How does Schiaparelli's guidance system work?

Question

I was not able to find any technical documentation on the closed loop guidance system that was supposed to accomplish the powered descent of ESA's Schiaparelli Entry Demonstrator Module. Several publications mention that Doppler Radar altimeters and inertial measurement units are used as sensors, but it is not obvious how they wanted to null horizontal movement.

Any hints about the employed control algorithms, performance estimations, region of convergence and stability, and fallback mechanisms would be welcome.

Answer 1

The 2011 Thales Alenia Space presentation on EXOMARS-2016 GNC Approach for Entry Descent and Landing Demonstrator might have something that may help.

From that:

1. The parachute is deployed based on an acceleration trigger, which is chosen to keep the deployment within the required Mach-dynamic pressure box over the dispersed entry conditions. The parachute is deployed at about 10 km above ground level, at Mach 1.95 and a q of about 700 Pascals.

2. The heat-shield is deployed a fixed time from parachute deploy, shortly after which the RADAR Doppler altimeter is turned on.

3. The RADAR-determined altitude and velocity is used to determine when to deploy the backshell with parachute. This occurs (was supposed to occur) at 1.6 km above the ground at about 70 m/s vertical velocity.

4. The descent engines fire up one second later, and execute a short backshell-avoidance maneuver sideways, to make sure that the surface module does not just accelerate right back up into the backshell and large parachute that was just separated above.

5. The Doppler altimeter combined with inertial measurement data feeds the state determination, which then controls the throttling of the engines to perform a gravity turn to the surface, nulling the vertical and horizontal velocities both to less than 1 m/s. A constant deceleration is targeted. The thruster command rate is 10 Hz. This phase is 22 to 30 seconds, and goes from 1.3 km above the ground to 2 meters, and 70 m/s to 0.5 m/s. Starting at 10 meters above the ground, the state determination ignores the altimeter, using only inertial guidance from that point on.

6. At 2 meters above the ground, the thrusters are shut off and the surface module free-falls to the ground. It impacts at 4 m/s vertical and less than 1 m/s horizontal velocity. The crushable material on the bottom of the surface module absorbs the impact.