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)