I wonder if it's the right place to put it, but since the title of this category also says "tech", i'll give it a shot:

In "roving Mars", Steve Sq. tells about the DIMES and TIRS systems and how it came to be. It is (was) used in the last minute of decend to the surface and it tells a system behind transverse rocket motors how long and strong they should burn in order to kill horizontal motion over the surface, by comparing two vertically (downward) pictures that are taken within a time interval. In this way the crafts knew how fast and in what direction their motion across the surface was before impact.

What he did not explain however, is: how does the craft know that it did not spin, swing or change (like change in wind direction) movement between image aquisition and the correction burn. I really don't understand and has been a question for me since i first saw the Daniel Maas animation.....
The EDL images from down the craft were from about a mile height, so it must have taken at least half a minute until touchdown. What happens in between ? Is the spacecraft attitude and speed control system so advanced, that it can actually remember in which "wind" direction (north/east/south/west) it should correct for, continually monitoring the rotation/position of the backshell within the last mile ? Or was it much more "brute" and it just fired the opposite direction of what it calculated from the images, supposing that it did not turn or swing at all ?

Maybe very detailed question, maybe too many words.....but i want to know

Well - they also had Gyroscopes and Accelerometers on board that could tell the computer how much it's rotating and how much it's accelerating along or around any axis ( the data sets for this are available online )

So - they could calculate rotation, and pull that out of the dimes imagery, and from THAT - calculate how fast and in what direction they were moving when they took the dimes pictures - sort of a 'baseline' of motion at a specific point in time.

From THAT - you then can use accleration and rotation from the gyros and accelerometers to keep your motion updated - so that you can figure out in which direction and how fast you're drifing at the moment you need to fire TIRS.

Now - I'm actually guessing most of that - but they accel and rotation data is on the web
http://atmos.nmsu.edu/PDS/data/merimu_1001/ and it's the only way I can think of it.

Doug

Thanks for that ! I did not understand how they could possibly anticipate on any changes in between: now i know. They monitored the movement after the images are made and correct for that. They would have had a big problem however, considering landing in a DD (with winds coming from all directions) or a sudden gust of wind.....which (fortunately) did not happen.