MER-A's nominal mission would have "entered" the Martian atmosphere at an altlitude of approximately 126 km (the definition of "entry" is arbitrary). The atmosphere relative velocity at entry is 5.4 km/sec with a free stream density of about 1.6e-08 kg/m^3. Dynamic pressure is defined as:

dynamic pressure = (1/2) * [free stream density] * [relative velocity]^2

Initially the free stream density is rising rapidly along with the velocity because there is little drag. Then the atmospheric drag becomes significant causing the velocity to noticeably reduce. However the free stream density is an exponential function of altitude. Consequently the free stream density is increasing faster than the velocity squared is reducing. Therefore the dynamic pressure increases monotonically until it reaches a peak value of 4720 Pascals at an altitude of 33.6 km which occurs 119 seconds after entry. The autopilot remembers when peak dynamic pressure occured. After peak dynamic pressure, the velocity squared is reducing faster than the free stream density is increasing. Deceleration is proportional to dynamic pressure. Deceleration can be directly measured by the IMU (Inertial Measurement Unit). When the dynamic pressure reduces to 750 pascals then the parachute is shot out of the aeroshell using a mortor (no pilot chute). The time from peak deceleration to parachute deployment can be predicted based upon an assumed atmospheric model. My code predicts parachute deployment at Mach = 1.7 which occurs at an altitude of 8.9 km or 236 sec. after entry. MER prefers to deploy its parachute based upon a measured deceleration rather than a time interval. However if the time after peak deceleration grows too large then the autopilot starts to get anxious and finally says "I've done this long enough!". It's important that the parachute not be deployed too soon. Too much dynamic pressure will shred the parachute. MER also has a radar altimeter which is used in conjunction with the air bag and RAD rockets. I'm not familiar with the radar altimeter and don't know if the autopilot uses it as a backup for parachute deployment (probably doesn't).

Gary