Two types of radar systems are used to aid air traffic controllers in providing advisory and separation services to pilots. They are primary and secondary radar.
Primary Radar
The primary radar system transmits pulses of radio waves, then listens for these pulses to be reflected back and received. Metal objects, like airplanes, reflect this radio energy well, and can be detected by primary radar. Since the radio waves are transmitted in a particular direction, the direction of the primary radar reflection is known. The range of the radar return is determined by marking the time between the transmission and reception of the radar return. Since radio waves travel at the speed of light, the distance to the radar return can be calculated. The radar antenna moves in a circular sweep. The data from the radar is then fed into a computer for processing, and displayed to the air traffic controller on a radar display.
Primary radar may depict additional or erroneous information, such as ground clutter. Primary radar may also be blocked by terrain or affected by precipitation and other atmospheric conditions.
Secondary Radar
Secondary radar operates by communicating with transmitter/receivers onboard airplanes called transponders. The secondary radar antenna usuallyy rides on top of the primary radar antenna, the two sweeping together in a circle. As the antenna turns, a secondary radar component, called the interrogator, sends out radio interrogations. When the transponder in an aircraft receives this interrogation signal, it replies by transmitting its four digit transponder code. The secondary radar antenna receives this reply, and forwards the code, azimuth, and distance to the computer system for processing and display on the radar screen.
The four digit transponder code can be set by the pilot. An air traffic controller may request a specific code by stating "squawk" and the requested code. For example, "Cessna four two papa, squawk four two seven one". Each of the four digits are selectable between zero and seven, allowing 4,096 possible codes.
If on a VFR flight, when air traffic control has not assigned you a specific transponder code, set your transponder to code 1200. Code 1200 is the standard code used by all VFR aircraft not assigned a specific code. Since this code is used by VFR aircraft, an air traffic control instruction to "Squawk VFR" is an instruction to set your transponder to code 1200.
Three other standard transponder codes are for emergency use. They are 7500 for hijacking, 7600 for radio failure, and 7700 for emergency. If a pilot selects one of these codes, all air traffic controllers who receive the code will immediately take appropriate action. For example, if a pilot selected code 7700, every radar facility in range would receive an alert of an aircraft in distress at that location. Since 7700 is the emergency code, an air traffic control instruction to "Squawk emergency" is an instruction to set your transponder to code 7700.
Be mindful of codes 7500, 7600, and 7700 when selecting a transponder code. Many pilots switch their transponder to the standby setting while changing codes, in order to make sure one of these codes is not accidentally transmitted to air traffic control.
Transponder Mode C
Mode C refers to the altitude reporting function on many transponders. A mode C transponder transmits altitude information to the secondary radar system in addition to the beacon code. This allows air traffic control to receive altitude updates from your airplane during the flight.
Mode C is activated on most transponders by selecting the "ALT" setting. The "ON" setting transmits the beacon code without the altitude information. This is known as Mode A.
An air traffic controller can request a pilot to start or stop altitude reporting with the phrases, "Squawk altitude", or "Stop altitude squawk", respectively. The phrase, "Squawk standby" is an instruction to place the transponder in the standby position, in which case it will not reply to air traffic control radar.
IDENT
When the pilot presses the ident button on the transponder, the transponder sends an ident signal to the secondary radar system. The radar display momentarily changes the look of the secondary radar target, allowing the air traffic controller to verify the location of an aircraft. Press the ident button when requested to do so by air traffic control. For example, "Cardinal four two papa, ident".
Transponder Equipment and Use Requirements
This regulation states that all aircraft must have an operable Mode C transponder to enter Class A, B, and C airspace areas.
A Mode C transponder is also required anytime you are within 30 nautical miles of specific airports, which are listed in CFR Title 14, Part 91, appendix D, section 1. For airports listed in section 2, a Mode C transponder is required when within 10 nautical miles, unless you are outside the lateral limits for the particular airspace and remain below 1,200 feet AGL.
Additionally, a Mode C transponder is required anytime you are operating above a Class B or C airspace area.
Finally, a Mode C transponder is required anywhere in the continental United States and the District of Columbia when at or above 10,000 feet MSL and also above 2,500 feet AGL.
ยง 91.215 ATC transponder and altitude reporting equipment and use.