# Re: Mean motion in old elements

From: Bjoern Gimle (b_gimle@algonet.se)
Date: Tue May 08 2001 - 10:34:36 PDT

• Next message: Leo Barhorst: "Obs Apr 28 + May 7"

```According to Alan's SatEvo -E, the equator crossing times are very close to
the Epochs given in the elsets, which I believe was the standards at that
time.

The AOP does not vary linearly, but over the entire period of 126.26 days it
has decreased by 322.2 degrees. So, it has completed 0.895 MORE anomalistic
orbits than nodal orbits.
So, if the anomalistic mean motion is 13.661 it should complete 1724.8
orbits perigee-to perigee, and 1724 orbits node-to-node.

The average NODAL mean motion over the 126.26 days is then
(110.37564562+365-349.11300369)/1724 = 13.65407831, which is quite close to
the MM of elset #25 (and one of the #22)

1 00424U          63349.11300369 -.00000170           00000-0 0   203
2 00424  80.4713  91.6706 0017354 275.3552  84.4470 13.66095625 60302
...
at/Z60S
1 00424U          64110.37564562  .00000443           00000-0 0   252
2 00424  80.4721 327.6648 0022611 313.1602  46.6510 13.65339186 77537

> OIG orbit elements include "mean motion" in revs/day.
> This is normally revs measured from perigee to perigee, but in
>  some old elements the data does not fit this definition.
>
> Example from
> http://hea-www.harvard.edu/QEDT/jcm/space/elements/disk1/00400/S00424
>...
> The epochs clearly show that the mean motion is about 13.653 as seen
> on 64020.60696148 and 64110.37564562 (and all later elements).
>
IMHO, 13.661 is correct perigee-to-perigee, as per current definition,
13.653 is OK node-to-node.

/Björn

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