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# Space probes communications

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#### sram

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I was reading about how communication is made with space probes too far away in the deep space. I came to know that it is done via the DSN (Deep Space Network run by NASA) using huge dish antennas (For maximum gain and directivity that is). The dish antennas are 34 meter and some are 70 meter dish antennas, which are found on 3 places on earth (California, Madrid in Spain, Canbera in Australia for best coverage). I read that the output power of such antennas can reach 400 Mega watts - if I remember correctly- so that it can reach the deep space. It all made sense but something is bothering me. Space probes can be as small as a car. The hope probe launched by UAE not very long ago was about the size of a car. Its Antenna is a 1.5 meter dish antenna. Now the earth station can up the watts to whatever they want so that the signal will not die out and reach distances hundreds of millions of kilometers away, but what does the probe itself do? It can't use the same amount of output power because of the different limitations imposed by size....etc. Or is it because the system is designed so that the combination of the two antennas (earth station/probe antenna) work? Meaning the signal getting out of the probe can only be seen by such a large highly sensitive antenna?

In other words, can we say that one size of an antenna will compensate for the other, since we can't have both of them to be large. So one will be large enough, and the other will be too large to make up for the size installed on the probe. Something like saying instead of having a system with two 20-meter dish antennas, let's have a 1 meter dish antenna and a 30 meter dish antenna.

Sorry if I didn't put it in good words.

You already know the answer, so why ask? - the system is simply designed accordingly to what they need.

I think you have the right idea. The ability of a link to work is based on a few basic things. The Friis equation provides the basic computation of how much attenuation a link will present in free space. Knowing this, you can work out how much power is being received when you know how much power is being transmitted. You then have to work out the signal to noise ratio on the output of the receiver to understand what error rate you might suffer. Since the link equation always involves both receive antenna gain and the transmit antenna gain, the two become interchangeable for the sake of your question. So, when the satellite must be small and the antenna gain on the satellite might not be all that much, you have the option of enlarging the array on the earth station to increase the link gain, and this will benefit signals going from earth to spacecraft the same as going from spacecraft to earth.

When you say the " output power of such antennas can reach 400 Mega watts " I assume that you are talking about the ERP (Effective radiated power.) As these antennas have a very large gain the ACTUAL power from the transmitter fed to the dish will me very much less. It is the power that would have to be fed into an isotropic antenna to achieve the same field strength. An isotropic antenna radiates it's input power equally in all directions. It is a theoretical concept rather than an actual antenna.

Les.

Overall signal gain is the sum of both antennas. While a big dish can transmit large ERP, the satellite with its small dish receives a weak signal. The same the other way around, the satellite transmitting a weak signal requires large dish antennas on earth to receive the signal. Overall, the gain of the system is the same with both antennas.
The only difference is that a satellite will have less transmit power, and it may take two or more earth dishes to capture that signal properly. (or one really big dish). It may be that receiving dishes used are bigger than the transmitting dishes...

You already know the answer, so why ask? - the system is simply designed accordingly to what they need.
It was bothering me yesterday. I only thought about the possible solution while typing the question and wanted to be sure.
Thanks to everyone. It is clear now.

It was bothering me yesterday. I only thought about the possible solution while typing the question and wanted to be sure.
Thanks to everyone. It is clear now.
Just remember that this space communication is 2 way . Your 1 metre dish out there receives signals from the large antenna from earth.

Directionality is the key. The probes can't be wasting radio transmissions in all directions, since they only need to communicate with Earth. Directional antennas have much improved gain. Also, there's not much out there in space to interfere with the transmission, so a small signal can indeed make it all the way back here.

It helps that we know what frequency to look for ... The baud rate is usually low, because the the signal is sampled over and over again to increase the signal to noise ratio by mathematically canceling out any noise.

Here is a project I did several years ago for audio but could just as easily be applied to a radio signal as a proof of concept...

Propeller Application DEMO: Spectrum Analyzer (for Audio)

The baud rate is usually low, because the the signal is sampled over and over again to increase the signal to noise ratio by mathematically canceling out any noise.
iirc they use a lot of forward error correction in the protocol as well...

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