17, 1991 COMMBETW.TXT COMMUNICATING BETWEEN EARTH AND ALPHA CENTAURI INTRODUCTION The rate at which Earth or Alpha Centauri should transmit information depends on the transmitter power and the size of the antennas at either end. Transmitting too many bits per second increases the error rate to unacceptable levels -- a major problem when talking to a species you have never met! Transmitting too slowly avoids errors, but means you don't communicate as much or as rapidly as you could. For these reasons, the transmitting civilization needs to know the size of the receiving antenna. The transmitting civilization would also like to know whether there are times when the receiver is not available. Otherwise, it can't assume that everything it transmits is received. To fill in possible holes, it would have to repeat vital material. Again, this means not communicating as much or as rapidly as possible. If the aliens have observed TV carriers broadcast from Earth, they know that we have microwave technology, and they might estimate the size of our TV transmitter antennas. They could use that size to estimate a rate we can surely receive. They should transmit information about their receiver, so that we can calculate the bit rate at which to reply. Once the two civilizations are communicating, they can optimize communications by advising each other of preferred bit rates, receiver schedules, and methods of data compression and error correction. They may also build more or larger antennas to increase transmission rates and add more channels. OBSERVING EARTH'S TV FROM ALPHA CENTAURI How big an antenna would the aliens need to detect Earth's microwave broadcasts, and what could they learn about the Earth from them? Sullivan, Brown, and Wetherill discuss the information interstellar observers could deduce about the Earth from its TV and FM radio emissions in their 1978 article in Science (vol. 199, pp. 377-388). They say that strong video carriers could be detected at about 25 light years with an antenna like Project Cyclops, one having an aperture equivalent to 3 km. Since detection depends on the power received across the area of the antenna, observers in the Alpha Centauri system only 4.3 light years away could detect strong video carriers from Earth if their antenna has only 3% the area of Project Cyclops. They could detect strong video carriers with a dish 520 meters in diameter, but they could not detect program material in our TV broadcasts even if their antenna was as big as Project Cyclops. By carefully observing individual TV stations, they could determine Earth's size, orbit, length of sidereal and solar days, and axial tilt with respect to Alpha Centauri. They could also estimate Earth's temperature from its orbit and assign a reasonable density to estimate Earth's mass and surface gravitation from its radius. They could only detect stations south of 31 degrees N latitude, but they could map these stations onto a globe. If they measured antenna lobe patterns and assumed that the apertures were diffraction-limited, they would find that Earth's TV transmitter antennas are typically 15-20 meters across. TRANSMITTING FROM ALPHA CENTAURI TO EARTH The rate at which Earth can receive information sent from Alpha Centauri depends on the equipment at either end of the link. Figure 5-8 in NASA's Project Cyclops design study shows bit rates for 3-cm microwave transmissions between antennas of the same size. I read off the following values for a distance of 4 light years: CLEAR APERTURE BIT RATE COMMENTS 30 meters 10 bits/sec (bps) 100 meters 125 bps 300 meters 10^4 bps 1 kilometer 10^6 bps Better than good-quality audio (3x10^5 bps) 3 kilometers 10^8 bps Better than studio- quality TV (6x10^7 bps) The 3-km aperture is the Project Cyclops design, a phased array with 900 individual 100-meter dishes each drawing 10^5 watts and beaming 1.1x10^13 watts of effective radiated power. The array is actually spread out to a diameter of 6.4 km to prevent the dishes from shadowing each other when pointed to low elevation angles. The bit rate that can be received depends on the power received across the area of the antenna. It is inversely proportional to the square of the distance of the transmission, and it is directly proportional to the area of the antenna and the effective power radiated by the transmitter. The effective power radiated (EPR) depends on how wide the beam is, and that depends on the aperture of the transmitter. The aliens must decide at what bit rate to transmit, but they don't know the size of the receiver on Earth. All they know is that many of our TV transmitters are 15-20 meters across. They may decide to use 15 meters as a safe estimate of Earth's receiver size. If so, and if their transmitter happens to be comparable to Project Cyclops, they should send only 2500 bits per second. This is a hundredth the rate of good-quality audio (3x10^5 bps) and less than a ten-thousandth the rate of studio-quality TV (6x10^7 bps). At this rate the aliens could send a diagram of 50x50 on/off pixels in one second. If the aliens' transmitter is smaller or less powerful than Project Cyclops, they would transmit even more slowly to an assumed 15- meter receiver. The table on page 6 lists bit rates I calculated for various combinations of antennas. In it the transmitters all consist of one or more 100-meter elements from Project Cyclops. TRANSMITTING FROM EARTH TO ALPHA CENTAURI If the Earth wants to reply to Alpha Centauri, it will do so from the southern hemisphere. To avoid noise (and shadowing in an array), we should transmit when the star is at least 20 degrees above the horizon. Since Alpha Centauri has a declination of - 60.63 degrees (60.63 degrees south of Earth's equator), it is visible some of the time at locations between 30 degrees N latitude and 30 degrees S latitude, and it stays above the horizon in locations south of 30 