Secondary surveillance radar (SSR) has been around since World War II, based on the military’s “identification friend or foe” radar system. While the radar systems have evolved, many of the test challenges have remained constant. Whether designing, characterizing, installing, maintaining or troubleshooting an SSR, one may have different requirements for testing the RF transmission. As a result, different instrumentation is often used, based on the task, or the same equipment is used throughout, requiring users to accept compromises. The former leads to higher equipment cost, the latter inefficiency and lower productivity. This article describes a new approach to these test challenges, where the same test equipment can be used without the compromises.
SSR is used in air traffic control to complement the primary radar system (see Figure 1). The primary radar measures the bearing and distance of aircraft or other targets using the reflections of transmitted radar signals. The SSR provides additional information, such as an aircraft’s identification code or its altitude. Unlike primary radar, which only depends on signal reflection, SSR systems require the aircraft to have transponders - a transmitter responder, which receives a signal, then transmits a response. An SSR sends an interrogation signal to the aircraft requesting specific information. The interrogation signal is received by the aircraft’s transponder, which responds with an encoded signal containing the requested information. Correspondingly, an SSR system has two transmitters to test: the interrogator and the transponder.
The interrogation signals are categorized by modes, primarily A, C and S. The interrogation signal parameters depend on the mode (see Figure 2), where mode A requests identity, mode C requests altitude in 100 foot increments and mode S is multi-purpose. The time between P1 and P3 of the mode A/C and S interrogation signals are 8 and 21 µs, respectively. Mode S comprises P1 and P2 in a preamble followed by a data block of 56 or 112 bits modulated with differential phase-shift keying. Mode A and C transponders respond with 12 and 11 pulse replies, respectively, and the mode S reply includes a four pulse preamble followed by a data block of 56 or 112 bits (see Figure 3). Mode C does not use the D1 pulse.