- Q:
As far as I could learn from your homepage, the accuracy of the DEOS
precise orbits is about 5-6 cm radially for ERS-1.
Can you tell me the accuracy for ERS-2 (until Jan. 2000)?
-
ERS-2 orbit accuracies are the same as for ERS-1: about 5 cm radial and
about 15 cm in along- and cross-track direction. After Nov 1999 the quality
degrades from time to time in the event of solar flares (periods of high solar
activity).
We are working on improvements of the drag modelling, which will be
particularly benificiary for those periods.
- Q:
How much accuracy degradation can be expected for the fast-delivery
and the preliminary orbits compared to the precise orbits?
-
Any answer to this is, of course, a general one. There will always be cases when the
reality is different from general practise. So in general, I can say that our fast-delivery
orbits are remarkably close to our final orbits. The differences between the fast-delivery
and precise orbits have even reduced during the ERS-2 mission, and are now roughly 2 cm RMS
in radial, 8 cm in along-track and 6 cm in cross-track direction. Obviously, there are
exceptions, especially near orbital manoeuvres, when the lack of tracking data and time
constraints on the orbit production lead to larger errors.
More care has been spent on the preliminary orbits. They differ by approximately 1, 4 and 3
cm RMS in radial, along-track and cross-track direction.
- Q:
Since you do not provide precise orbits for ERS-1 in 1997, can you
estimate the error of the CEOS-orbits?
-
Yes, orbit determination for ERS-1 in 1997 is imposible. I'm not sure
how well the CEOS orbits do, but they will have a similar quality as earlier
periods. Generally, the errors in those orbits are between 2 and 10 metres.
- Q:
I need the "best" orbit information
because I do some interferometric analyses with these ERS data and
the accuracy of the baseline (vector between satellite acquisition
positions) is a very critical point. The difference between the DEOS precise
orbits and the information in the CEOS headers is several metres. Which orbit can I trust?
-
The orbit information stored in the headers is a very rough guess of
where the satellite is. As far as I have been told it is based on the
propagation of the state vector (position and velocity) at the latest
ascending equator crossing. And even that state vector is based on a
very crude orbit determination. Needless to say the orbit in the header
can be wrong by several meters.
The DEOS orbits are accurate to about 4-5 cm in vertical and 10-15 cm in
horizontal directions (both cross-track and along-track). Some of this
error is, however, geographically correlated. This means that it is
repeatible from cycle to cycle (and will be the same for ERS-1 and
ERS-2 along the same gound-track). So this part (which can de several
centimeters) cancels in the orbit differences that you compute for
InSAR applications.
- Q:
What happens in getorb when there is a manoeuvre?
-
All ODRs are continuous in the sense that there are no gaps. When there
is an in-plane manoeuvre, we calculate right through them. This ensures
also a continuous orbit in the sense that there are no magic jumps.
However, when there is an out-of-plane manoeuvre (every 2-3 months) we
can not calculate through it. That is not so much because we can't
handle accelerations in out-of-plane directions, it is simply that the
delta-V's are so large and uncertain that the prerequisit for linear
least squares estimation (small corrections) fails.
Thus we cut the start and stop the orbit computation at out-of-plane
manoeuvres and then paste the two pieces together to form one ODR.
There will be a discontinuity at that point, but the interpolation
routine is able to go through that, although it will produce worse
orbits around that point.
Secondly, there is generally no tracking data between the out-of-plane
manoeuve and the next in-plane manoeuvres (a few hour later). In
between the orbit is seriously degraded, but since the altimeter is
often switched of during that period, this is not always a problem.
Look at the manoeuvre files for a history of
ERS-1 and
ERS-2
manoeuvres.
- Q:
The getorb routine sometimes says "outside precise range". What does this mean?
-
the file you want to use for the interpolation. If you specify in the
getorb call the directory name instead, it should automatically jump to
the appropriate ODR file when it gets "outside precise range".
This, by the way, only means that you request the interpolation of the
orbit outside the middle 50 or 51 revolutions (roughly 3.5 days) of the
ODR file. The ODR file contains 5.5 days of orbits, but only the middle
part should be used, since the outer limits get less accurate. It's the
free end of the solution, like a plank suspended over a couple of
trusses. The ends that stick out are much more flexible than the inner
part.
- Q:
How good or bad are the orbits "outside precise range"?
Would they have slope errors of greater than 2 microradian?
-
If you only go slightly beyond the middle 3.5 days, you're fine. Its
basically the outer 6 hours or so that get slightly less accurate, but
not even close to the 2 microradian level (even a 1 metre orbit error is only
0.025 microradian).
