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RE: RE: [ans] ANS 090 (Incorrect explanation of Arg. Peri.)



I stand corrected.  Somehow I got 360-ArgP confused with
ArgP.  I was measuring from perigee to ascending node
and the actual definition is from ascending node to
perigee. The ANS article is correct and I was wrong.

Bob


-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of i8cvs
Sent: Monday, April 01, 2002 11:04 AM
To: Robert McGwier; AMSAT-BB
Subject: R: [amsat-bb] RE: [ans] ANS 090 (Incorrect explanation of Arg.
Peri.)


Bob,

I don't aegree with your Argument of  Perigee explanation because
by definition ArgP is the angle between the ascending node and
perigee as MEASURED IN THE SENSE OF SATELLITE MOTION..

So if  the apogee satellite subpoint  occurs in the Northern hemisphere
after the equator crossing heading north,than the ArgP is greater than
180 deg

If  the apogee satellite subpoint occurs in the Southers hemisphere,after
the Equator crossing heading north,than the ArgP is less than 180 deg

Actually the AO40 ArgP is around 32 deg and at MA=128 or at apogee
the AO40 satellite subpoint  occurs in the southern hemisphere.

The equation to compute the SSP latitude at perigee is the following:

Lat(perigee) = arc sin ( sin i  x  sin ArgP )

where:

i = orbit inclination in deg

ArgP = Argument of perigee in deg

To get the apogee latitude needs only to change signe to it.

73" de i8CVS Domenico


----- Original Message -----
From: Robert McGwier <rwmcgwier@comcast.net>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, April 01, 2002 3:48 PM
Subject: [amsat-bb] RE: [ans] ANS 090 (Incorrect explanation of Arg. Peri.)


>
> Maybe a better drawing would have gotten Argument of Perigee
> correct.  Think of putting your pencil on the globe at the
> subsatellite point of any satellite at its perigee.  Follow
> the trace that is on the ground in the plane of the orbit
> as it cuts the surface of the earth.  If the earth were a
> sphere, this would be a circle and for our purposes, it is
> close enough. From the subsatellite point of the perigee to
> the subsatellite point of the apogee, 1/2 of the circumference
> of a circle is drawn on the globe or surface.  Also starting
> at perigee, you follow along the trace on the surface
> on the earth made by the orbital plane until you get to the
> subsatellite point where the satellite orbit cuts across the
> equator going from south to north.  This is the ascending
> node.   If the apogee subsatellite point occurs after this,
> then the argument of perigee is LESS than 180 degrees and
> thus the apogee is in the NORTHERN hemisphere and the
> perigee is in the southern hemisphere.  Similarly, if the
> subsatellite point for the apogee occurs BEFORE we have
> reached the ascending node, then the argument of perigee
> is greater than 180 but the apogee is in the SOUTHERN
> hemisphere and the perigee is in the northern hemisphere.
>
> If you project a line from the center of the earth, through
> the ascending node to the stars, then since the ascending
> node is on the earth's equator,  this will go out and touch
> the celestial equator.  The coordinates touched on the
> celestial equator, which is called Right Ascension and is
> given in hours by astronomers and degrees by orbital mechanics
> types is thus the Right Ascension of the ascending node.
> Zero degree Right Ascension means the satellite goes north
> across the equator at exactly the same spot the sun's "subsat"
> point goes north of the equator.
>
> I hope this throws me light on the subject than dust in the
> air!
>
> Bob
> N4HY
>
>
> -----Original Message-----
> From: owner-ANS@AMSAT.Org [mailto:owner-ANS@AMSAT.Org]On Behalf Of Dan
> James
> Sent: Monday, April 01, 2002 12:37 AM
> To: 'ANS Release'
> Subject: [ans] ANS 090
>
>
> AMSAT NEWS SERVICE
> ANS 090
>
>
> SB SAT @ AMSAT $ANS-090.01
> AO-40 UPDATE
>
> AMSAT NEWS SERVICE BULLETIN 090.01 FROM AMSAT HQ
> SILVER SPRING, MD, MARCH 31, 2002
> TO ALL RADIO AMATEURS
> BID: $ANS-090.01
>
> AO-40 ground control station W4SM recently reported on the latitude drift
> of AO-40's sub-satellite (apogee) point. The sub-satellite point of
latitude
> at apogee is a function of argument of perigee and the inclination of the
> satellite's orbit. The greater the inclination, the more the variability.
>
> Argument of Perigee (ArgP) needs a drawing to fully visualize, but it
> represents the angle between the perigee of the orbit and the point
> where the orbit crosses the equatorial plane headed north (ascending
> node). If ArgP = 0 or 180, then apogee is over the equator. When ArgP
> is less than 180 degrees, the apogee is in the southern hemisphere.
> When ArgP is greater than 180 degrees, apogee is in the northern
> hemisphere. For AO-40, ArgP is currently ~32 degrees, and increasing
> 0.3251 degrees each day. The duration of a full cycle is a little over
> 3-years.
>
> Inclination is a measure of the tilt of the orbital plane with respect to
> the Earth's equatorial plane. For satellites with highly elliptical orbits
> such as AO-40, the inclination is subject to significant solar/lunar
forces
> which tend to alter it in a non-linear fashion. AO-40's inclination has
> been increasing from about 5.2 degrees in mid-2001, to the current value
> of 7.3 degrees. Orbital element integration, factoring in solar, lunar,
and
> terrestrial forces show that inclination will continue to increase until
> it peaks at approximately 10.3 degrees in the spring of 2004. As AO-40's
> inclination and eccentricity change due to these forces, the rate of
> change of ArgP will fluctuate very slightly as well.
>
> From a northern hemisphere perspective, the low point for elevation of
> AO-40 at apogee will occur in the fall of this year. Apogee elevation will
> then improve, peaking 18 months later (in the spring of 2004).
>
> AO-40 is currently experiencing eclipse periods of nearly 30 minutes.
> During a recent eclipse period, both the magnetorquers and beacon
> were on when the IHU detected that the battery was running low,
> triggering the low-voltage software-which turned the S-2 transmitter off.
>
> For the last three orbits, the battery voltage during the final 45-seconds
> of  the eclipse period has dropped to the point where it has triggered
> S-2 shutoff.
>
> The shutoff is set very conservatively and the scheduling software has
> restarted the S-2 transmitter at MA-15 nominally each time. W4SM
> lowered the shutoff threshold by one count, and this should be
> sufficient to prevent further eclipse shutoffs for this cycle. If not,
> additional adjustments will be made.
>
> The current perigee eclipse cycle runs through June 19th. From
> August 13th to September 18th, AO-40 will experience much longer
> non-perigee eclipses that peak at 165 minutes duration. During these
> eclipses the S-2 transmitter will be turned off by the scheduler.
>
> [ANS thanks the AO-40 team for this information]
>
> /EX
>
>
>
> ----
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> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>




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