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Re: AO-40 MA, ALON/ALAT and squint angle ??

In a message dated 12/31/00 2:06:23 PM Eastern Standard Time, 
cbuttsch@slonet.org writes:

> I'll take it on faith that one must place a
>  negative in front of the BLAT and subtract 180 from the BLONG, but surely
>  we want to know WHY, rather than the antennas are on the other side from
>  AO13!   We stand the risk of mass confusion changing the old BLONG/BLAT
>  system used on AO13 unless it is explained well.  Cliff K7RR

Here's the explanation from Stacey, W4SM. I don't think we'll get a better 
Please note that NOVA for Win is the only program I'm aware of that _does 
not_ need this fix.
>From W4SM:
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.


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.

Mike, N1JEZ
AMSAT #29649
Local Area Coordinator
"A closed mouth gathers no feet."
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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