It was then supposed to go through a period of aerobraking, skimming though the upper atmosphere for weeks to gradually lose speed.īut it didn't. Once there, MCO was set to fire its main engine for the orbit insertion burn that would put it into an elliptical orbit around Mars. Nevertheless, the spacecraft made it to Mars in September of 1999, nine and a half months after launch. This means that every impulse measurement done by the Angular Momentum Desaturation software underestimated the effects of the thrusters on the spacecraft by a factor of 4.45 (1 pound of force is equal to 4.45 Newtons).
So metric onboard the spacecraft, imperial on the ground. The ground computers, on the other hand, used software that used imperial pound-seconds to measure impulse. The Software Interface Specification that was used to define the Angular Momentum Desaturation file on board the spacecraft relied on the widely used-and metric-Newton-seconds as the units for measuring impulse (which is defined as force multiplied by time). It turned out that the two systems were using different units of measurements. The numbers from the spacecraft and the ground computers didn't match. The MCO team didn't start using those ground computers' data to adjust the spacecraft's trajectory until four months into the mission, but almost immediately it was apparent that something wasn't working right. The operations navigation team from JPL then used the information in that file to model the forces acting on the spacecraft and adjusted its trajectory by firing its thrusters as needed.Īfter every thruster firing, the change in velocity was measured and calculations about its performance carried out both by the spacecraft and by support computers on Earth. The data is processed then placed into an Angular Momentum Desaturation file.
First, spacecraft data is transferred to the ground and processed by a software program in the case of MCO the program was called SM_FORCES (small forces).
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