The S-1403DL/MLD Mode S accessory unit supports the Mode S Data Link and ADS-B (extended DF17 squitter). The “DL” is backward compatible with the “C” model. Current ATE programs implemented on the S-1403C will operate with the S-1403DL/MLD without program changes. The unit provides additional pulse code modulation, as specified by RTCA/DO-181 to the ATC-1400A for testing ATCRBS and Mode S transponders, including Data Link, ADS-B and MLD (Multi Level Diversity) requirements. The S-1403DL/MLD may also be used as a stand-alone unit to perform limited Mode S tests.