# Re: When does the orbiter significantly change its orbit?

From: Ed Davies (edavies@nildram.co.uk)
Date: Mon Aug 20 2001 - 17:05:42 PDT

• Next message: Chris Olsson: "Re: Geodetic precision"

```Walter Nissen wrote:
> Can anyone point to an explanation for the following, particularly how
> one detects the time of the major impulses?  Or provide such an
> explanation?
> ....
> IMPULSIVE TIG (GMT)   M50 DVx(FPS)      LVLH DVx(FPS)      DVmag(FPS)
> IMPULSIVE TIG (MET)   M50 DVy(FPS)      LVLH DVy(FPS)      Invar Sph HA
> DT                    M50 DVz(FPS)      LVLH DVz(FPS)      Invar Sph HP
>
> -
>
> 232/16:11:26.402           0.5              -6.0              6.0
> 009/19:01:12.420          -5.1               0.0              218.9
> 000/00:00:24.839           3.0               0.1              205.4
>
> 232/20:40:21.863          -3.6             -11.0              11.0
> 009/23:30:07.880          -6.3              -0.0              218.5
> 000/00:00:15.761           8.2               0.1              199.7

OK, let's guess.  Presumably "GMT" stands, rather quaintly, for Greenwich
Mean Time - so they probably really mean UTC (which can be up to 0.9 secs
different from GMT but let's not start another pedant war).  So the impulse
probably happened (presumably started) this year on day number 232 (i.e.,
20th of August) at just about tea time.

MET will mean mission elapsed time - so this is 9 days, 19 hours and a
minute or so after lift off.

I'd guess DT is delta time, the length of the burn, so the first burn
presumably lasted nearly 25 seconds and the second just over 15 and a
half seconds.

The other columns presumably specify the magnitude and direction of the
burn in three different reference systems.  The first two are cartesian
magnitudes (in feet per second).  I've seen some reference to the M50
frame somewhere before - is it an Earth fixed or inertial frame or what?
LVLH refers to the spacecraft orientation relative to its orbit direction -
I can't remember exactly what it stands for.

Applying pythagorous (sp?) to the first two columns (sum the squares
then take the root) gives total velocity changes of about 6 and ll
feet per second which match well with the delta V magitudes in the
first row of the last column.

Presumably the other numbers in the last column are the spherical
coordinates ("lat/long") relative to some frame of the impulse
direction.

How's that so far?  Maybe a lot of us would learn something from the
gaps above being filled in.

Ed.

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