About accuracy of the INTERBALL spacecraft motion
calculation and orbital elements modification
Dear colleagues!
Let us draw your attention to the following:
The accuracy of calculation of both INTERBALL spacecraft (SC)
motion has been estimated at the IKI. For this estimation, we used
the initial orbital elements, determined at the Keldysh Institute
of Applied Mathematics (KIAM) of Russian Academy of Sciences
during the treatment of the inflight SC trajectory measurements.
The 30 days measuring intervals were used during the Tail probe (TP)
orbital data treatment, and 15 days intervals - during the Auroral
probe (AP) orbit determination. Obtained orbital elements are presented as a
state vectors, which include the SC Cartesian coordinates and velocity
components in the J2000 reference. These state vectors correspond
to the last points of measuring intervals. The orbit
determination mean square error of 30-40 km inside the measuring
intervals was estimated by KIAM.
During the forward in time extrapolation of SC motion outside the measuring
interval the additional error appear as the SC orbiting period error. The mean
square error of TP orbiting period is 13.6 s. In one month it leads to the mean
square errors of 550 km at the SC position near the perigee region, and of 80 km
near the apogee. For the AP, these values respectively are 0.91 s, 550 km and
145 km, but in the 15 days. The reason of these errors is the non-determined character of the orbit perturbation, caused by the attitude control system outside the measuring interval.
During the aposteriory orbital calculations, there is a possibility to
completely avoid the methodological errors described above, using the
interpolation of the SC motion inside the measuring interval only. For this
purpose, it is necessary to perform integration of the SC position backward in
time. In this case we obtain as a result the accuracy of SC position calculation
of 30-40 km. This accuracy corresponds to the one obtained during orbit
determination.
The same effect may be achieved using the standard procedure
of integration forward in time, but taking as an input the new
modified orbital elements produced at IKI, which now correspond
to the initial points of the measuring intervals. These modified
orbital elements were obtained by means of backward integration
of the both probes motion from now up to the launch.
The modified orbital elements, as well as the original ones,
are available now in the same format from IKI Satellite Situation
Center (SSC) Web site. Please use the following URLs:
The ballistic center at KIAM continue its efforts to improve
the orbit determinations accuracy by the after-flight reconstruction
of the INTERBALL orbits on the basis of the full set of the
performed orbital measurements. The accelerations caused by the
attitude control system and thus impossible to model before the
flight are now being included into consideration. This will
allow to significantly improve the accuracy of the INTERBALL SC
motion calculations and reduce the errors down to only several
kilometers.
The first results of the orbit reconstruction are being added to the test IKI
database. Presently, testing and comparisons of this new
dataset with the existing data are on the way. After this strict
analysis, if necessary, we will need to make a decision on the
aposteriory calculations modification.
Scientific Coordinator
of the INTERBALL Project
Prof. Lev Zelenyi
lzelenyi@iki.rssi.ru
25 July 1999
IKI Satellite Situation Center
Prof. Ravil Nasirov,
rnazirov@rssi.ru
Dr. Victoria Prokhorenko,
vprokhor@iki.rssi.ru