The following options are available for controlling filter or simulator start and stop times and process noise updates:
Process Control  

Option  Description  
StartMode  Specify whether this is the Initial run, a scheduled Restart or an AutoRestart.  
(Re)Start time options 
Depending on the selection made for StartMode, one of the following will appear:


Direction  Filter direction (in time)
= Forwards  Backwards (default = Forwards) when "Backwards" is selected the following are disabled:
when "Backwards" is selected times are "like Smoother" timing


StopMode  Select one of the following:


TimeStep (Simulator only) 
Defines the minimum allowable time between measurements, as well as the grid on which ephemeris will be written if requested. The time step defines the measurement time grid in the absence of custom tracking intervals.  
ProcessNoiseUpdateInterval (Filter only) 
Enter a time value in the selected time unit to specify how often process noise is to be updated between measurement passes (see note below). The filter will output data on a union of this uniform time grid and the measurement time grid. The time update grid ensures that process noise is added between measurement passes. This attribute defines the uniform grid used for ephemeris output, if that option is selected. If the Scenario.Units.DateFormat attribute is set to UTCG, the time upgrade grid will be on an even UTC grid; if it is set to GPSG, it will be on an even GPS grid. 

MeasurementProcessingMode (Filter only) 
Measurements may be processed in either the scalar mode or the simultaneous mode during filter operation. In the scalar mode, a separate measurement update operation is performed for each measurement processed at a given time. In the simultaneous mode, all measurements at a single time are processed together in a single measurement update operation. The simultaneous mode typically improves the run time performance of the filter, but has been seen to exhibit some instability during the filter initialization period if the initial uncertainties are large. You can switch from one mode to the other when running the filter in either initial mode or from a restart record. (See 2nd note below)  
ModelMeasOnlyWithRefEphem (Filter only) 
If set to true (default is false), running the filter will (only) process the measurements, computing residuals and the corresponding measurement error variances using Satellite Reference Trajectories (*.e files) for orbit and orbit covariance values. No estimation will be performed. The measurements are modeled using the nominal measurement bias and modeling values. Measurement error variance values only consider orbit errors and measurement white noise. For all satellites in the Filter.SatelliteList the Satellite.EstimateOrbit flag must be set to false and a Reference Trajectory file must be input. Selecting this option will cause the Filter StartMode to be set to Initial, the OptionalSolveForParms.MeasBiases to be set to false, the SmootherData.Generate flag to be set to false, and the STKEphemeris.DuringProcess.Generate flag be set to false. Therefore turning this option off (after selection) will require the user to reset these flags. The measurements will be modeled by interpolating the reference trajectories and propagating the reference covariance as necessary to the time of interest. It is recommended that the user select a smoothed reference ephemeris instead of a filtered reference ephemeris to avoid interpolating across possible discontinuities caused by measurement updates. If the reference trajectory file does not contain velocity covariance then the covariance will be propagated assuming a velocity covariance of zero. If the reference trajectory file does not contain any covariance, then a zero orbit covariance is assumed. Note: Using this feature allows a reference ephemeris with covariance to be used in residual calculation and residual covariance calculation. The attribute will cause the filter to stop saving restart files and turn of smoother rough file generation, among other actions. Each action yields a Message Viewer message, but the user is required to reset quantities after making a filter run. Warning: Manual setting of this field can lead to errors in filter settings. It is recommended that the user employ ResidualsVersusReference.htm, which will automatically remember and restore settings that were changed by this attribute. 

NumOrbitsAllowedForEpochAlignment  Specify the maximum number of revolutions that a satellite orbit may be propagated to align the satellite initial state with the start time of the process. This setting is also used to determine the allowable difference in epochs between a spacebased GPS receiver clock state epoch and the initial condition epoch of its host satellite.  
GlobalAtmosphericDensityEstimation (Simulator only) 
Controls for simulating deviations in selected atmospheric density model parameters.  
EnableVLS  If true and the filter has one or more child VLS objects with their ProcessControl.Generate flag set to true, then those VLS will be executed in conjunction with the Filter execution.  
MeasUpdateOrbitStateRepresentation (Filter only) 
Select the coordinates to be used when updating the nonlinear orbit trajectory during the measurement update operation. Choices are Cartesian coordinates and equinoctial elements. The state update is always computed in Cartesian coordinates but can be linearly transformed to an update in equinoctial elements. For closed orbits where the motion is dominated by the twobody dynamics the equinoctial option typically leads to improved filter accuracy and an increased convergence region. Cartesian coordinates can be used for all classes of orbits including open orbits. Note: This option is currently incompatible with variable lag smoothing. 

