# Notes on Squint Angle & ALON/ALAT

• Subject: [amsat-bb] Notes on Squint Angle & ALON/ALAT
• From: "Stacey E. Mills, M.D." <w4sm@xxxxxxxxxx>
• Date: Sat, 02 Dec 2000 19:01:14 -0500

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There may be  some confusion among AO-40 users and software writers
(updaters) about the differences between ALON/ALAT and squint angle on
AO-40 vs. AO-13.  As described recently on the amsat-bb, squint or
"offpointing" angle is the angle between the tip of the spacecraft (S/C)
antenna and your QTH.  Note that this is where the S/C antenna is pointing
and has nothing to do with your antenna, which is assumed to be pointing at
the S/C.  The only way you can improve your squint angle is to move your
QTH or wait for the S/C to move.  A squint of 0 degs. means the tip of  S/C
antenna is pointed right at you. For the higains, you should have optimal
signal.  For the omni's, optimal signal is closer to a squint of 90 degs.,
looking at the side of the omni element, rather than its tip.

Squint is calculated in tracking programs based on ALON/ALAT and ALON/ALAT
is defined in orbital coordinates, based to the
direction of the +Z axis on the S/C, with regard to the the semi-major axis
and plane of the orbit.  The +Z axis passes through (and is orthogonal to)
the side of the S/C containing the 400N motor.  At apogee, if the +Z axis
is pointing directly at the subsatellite point on
the earth, the ALON/ALAT is 0/0.  On AO-13 the higain antennas were on the
opposite side of the S/C from the 400N motor, so in order for AO-13's
antennas to point at the subsatellite point at apogee (optimal), ALON/ALAT
was 180/0.  Because AO-40 has the higains on
the same side as the 400N motor, the equivalent for AO-40 is ALON/ALAT = 0/0.

The direction of "rotation" for the ALON coordinate is such that an
ALON/ALAT of 270/0 means that the +Z axis points at the earth shortly AFTER
perigee, and with an ALON/ALAT of 90/0, it points at the earth shortly
BEFORE perigee. It's easier if you remember lower
numbers (<180) come before (perigee) higher numbers (>180) come after.  A
+ALAT points north of the orbital plane and a -ALAT points south of the
orbital plane. A hand drawing or a simple model will help at this
point.  See the following drawing.

www.cstone.net/~w4sm2/software2/ALON.gif

If you want to fool your AO-13-based software into giving you a proper
squint angle for the higains on AO-40, add or subtract 180 from the ALON
(whichever gives a 0 to 359 value) and change the sign of ALAT. Enter these
new values into your program.   Alternately, subtract the AO-13-style
squint value reported by your software from 180 degs to get the true value
for the higains (see below).

The other important distinction between the two satellites is with regard
to the omnis.  On AO-13, the omnis were on the same side of the S/C as the
higains. On AO-40, they are on the opposite side.  Thus, the squint angle
with respect to the omnis on AO-40 is really 180-squint for the
higains.  If you've followed my ramblings thus far, you realize that the
unaltered ALON/ALAT, entered into an AO-13 style program will
give you squint with regard to the omnis.  If you want to leave your
entered ALON/ALAT as given, without doing the changes above, then you can
simply subtract 180 from the indicated squint (which is really omni squint)
to get the high gain squint.

Keep in mind that  if not disturbed by magnetorquing, etc. a spinning
satellite always points to the same spot in space.  However, ALON/ALAT are
defined in orbital coordinates, so as the orbit precesses (RAAN and ArgP
change) over time,  the ALON/ALAT values change because the reference
system has changed.

I hope this helps clarify things.

--
_______________________________________________________________________
Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key
_______________________________________________________________________

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