From: Matti Aarnio (hwy.tsbqwhci@fraghaus.com)
Date: Thu May 02 2002 - 10:30:33 EEST
On Wed, May 01, 2002 at 05:33:54PM -0700, James Cutler wrote:
> > Most WLAN cards have extremely wide receivers, which drown out their
> > AGC when they receive powerfull RF within 200-400 MHz. Narrow Rx
> > pre-filter helps, but does not appear at commodity pcmcia cards.
> > For that matter, separate Rx/Tx paths, and PTT signal are not
> > available at commodity stuff either.
>
> Anyone know if WLAN cards can handle doppler shift say from a LEO
> satellite?
Possibly can. It all depends on the oscillator if it can be
programmed in small enough steps -- and having suitable firmware
that can be commanded. Lets see more:
Spacecraft downlink can be at 2.4 GHz, but uplink must be somewhere
else, say 5.6 GHz (national regulations vary a bit again, but
the uplink can't be at 2.4 GHz.)
8 km/sec speed (average LEO) translates to doppler as:
2.4 GHz: 64 kHz
10.5 GHz: 280 kHz
Which of course means that it is coming straight at you, while
in reality it passes by -> radial speed is always lower.
For link-budget S/N reasons there most probably is need to:
- Narrow the bandwidth ( = bitrate)
- Send shorter frames to improve changes of sending correct frame
- use non-IEEE spreading-code (one of those ARRL codes?)
- Center frequency tuning by 100 or 1000 Hz steps.
- Separated Rx/Tx/PTT signals
Which all boils down to: new firmware
and possibly custom hardware.
I have wondered what would be the interesting technical thing for
a talked about AMSAT-OH satellite, a Spread-Spectrum datalink would
be a first one (although AO-40 should be able to do it too).
(And no, we are just beginning the talks..)
/Matti Aarnio -- OH2MQK
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