DynamicStateSpace (Filter Only) 
The filter can be configured to add/drop particular types of states as they are needed. This feature can be used to improve the processing speed of the filter, improve the processing speed of fixed interval and variable lag smoothing and reduce output file size which results in faster reporting and graphing operations. For each type of state, select between Manual (all possible states are included in state space) and Automatic (states are added when needed at the initial epoch and restart points and dropped when not needed). The following table describes the Automatic mode behavior.


InstantManeuverEstimation (Filter only) 
Corrections to modeled instant maneuvers can be estimated during the filter process. These estimates are facilitated through the use of fixed epoch smoothers where two smoothed states are estimated at the time of the instant maneuver. The first smoothed state does not contain the velocity change associated with the maneuver and the second smoothed state, at the same epoch, does contain the velocity change. The maneuver estimate is derived by differencing of the two smoothed states. Maneuver estimates which have not completed based on the completion criteria listed below may still be reported at the end of the filter run, but will continue in a subsequent filter run which is initialized from a restart record. A maneuver estimate is considered to be complete when any of the associated completion criteria are met. The following options are available for the estimation of the instant maneuvers during the filter process.


HigherOrderCorrections (Filter only) 
Special algorithms to account for higher order effects are available to augment the ODTK optimal sequential filter. These algorithms consist of the Gauss 2nd order filter and two underweighting schemes: residual variance scaling and a covariance reduction limit. All higher order augmentations are specific to the ODTK Filter. These options for the mitigation of higher order effects should not be required in most normal circumstances. Second Order Gaussian Filter The capability to add in the additional terms associated with the Gaussian second order filter is accessible through the HigherOrderCorrections scope of the Filter attributes. Set the MitigationMethod to SecondOrderGaussian to expose the attributes. The Gaussian second order filter provides augmentation of measurement processing in terms of an adjusted residual and additional measurement deweighting and an augmentation of the time update in terms of an acceleration bias. Controls exist to allow for the inclusion of any or all of these augmentations. The selected second order effects can be included all the time or only when needed based on an automatic detection scheme. The inclusion of second order terms in measurement processing requires the computation of second order partial derivatives (Hessians) of the measurement models. These computations are provided analytically for the most commonly used measurement models in ODTK. Measurement Hessians are computed numerically for those measurement models which do not support second order derivatives analytically. The need to compute measurement Hessians results in slightly slower execution times. The second order effect on the time update is accounted for through the computation of an acceleration bias on all orbit substates. The complete acceleration bias representation would require the computation of Hessians for all acceleration models involved in the trajectory propagation. The approach taken in ODTK is to only compute the Hessian of the twobody acceleration since all other accelerations are typically at least 3 orders of magnitude smaller than the two body acceleration. Note: In all cases examined to the time of this writing, the inclusion of second order augmentation in the measurement update, specifically the measurement deweighting, has proven to be much more important than the inclusion of second order effects in the time update. Underweighting The capability to augment the linear residual error variance with additional underweighting is accessible through the HigherOrderCorrections scope of the Filter attributes. Set the MitigationMethod to Underweighting to expose the attributes. There are two underweighting schemes implemented: residual variance scaling and a covariance reduction limit. The concept behind underweighting is to decrease the corrections made during measurement processing when the effects of nonlinearity are significant. The main effect of underweighting is to slow the contraction of the covariance which can help prevent the filter from "over correcting" and subsequently rejecting valid tracking data.

NOTE: If you set the process noise update interval too large, problems may arise in subsequent calculations.
NOTE: The ODTK filter processes all measurements up to and including a restart record. Upon restart, ODTK does not process measurements at the restart time. In Scalar mode only measurements at exactly the restart time are not processed; in Simultaneous mode measurements at the restart time + epsilon are not processed where epsilon is 0.01 sec.
ODTK 6.5