Rendezvous Part 8 – Getting the Details Right

In my book Shuttle, Houston: My Life in the Center Seat of Mission Control, I bring readers deep into the middle of how Mission Control worked during the Space Shuttle era. I give readers a glimpse of both the technology and the humanity required to put humans in space on a routine basis. There are many parts to this thirty-year story, and only a small portion of the tales fit into the book – so here is an excerpt from an unused chapter on the process of rendezvousing the shuttle with other objects in orbit. 

Rendezvous operations were one of the places in orbit flight control where time really mattered. Trajectories were such that you had to do certain burns, and get certain data at the right time, or the whole thing was blown. There were hard deadlines that had to be met, and where margin on the clock was one of the most important things you could keep in your pocket. My flight controllers probably got tired of hearing me say “if you’re not early – you’re late!”, but it was true. The most routine looking (and sounding) rendezvous were those where everyone stayed ahead of the game, giving their Go’s early and looking ahead. There were, of course, things that you couldn’t do until you got to the right moment – but in general, getting at least a little ahead was important – if for no other reason it gave you extra time to deal with the unexpected.

A rhythm built up in the Control Center during a rendezvous. Routine burns had to be executed on time, so the clock ticked, the timeline ran, and the crew and controllers handled those as they came along. All the while, as failures or dispersions came up, you’d find time to handle them while not missing the next burn or sensor pass. Sensor passes were where you refined your relative position to the target. You could miss one, or two (and occasionally, you lost most of them), but the result was to have more dispersions in the navigation solution, and thus you burned more propellant when you burned because you didn’t really know where you were.

Dispersions were the errors between where we thought we were, and were we actually were. Dispersions crept in because of sensor inaccuracy, alignment errors, and all sorts of little mathematical unknowns. The way the navigation worked was that the Inertial Measurement Units tracked the accelerations of the vehicle in all three axes. It knew where we “started”, and then integrated the various accelerations to keep track of where we were at any time. But, all measurements have errors, much of it related to how small a unit of measurement you could achieve. So, every time you measured an acceleration in a specific direction, a tiny error crept in to the current knowledge of where you were. The position errors were three dimensional and created an error ellipse around the vehicle. You knew that you were somewhere inside that football, but you weren’t exactly sure where.

The way you shrank the error ellipse was to get some sort of truth data. You could start with ground tracking – that was a good way to initialize the navigation solution. This starting point got you close enough that you could point sensors at the target and take more measurements to try and make the solution better. One type of sensor was optical – the star trackers we used to align the spacecraft to the stars worked well for this, because if the target was in sunlight, it reflected that light and looked like a star. You could tell that it was the target, and not a star, because it moved relative to all the rest of the stars in the field of view. Star Tracker navigation worked a long way out, as long as you had a good reflective target and it was a good way of shrinking the error ellipse. But when you got in closer, it just wasn’t as precise. Fortunately, we had radar – when you got close enough.


We’ll bring you the next portion of “Rendezvous” next week right here! If you enjoyed this look inside the Shuttle program, you can find many more details and stories in the book – look for it wherever you buy your paper books, or add it to your favorite E-reader or audio book account.

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