How are we able to send a satellite signal so far out to space?

[jtvang125]

I was watching Roving Mars, the documentry about sending the 2 rovers to Mars, the other day and I couldn't remember how far Mars was from Earth but it said that it will take 7 months for a rover to reach Mars. There was also a 5-6 minute gap of communication delay when a signal is sent to the rover or a signal coming back from one.

Is the lack of air in space allowing us to send a signal much farther than possible than here within earth? Also what type of bandwidth are we looking at here to allowing sending back pictures and live video?

 

[f95toli]

The atmosphere does attenuate radio signals somewhat. However, the main reason why the range is so limited here on earth is simply that our planet is a sphere. Hence, in order to send a signal to something which is significantly below the horizon you need to bounce it off the ionosphere (a thin layer in the athmosphere, which fortunately reflects radiowaves). This only works for certain (long) wavelenghs and can't be used for e.g. mobile phones. However, you CAN use it for e.g. radio transmission which is how shortwave radio works (which is why you can listen to e.g. BBC everywhere).

Hence, it is in many ways easier to send signals to Mars as long as you have a free line-of-sight than to send them to the other side of the planet, for the latter you need a relay station of some sort.

 

[Modelworks]

Mars rover communication isn't all that tough, they had a good example to follow with the voyager probes.

The rover cameras are 1 megapixel
Transmission of 42megabits took 6 minutes , so roughly 117Kbits/sec, or 14KB per sec.

I find the voyager probe signal even more interesting.

The spacecraft are now so far from home that it takes nine hours and 36 minutes for a radio signal traveling at the speed of light to reach Earth,"said Ed B. Massey, project manager for the Voyager Interstellar Mission. "That signal, produced by a 20 watt radio transmitter, is so faint that the amount of power reaching our antennas is 20 billion times smaller than the power of a digital watch battery,"

Imagine sending a command and then waiting 19+hours to see the result.

 

[JohnCU]

Originally posted by: Modelworks
Mars rover communication isn't all that tough, they had a good example to follow with the voyager probes.

The rover cameras are 1 megapixel
Transmission of 42megabits took 6 minutes , so roughly 117Kbits/sec, or 14KB per sec.

I find the voyager probe signal even more interesting.

The spacecraft are now so far from home that it takes nine hours and 36 minutes for a radio signal traveling at the speed of light to reach Earth,"said Ed B. Massey, project manager for the Voyager Interstellar Mission. "That signal, produced by a 20 watt radio transmitter, is so faint that the amount of power reaching our antennas is 20 billion times smaller than the power of a digital watch battery,"

Imagine sending a command and then waiting 19+hours to see the result.

Wow, the sensitivity of whatever is trying to receive the voyager signal must be insanely awesome. What's that, like, .001 femtoamps?

 

[silverpig]

I'm guessing the receiver needs to be pretty cold in order to work.

 

[JohnCU]

Originally posted by: silverpig
I'm guessing the receiver needs to be pretty cold in order to work.

To eliminate thermal noise?

 

[f95toli]

Originally posted by: JohnCU

Originally posted by: silverpig
I'm guessing the receiver needs to be pretty cold in order to work.

To eliminate thermal noise?

Among other things.

However, as far as I know NASA operates a whole network of reciver dishes spread around the world so the signal is not recieved by just a single detector.
Also, using modern detectors you can detect VERY small powers and I doubt that any of the detectors used for this are anywhere near that limit of what is possible; detectors used for radio astronomy are several orders of magnitude more sensitive.
20W might not sound like much but it is actually a significant amount of power for a radio transmitter (it is over 40 dBm).

 

[silverpig]

Originally posted by: f95toli

Originally posted by: JohnCU

Originally posted by: silverpig
I'm guessing the receiver needs to be pretty cold in order to work.

To eliminate thermal noise?

Among other things.

However, as far as I know NASA operates a whole network of reciver dishes spread around the world so the signal is not recieved by just a single detector.
Also, using modern detectors you can detect VERY small powers and I doubt that any of the detectors used for this are anywhere near that limit of what is possible; detectors used for radio astronomy are several orders of magnitude more sensitive.
20W might not sound like much but it is actually a significant amount of power for a radio transmitter (it is over 40 dBm).

I had an astro lecture once on radio astronomy where the prof was telling us about a conference he went to back when radio astronomy was in its infancy. All the conference attendees went to the lecture room and sat down at their seats to find the program book sitting on the tables in front of them. The front page had the title of the conference and a picture of a radio telescope. Upon flipping the first page they saw some text printed on page 2:

"By turning this one page you have used more energy than the earth has received in total radio power in the bands we are interested in over the course of earth's entire existence."

