Advanced Communications Systems - more efficient communications

sethmckiness · 2002-09-24 14:52:42

Anyone thought about any ideas for improving communications. ie bandwidth increases, bandwidth effiecency 128-PSK and crazy stuff like that or polarity modulation(Is it feasible). Would like some brain teasers on this point. Also, anyone know any equations for figuring power for communications systems, was thinking about it the other day how much power it would take to transmit a one Mhz wide signal in the SHF range from 30 light years.. Ie, how much power it would need for Seti to pick up a signal.

sethmckiness · 2002-09-28 00:17:59

Also another point to be brought up is communication on Mars. Does anyone know where the Clarke belt on Mars is? Also with the atmosphere being the way it is, I doubt there will be much of chance for Troposcatter communications.. which is still relied upon here for short runs ( ~250 miles). I am also curious on the noise floor on Mars.. on what is filtered out by the atmosphere.. How well hf and Vhf radio may work.. Just thoughts.. ???

turbo · 2002-09-28 09:59:56

I've got one book on comms with equations all over the place, but nothing about power needed over distance that I can find.

You definately have me wondering seth, since I imagine the HF and VHF bands would probably be very dirty.

Clarke belt needs a little calculation. Mighty text book shows Kepler's third law to show Clarke belt for earth to be 35,768 km. Calcs I've done shows belt would be 41,362.49 km minus the equatorial radius of Mars, which I don't have handy.

On the bright side, I had found that Viking used an S-band transmitter with a whopping 20 Watts of power to send data to Earth. NASA website also says the orbiter part carried a relay radio of 30 Watts, and transmitted to earth at 381 MHz, in the UHF band. I didn't see any mention if the thing used PSK, FSK, or some other scheme. (For Cindy and all those without fancy textbooks on comms, PSK = Phase Shift Keying and FSK = Frequency Shift Keying).

Hmmm, I wonder if any of those sats that were mentioned before (the nuke powered that never turned on) are set up for TDMA? Might not need multiplexing in the first years, but would be nice to have the capability.

Gotta find that equatorial radius........
turbo, whose advanced comms text is copyrighted 2001

sethmckiness · 2002-09-30 10:21:18

Yeah... what I am worried about it the lack of bounce capability. let a lone the free space lost accompanied with a dust storm. the power consumption you talk about also must realize that some of those signals where going to DSN (Deep Space Network) Sites. even though S-Band is still UHF(very high UHF) Someone could use a dish if they wanted to, I suppose, don't know what would be more efficient, let alon gain issues. but the directionality would also be key. an antenna with .5 degrees look angle would need 3 db less power of an antenna that had a .7 degree look angle. If you see where I am going with this. Some antennas are shotguns. Satellites are huge shotguns, some most likely in excess of 10-15 degrees. The other thing with that 20-30 watts is bandwidth. I guarantee it is not wide enought to do voice. So.. if you could tell me how much bandwidth we could do some calculations. Also, how much above the noise floor was it? did it say?

thanks

turbo · 2002-09-30 11:54:53

I've seen nothing describing the bandwidth of Viking's transmitters, and no mention of noise floor. We have to get creative, so back in the mighty text, I see under Shannon Limit that a standard voice band comms system has a 2.7 KHz band width and a 30 dB SNR. Text gives specific mention that digital comms won't work there, so thinking 1970s I'm hoping our little space probe was analog. Otherwise, look at using 6 KHz for minimum BW.

If the Viking orbiter's 30 Watt transmitter could be returned to service, at least Aricebo would hear, right?

t

sethmckiness · 2002-10-01 00:16:54

well hmm.. remember, it wasn't voice, it was data. I was thinking like 200-300 bps at 30 Watt would make it recievable at a DSN Antenna (see "the Dish" with Sam Neil, good fairly accurate movie) the power will increase proportionally with with band width. I know it is not over 4kbps (kilobits per sec) cause of the age, and timing issues. So, lets say it was 4 kbps, if we were to sent a E-1 STD or a 2 Mbps signal with 8psk, so an approx 500 kilohertz bandwidth if we uses 8/8 encoding. if it's 7/8 encoding assume aroun 750? Khz. so we will be pushing over 3.7 kW of power at the Antenna. pending noise floor issues. maybe some gain or loss by different frequency and or modulation. I would assume would take more power in lieu of need to transmit in SHF. may consider 40 + ghz from Mars to Earth Clark belt then SHF downlinks in the Ku band. If anyone want a basic explanation of PSK BPSK QPSK 8PSK 16PSK etc. and the requirement please ask. If properly engineered the communications for this project could be relatively inexpensive or insanely expensive! CVSD encoding can be used to reduce B-widths to 16 kbps instead of the the 64kbps standard for PCM(8 bit 4000 hz sample ranged at twice max sample frequency) A fairly large Antenna(SHF Range) Could be installed inexpensively in orbit on most craft with minimum of effort and minimum impact. I need to learn more on Antenna Desigins, I can quote existing systems but need to learn how to figure net gain of an antenna.

Ronman · 2002-10-06 12:07:10

Why is this transmitter Mars based? Why not use a Geo sat and a ground station that transmits to the sat? Could also use the Sat for geonav functions. Actually need a few sat anyway for ground nav function.

sethmckiness · 2002-10-06 23:00:05

-Nav satellites are typically not in geo synch orbit. Yes you could run a relay in space from geo synch to geo synch back to earth. For any kind of b-width you will use lots of power. I do not know if you could run a full time master slave over such a distance such that you could distribute timing so that you could use a 32 or 64 psk to save on bandwidth(teachers always say I was bad with run-on sentences). Once an intial bandwidth was decided upon or a number of digital voice channels then we could sit down and calculate how much power a satellite would need. I know that one would need less then 5 watts with a 50+dB gain dish for a 10-20mhz b-width. But the power consumption of a system like that is fairly high. Atleast 20 K-watts. Think of a Medium Terminal. I imagine with less b-width you maybe able to knock power consumption down to 5k-watt, still keeping reasonably priced sat terminals.

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#1 2002-09-24 14:52:42

Re: Advanced Communications Systems - more efficient communications

#2 2002-09-28 00:17:59

Re: Advanced Communications Systems - more efficient communications

#3 2002-09-28 09:59:56

Re: Advanced Communications Systems - more efficient communications

#4 2002-09-30 10:21:18

Re: Advanced Communications Systems - more efficient communications

#5 2002-09-30 11:54:53

Re: Advanced Communications Systems - more efficient communications

#6 2002-10-01 00:16:54

Re: Advanced Communications Systems - more efficient communications

#7 2002-10-06 12:07:10

Re: Advanced Communications Systems - more efficient communications

#8 2002-10-06 23:00:05

Re: Advanced Communications Systems - more efficient communications

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