[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

RE: commutating antennas (was Re: Canted S-band antennas on satellite)



Bill Tynan introduced the concept of such a spacecraft at an AMSAT Symposium
several years ago.  What Bill suggested was a simple spacecraft with no
mechanical parts and thus electronically switching to the antennas facing
Earth.  The postulated orbit was Geosynchronous or another high circular
orbit to keep it simple (i.e., constant switching rate).  However, there is
nothing inherent in the concept that precludes operation in an elliptical
orbit as long as it is understood that that the switching rate must vary
depending on where you are in that orbit.

The technical challenge is having accurate, real-time knowledge of which
side of the spacecraft if facing the Earth.  I've been pondering
implementations of inertial sensors to make such a determination.  Such a
setup would, however, require a backup set of omni antennas for cases in
which the switching electronics wasn't working correctly.

Ken Ernandes, N2WWD







-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Tom Clark (W3IWI)
Sent: Friday, May 03, 2002 11:11 AM
To: 'Franklin Antonio'; 'Ronald Long'
Cc: amsat-bb@AMSAT.Org
Subject: RE: [amsat-bb] commutating antennas (was Re: Canted S-band
antennas on satellite)


Franklin, N6NKF opined:

> This is related to something I have been advocating for
> awhile now.  AO-40
> (while in its present spin mode), like its cousins AO-10 and
> AO-13 before
> it, has the antennas pointing parallel to the spin axis.
> That causes
> the operators put the spin axis near the plane of the orbit.
> That causes a
> geometric problem at some times where the sun has bad angles
> to the solar
> panels (which are on the sides).  It also makes the antennas
> point toward
> Earth during only one part of the orbit.
>
> A different approach would be to put antennas on the sides,
> (perhaps 4 or 6
> sides), and let the satellite spin axis be roughly
> perpendicular to the
> orbit plane.  For a low inclination orbit (like AO-40's
> orbit), this would
> give good sun angles all the time.
> [snip]

Historically, one of the serious problems with this approach is spin-induced
phase modulation. The antenna phase center -- the point (or set of points)
from which the energy appears to start -- is not coincident with the spin
axis. If the radius of the antenna element is R, then you can get a phase
shift of a significant fraction of R wavelengths as the satellite spins; a
50 cm sized satellite would have multiple wavelengths of phase shift at a
wavelength like 13cm.

If you are using a mode where phase is of little importance (SSB, CW, FM)
then you will hear a whoompa whoompa modulation. If you are using a digital
mode where phase is of importance, the PLL will be pulled with this phase
modulation. The demod PLL's will have to be designed to track thru this
modulation. I'm sure Karn will suggest that proper application of
coding/interleaving can make the data losses be tolerable.

I believe that this summer is the 40th anniversary of the use of this
technique on TELSTAR 1; this was the first satellite to send video from US
to Europe. A couple of fun references can be found at


http://www.ieee.org/organizations/history_center/milestones_photos/telstar.h
tml
and
  http://www.retrofuture.com/telstar.html

The former includes a picture showing the equatorially mounted antennas,
while the latter is about the 60's Rock 'n Roll group Tornadoes and the hit
song].

73 de Tom, W3IWI


----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org


----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home