Rendezvous Part 9 – The Clever Save

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. 

At about 50,000’ distance, you could lock on with the radar, and determine the range and range rate, plus the angles to the target. Basically, this gave you everything you need to know about determining relative position…so long as the radar worked. You could make the rendezvous work without it, using techniques developed way back in the Gemini program. I was fortunate in that in my very early days at the Johnson Space Center (as a Coop student, in fact), I shared an office with one of the designers of the operational procedures for Gemini rendezvous, Will Fenner. I never learned any math from Will, but I listened to his stories and gained a knowledge of how the rendezvous procedures actually worked – an intuitive education, I guess. Much like everything I learned back in school, I had no idea what to do with that knowledge until much later in life, when suddenly I had a need for it, and it made sense! I guess that those early stories from Will set me up for my later Flight Director job – interesting how that worked out.

Now one of the problems with using sensor data to refine your knowledge of where you are and where you’re moving is that you can get corrupt data. You can start with a great, accurate state vector, and you carefully track all the changes you make to your velocity and position — and then, all of a sudden, the star tracker locks onto a star instead of the target, or the radar has a glitch and starts to rapidly corrupt the state vector – your knowledge of where you are and where you’re going.

Not to worry though – one of the clever bits of software in the Shuttle’s navigation system allowed you to save the vector any time you wanted. So what we did, to avoid losing an entire days of accurate propagation of the vector, was to save the current vector before adding new sensor data. That way, if something bad came in to the propagated vector, you could essentially erase that, and go back to a spot in time where you actually were fairly certain that you knew where you were. Sure, you lost the advantages of the sensor pass that you were trying to get, but at least you weren’t any worse off than if the sensor hadn’t acquired the target at all. You could always start a new sensor pass or wait for an opportunity to use the radar when you got closer.

That’s a lot of detail to basically point out that while a real rendezvous, with no real problems, might seem terribly quiet and boring (as most of them were), there were thousands of little details that the Flight Director and the entire team had to understand and practice. It was not uncommon for the training team to actually mess with the software constants that were loaded for each flight and create errors in burn solutions – or make the software fail to find a solution for a particular burn. The actual nominal rendezvous procedures took about twenty pages to cover the six-hour rendezvous process. But, the book was usually about ten times that thick so that it included all of the various contingency procedures and modes to accomplish the rendezvous – or to break out and set yourself for another day – or just call off the rendezvous and safely transfer to an orbit from which you could just come home.


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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