 

[SlicedBread27]

Originally posted by: silverpig

Originally posted by: f95toli

Originally posted by: JohnCU

Originally posted by: silverpig
I'm guessing the receiver needs to be pretty cold in order to work.

To eliminate thermal noise?

Among other things.

However, as far as I know NASA operates a whole network of reciver dishes spread around the world so the signal is not recieved by just a single detector.
Also, using modern detectors you can detect VERY small powers and I doubt that any of the detectors used for this are anywhere near that limit of what is possible; detectors used for radio astronomy are several orders of magnitude more sensitive.
20W might not sound like much but it is actually a significant amount of power for a radio transmitter (it is over 40 dBm).

I had an astro lecture once on radio astronomy where the prof was telling us about a conference he went to back when radio astronomy was in its infancy. All the conference attendees went to the lecture room and sat down at their seats to find the program book sitting on the tables in front of them. The front page had the title of the conference and a picture of a radio telescope. Upon flipping the first page they saw some text printed on page 2:

"By turning this one page you have used more energy than the earth has received in total radio power in the bands we are interested in over the course of earth's entire existence."

Exactly. The amount of energy a wave contains is inversly proportional to its wavelength, or directly proportional to its frequency - whichever you prefer. The waves that we observe with radio telescopes and the ones that we use for interplanetary communication have massive wavelengths (I think ~1km, correct me if I'm wrong) and therefore contain incredibly miniscule amounts of energy.

 

[Born2bwire]

Originally posted by: SlicedBread27

Originally posted by: silverpig

Originally posted by: f95toli

Originally posted by: JohnCU

Originally posted by: silverpig
I'm guessing the receiver needs to be pretty cold in order to work.

To eliminate thermal noise?

Among other things.

However, as far as I know NASA operates a whole network of reciver dishes spread around the world so the signal is not recieved by just a single detector.
Also, using modern detectors you can detect VERY small powers and I doubt that any of the detectors used for this are anywhere near that limit of what is possible; detectors used for radio astronomy are several orders of magnitude more sensitive.
20W might not sound like much but it is actually a significant amount of power for a radio transmitter (it is over 40 dBm).

I had an astro lecture once on radio astronomy where the prof was telling us about a conference he went to back when radio astronomy was in its infancy. All the conference attendees went to the lecture room and sat down at their seats to find the program book sitting on the tables in front of them. The front page had the title of the conference and a picture of a radio telescope. Upon flipping the first page they saw some text printed on page 2:

"By turning this one page you have used more energy than the earth has received in total radio power in the bands we are interested in over the course of earth's entire existence."

Exactly. The amount of energy a wave contains is inversly proportional to its wavelength, or directly proportional to its frequency - whichever you prefer. The waves that we observe with radio telescopes and the ones that we use for interplanetary communication have massive wavelengths (I think ~1km, correct me if I'm wrong) and therefore contain incredibly miniscule amounts of energy.

The amount of energy in a wave is dependent upon the amplitude of it's fields, and in no way on its frequency. The energy of the photons in the wave are dependent upon frequency. In terms of your transmitted power, 20W from infrared is the same as 20W from microwave and the radiated fields will have the same amplitude regardless. The problem is the free-space loss affecting the link budget, which is due to the inverse square law and the receiving aperture. That is, power drops off as the square of distance independent of frequency. The receiving aperture is dependent upon frequency but is a property of the receiving antenna and not the wave itself. The enormous distances that the radio sources in space have to cover to get here is the reason why the received power is so low.

 

[Modelworks]

Did some digging and found some of the real technical docs on the probes.
Definitely not a broadband connection




Earth to Spacecraft command on S-band carrier of 2.115Ghz
Downlink modulated on S-band carrier of 2.295Ghz
Data rates between 8 and 2048 bits/sec
During interplanetary data rate is 1024 bits/sec
Antenna dish of 109cm(43 in) diameter
Two receivers , if one fails to receive a command from earth in 36 hours they automatically switch.

Two data storage units
Each unit has a capacity of 524,288 bits

Commands are sent via PCM/FSK at a fixed rate of 4 bits/sec
Each command word consist of 48 bits , including 13 bits for synchronization
a total of 204 commands implemented

 

[Throckmorton]

Originally posted by: Modelworks
Mars rover communication isn't all that tough, they had a good example to follow with the voyager probes.

The rover cameras are 1 megapixel
Transmission of 42megabits took 6 minutes , so roughly 117Kbits/sec, or 14KB per sec.

I find the voyager probe signal even more interesting.

The spacecraft are now so far from home that it takes nine hours and 36 minutes for a radio signal traveling at the speed of light to reach Earth,"said Ed B. Massey, project manager for the Voyager Interstellar Mission. "That signal, produced by a 20 watt radio transmitter, is so faint that the amount of power reaching our antennas is 20 billion times smaller than the power of a digital watch battery,"

Imagine sending a command and then waiting 19+hours to see the result.

