Final answer:
The biggest upgrade Monopulse Secondary Surveillance Radars (MSSR) have over older beacon radars is that they use a technique called monopulse tracking and pulse compression to improve sensitivity and detection at longer ranges.
Step-by-step explanation:
The biggest upgrade Monopulse Secondary Surveillance Radars (MSSR) have over older beacon radars is that they use a technique called monopulse tracking. Monopulse tracking allows the radar to determine the angle and range of a target more accurately, leading to better sensitivity and detection at longer ranges. Another improvement in MSSR is their ability to distinguish multiple targets in a congested environment. This is achieved through the use of pulse compression techniques, which allow the radar to separate and identify individual targets by their unique radar signatures.