Hi Orekit community! I’m using Orekit library to develop an Orbit Determination and Time Synchronisation (ODTS) prototype for a GNSS constellation.
The algorithm process is as follows:
1- Configuration of the satellites is made using nav messages at AbsoluteDate 2011-03-07T01:00:00.000, corresponding with the first epoch of estimation.
2- Configuration of the ground stations is also made and all observables (from 2011-03-07T00:00:00.000 to 2011-03-07T23:59:59.000) from the stations are processed.
3- A BLSEstimator is built with the propagator builders configured at current epoch.
4- Observables are added to the estimator, from the start of data date (2011-03-07T00:00:00.000) to the current epoch date (in this case 2011-03-07T01:00:00.000).
5- Parameters are estimated using Gauss Newton optimizer, converging in 2 iterations.
6- Epoch date is updated shifting the latest one by 10 minutes. Steps from 3 to 5 are repeated using the new epoch date.
Once the process I am following has been explained, I would like to
ask you some questions:
First of all, regarding the arc of observations, I have set a maximal length of 3 hours, so that if we are placed in epoch date 2011-03-07T04:20:00.000, for example, all observations before 2011-03-07T01:20:00.000 must not be considered. And the question is, is there any way to erase old not interesting measurements already included in the estimator? I tried by searching these old measurements and calling the method ‘ObservedMeasurement.setEnabled(false)’, but then at the time of estimation there is a problem of matrix dimensions from the LMS.
By solving this problem, I would be able to create an estimator at first epoch and, then, I would only need to add current epoch date measurements instead of adding all measurements from the start to the end of the arc at each step. In conclusion, it would be really useful.
Then, I would like to know how the derivatives of the parameters to estimate are calculated. Since I created my own EstimationModifiers, I’m afraid that this might perturbate the parameter derivatives. Besides, regarding the fact that I create a new BLSEstimator at each epoch (for the moment, until the first question is solved), does it have any impact on the derivatives calculation?
Thanks in advance,