# How early?

dmbrierley@taz.dra.hmg.gb
Thu, 5 Feb 1998 09:40:02 +0000

```Dear SeeSaters,

For those of you who have been trying to find Molniya 3-16, with its
extremely large drag term as in the elset below, I recommend the
following handy rules for estimating how early it might be.  You need
to look at the values of ndot and n (.29301886 and 5.51805563
respectively in the elset).

1) An error in ndot of .01 causes a satellite doing 14.4 revs/day
(n=14.4) to be 1 minute early/late after 1 day.

2) An error in ndot of .01 causes a satellite doing 14.4 revs/day
(n=14.4) to be 4 minutes early/late after 2 days, 9 minutes early
after 3 days, and pro rata.

3) If the satellite is doing only 7.2 revs/day it will be twice as
early/late, and pro rata, i.e. if n=X revs/day, multiply the result
by 14.4/X.

4) With a very low perigee object, variations in air drag are
typically within +/-10 per cent - unless the Sun is very active, so
start by assuming a 10 per cent error in ndot.

Applying the rule to the elset below, you could expect the Molniya to
be early/late by (.0293/.01)*(14.4/5.518) minutes after 1 day.  This
equates to as much as 7.6 minutes, and no less than 30 minutes after
2 days.  Clearly it's vital to use the freshest elset you can find.

However when the apogee is as high as 14219 km, the perturbations
caused by the Sun and Moon can cause the perigee to rise or fall
quite rapidly, and ndot can change very rapidly, causing confusion in
NORAD's tracking system.  If the true value of ndot is really twice
.29301886, the satellite will be 76 minutes early after a day.

Observers, be warned.....

Molniya 3-16                                     14219 x 80 km
1 12512U 81054A   98032.88694387  .29301886  22688-1  15135-2 0 90580
2 12512  62.1634  89.8394 5225721 272.5383  31.4297  5.51805563123774

David Brierley
Malvern, Worcestershire, UK
Station 2675, 52.1358N, 2.3264W, 70m
```