Re: Favored decay latitude?

scribble (
Thu, 3 Dec 1998 11:40:21 +1100


The Orbit of the sat may be 'roughly circular' but the earth is not.  The
Earth experiences 'Flattening' due to it's rotation.
Flattening (f ) is defined as the difference in magnitude between the
semimajor axis (a) and the semiminor axis (b) divided by the semimajor axis,
or f = (a - b)/a. For the Earth the semimajor axis and semiminor axis differ
by about 21 kilometres, and the flattening is about one part in 300.  This
would put the sat at's it's lowest altitude on equator crossings, unless
it's orbit was sufficiently eccentric to counteract the 'rise' of the
Earth's surface as it approached the lower latitudes.

Furthermore, I would guess that, satellites with a high inclination or
retrograde orbits would experience higher drag over equatorial regions as
the difference between geodetic & satellite velocities would be at maximum.
Therefore it should follow the same would be true of atomospheric &
satellite velocities.

-----Original Message-----
From: Mark Hanning-Lee <>
To: SeeSat-L <>
Date: Thursday, 3 December 1998 10:19
Subject: Favored decay latitude?

>I've noticed that many reentry predictions on the list seem to predict
>decay soon after an equator crossing.
>Why would a sat in a roughly circular orbit tend to decay at any
>particular latitude? Does a decaying sat feel significantly more drag at
>low latitudes? If so, why is the atmosphere number density higher there;
>is it just the greater insolation?
>(I appreciate that a sat in a very eccentric orbit decays near perigee,
>which makes sense to me.)
>Thanks for your replies, Mark