[amsat-bb] Re: 2.4GHz broadband router on satellite?
terrando at tiscali.it
Fri Feb 22 00:14:06 PST 2013
wifi over long distance is a reality on earth-based point-to-point links,
with distances up to 200miles. The long distance means long latencies, thus
the standard wifi protocol needs to be modified. It exists today
implementations of the modified protocols for the WRT54G router.
To cope with the link budget you need higher power (i.e power amplifier)
and high gain antennas, and this should not be a big deal even on a
spacecraft, although I am a bit concerned on the possibility of fitting a
high gain antenna on top of a cubesat. Maybe it would be feasible at 5GHz,
but in any case you would need 3 axes stabilization of the spacecraft.
What looks more difficult to me is the Doppler shift compensation, as the
wifi transceivers do not have (at least to my knowledge) the possibility to
fine tune the channel frequency.
I know that long range wifi has been investigated on south america to help
connecting isolated villages in the jungle, and by a group of Italian
researchers (http://www.ixem.polito.it/index_e.htm), and a high datarate
link has been established with a stratospheric balloon.
It is an interesting discussion topic, and I hope somebody with more
experience on the topic will jump into the discussion.
2013/2/22 Greg D <ko6th.greg at gmail.com>
> Lee Maisel wrote:
>> James French wrote:
>>> What are the possibilities of building a satellite that uses a Linksys
>>> WRT54GL router with a modified DD-wrt or HSMM-Mesh software as a store
>>> and forward BBS, to route a received request from one station to another
>>> station, or even to connect to a on board networked camera to receive
>>> What kind of uplink power would be needed from the home station?
>>> How fast could the speed(s) get theoretically?
>>> How big would the antenna have to be on the craft and for the ground
>>> station to even be able to do this adequately?
>>> Would the doppler be too much to even consider this?
>>> Would the space environment be too harsh for something like this?
>>> This is just something I was thinking about this morning and thought I
>>> would toss it out.
>>> James W8ISS
>> THAT is an AWESOME Idea!
>> I don't see why it wouldn't work, I don't know if doppler is an issue
>> though, it may not be if the modulation is FM.
>> The antenna would not have to be big, it's 2.4Ghz
>> Why don't you post this on the HSMM-MESH.org web forums and get ideas?
>> Hi James, Lee,
> If you are thinking of using standard Wi-Fi as the link protocol, be aware
> that the timers that drive the protocol don't well work over long distances
> (few miles). Something about the speed of light not being fast enough.
> Real bummer. These would need to be adjusted, though I think the
> implications for a point-point connection may not be too severe.
> Besides doppler shift, which could be a problem depending on how agile the
> ground station is, the modulation scheme (it's NOT simply FM) uses about 20
> mhz of bandwidth, so you will need significant power to get the 20db S/N
> needed to decode anything halfway reliably. Remember, a typical AP runs
> 100mw on 2.4 ghz, and gets reliable communication over distances of 100's
> of FEET with omni antennas. Add some gain on both ends (so now you need
> attitude control on the satellite!), and you can go a few miles. But 100's
> of miles to orbit? I need someone to "do the numbers", but I bet it's not
> too good.
> Greg KO6TH
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