degrees S latitude. This means it is always at least 20 degrees above the horizon for locations south of 50 degrees S latitude. Between 10 degrees N latitude and 50 degrees S latitude it sometimes rises higher than 20 degrees above the horizon, but only briefly in the northern part of this range. Humid air, high winds, and snow loads all degrade microwave transmissions. Sites with good weather south of 50 degrees S latitude are scarce or nonexistent, so Earth will probably transmit from one or more sites north of 50 degrees S latitude. Unless there are several transmitters with overlapping coverage, transmissions will not be continuous. We could also transmit from space or the moon when we get large enough radio telescopes there. I don't know what radio telescopes would be available in the southern hemisphere to reply to Alpha Centauri. The bit rate we send would depend on the transmitter's power and aperture, as well as the area of the antenna at Alpha Centauri. For example, a 100- meter antenna similar to a single element of Project Cyclops could send 10^5 bits per second (almost good-quality audio) if we assumed a receiver at Alpha Centauri comparable to Project Cyclops. For a smaller receiver the bit rate is lower in proportion to the area of the receiver. We could only send 125 bps if we thought our 100- meter dish was transmitting to another 100-meter dish! How can Earth decide what bit rate to transmit? We don't know the size of the receiver at Alpha Centauri unless the aliens tell us. We might guess that it is at least 500 meters in diameter because that size is needed to detect Earth's video carriers. All we know initially is the power per square meter we receive from the aliens' transmitter. Let's hope the aliens tell us their receiver's size! We can estimate the aperture of the aliens' transmitter by analyzing its antenna pattern if the beam from Alpha Centauri drifts enough for us to measure the width of the primary lobe. If we assume that the aperture is diffraction-limited, we can then calculate the size of the alien's transmitter. If we assume their receiver is the same size as their transmitter, we can determine the bit rate to transmit. If and when Earth and the Alpha Centauri system each have a Cyclops-sized antenna, they can send and receive one channel of information at TV rates. But the aliens won't know we have a Project Cyclops antenna until we tell them, either explicitly or by transmitting with it. They might wait to send at TV rates until they hear from us, or they might occasionally send blocks of information at that rate in the hope that we do have a suitable antenna. Earth might also tell them that we are building a Project Cyclops antenna array so that they can begin transmitting at TV rates four years before we finish the project. That way, Earth can expect to receive information at the faster rate immediately upon completion, rather than waiting 8.6 years of round-trip travel time. SUMMARY The optimum bit rate for transmissions depends on the distance and the equipment at each end. Initially, neither Earth nor Alpha Centauri knows the size of the others microwave receiver, although either may estimate the size of the others transmitter. Without knowing the size of the receiver, neither knows the maximum rate at which it can transmit to the other. If each civilization knows that the other has the equivalent of Project Cyclops, they can transmit at television rates. But a Project Cyclops must transmit more slowly if the receiving antenna is smaller, or if they think the receiver is smaller. For example, if Alpha Centauri has the equivalent of Project Cyclops and thinks that Earth has only 15-meter receivers, they would transmit only 2500 bits per second. To optimize communications, each civilization should tell the other its receiver size and schedule. They can also improve communications by using data compression and error-correcting codes once they have described them to the other. BIT RATES FOR VARIOUS COMBINATIONS OF TRANSMITTERS AND RECEIVERS: CYCLOPS EFF. POWER TRANSMITTER RECEIVER BIT ELEMENTS RADIATED APERTURE APERTURE RATE 1 1.1x10^13 watts 100 meters 15 meters 3 bps 30 meters 11 100 meters 125 300 meters 10^3 1 km 10^4 3 km 10^5 9 9.9x10^13 watts 300 meters 15 meters 25 30 meters 100 100 meters 10^3 300 meters 10^4 1 km 10^5 3 km 10^6 100 1.1x10^15 watts 1 km 15 meters 280 30 meters 10^3 100 meters 10^4 300 meters 10^5 1 km 10^6 3 km 10^7 900 9.9x10^15 watts 3 km 15 meters 2500 30 meters 10^4 100 meters 10^5 300 meters 10^6 1 km 10^7 COMMENTS FROM CTEIN TELEVISION Usable television can be sent at bit rates less than the 6 x10^7 bps listed on page 2 for studio-quality TV. Cyclops-sized antennas on Earth and Alpha Centauri could support maybe 10 channels of usable television. Antennas with 2-km apertures at each end of the link could support a channel of usable TV. If we can decode an interstellar television signal we receive with an antenna 10's of kilometers across (or smaller), we can be sure that the signal was intentionally sent to us. Unless the signal was beamed at us, it would spread too much for us to decode the information, and we could only detect the carrier. Aliens in the Alpha Centauri system would need a planet-sized antenna to decode commercial television broadcasts from Earth. If they built that large an antenna, they must also have the technology to send Forward probes (or possibly larger probes) to our solar system. In fact, sending a Forward probe would be one of the few reasons to build such a large antenna. SPECTROSCOPY Spectroscopy is the most efficient way to look for life in other star systems. It will reveal reactive species out of equilibrium, which would imply that something (presumably life) is generating them. The Hubble telescope may not be quite large enough, but it might be done with a suitably-equipped optical telescope with 100 meter aperture.