Description of Preliminary MAG-L Data Level 2.3
The preliminary MAG-L data are the first, official Ørsted magnetic
vector data released. The data are preliminary since they are not
yet decimated to the planned 1 Hz rate, and the data may still contain
some errors. Also, not all known disturbances have been corrected for.
The vast majority of the data have an accuracy of 7 nT or better, see
section on accuracy below.
The data are distributed in what is expected to be the format of the
final MAG-L data, but note that the preliminary data are not exactly
as described in the MAG-L Format
Description. See section below for
details of the differences. The last section of
this document contains a list of known errors and deficiencies of the
preliminary MAG-L data.
What's new with regard to Level 2.2
It is possible to fit a degree/order 13 field model to the vector data
with an rms misfit of about 5 nT, which indicates that the data are
within specifications. However, the 5 nT rms data noise (corresponding
to 9 nT rss) is not equally distributed over the three
components. Almost all the noise (8 nT rms) is in the component
perpendicular to both the bore-sight of the star imager and the
direction of the main field. (For nominal satellite attitude the
component of maximum noise is the radial direction at low latitudes
and to North-South-direction at high latitudes).
- The SIM data have now been corrected for small scale timing errors up to
approximately 15 ms. The SIM timing accuracy should now be better than
0.5 ms, with occasional errors of 1 ms.
The source of this noise in the vector data is probably the
intrinsic noise (of about 30 arcsecs rms) in one of the attitude
angles measured by the star imager. We are currently working on
methods of data pre-processing (filtering/smoothing of the attitude
data) to reduce the effect.
The user of the Oersted vector data should be aware of this noise
which might result in relatively large (10 nT amplitude or more)
point-to-point variations in one of the vector components.
The preliminary data consists of Stark filtered magnetic vector samples
given at the times of the Star Imager (SIM) attitude samples.
The SIM sampling rate is 1.135 seconds hence this is the basic
sample rate of the preliminary MAG-L data.
- The scalar magnetic data (F) from the OVH instrument
are not compensated for spacecraft disturbances, which is also noted in the
- There are no error estimates
(Bsigma) nor magnetic signal
intensity (Bac) computed for the
preliminary MAG-L data.
The timestamps of the SIM data have been intensively processed in
order to obtain the correct timing. The SIM timing accuracy should now be better
than 0.5 ms, with occasional errors of 1 ms. Finally, a static timeshift have
been applied to the SIM data based on theoretical and practical studies.
Temperature Correction of Vector Magnetometer
Due to the regular lack of house-keeping data, before July 14, 1999, only 60% of
the vector magnetometer (CSC) data before this date are properly temperature
corrected.This error is assumed to contribute less than a few nT.
Other Attitude Effects
The noise in the SIM data is an-isotropic causing a 4 to 7-times larger
error in the rotation around the boresight angle compared to the
rect-ascension and declination angles of the boresight axis itself
(as is also described above).
This contributes to up to 5 nT rms noise in the vector components.
Boom vibrations are causing aliasing effects in and possibly slight
degradation of the attitude. This is probably negligible at
night-side, but typically a few effects (10 nT peak-to-peak) per orbit
are seen at the day-side.
The Euler angles of the SIM-CSC transformation are expected to be
known with an accuracy of 20" or better, which corresponds to a
vector uncertainty of less than 5 nT.
Charge Particle counts
CPD counts have been
calibrated with the following factors to get UNITS of
counts/s cm2 ster.
|Pe,med and Pe,high
||0.0553 cm2 ster
|Pi,med and Pi,high
||0.273 cm2 ster
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Last modified 010924 by Lars Tøffner-Clausen.