Hi @MaximeJ and @petrus.hyvonen,

thank you so much for your replies, now that line works! And I am finally able to run the entire code.

To test the code, I firstly set the estimated values in the *theoreticalEvaluation* method to be equal to the observed values. As we imagine, this returns the orbit we give in input. However, this happens only if the number of measurements is <= 4. In fact, if I consider > 4 measurements I get an **org.orekit.errors.OrekitException: unable to solve: singular problem**.

If instead I do not put the estimated values to be equal to the observed ones (i.e. considering the true estimated values) I get either the aforementioned exception or a non existing orbit (e.g. a non valid value for the true anomaly). What could be the problem here? I have followed the Satellite Laser Ranging Orbit Determination written by @yzokras so I guess the problem is in my extension of the of the *PythonAbstractMeasurement* classā¦

Here it is the extended class: the difference between my class and *Range* is that the measurement consists in the distance between the satellite and the Solar System Barycentre (SSB) along a given unit vector *n* . Since I am considering an ICRF frame (centered on the SSB), I have defined my measurement as *numpy.dot(sat_position, n)* = n1*x + n2*y + n3*z i.e. the dot product between the satellite position and *n*. Notice that the vector of derivatives of the measurement with respect to x, y, z, vx, vy and vz is [n1, n2, n3, 0, 0, 0] i.e. the three velocity components do not impact on the āistantaneousā range measurement.

```
class NewRange(PythonAbstractMeasurement):
def __init__(self, date, observed, sigma, base_weight, satellite):
super().__init__(date, observed, sigma, base_weight, satellite)
def theoreticalEvaluation(self, iteration, evaluation, states):
state = states[0]
pvaDS = super().getCoordinates(state, 0, 6)
n = [np.sin(90-22.0144)*np.cos(83.6292), np.sin(90-22.0144)*np.sin(83.6292), np.cos(90-22.0144)]
sat_position = [state.getPVCoordinates().getPosition().getX(), state.getPVCoordinates().getPosition().getY(), state.getPVCoordinates().getPosition().getZ()]
pSSB = FieldVector3D(Gradient.constant(6, 0.0), Gradient.constant(6, 0.0), Gradient.constant(6, 0.0))
vSSB = FieldVector3D(Gradient.constant(6, 0.0), Gradient.constant(6, 0.0), Gradient.constant(6, 0.0))
aSSB = FieldVector3D(Gradient.constant(6, 0.0), Gradient.constant(6, 0.0), Gradient.constant(6, 0.0))
pvaSSB = TimeStampedFieldPVCoordinates(state.getDate(), pSSB, vSSB, aSSB)
# CREATE THE ESTIMATEDMEASUREMENT OBJECT
new_range = NewRange(self.getDate(), float(self.getObservedValue()[0]), float(self.getTheoreticalStandardDeviation()[0]), float(self.getBaseWeight()[0]), self.getSatellites())
estimated = EstimatedMeasurement(new_range, iteration, evaluation, [state], [pvaDS.toTimeStampedPVCoordinates(), pvaSSB.toTimeStampedPVCoordinates()])
# SET THE ESTIMATED VALUE FOR THE MEASUREMENT
estimated.setEstimatedValue(float(np.dot(sat_position, n)))
# SET THE DERIVATIVE OF THE ESTIMATED MEASUREMENT WITH RESPECT TO THE CURRENT STATE (CARTESIAN)
derivative = [float(n[0]), float(n[1]), float(n[2]), 0.0, 0.0, 0.0]
derivative_jarray = JArray('object')(1)
derivative_jarray[0] = JArray('double')(derivative)
estimated.setStateDerivatives(0, derivative_jarray)
return estimated
```

Do you see something strange in my code? Another thing I was thinking about is that maybe I do not have enough measurements (the sigma here is very large, between 1 and 3 km) or maybe I have to divide the *range_weight* variable (used in the laser ranging example ) by the number of observations? I havenāt understood if it is done automatically or notā¦

Thank you in advance.

Samuele

P.S. I am giving the true orbit as a guess orbit in these first experimentsā¦