I t's getting to be almost impossible to fly anywhere without a Mode C transponder. This equipment is now required in all Class A, B, and C airspace, within 30 miles of primary Class B airspace airports, and anytime you're above 10,000 feet. And if the FAA has its way, that altitude will soon drop to 6,000 feet. ATC is watching you ATC radar facilities presently have both both primary and secondary radar. It's the same sort of radar that was used in World War II to detect enemy aircraft.
Primary radar has lots of limitations.
Its range is limited by terrain and precipitation. And it only reports a target's azimuth and range, not its altitude. Secondary radar was invented to overcome these limitations. Today, virtually all ATC radar installations are equipped with both primary and secondary radar capability. However, the FAA is threatening to decomission primary radar from its enroute facilities.