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

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



Hi Steve,

Maybe this way of thinking about it might help...

AO-40 is whizzing around the earth in an elliptical orbit.  At the moment,
the satellite is spin-stabilised.  This means, that the "front" of the
satellite is always pointing at a particular fixed point in space,
regardless of where in the orbit it is.  

The whole issue of "ALON/ALAT", or "satellite attitude", or "Bahn LAT/LON"
or "BLAT/BLON" is to describe which particular direction the front of the
satellite is indeed looking at.

Satellite Attitude is measured when the satellite is at apogee (furthest
point from the earth).  Think of the Attitude Longitude as equivalent to
Azimuth.  

If the front of the satellite (the +z facet) is pointing straight towards
the earth, then ALON = 0 degrees.

If the +z facet is facing straight back to where it has just come from
(strictly speaking, facing backwards to the tangent of the orbit), then
ALON = 90 degrees.

If the +z facet is facing directly away from the earth, then ALON = 180
degrees.

Finally, if the +z facet is facing directly forwards (along the tangent
line), then ALON = 270 degrees.

Remember, a spin-stabilised satellite (like AO-40 is now) keeps pointing to
the same point in space...  so with a ALON of 270 degrees, whilst the +z
facet is pointing straight forward at apogee, it will be pointing straight
backwards at perigee.  AO-40's 400N motor is on the +z facet;  by having it
pointing backwards at perigee, the thrust adds to the satellite's orbital
energy.  (This is how the approx 6 minute burn lifted the apogee out to
60,000 km).

There's a pretty good diagram showing ALON of 270 degrees at:
http://www.cstone.net/~w4sm2/software2/ALON.gif


As for ALAT:
Think of ALAT as the satellite's equivalent of Elevation.  If ALAT is
positive, then the +z facet is pointing up, with respect to the orbital
plane.  If ALAT is negative, then the +z facet is pointing down with
respect to the orbital plane.


AO-40's current attitude
========================
I'm using a figure of 260/+2.2.  This means that at apogee, the +z facet is
pointing slightly (2.2 degrees) up, and slightly (10 degrees) to the right
of straight ahead.
At perigee then, the motor and antennas are pointing almost straight
backwards, but with a slight (10 degree) angle away from the earth.


What's Attitude got to do with Squint Angle?
============================================
Attitude tells us where in space the satellite is pointing.  As the
satellite moves through its orbit, the earth may or may not come between
the satellite and that point in space.  Most of the time, the earth is
_not_ in front of the satellite...  it's somewhere off to the side.
Depending on the satellites attitude, and the geometry between the
groundstation and the satellite, the squint angle can be calculated.

Most satellite tracking programs that can do this date back to AO-10 and
AO-13 days.  AO-10 and -13 had the high-gain antennas mounted on the
"bottom" of the satellite (the -z facet)... but AO-40 has the high-gain
antennas on the +z facet.  This is why you have to modify the published
ALON/ALAT figures prior to putting them into InstantTrack, etc.

(Figure AO10-1, page C-2 of The Radio Amateur's Satellite Handbook seems to
be in error here.  It shows AO-10's antennas as being on the +z facet).

If the published figures are 260/2.2, then obviously the other end of the
satellite is pointing the other way;  

Corrected_ALON = (Published_ALON + 180 degrees) MOD 360.  
eg = (260+180) MOD 360 = 80

Corrected_ALAT = Published_ALAT * -1
eg = +2.2 * -1 = -2.2

So for Instanttrack, use a figure of -2.2,+80  (IT takes the attitude in
the order of ALAT,ALON).



I hope this somewhat lengthy explanation has helped a little.


Regards,


Chris vk6kch





At 22:30 31/12/2000 EST, you wrote:
>In a message dated 12/31/00 4:19:48 PM Eastern Standard Time, Mike73@aol.com 
>writes:
>
><< A hand drawing or a simple model will help at this 
> point.  See the following drawing.
> 
>     www.cstone.net/~w4sm2/software2/ALON.gif >>
>  
>I wonder what an ALON/ALAT of 0/270 would look like.
>That might help my understanding of the range of possible
>attitudes.
> 
>73, Steve
>K5PK
>----
>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