I don't have to imagine. It's called playing EA games online.

 

[OS]

All satellites have a small, finite amount of power to send a transmission and the transmissions require extensive signal processing to recover the original signal.

I think my prof once mentioned with modern techniques you could typically recover a signal that was 10,000 times weaker than the strongest present interference.

 

[playaveli]

I want to send a wireless binary code to a location in this earth, can anyone help me accomplish this? I have a specific binary coded message that i already have encoded that must be sent to a certain location on the map, I am able to " compensate " anyone able to help. by any means necessary.

 

[inachu]

Funny thing about these signals is one time NASA scientists discovered a stray signal thats has been bouncing along the upper layer of our stratosphere and the signal was that of I LOVE LUCY.

So just imagine the radio noise we are saving up there.

 

[AD5MB]

the parabolic dish for the earth station was 210 feet across. then they added to it:

http://eecue.com/a/1581/Goldstone-NASA-Deep-Space-Network.html

a story you won't hear anywhere else:

that bright white thing is a subreflector. the antenna is a catadioptric, AKA a Schmidt-Cassegrain, a reflector with a subreflector in the middle.

After the Landers earthquake they took a ride up there in a manlift. that subreflector had 8 tabs welded to it, 8 cables to tabs on the framework thet holds the subreflector. It was hanging by one safety wire, the other 7 broke. they looked in the documentation to find out who engineered the system, tell him he got it just right.

it wasn't in any drawings. someone just went up there and did it. no documentation, no permission.

the answer to the inevitable question: I worked there at the time. At a different site for the same contractor.

 

[Abwx]

Mars rover communication isn't all that tough, they had a good example to follow with the voyager probes.

The rover cameras are 1 megapixel
Transmission of 42megabits took 6 minutes , so roughly 117Kbits/sec, or 14KB per sec.

I find the voyager probe signal even more interesting.

The spacecraft are now so far from home that it takes nine hours and 36 minutes for a radio signal traveling at the speed of light to reach Earth,"said Ed B. Massey, project manager for the Voyager Interstellar Mission. "That signal, produced by a 20 watt radio transmitter, is so faint that the amount of power reaching our antennas is 20 billion times smaller than the power of a digital watch battery,"

Imagine sending a command and then waiting 19+hours to see the result.

Emitter power is only 20W but a very high gain parabolic antenna is used, power is extremely focused to earth direction such that the apparently radiated power is about 1.3MW, that would be the necessary power to get the same signal on earth if the Voyager antenna did radiate equally in all directions, attenuation of the signal due to distance is about 314dB, that s a 10^15.7 ratio.

 

[disappoint]

I had an astro lecture once on radio astronomy where the prof was telling us about a conference he went to back when radio astronomy was in its infancy. All the conference attendees went to the lecture room and sat down at their seats to find the program book sitting on the tables in front of them. The front page had the title of the conference and a picture of a radio telescope. Upon flipping the first page they saw some text printed on page 2:

"By turning this one page you have used more energy than the earth has received in total radio power in the bands we are interested in over the course of earth's entire existence."

We haven't been listening to radio signals over the course of Earth's entire existence. How does the author of that snippet of bologna know we didn't receive a signal before we were listening? We've only been listening for a short while.

So...what putting "since we've been listening" rather than the malarky "over the course of earth's entire existence" doesn't trigger enough neurotransmitter release to trick the subconscious into accepting that as true? So you have to lie to get people to believe? Now I see why religion is still prominent. People are sometimes more likely to believe a lie than the truth.

I can see the thought process now: "Oh I believed them because it seemed so amazing and I wanted to believe amazing things were still possible* and it FELT so good to believe without actually having to think critically."

I've said it before and I'll say it again because it's still true: The subconscious cares not what is real and what is unreal. It's the emotional part of the brain that only cares what something feels like. Humans are going to have to understand their minds better if they're going to get a handle on what ails them.


*amazing things are still possible. The universe is doing amazing things unbeknownst to 99% of the population because they're mired in bullshit instead of learning how things really are.

 

[John Connor]

I once read a book on satellites and the author said something like the signal is basicly non existent. Refering to things like DirectTV and GPS.

In amateur radio you can do what is called a moon bounce. Where you bounch a signal off the surface of the moon. Amateur's also have sats to talk on.

 

[NTMBK]

I want to send a wireless binary code to a location in this earth, can anyone help me accomplish this? I have a specific binary coded message that i already have encoded that must be sent to a certain location on the map, I am able to " compensate " anyone able to help. by any means necessary.

Need to get a message to a precise co-ordinate fast?