Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: moccacake on 05/12/2006 19:47:49
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How can the Voyager Space Probe send signals back to Earth even though it's billions of miles away from our planet?
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See revised version
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I gave this figures of the top of my head When I browse through the published materiel I find that I have have rather underrated the radiated power, Voyager has a rather powerful transmitter of 23 Watts operating at 8 GHz with a 3.7 meter antenna.
Allowing for an antenna efficiency of 40% this would give a gain of 45.6db and an ERP of 0.85MW this assumes of course that you have perfect alignment with the Earth
For reception NASA have the deep space network of 70 meter antenna's equipped with low noise (20k) maser type amplifiers and computer enhanced receivers.
I will update this when I have found more information on receiver techniques
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I thought it may help if I explained some of those terms:
The gain of an aerial is basically how directional it is. An aerial that radiated power in all directions equally would have a gain of 1.
[diagram=39_0]
If the power is 5x as powerful in one direction than the average it would have a gain of 5
[diagram=40_0]
The gain has been quoted in strange units called decibels (dB), 85dB is equivalent to 10 times itself 8.5 times or about 300 million, so the 23W are very tightly channeled towards the earth and it's signal is 300million times stronger than you would expect from the simplest possible aerial.
The receivers in the deep space network can be a dish up to 70m across, this will be even more directional than the voyager antenna, so the signal that they receive will be 10 billion or so times stronger than you would get from the simplest possible aerial on earth.
Between these and some very sensitive amplifiers at this end it is possible to communicate, if quite slowly.
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It would be nice to think that you could fly an antenna with a gain of 85db but I don't think it is possible at 8GHz although no doubt it would be at optical frequencies if the pointing problem could be overcome.
The gain I quoted was 45.6db which gives a hefty effective radiated power of 0.85 megawatts.
Here is a very good article on communications with space craft
http://www2.jpl.nasa.gov/basics/bsf10-1.html
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THe simple reason is that in the low microwave region receivers can be fantastically sensitive and quite directional. space is cold and does not create much interference and quite moderate power signals can be received from vast distances This is what SETI is all about.
Using equipment we have today it would be possible to detect the TV and radar transmitters on the earth at distances up to several light years and specifically directed high power signals at distances several orders of magnitude greater than this