HYDERABAD: The Indian Space Research Organization ( Isro ) has come up with an explanation on the Mars Orbiter Mission's fourth orbit-raising operation that could not take the apogee of the spacecraft to the intended one lakh km."In the fourth orbit-raising operation conducted this morning (November 11, 2013), the apogee (farthest point to Earth) of Mars Orbiter Spacecraft was raised from 71,623 km to 78,276 km by imparting an incremental velocity of 35 metres/second (as against 130 metres/second originally planned to raise apogee to about 100,000 [1 lakh] km). The spacecraft is in normal health. A supplementary orbit-raising operation is planned tomorrow (November 12, 2013) at 0500 hrs IST to raise the apogee to nearly 1 lakh km," Isro said.During the orbit-raising operations conducted since November 7, 2013, Isro has been testing and exercising the autonomy functions progressively, that are essential for Trans-Mars Injection (TMI) and Mars Orbit Insertion (MOI).During the first three orbit-raising operations, the prime and redundant chains of gyros, accelerometers, 22 Newton attitude control thrusters, attitude and orbit control electronics as well as the associated logics for their fault detection isolation, and reconfiguration have been exercised successfully. The prime and redundant star sensors have been functioning satisfactorily. The primary coil of the solenoid flow control valve was used successfully for the first three orbit-raising operations.Isro said during the fourth orbit-raising operations held today (November 11, 2013), the redundancies built-in for the propulsion system were exercised, namely, (a) energizing the primary and redundant coils of the solenoid flow control valve of 440 Newton Liquid Engine and (b) logic for thrust augmentation by the attitude control thrusters, when needed."However, when both primary and redundant coils were energized together, as one of the planned modes, the flow to the Liquid Engine stopped. The thrust level augmentation logic, as expected, came in and the operation continued using the attitude control thrusters. This sequence resulted in reduction of the incremental velocity," Isro explained.While this parallel mode of operating the two coils is not possible for subsequent operations, they could be operated independently in sequence, Isro said.