[amsat-bb] Re: Eagle U/V modes
kb5mu at amsat.org
Sun Oct 8 09:39:56 PDT 2006
At 10:23 AM -0400 10/8/06, McGrane wrote:
>Could someone please explain further the capabilities of the U/V text
>messaging mode. Would this incude the use of typical packet equipment?
No. In order to transfer information (even at low data rates) with
very small antennas and low power, it's necessary to use every trick
in the digital optimization book, so it won't be compatible with
existing packet equipment.
By very small, it's meant that the radio might clip onto the back of
a handheld PDA, with an antenna system small enough to be manageable
handheld. Probably not as small as a rubber duckie, but something
that doesn't need pointing. Exact details are still to be designed.
There's not a lot of detail yet about how the system will work from a
user standpoint. The work so far has been on making the links work
and determining the spacecraft requirements. For higher level
architecture, one idea is to use Jabber, which is an open protocol
for instant messaging (keyboard chat) used on the internet. It's
thought that many of the applications that work on APRS would work
great on this system (not just positioning).
Now would be a good time to throw out your ideas for applications for
a service like this, so the system designers can try to accommodate
(The above info is based on what I heard at the Space Symposium and
not to be considered official in any way.)
>Could some old timers describe a typical AO-13 or AO-40 mode U/V station
>for my education and to help me build a station.
The baseline AO-13 Mode B station was a 20-foot-long circularly
polarized cross-yagi for 2m, plus a 14-to-20-foot circularly
polarized cross-yagi for 70cm, mounted for azimuth and elevation
rotation. Rather short low-loss coax feed (e.g., Belden 9913) or a
mast-mounted low-noise preamp on the downlink. On the uplink,
operators who wanted to work under all conditions had about 100 watts
available, but under good conditions much less power was needed.
Continuously variable uplink power was considered mandatory since
being too loud is bad practice and being too weak meant marginal
stations couldn't hear you. SSB and/or CW capability on the radios.
Most conveniently, a single-box "satellite" rig would allow the use
of a single knob to tune around the transponder, but separate
transmit and receive rigs were also common.
Seriously hard-core stations who wanted to hear down to the
transponder noise floor even when conditions were poor would phase
two or more of the 20-foot cross-yagis. It was good to have a few of
those stations around to pick out the very weak uplinks, but it
wasn't really necessary for most users to have that much gain.
Computer control of the rotators was convenient but not necessary,
since the satellite moves slowly across the sky. Likewise computer
control of radio frequency was generally not required, since the
Doppler shift changed rather slowly.
kb5mu at amsat.org
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