The primary cause of this discrepancy was that one piece of ground software supplied by
Lockheed Martin produced results in a
United States customary unit, contrary to its Software Interface Specification (SIS), while a second system, supplied by NASA, expected those results to be in SI units, in accordance with the SIS. Specifically, software that calculated the total
impulse produced by thruster firings produced results in
pound-force seconds. The trajectory calculation software then used these results – expected to be in
newton-seconds (incorrect by a factor of 4.45)
[15] – to update the predicted position of the spacecraft.
[16]