Re Ariane launch visible from E US

From: Michael Waterman (
Date: Wed May 08 2002 - 15:04:24 EDT

Answers to questions.

Kourou is in Guyane in S America: 5.2N,52.7W.
All Arianes are launched from there.

I am part of a team who wrote the flight program for
the on-board-computer on the Ariane 4 (and 1,2 and 3).
This does the navigation, guidance, attitude control
etc from a few secs before liftoff to a few minutes after
payload release. We also update the program, run a
standard set of tests for each launch, and analyse the
telemetry after the launch. We have nominal position and 
velocity in earth-centred inertial coordinates; for
the Spot launch from -9s to 1600s. These I converted
into the table in my 1 May email.

>From Bjorn Gimle (thanks) I deduce that a "trj" file 
is 4 columns with time(sec) Lat Long Height(km), so I will 
provide that in the future. 
Do the number of spaces between columns matter? 
Are further columns to the right permitted in a trj file?

I was (first time for me) at the "Jupiter" control centre
about 15km from the launch pad, and saw the launch
up to about liftoff+5min, when I suspect it went behind clouds,
but possibly the third stage (burning Lox & LH2) is less

Payload separation was scheduled at launch+19m 15s = 015101, but
the actual time depends on the time of cutoff (typical variation
5 or 10secs) and duration of the reorientation manoevres after 
stage 3 cutoff, and this depends on the angular rates at cutoff, 
and this depends critically on many things in the last few seconds 
before cutoff. Variations of 10 or 20 secs are common. I do not 
have actual flight details yet.

I see that many observers saw the fuel dump(s), and some saw
the rocket.
There is almost always some unused fuel when orbit is reached.
Fuel dumps are done to avoid later explosions of rockets; most
rockets do fuel dumps.

>From the mission data Lox, LH2 and in-orbit orientation propellant
Lox & LH2 dumping (main stage 3 propellants) starts about 5min 
after cutoff, and in-orbit orientation propellant dumping a few 
minutes later. Times uncertain by tens of secs as above.

Some parts of Spot are covered with shiny orange material.
Spot 5 is externally identical to its predecessors, the rocket
is identical.
Spot will do several small manoevres (I dont have details, we
just put it into orbit): so may be a minute or more away
from predictions for one or two weeks.

The IDEFIX payload (French amateur comms) will remain attached 
to the rocket stage.

For Spot the orbit that our guidance is trying to reach is
Apogee=812612.6  Perigee=793913.5 relative to earth radius 6378135m
Incl=98.744   Arg of perigee=120.0589
Longitude of ascending node=4.68847
This longitude is relative to the longitude of the launch site
(-52.77583) at the time of platform release. Platform release is
when the navigation reference is defined = H0-9s = liftoff-13.4s.
For most missions the longitude of ascending node has the same
value whenever the launch occurs, but for sun-synchronous missions
(like this one) the longitude is fixed inertially: so that if the
launch is 10min late then the trajectory over the earth is more
West, but the orbit has the required RA of ascending node.
Although the navigation uses the J2 gravity coefficient (equator
bulge) the guided orbit above is calculated from position & velocity
but no J2 (I think this is called an "osculating" orbit). 

The guidance optimises the point at which the rockets reaches the
required orbit, so the mission data does not have a required
time of equator crossing (or equivalent time).

The actual orbit will be slightly different, mostly due to
errors in the navigation reference (laser gyros & accelerometers),
typically by about 1m/s. Fuel dumps may affect the orbit of the rocket
by a similar amount, but not the orbit of the payload. (work) (home: not checked very often)

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This archive was generated by hypermail 2b29 : Mon May 20 2002 - 18:25:35 EDT