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Comment by tremon | Hacker News Reader

The more interesting question is why it crashed, or rather:

- why did it eject its parachute too soon?

- why did the brake thrusters activate for a shorter time than planned?

Does anyone here know if the landing steps were timing-driven or telemetry-driven? Was it supposed to eject its parachute after N seconds, or when reaching M meters above the surface?

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The radar was only activated after the front shield was ejected, so there was no altitude data prior to that event. It could be that accelerometers on board sensed the dynamic pressure, so that parachute deployment was not purely open-loop. I'll try to dig through publications to find out what the EDL (Entry-Descent-Landing) strategy was.

EDIT: Here are some papers worth persuing:

[1] https://solarsystem.nasa.gov/docs/Bayle_ExoMars_EDM_Overview...

[2] http://ippw2016.jhuapl.edu/docs/abstracts/Missions-abstracts...

[3] http://arc.aiaa.org/doi/abs/10.2514/6.2011-6341 (behind journal paywall)

Thanks! So it looks like the entire descent was telemetry-based (source [1], section 3.4):

The functions of the EDM GNC [Guidance, Navigation and Control system] are (1) to detect the conditions adequate for the deployment of the parachute and (2) to command and control the final powered descent phase. The GNC sensors aboard the EDM are the Inertial Measurement Unit (IMU) and the Radar Doppler Altimeter (RDA). A main challenge for the EDM GNC relates to the fact that the EDM needs to go into a hibernation mode during the 3-day long coast phase. During this phase, the inertial reference of the IMU is lost and an additional sensor is necessary in order to rebuild the inertial attitude shortly before entry.

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