Optimal filter
A state-of-the-art optimal sequential filter estimates orbits with measurement updates similar to a Kalman
filter and physics-based process noise models for the time update, enabling the calculation and propagation of
the realistic error covariance. The filter also calculates time-varying biases on the tracking system as well as
time-varying corrections to the force models. The filter uses a flexible state space, which allows operational
users to add or remove elements from the filter state space (such as satellites, facilities, biases, and
finite maneuvers) without forcing a filter restart, thus reducing operational risk. The filter also includes a
variable lag smoother, which provides a smoothed ephemeris as the filter runs. A fixed interval smoother can
post-process the filter output to create a more accurate and continuous definitive spacecraft ephemeris.
The result is an estimate of the orbit solution and biases with a realistic understanding of their accuracy.
The realistic error covariance is also used as a rigorous test for autonomous measurement editing. A detailed
discussion of these methods can be found in the
Orbit Determination Tool Kit white paper.
High-fidelity numerical integration
A Variation of Parameters numerical integrator is used with a full force model to propagate the orbit and form
the state transition matrix. The force model includes a precise spherical harmonic gravity model; solid Earth
and ocean tides; a Jacchia atmospheric drag model; the effects from solar radiation pressure; and the third
body effects of the Sun and Moon.
Orbit-parameter estimation
ODTK estimates orbit position and velocity for single or multiple satellites. It also calculates time-varying
measurement biases, transponder biases, corrections to the atmospheric density, ballistic coefficient and the
solar radiation pressure coefficient.
Realistic error covariance
ODTK calculates a realistic accuracy assessment of the estimated parameters. This error covariance of estimated
parameters can be used to predict possible close approaches with other spacecraft using
STK/CAT,
or to determine whether the orbit meets mission accuracy requirements. The position covariance can be displayed
as an error ellipsoid when used with STK.
Fixed interval smoother
The fixed interval smoother combines filtered state and covariance information to calculate a best post-fit
estimate of all estimates states. This capability will support programs with high accuracy post-fit requirement.
Maneuver processing
ODTK estimates orbits that include maneuvers. Individual thrusters or thrust segments can be modeled and
calibrated over multiple maneuvers, which is particularly useful for XIPSTM thrusters. Maneuver plans created
in
STK/Astrogator can be included to improve the filter performance. The fixed interval smoother can
reconstruct unknown maneuvers or calibrate planned maneuvers.
Initial orbit determination
The tool kit includes a Herrick-Gibbs algorithm to calculate an initial orbit from range and angle
measurements, a Gooding algorithm to calculate an initial orbit from angles-only measurements and a navigation
solution algorithm to calculate an initial orbit from GPS pseudo ranges. A least squares algorithm is also
included to refine this orbit, resulting in an orbit state suitable for filter initialization.
Tracking data simulator
ODTK generates simulated tracking data for a specified tracking schedule. An interface is provided so that
sensor constraints can be evaluated with STK and realistic access windows can be used in the simulator. The
controls for all random deviates are provided to the user—including random number seed—enabling Monte Carlo
analyses. The simulator is useful for pre-mission system design studies, as well as scheduling impact assessment during operations.
GPS orbit and clock estimation
The ODTK filter and smoother can be used to estimate orbit parameters and clock states for the entire GPS
constellation and estimate clock states for monitor stations. ODTK applies Brown’s Composite Clock model to
define GPS Time, provides the capability to steer GPS Time to maintain a fixed offset from UTC, and provides
processes to model clock events such as a phase reset or a frequency reset.
TDOA & FDOA orbit estimation
ODTK simulates and processes TDOA (time difference of arrival) and FDOA (frequency difference of arrival) data
to perform orbit determination for relay satellites, and can also be used to support geolocation of terrestrial emitter sources.
Mission flexibility
ODTK supports mission analysis for a whole range of orbit classes, from LEO to MEO to GEO and even to lunar and heliocentric orbits.
Automation and scripting points
Users can plugin custom scripts that are called during the estimation process. These scripts can be used to
notify operations personnel of problems with tracking data or completion of routine tasks. The tool kit can be
automated and integrated into ground systems using standard Component Object Model (COM) automation.
Measurement editing and thinning
ODTK provides users with the ability to use custom data editing attributes and rules to achieve significant
control over the orbit determination process.
Reporting and graphing
The tool kit can be used to create dynamic graphs that show filter performance while data is processed, as
well as dynamic reports that display tables of numerical data. After orbit determination, a wide variety of
data is available for reporting and graphing with user-customizable report forms. Static reports and graphs
can be exported in a wide variety of standard formats for use in other industry-standard analysis tools.