Entry Refsmmat

With the entry PAD recorded, the guidance platform could be realigned to an orientation that was suitable for the entry. Around the time the PAD was being passed to the crew, flight controllers were granted direct uplink access to the onboard computer's memory to change the numbers that defined the REFSMMAT.

A brief recap of the REFSMMAT might now be appropriate. The computer's idea of the direction in which the spacecraft was pointed was always given with respect to the orientation of the stabilised platform inside the IMU. However, to make any sense, the platform had to be aligned to some known reference - one that was related to the universe around the spacecraft. The REFSMMAT numbers defined such an orientation in space to which the guidance platform could be aligned and the flight controllers could choose them arbitrarily.

For re-entry, an orientation was chosen that would help the crew to make sense of their 8-ball displays, essentially turning them into artificial horizons that would show attitude relative to the ground below. It was based on the point of entry interface, whereby the x axis was aligned along the azimuth of their flight path but parallel to the horizontal plane. The z axis was parallel to a vertical line at the point of entry interface. In other words, at entry interface, the z axis would be pointing towards the Earth's centre. By default, the y axis was aimed to the right of the flight path. The upshot of this arrangement was that at entry interface, with the spacecraft presenting the heatshield forward and the crew heads-down, the FDAI display would read 0 degrees for roll, 180 degrees for pitch and 0 degrees for yaw. In this way, the display was much easier to interpret.

It was usual throughout a flight that, when the orientation of the guidance platform had to be changed, it would be realigned twice. The first would be according to whatever orientation had been in use up to that point, which, prior to re-entry, was likely to be the PTC REFSMMAT. This was simply to determine how far the platform had drifted since its last realignment, to give engineers an additional 'data point' in their record of the mechanical characteristics of the IMU, in case this data had a bearing on subsequent flights. The second realignment swung the platform around to its new orientation as defined by the entry REFSMMAT.

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