Hi guys

Is there any different between the RF use in earth than the RF used in space? I mean Voyager 1/2 are bloody far away from earth, and we can still get data off them. Is it because in vacuum, there is no RF signal attenuation or something?

Just an interesting question.

 

There is some attenuation simply due to distance. That, to my mind, brings two issues:
1. Solar winds. But that can be overcome by repeating the signal.
2. Even if they have implemented some signal stearing/directionality, even if they are sending signal directly at Earth so that we can receive as much of it as possible, the arc that the signal (wave) makes by the time it reaches Earth is huge so only part of the signal actually "hits" the Earth. On the other hand Earth is moving through the arc so we can catch parts of it, and if we catch enough parts, we can put them together to get the whole or at least most of the original signal. And again, repeating the signal helps, a lot.

The really interesting thing is what is going to happen to the Voyagers once they get beyond the Sun's (star's) gravitational "field?" One theory is that gravity does really weird things when you get past nice and organized gravity that large bodies like stars provide. A mention of gravitational standing waves have been made...

 

It is the same RF and it has to go somewhere. What is does from a faraway source is spread out. Modern receiver and antenna technology can take extremely small amounts of power and pull information out of the noise floor by knowing what they are looking for and using slow data rates.

 

Thanks for the replies guys

 

Hi guys

Is there any different between the RF use in earth than the RF used in space? I mean Voyager 1/2 are bloody far away from earth, and we can still get data off them. Is it because in vacuum, there is no RF signal attenuation or something?

Just an interesting question.

It is the same signal and it has the same distance-squared attenuation. So you do three things to overcome the problem. You use a high-gain (i.e., highly directional) antenna on the spacecraft, you use a REALLY high gain antenna on Earth (whose effective aperture size is as close to the size of the Earth itself as you can get, or potentially even bigger using satellites, but I don't know if that's actually been done yet).

In general space missions can't repeat data. Because of the time lag involved and the opportunity costs it represents, most deep space missions, particularly if they are doing flybys like the Voyager probes, fill their data buffers to capacity and then turn their high-gain antennas toward Earth (often the same antenna they used as part of the data gathering, so they can't collect data while they are transmitting to Earth) and transmit their data. They then begin overwriting the data with new data because they usually have a very short window of time to collect data during the encounter. Of course, mission planners take care not to put the encounters during periods when the sun or other big objects will be between Earth and the spacecraft; but even so, if the data is not successfully received, possibly hours later, on Earth, then it is gone forever;. That is where the third technique comes into play -- channel coding. Error correcting codes are employed (along with other forms of channel coding) so that a significant portion of the data can be missed or corrupted and the actual data still successfully recovered from it.

 

2. Even if they have implemented some signal stearing/directionality, even if they are sending signal directly at Earth so that we can receive as much of it as possible, the arc that the signal (wave) makes by the time it reaches Earth is huge so only part of the signal actually "hits" the Earth. On the other hand Earth is moving through the arc so we can catch parts of it, and if we catch enough parts, we can put them together to get the whole or at least most of the original signal. And again, repeating the signal helps, a lot.

What arc does the signal make? Radio signals aren't subject to gravity are they?

 

What arc does the signal make? Radio signals aren't subject to gravity are they?

The signal propagates as a sphere. When you take cross section of the sphere, you get a circle. A section of a circle is an arc.

Now. I assume that Voyagers are not actually broadcasting in all directions (sphere). I assume they have directional antenna that is pointed at Earth. So the actual signal is more of a cone. So. Now imagine this cone... It is hundreds of millions of kilometers in height and its base is tens or more millions of kilometers in diameter. Earth is almost like a small pebble when you are dealing with those dimensions.

 

Oh, I interpreted "the arc that the signal makes" as "the arc that the signal travels". Thanks.

 

Wbahn beat me to many of the punches... but... when I was working in the field we were still using vacuum tube output stages on the RF high gain antenna. 100W output power - highly directional. That doesn't sound like a lot of power, but when you consider most of the spacecraft were around 400W max power consumption, that's a significant portion of the power budget.

There are 3 places on earth that form the "Deep Space Network". Goldstone, CA. Madrid, Spain. Canberra, Australia. Each are equally spaced around the earth and have approximately 120 degree coverage so they can see all around the earth. These each have very large parabolic antenna so they can receive very small signals from very large distances. Time on the DSN is extremely valuable. The spacecraft I worked on usually had a couple hour window that we recieved engineering telemetry every day or two. There are a LOT of deep space probes out there and they all want time on the system.

Time on the DSN is extremely valuable. One time we nearly lost a spacecraft. We were literally hours from total spacecraft loss. We had to call another manager of another spacecraft and ask permission to use the DSN during their timeslot. They could have said no. But they didn't. It's rare that you get into a tight situation like we did and another program isn't sympathetic to your situation and lets you have their time. It's also somewhat rare that you get so close to losing a spacecraft prior to mission completion. We made some quick changes and were able to save the spacecraft to live another day.