The simulator attempts to create a set of realistic measurements by applying different deviations to initial conditions of satellites and measurement biases, and by adding noise to measurements based on the defined measurement statistics for each station. ODTK provides an ability to control which of these deviations are applied. If no deviations are applied, the resultant measurements should provide zero residuals when run through the filter (depending on the precision of the tracking file to which the measurements are saved).

The following table lists the error modeling options. In addition, you can assign scaling factors to the attributes that you choose to deviate.

In addition to opting whether or not to deviate a value, you can apply a scale factor to the deviation. The default ErrorScaling value is 1, or 1.00000. This is a sigma multiplier. When a value is deviated (see above), the sigmas normally used to create the initial deviation are multiplied by the error scaling factor.

As an illustration, suppose you want to put a lot of noise into the simulated tracking data to see how the filter will respond given "optimistic" measurement biases. For example, suppose you want to increase all measurement biases by 50%. To do this, simply change the ErrorScaling factor in the simulator to 1.5, and all measurement biases used by the simulator will be increased 50%. After you run the simulator, the Sim Tracker Initial State report will show the sigma values actually used in the simulation.

**NOTE:** The satellite's drag model attributes
include DensityCorrSigmaScale. This scale is applied when using the
simulator as well.

The user has the option to steer the simulated stochastic sequences for some state parameters so as to keep the "total estimate" for that parameter positive. This includes the satellite density, the satellite ballistic coefficient, the satellite solar pressure (Cp and K1), the transponder bias, and laser retroreflector delay parameters. The "total estimate" means the addition of the "constant" value with the current error estimate. Steering the sequence to be positive makes sense physically, but steering also results in a sequence that may be non-random and possibly violates the statistical specifications of the sequence itself, complicating one's ability to interpret the results based on the simulation.

Prior to this option being added to ODTK, these parameters were always steered to be positive. With the addition of this option the default for new scenarios is to not steer these parameters. Scenarios built prior to this option will still default to enable the bounds checking. Note that the steering algorithm assumes that the initial total estimate is positive. If it is not positive odd sequences may be simulated.

ODTK 6.5