Interplanetary communications

By Miguel R. Ghezzi (LU 6ETJ)
www.solred.com.ar/lu6etj
SOLVEGJ Comunicaciones
www.solred.com.ar/solvegj

It was my intention to transcribe an old bulletin sent to LATNET some years ago via packet radio. Regrettably I have lost it, so that I have re-written the fundamental concepts settled down in that (although without the Science Fiction literary part of that accompanied it to make it a little more funny).

Space communications topics it is not habitual in the Argentinean amateur radio, excepting those directly related with amateur satellites or, very sporadically, with moonbounce ones.

Some days ago (1.999), commenting possible capture of Mars Global Surveyor Relay signals with relatively simple antennas and equipment, I was surprised for the incredulity level that caused such a comment. The intuitive conviction of radio meeting participants were that it is necessary for such purpose enormous antenna arrays, giant parabolas and technologies based on supercooled systems...!

It is obvious that, at least at the local radio amateurs "popular level"  there is not a good knowledge of what is possible in this sense. Identical situation probably happen with the "Argus" Amateur SETI Project. Surely few of them will believe that some more or less interesting probability exists that, by means of amateur equipment, we have conditions to capture signals sent it from hipothetical extraterrestrial intelligence and that that intelligence it could be located outside of our solar system at several light years distances. 

Is it possible to communicate with other worlds?

That is more than obvious, we have seen that on TV, in reference to all the probes sent to explore our solar system, the question then is: Could we communicate with other worlds with our radios?

Surely many suspect that yes, but it is necessary to specify even more the question: Is it possible communicate with other worlds with our rigs without it means it an prohibitive economic investment or extremely complex technologies?

The categorical answer is yes, yes, yes...!

To verify it, we will get on hand some theory and, being ahead to somebody who could affirm the theory doesn't serve, I respond her/him from already: how she/he believes that NASA boys sends a ship to Mars or Jupiter and they does communicate with it? practicing...?

Free space propagation... 

It can be found in any basic radio technical book a simple formula that allows to calculate signal attenuation in the free space, is the following one

A0 = 32.4 + 20 log d + 20 log f   where:

With this simple equation we can calculate UHF (440 MHz) radio path attenuation between Earth and a orbiting Mars spaceship at the moment of maximun approach to Earth (approximately 55 millions of kilometers). The result is (You verify it with your electronic calculator):

A0 =  239,9 DB         <= This is truly a great attenuation...!

The number is important and we will associate it with something more intuitive, for example a coaxil cable type RG 213U employee on the same frequency have an attenuation of 0,2 dB for meter (6,1 dB / 100ft, according to ARRL Handbook 1.996 pag 19.5), thus:

RG 213U length = 239,9 [dB]/ 0,2 [dB/m] = 1.199 m

it is to say that a line of little more than a km has the same attenuation that Mars's attenuation path so that the matter it doesn't seem so difficult, after all, it can't be an important challenge to make arrive a "poor one Watt" at the end of an 1 km cable... But not all that shines it is gold, we will verify to see if our intuition is correct.

Let us see what represent these 240 dB of attenuation. Remembering that:

Relationship of powers in dB = 10 x log (P2/P1)

If on output of such cable we want to get 1 W, as much as we should inject in the transmitter end? We see:

240 dB = 10 x log (1.000.000.000.000.000.000.000.000 W / 1W)

If I didn't make a mistake this is: A cuatrillion Watts... How much...!!!???. (I believe on english notation a cuatrillion it is not the same that in latin notation: 1024, use numerical value)

Not, something should be bad how we will need so much power!, it is more, these numbers indicate that it is not possible to output one watt trough 1 km cable, neither a miliwatt, not even a microwatt neither anything that it seems. It is contrary to the common sense!!!, well... that is, in fact, just what I wanted demonstrate with this example.

Neither sense common, neither the intuition are reliable tools to manage radio numbers...!

AND, as well as our intuition fail doing us believe that it is possible to get one watt at the end of 1 km RG 213 line in UHF, it also a mistake believe 55.000.000 km communications is almost impossible for the Ham...

All our problem is find which electric conditions make possible this contact type. For experience we know we have the following ones elements to play.

There are another item, not so common, but will be fundamental in this question and it is: Information bandwidth that is exchanged, and related receiver bandwidth .

Our objective will be to discover how we can manipulate these variables so that we be able to reach desired results and also to find what kind of Ham communication equipment is it possible to imagine that a space ship will take for this purpose in a future piloted (or not) mission.

At the present nothing prevent us to imagine the ship cannot transport a transmitter with 100 W available for contact with radioamateur, neither that it doesn't have antennas of important gain to transmit another type of information to the Earth, and that they are compatible with frequencies assigned to our service.

Then, we think an antenna system about 23 dBd gain in both ends. An antenna of 23 dBd in UHF is hardly a simple array of four phased yagis about 20 elements per antenna (booms of less than four meters) that anyone can build for a few $.

For preamplifier we can build one able to contribute with a Noise Figure about 0,5 dB, and here the surprise comes...!

In such conditions, it will be possible to establish a CW contact with a common 250 Hz BW filter... with a signal to noise ratio of almost 9 dB...!

But somebody could even object that an antenna of these characteristics is not within a medium possibilities, I say yes, unless the man were very, but very lazy. Well, we can accept it, we propose then one elementary antenna of hardly 13 gain dB. In this case the contact could be carried on equally reducing receiver bandwidth to about 25 Hz. This, today is gotten very easily with DSP (software or hardware)  just as the DSP Blaster (or simple active filter) and today any ham has a compu with a sound card!

Undoubtedly the question it is not solved so easily, somebody could say: How can I know that you say is truth ?, after all, this statement is yours...

OK, that is true, but to understand these calculations is necessary to have some basic mathematical knowledge about such things as "Decibels", "Receiver Noise Figure" or "Noise Temperature" and other technical stuff that, very blended in the cocktail shaker, allows to arrive at these results.
Certain things are not able only be explained with words and it is need to appeal to the mathematics and circuits theory to make them clear. But, making click here You will be able to discharge an MS Excel sheet will allow you to corroborate the numbers changing data in cells and, on this way, verify results under different conditions of power, gain, bandwidth etc.

Bandwidth importance

Is worthwhile to highlight the importance of bandwidth. Today, Digital Signal Processing (DSP) is within hams possibilities. It is easy to get programs capable to recover signals that are under receiver noise floor level ! and we have already seen how the bandwidth reduction it diminishes the necessary antenna gain (or the transmitter power that is the same).
The price paid for it is it the quantity of information that you can transport as you reduces the bandwidth. For example, reducing bandwidth to about 2,5 Hz, we could still copy a CW signal when space ship transmitter operates with only 10 W (with the 13 dB antenna), but transmission speed should decrease very much, a matter of fact with a bandwidth of only 0,2 Hz trasmitter power could be about 1 W but the only way of recovering such CW transmission would be basically drawing it on PC screen due its slowness.

Some experiences, for example on Mars Surveyor Relay Test carried out by NASA with radioamateur stations has been used bandwidth in order of 0,013 Hz...!
(see http://ourworld.compuserve.com/homepages/demerson/marsspec.htm) or AMSAT Journal, Volume 20, Not. 1, Jan / Feb 1997. pp. 1,4-11.
In that site you will find links to other related sites about the same topic.

I sincerely wish you a lot of success in your interplanetary DX's.

73's

PD: An interesting experience can be made calculating attenuation that presents a signal that should cross a distance of 4,2 Light Years, that is the one that separates us of our nearer star: Proxima Centauri. The Excel sheet has the necessary calculations facilitated for distances expressed in years light.

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