mod: if bstar term goes negative using estimate_batch_orbit_sgp4_bsta…

…r(), automatical use estimate_batch_orbit_sgp4() instead with no optimisation of bstar drag term.

BatchEstimator.py

@@ -591,7 +591,8 @@ def sgpsolver(para,tle,time):
 
     def estimate_batch_orbit_sgp4_bstar(self):
         """!@brief  Calculates the batch orbit estimates using SVD from a set of position and
-                    velocity measurements, where the SGP4 propagator is used.
+                    velocity measurements, where the SGP4 propagator is used. Optimises all six
+                    orbital elements as well as the bstar drag term.
             
             @return success result or return error code of integration tool
         """
@@ -680,11 +681,13 @@ def sgpsolver(para,tle,time):
 
             iter_count = iter_count + 1
 
-
-        self.t0 = self.time[0]
-        self.para = y[0:6]
-        self.bstar = y[6]
-        self.write_to_tle()
+        if y[6] < 0:
+            self.estimate_batch_orbit_sgp4()
+        else:        
+            self.t0 = self.time[0]
+            self.para = y[0:6]
+            self.bstar = y[6]
+            self.write_to_tle()
 
         return 1