The purpose of this page is to allow you to determine the range at which a possible alien radio transmission, such as from Gliese 581 or SHGb02+14a, can be detected, using your choice of assumptions about frequency, antenna dish size, receiver bandwidth etc. Similarly you can calculate and model ideas for sending signals to, or receiving signals from, places like OGLE-2005-BLG-390Lb
or HD 189733b or 55 Cancri. The Search for Extra Terrestrial Intelligence SETI involves using sensitive radio telescopes to search.
You can help the SETI search at home using your own PC to analyse the recorded signals - which are mostly just random noise.
Complete all the light grey boxes below (by modifying the default values shown) and then click the calculate button to obtain results in the green boxes.
Input your idea of what the alien transmitter might use in terms of: Frequency (in GHz) and Transmit Antenna Diameter (in metres). Be cautious about combinations that give extreme gain (more than 80 dBi) as it may (or may not) be difficult for aliens to make such dishes with sufficient surface accuracy. Input an alien Transmitter Power (in watts). Your receive system will have a bandwidth (in Hz). Choose something more than 0.1 Hz to avoid dispersion and phase noise and to give a chance for some useful information to get through. Input also your dish diameter (in metres) and system noise temperature (in deg Kelvin). Use the nominal 50K if you don't understand this. Input what Carrier to Noise ratio you think you need to detect something. If you have time to do long averaging of a CW (continuous wave i.e. un-modulated) source then a C/N = 0 dB might be adequate. You would detect a 3 dB hump in the noise floor ((C+N)/N) once the short term noise was averaged out. If you need quick detection with a moving antenna or moving source then a detection threshold C/N = 5 dB or higher might be better.
Press calculate and see what is the resulting maximum range, if a signal is to be detected.
Repeat the range calculation several times, experimenting with different powers, bandwidths, antenna sizes etc.
Have fun and good luck.
The default parameters above (frequency, dish sizes, transmission power etc) are set to achieve a range of about 23 light years which gives a little margin for Gliese 581 at a range of 20.4 light years. Gliese 581 has 3 planets around it Gliese 581c and Gliese 581d are both bordeline for being in the habitable zone, marginally too hot or too cold, so there is a chance and it is worth studying this further. The range is reasonable also at 20.4 light years, less then the default communications link parameters above.
55 Cancri is at a range of 41 light years.
The bandwidth is very limited, to about only 1 Hz, giving a transmission bit rate of only 1 bit per second. This means that a single letter might be transmitted every 8 seconds using ASCII code. If the more efficient morse code was used then perhaps one letter every 3 seconds might be possible. Please try higher bit rates, but notice that the range reduces significantly unless you use bigger dishes.
You may speculate what coding method might be used and what format of message might be sent. One idea is to send a picture comprising a number of dots. Each dot is 1 bit, so provided the image is small it does not take too long to send the complete image.
Possible SETI message to send to the aliens on Gliese 581 !
The image on the right is an example of what might be sent. It is like a TV picture, made up of lines, each with 23 dots along each line. The image is supposed to show some binary numbers, the DNA structure, a human figure, the sun and planets and the SETI parabolic dish at Aricibo. There are 69 lines, so in total there are 1587 bits to transmit, taking about 26 minutes. If someone receives this signal then hopefully they will detect that it repeats every 26 minutes. They will then work out that the frame length is 1587 bits and realise that this is a multiple of 23 x 69 and be able to plot it out as a picture.
A message like this was actually transmitted on 16th Nov 1974 towards the M13 star cluster at a range of 25,000 light-years. I don't think there is any hope of them receiving that distance. Even 21 light years is rather optimistic.
I am interested to know if this page gives correct results. If you agree or disagree please send details to me by e-mail Eric Johnston
Copyright (c) 2002 Eric Johnston, All Rights Reserved.
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