I'm not sure if I am following you. Would you please re-state that?

3.7km in a sec? Do you mean 3.7km/sec? Or duration? reference below ground?

And do you mean that the sensor info should only be accepted if it is within a range of a predicted trajectory?

I don't know what the negative altitude reading was. It could have been -100,000 km at the trigger point for all I know so it's all of the above.

I'm saying the software decision to start final descent looks like a one-sided comparison where some negative number also triggers the sequence.

 

Are you saying that the computer stream was reporting the craft was below ground level?

 

Are you saying that the computer stream was reporting the craft was below ground level?

Yes. The signal from the IMU corrupted the altitude data somehow per the report from ESA.

When merged into the navigation system, the erroneous information generated an estimated altitude that was negative – that is, below ground level.

 

I got you now. Thanks. Yeah that's a boo boo.

 

I got you now. Thanks. Yeah that's a boo boo.

It's a common problem, sensory saturation forced a premature release...

 

It's a common problem, sensory saturation forced a premature release...

Perfectly understandable. 50 years ago.

 

It sounded to me that they didn't give the IMU time enough to reset. Evidently they knew the IMU needed to reset after chute deployment jerk. It just took longer than they guessed.

Timer setting.

BR, with all due respect. I very much doubt that a mission of this magnitude would rely on timers instead of precise sensor reports to deploy whatever artifacts are needed for its successful landing.

 

BR, with all due respect. I very much doubt that a mission of this magnitude would rely on timers instead of precise sensor reports to deploy whatever artifacts are needed for its successful landing.

You could also doubt that a mission of this magnitude would rely on software that doesn't have a method to disregard a negative number for altitude, but the first thing to doubt is that the mission can be done at all, considering the thousands of things that simply must function properly. What is left after that thought is, which error will be fatal? or, We will find out which error is fatal, right after the crash.

This isn't the first mission which demonstrated that it's nearly impossible to plan for everything that can go wrong.

 

You could also doubt that a mission of this magnitude would rely on software that doesn't have a method to disregard a negative number for altitude, but the first thing to doubt is that the mission can be done at all, considering the thousands of things that simply must function properly. What is left after that thought is, which error will be fatal? or, We will find out which error is fatal, right after the crash.

This isn't the first mission which demonstrated that it's nearly impossible to plan for everything that can go wrong.

I'm wondering if they did any field testing of their Mars lander (e.g. drop to earth).

 

The data says your're below ground but you can't be below ground at that point in the flight, how would you recover? Do nothing in the hope that the altimeter data will resume to reasonable values in time for a normal descent, trigger a limp home mode (integrate time and velocity into a dead reckoning altitude) like a car with bad EMU sensor data or some combination of both?

They were able to reproduce the issue in simulation so it wasn't a hardware glitch it was a software error that was missed in testing.

 

I'm wondering if they did any field testing of their Mars lander (e.g. drop to earth).

I would bet they did...then something changed.
Wrong copy of the software installed?
Test conditions not realistic?

Acceleration in a gravity field is well known, mathematically.
Even the density of the atmosphere is know in this case.
There are a lot of engineers right now screaming, "It should have worked!".
And probably a few unemployed today.

A human on board (with spare underpants) probably could have avoided this crash, but I guess they have to 1) practice without a pilot until they get things very close to correct...and 2) provide for the pilot to survive for a while on Mars, and 3) provide for a return trip. Parts 2 and 3 seem every bit as difficult as part 1. I guess it's going to be a while before we get this right. (Sigh)

 

The guys who built the RTPU lander system.

http://dkmat.dk/wp-content/uploads/2016/09/Kim-Plaugborg-Matematikken-viser-vej-til-Mars.pdf

 

how would you recover?

I would say, go to dead reckoning from the last known position, check ambient atmospheric pressure for an altitude approximation, check velocity pressure of the atmosphere (if that's possible without melting the sensor), and keep checking for a valid altimeter input so as to revert to that if possible.

Much more expensive, difficult, heavy, and likely to failure would be optical methods of estimation, like a human would do with their eyes and brain, but that's probably impractical.

"Houston to lander: Video images show you should have deployed parachute 28 minutes ago."

 

go to dead reckoning from the last known position, check ambient atmospheric pressure for an altitude approximation, check velocity pressure of the atmosphere (if that's possible without melting the sensor), and keep checking for a valid altimeter input so as to revert to that if possible.

Hey... when does the timer come into play?

Video images show you should have deployed parachute 28 minutes ago

Ahhhh... there it is!

 

There are fixed timers and conditional timers. A program loop can be used as a conditional timer.

Golly Ned.

 

There are fixed timers and conditional timers. A program loop can be used as a conditional timer.

Golly Ned.

... touché ...

 

... touché ...

Touché? Doesn't cmartinez say "toque"

 

Touché? Doesn't cmartinez say "toque"

Actually, it's said: "tocado!"

 

Actually, it's said: "tocado!"

I thought it was taco.

 

I thought it was taco.