# MA and Phase [was:Sorry...I wasn't paying attention]

• Subject: [amsat-bb] MA and Phase [was:Sorry...I wasn't paying attention]
• From: Margaret Leber <maggie@xxxxxxxxxxxx>
• Date: Tue, 08 May 2001 23:06:06 -0400
• User-Agent: Mozilla/5.0 (X11; U; Linux 2.2.16-22 i686; en-US; 0.7) Gecko/20010119

```RogerKola@aol.com wrote:

> Can someone recommend one or several tracking programs which directly
> indicate MA's?...do MA's have another name that I might be overlooking like
> Predict's use of "Phase"?

Phase and Mean Anomaly (MA) are basically the same thing, except that
"Phase" explicitly means in units of 0-255, and Mean Anomaly is
*supposed* to be in degrees.

But see the following explanation from

"Keplerian Elements Tutorial"

http://www.amsat.org/amsat/keps/kepmodel.html

"Anomaly is yet another astronomer-word for angle. Mean anomaly is
simply an angle that marches uniformly in time from 0 to 360 degrees
during one revolution. It is defined to be 0 degrees at perigee, and
therefore is 180 degrees at apogee.

"If you had a satellite in a circular orbit (therefore moving at
constant speed) and you stood in the center of the earth and measured
this angle from perigee, you would point directly at the satellite.
Satellites in non-circular orbits move at a non-constant speed, so this
simple relation doesn't hold. This relation does hold for two important
points on the orbit, however, no matter what the eccentricity. Perigee
always occurs at MA = 0, and apogee always occurs at MA = 180 degrees.

"It has become common practice with radio amateur satellites to use Mean
Anomaly to schedule satellite operations. Satellites commonly change
modes or turn on or off at specific places in their orbits, specified by
Mean Anomaly. Unfortunately, when used this way, it is common to specify
MA in units of 256ths of a circle instead of degrees! Some tracking
programs use the term "phase" when they display MA in these units. It is
still specified in degrees, between 0 and 360, when entered as an
orbital element."

Short answer is: "Phase" as presented by PREDICT and SatTrack is really

Next lesson: "squint angle"...or "off-pointing angle", for the
fastidious. A lesson I'm learning right now. :-)

73 de Maggie K3XS
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/____ICQ 7161096_(_/_(_/__AMSAT 32844____/ <maggie@voicenet.com>

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