Range rate drift modifier

Orekit usage
1

Hi guys,

In my orbit determination software, I am using the ‘Bias’ modifier for my range rate measurements. Actually, measurements are doppler measurements converted as range rate, so that I could use already implemented classes.

I would like to also estimate a linear drift, on top of the bias, during a satellite pass over the ground station.
What would be the simplest way to achieve that ?
Thank you

2

Hi @Anthony-C31

Do you want to estimate a clock drift?

Best regards,
Bryan

3

This drift is initially due to the onboard clock so I guess yes

4

Let satellite be the ObservableSatellite used during the orbit determination process (i.e., the one you set when you initialize the RangeRate Measurement). Once, the satellite object initialized, you can solve the satellite clock drift during an orbit determination by doing.

satellite.getClockDriftDriver().setSelected(true);

Just note that doing this the initial value of the drift will be 0.0. You can override it by using .setValue(newValue) and .setReferenceValue(newValue) methods.

Best regards,
Bryan

1 Like
5

I am not sure how this will solve my problem for two reasons:

  • The frequency drift has to be estimated for each satellite pass over a ground station, on top of the bias
  • The clock drift is not linked to the range rate measurements, so I don’t think this will work at all
6

Hi @Anthony-C31,

Clock drift (and all other ParameterDriver objects) can be set by dates using their internal TimeSpanMap objects. If you add more span(s) to the driver then the estimation process will separate the driver values by span.

It is linked but only if the measurement is “two-way” (see here).
This is contrary to what’s done for the Range measurements, clock biases are only taken into account for 1-way measurements (see here)
@orekit_dev_team, is that an oversight, or did we set this up that way for a specific reason?

Cheers,
Maxime

1 Like
7

Thanks for your help,
I finally managed to do my own EstimationModifier class from the Bias class.
The only thing to do is to set the measurement partial derivative to “delta-T” instead of 1 for a bias. delta-T being the time difference with respect to a reference (beginning of the pass in my case, could be the center or any other date I guess)