Primary and SSR systems work hand in hand to gather important ATC information. Let’s review the basics of each:
Primary Radar: A ground-based radar antenna transmits a pulse toward the aircraft of interest and recaptures the reflected RF signals. The backscatter can reveal an aircraft’s range and bearing. While no onboard aircraft equipment is required, primary radar does not determine a target's identity, only its position.
SSR: At the heart of SSR is an aircraft’s onboard transponder, which is a device that takes in an RF signal and outputs a certain response. In SSR, the aircraft first receives an interrogation signal emitted from a ground station. The transponder decodes the interrogation to establish exactly what information is desired and sends an encoded reply. In this way SSR can determine an aircraft’s identification code and altitude.
An SSR antenna can transmit different kinds of interrogation signals to elicit a particular response from an aircraft. These interrogation signals are classified by modes.
For example, Mode A requests a target’s identification code. Mode C is similar and is used by ground stations to request an aircraft’s altitude in 100 ft increments. Mode S ups the complexity since it can request a variety of information from a target. As well as pulses, this interrogation signal includes a phase-modulated data block.