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Re: Notes on 2.4 GHz beacon



It is the amount of "mispointing" of the satellite antenna relative to the
receiving station (you). Since the satellite antennas are fixed (and several
are directional), they don't line up properly during all portions of the
orbit, depending on the "attitude" of the bird.

Further, until the final orbit is achieved, the squint angle is going to be
very bad at the wrong parts of the orbit. Ideally, the squint angle becomes
worse (larger) as the bird comes closer to the earth, where increased losses
can be tolerated.  Of course, there are systems on board most birds to
switch to lower gain or or omni antennas when the squint angle becomes a
problem near perigee.

Right now, because the orbit is merely temporary and the "attitude" of the
spacecraft is being changed, squint angles go all over the place. A squint
angle of 90 degrees is VERY BAD...you are seeing only the side of the
antenna (much like the front to side ratio of a beam antenna).

Traditionally the attitide of the spacecraft in the desired orbit results in
a squint angle of near  0 at apogee (the bird's highest position in orbit
35,000 Km or so). This way the high gain antennas are pointed right at the
earth when the two are furthest apart.

Most current sat tracking programs have an entry that will allow you to tell
how bad the squint angle is at any given time. These are called ALAT/ALON or
sometimes BLAT/BLON. This information on spacecraft attitude is published
frequently by amsat in bulletins, on the mailing lists, etc. Last time I
looked they were moving to 0,270 as soon as possible. I think my last entry
was something like -5, 255. (These are guesses from memory. The telemetry
coming down from the bird has these two variables in text strings, so you
can get the latest right from the bird.)

The results of a high squint angle (severe mispointing of the satellite
antenna) include, but are not limited to:

1. Weak signal
2. Sever spin modulation (rapid qsb)
3. Reversed polarity (Left Hand Circular instead of Right hand Circular)

Hope this helps.

73,

hasan, N0AN
schiers@netins.net

----- Original Message -----
From: Gene Harlan <atvq@hampubs.com>
To: Stacey E. Mills, M.D. <w4sm@cstone.net>; <amsat-bb@AMSAT.Org>
Sent: Wednesday, November 29, 2000 11:07 PM
Subject: RE: [amsat-bb] Notes on 2.4 GHz beacon


> What is squint angle? I can't say I've heard that term before and now
> everyone seems to be using it! If I were to guess I would say it is beam
> width of the antenna?
>
> Gene Harlan - WB9MMM
> Harlan Technologies - publishers of:
> Amateur Television Quarterly
> ATVQ@hampubs.com
> OSCAR Satellite Report
> OSR@hampubs.com
> http://www.hampubs.com
>
> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> Behalf Of Stacey E. Mills, M.D.
> Sent: Wednesday, November 29, 2000 12:53 PM
> To: amsat-bb@AMSAT.Org
> Subject: [amsat-bb] Notes on 2.4 GHz beacon
>
>
> Dear Group,
>
> The 400 bps GB (telemetry just like the V-Tx MB) has been set up on the S1
> transmitter (nominal freq. = 2400.2 MHz) from MA = 20 to 26. This is about
> 2.77x6 = 16.6 minutes.  The MA chosen is a compromise of reasonable squint
> angle and visibility.  The V-Tx will remain on during this time, as
>
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>

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