What must be done when using a pressure-sensor altimeter in a rocket that will not reach Mach 1?

When utilizing a pressure-sensor altimeter in a rocket that will not exceed Mach 1, ensuring that the altimeter has access to ambient atmospheric pressure is essential for accurate readings. Drilling one or more holes in the electronics bay allows for atmospheric access, enabling the altimeter to measure the surrounding air pressure effectively. The pressure change as the rocket ascends or descends indicates its altitude, and without these holes, the altimeter may not function correctly due to trapped air, which would lead to inaccurate altitude readings.

The other options suggest actions that are either unnecessary or irrelevant to the functioning of the altimeter under the specified conditions. For instance, if the altimeter is designed for environments below Mach 1, recalibration may not be necessary; it should already be configured adequately for its operational environment. Similarly, while having a backup system is good practice in many situations, it does not address the core requirement of enabling atmospheric access, which is critical for the primary altimeter to function correctly in the first place.