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EXECUTIVE SUMMARY
OF THE
MicroCosm® SOFTWARE SYSTEM CAPABILITIES
The MicroCosm® state-of-the-art precision orbit and geodetic
parameter determination software system substantially improves upon and
fully implements the NASA GEODYN II, version 8609, precision orbit and
geodetic parameter determination software system.
The capabilities of MicroCosm® include:
Numerical integration of satellite equations of motion for orbit
prediction, Orbit state vector estimation, Determination of parameters used to model atmospheric drag, direct
solar radiation and Y-bias effects on the satellite orbit, Determination of measurement and time tag biases, tropospheric
refraction scale parameters, and station and satellite clock
polynomials, Determination of gravitational coefficients, Determination of tracking station coordinates, Modeling and estimation of polar motion and Earth
rotation, Modeling and estimation of solid Earth and ocean tides, Modeling of tectonic plate motion, Modeling of ocean loading effects on tracking station
locations, Modeling of point-mass Lunar, Solar and planetary
gravitation, Modeling of Earth precession and nutation, Processing of satellite tracking data for the determination
of the above parameters, and Simultaneous processing of multiple satellites per data arc and
multiple data arcs per job.
MicroCosm® uses Cowell's method for numerical integration of the
satellite equations of motion and the variational equations for the force
model parameters which are to be adjusted.
Cowell's method as implemented in MicroCosm® uses predictor-corrector
formulas for the equations of motion and corrector formulas for the
variational equations. Optional orders and time step sizes may be selected
for the integration process.
Since measurements are unlikely to occur coincidentally with the
integration steps, MicroCosm® uses an interpolation scheme for computing
the instantaneous satellite position, velocity and adjusted force model
partial derivatives. This interpolation scheme is a variation of the
integration corrector formulas and thus maintains the same level of
accuracy as the integrator.
All of the measurements modeled by MicroCosm® have geometric
relationships to the instantaneous satellite position and/or velocity.
Consequently, MicroCosm® uses these relationships to calculate
estimated values of the measurements based on the set of parameters
describing the motion of the satellite and the locations of the tracking
stations.
Since variations in any of these parameters will result in differences
between the observed and the calculated values of the measurements used,
dynamical relationships can be obtained which will allow the estimation of
these parameters. These relationships are simply the partial derivatives of
the measurements with respect to the parameter set to be adjusted.
The set of measurements modeled by MicroCosm® include:
Topocentric right ascension and declination of the
satellite, Laser, radar and GPS range (including single, double and triple
differences), Range rates or Dopplers (including single and double
differences), Radar altimeter range, East and north direction cosines, X and Y angles of the satellite relative to the tracking
station, Azimuth and elevation angles of the satellite relative to the
tracking station.
In addition to these instrument measurements, MicroCosm® will also
accept as data, instantaneous Earth-fixed Cartesian or inertial Cartesian
or Keplerian spacecraft ephemerides.
The data recorded at tracking stations contains certain known
systematic errors which can be removed and thus eliminated from the
estimation process. The systematic error corrections performed by
MicroCosm® are:
Conversion of all measurement times to ephemeris time
(ET), Correction of measurement times for electromagnetic propagation
delays, Precession and nutation of optical measurements to true of date
inertial coordinates, Modeling of annual aberration, diurnal aberration and parallactic
refraction effects on optical data, Corrections to range measurements for transponder delay, ranging
ambiguities and carrier phase cycle slips, Corrections for offsets of antennas and reflectors from the
spacecraft center of mass, Antenna axis displacement corrections to range and range rate
data measured from X/Y and hour angle/declination mounts, and Tropospheric refraction propagation delay and bending
effects.
Bayesian least squares estimation is used by MicroCosm® for
parameter determination. The batch mode of estimation is used and
iteration is performed both to facilitate automatic data editing and
because of the non-linearity of the orbit determination problem. Because
Bayesian estimation is used, MicroCosm® requires the input of a priori
estimates and uncertainties for all parameters to be determined in the
solution.
A partitioned solution of the normal equations is performed by
MicroCosm®. This permits the separation of parameters into a global
set and an arc set thus allowing the processing of more than one arc at a
time. MicroCosm® has also been designed to permit the simultaneous
inclusion in a single arc of up to 99 satellites. This capability permits
the modeling and estimation of the entire Global Positioning System
constellation of satellites.
- Sim_Soft - Simulate GPS, laser or radar data,
- Res_Edit - Eliminate edited residuals,
- FormSP3 - Put trajectory in GPS SP3 format,
- NavMsg - Convert SP3 to RINEX Navigation,
- Orb_Diff - Difference trajectory files,
- Eclipse - Detect eclipse events,
- L2M & M2G - Laser data reformatting, and
- Site_Evo - Tectonic & tide displacements.
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