# Re: New Iridium orbits

Jim Varney (jamesv@softcom.net)
Fri, 15 May 1998 10:45:31 -0700

```Bjoern Gimle <bjorn@tt-tech.se> wrote:

> Jim Varney <jamesv@softcom.net> wrote:
>
> >
> Keith Stein <kstein@launchspace.com> wrote:
> >
> >> Delta/Iridium Nominal Orbit Data
> >>
> >>                                     SV-1 SEP         SV-2 SEP
> >> PERIOD (MIN):                       97.478           97.267
> >>                                     SV-3 SEP         SV-4 SEP
> >> PERIOD (MIN):                       97.172           97.113
>                                        SV-5 SEP         SECO-3
> >> PERIOD (MIN):                       97.107           92.734
> >> ...
> >(snip)
> My guess is that these periods are nodal. Since Iridium perigees
> precess about 3.6 degrees forward (correct me if I'm wrong on either
> of these assumptions) MM should be reduced by 0.01 from Jim's post:

Nope!  I didn't use the period to calculate the mean motion.  I used the
semi-major axis from Keith Stein's post
SV-1 SEP         SV-2 SEP
SEMI-MAJOR AXIS (KM):               6786.7975        7017.0787

and the equation

n = sqrt( mu / a^3 )   mu is Earth's gravitational constant

This is an energy equation and is not nodal; therefore, no MM adjustment
should be performed.

Ron Lee wrote:

> Thanks for the new elsets folks.  I ran MY elset (#98765) last night and
> compared the SECO-2 position against what mine predicted and there is a BIG
> difference.  Looking at the RAANs you see an eight degree difference. That
> is unacceptable and would cause a person to miss the objects.

Maybe one or more of the burns includes an out-of-plane vector to effect a
small plane change during ascent.  Your "launch pad" approach is only valid
if all of the booster impulses are in-plane.

Keeping the MM unchanged changes the perigee and apogee numbers to change
slightly.  Now we have all kinds of elsets to try out :)

Iridium SV-5 SEP                                         632 x 608
1 90005U 98999E   98135.95354167  .00000600 +00000-0  60000-4 0 00003
2 90005 086.579  265.2490 0017280 262.9760 193.2100 14.82899900000005
Iridium SV-4 SEP                                         634 x 607
1 90004U 98999D   98135.95064815  .00000600 +00000-0  60000-4 0 00000
2 90004 086.579  265.2510 0018820 241.5390 199.2790 14.82802449000003
Iridium SV-3 SEP                                         636 x 611
1 90003U 98999C   98135.94775463  .00000600 +00000-0  60000-4 0 00006
2 90003 086.579  265.2530 0017460 219.0050 206.4410 14.81905132000008
Iridium SV-2 SEP                                         638 x 618
1 90002U 98999B   98135.94486111  .00000600 +00000-0  60000-4 0 00004
2 90002 086.580  265.2550 0013920 194.3570 215.7060 14.80461050000009
Iridium SV-1 SEP                                         640 x 636
1 90001U 98999A   98135.93791667  .00000600 +00000-0  60000-4 0 00007
2 90001 086.583  265.2570 0003410 119.1820 253.8990 14.77260425000003

______________________________________________
Jim Varney
Member, Sacramento Valley Astronomical Society
www.softcom.net/users/jamesv/index.htm
```