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*Subject*: Re: [amsat-bb] AO-40 MA, ALON/ALAT and squint angle ??*From*: Chris Hill <CRH_Telnet@xxxxxxxxxxx>*Date*: Mon, 01 Jan 2001 21:32:21 +0800*In-Reply-To*: <b5.52655cd.278153da@aol.com>

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

**Follow-Ups**:**Re: AO-40 MA, ALON/ALAT and squint angle ??***From:*Woody

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