Chapter 14 Transponder Transponder Introduction Developed from the

Chapter 14 Transponder

Transponder Introduction Developed from the military system known as IFF (Identify Friend or Foe).

Ground Equipment The antenna shown has 2 parts: the lower portion is the older system that picked up aircraft skin reflections, the upper part is the beacon interrogator the signal from the aircraft. This example is an ASR (Airport Surveillance Radar) which has about a 60 mile range; these are known as “approach control” or Tracon (terminal radar approach control). Textbook page 95

Ground Equipment SSR Secondary Surveillance Radar PSR Primary Surveillance Radar Textbook page 95

Transponder Control-Display This example shows the transponder “squawking” code 1045. “Squawking” is when the pilot dials in a code. Pressing the IDENT button causes that aircraft’s “blip” on the air traffic controller’s radar scope to “bloom”. Mode A transmits ID information and Mode C adds altitude information. Textbook page 94

Textbook page 96

Mode S introduced to overcome the limitations of ATCRBS (Air Traffic Control Radar Beacon System). S stands for selective addressing. An ATC controller can request a specific aircraft to respond which reduces system clutter. This is accomplished by an aircraft address which is comprised of 24 bits.

Transponder Block Diagram The receiver picks up interrogations from the ground station at 1030 MHz and send to the decoder. The decoder measures the incoming pulses and identifies them; if they are recognized, they are sent to the encoder. The encoder creates the pulsed reply. The encoding altimeter converts barometric pressure to a signal for a Mode C reply. The Code Selector is used to dial in the code assigned by ATC. The modulator amplifies pulses that form the replay and sends it to the transmitter to transmit on 1090 MHz. The side lobe suppressor filters out side lobe signals (which would cause false position information on the ATC radar scope). The Suppression Circuit prevents interference from DME, also a pulsed signal. Textbook page 97

Transponder Antennas

Mode A Interrogation Textbook page 99 The ground radar sends out an interrogation signal of two pulses at 8 microseconds. The signal is made up of 3 pulses: P 1 and P 3 tell the airplane this is a Mode A interrogation by the 8 microsecond time period. P 2 helps overcome the side lobe problem by being a lower strength than P 1; if it is a higher strength, it would be a side lobe and the suppressor circuit would filter it out.

Mode C Interrogation Textbook page 99 A Mode C Interrogation has a 21 microsecond time delay. As the ground antenna makes one full sweep, it does a Mode A interrogation, then a Mode C interrogation on the next sweep.

Encoding Altimeter

Altimeter

Static Port and Static System Airspeed Indicator Vertical Speed Indicator Altimeter

Static Port

Blind Encoder

Mode C Altitude Reporting The altitude for encoding purposes is originates from an aneroid sensor that is factory preset to standard pressure (29. 92” or 1013 mb); this is to insure standardized altitude reporting from all aircraft. The altitude is then corrected for non-standard pressure by the ATC facility when it receives the Mode C reply from the aircraft. The altitude is converted to a binary code called “Gray” code in 100 foot increments from 1, 000 feet to 126, 700 feet. Textbook page 100

FAR 91. 411 § 91. 411 Altimeter system and altitude reporting equipment tests and inspections. (a) No person may operate an airplane, or helicopter, in controlled airspace under IFR unless— (1) Within the preceding 24 calendar months, each static pressure system, each altimeter instrument, and each automatic pressure altitude reporting system has been tested and inspected and found to comply with appendices E and F of part 43 of this chapter;

Transponder ID Code Textbook page 101 Here is the pulses of a transponder code of 1642. There are only 4096 possible codes in this system. If an “ident” (squawk) is requested, the pilot presses the IDENT button which causes that particular aircraft’s blip to “bloom” on ATC’s radar scope.

A is the first digit B is the second digit C is the third digit D is the forth A B C D

Binary to Octal Every digit has three pulses that are on or off. A 1 A 2 A 4 B 1 B 2 B 4 C 1 C 2 C 4 D 1 D 2 D 4 So first digit could be: A 1 0 0 1 1 A 2 0 0 1 1 A 4 0 1 0 1 Decimal 0 1 2 3 4 5 6 7

Transponder Codes Here is a Garmin GTX-327 Transponder. It is showing the code for VFR flight of 1200. There are special codes set aside for special situations: 7500 for Hijack. 7600 for loss of communications. 7700 for Emergency. 7777 for Military Interceptor. Textbook page 101

Chapter 14 Transponder Questions 14. 1 A transponder receives an ____ from a ground station and transmits a ____. 14. 2 The transponder operates in a system known as “SSR” What do the letters mean? 14. 3 How is an airline transponder labeled? 14. 4 How many digits are in a transponder ID code? 14. 5 What are 2 main advantages of transponder signals over primary radar, or “skin” return? 14. 6 The first secondary surveillance system is known as ____. The improved system is called ____. 14. 7 What is the main benefit of Mode S? 14. 8 What transponder information is carried by Mode A? Mode C? 14. 9 When a radar interrogator wants every aircraft in range to reply (ATCRBS and Mode S), it transmits____.

Gray Code Decimal 0 1 2 3 4 5 6 7 Gray 000 001 010 111 100 Binary 000 001 010 011 100 101 110 111 2 digits changed. 3 digits changed.

Mode S Discrete All Call All Mode S transponders reply with their 24 -bit address and ATCRBS aircraft do not reply. When the radar gets a Mode S address, it locks out the transponder from replying to further “all call” interrogations. Until the airplane leaves the area, it replies only when radar interrogates it selectively, however it is still tracked on the radar scope.

Octal Digits

ATCRBS, Air Traffic Control Radar Beacon System

Mode 1 –military only Mode 2 –military only Mode 3/A identification code for the aircraft, assigned by the air traffic controller. Mode 3/A is often combined with Mode C to provide altitude information as well Mode 4 – (military only) Mode 5 – provides a cryptographically secured version of Mode S and ADS-B GPS position. (military only) Mode C (Mode 3/C) – provides the aircraft's pressure altitude and is usually combined with Mode 3/A to provide a combination of a 4 -digit octal code and altitude as Mode 3 A/C, often referred to as Mode A and C. Mode S – provides multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24 -bit address.

From AIMs 4 -5 -6

IDENT #5 IDENT #20

Airline Transponder Components • The control-display, upper right is a Mode S transponder, it is also the control for the TCAS system. • The LRU, lower right is located in the E&E bay; fault indicator lights on the face of the unit monitor the transponder, antennas and control panel. Textbook page 98

Mode S Control Panel (GA) • • • The Reply light illuminates for several seconds while the reply transmits and also when an interrogation is answered. FLIGHT LEVEL shows the altitude, in thousands of feet; here 072 or 7200 feet is shown. ID CODE displays the code dialed in for squawking; this example shows 1200 which is used for VFR flights. FUNCTION SELECTOR OFF—turns off transponder. FLT ID for dialing in code assigned by ATC. SBY standby mode. TST lights up all the lighted displays. GND disables replies, used when on the ground. ON turns on transponder. ALT used during flight to allow for replies. The VFR button automatically sets code 1200 for VFR. Textbook page 96

Mode S Selective Address • If a controller needs to communicate with only one aircraft, it transmits only the Mode S address and only that aircraft replies. • This reduces clutter on the system and avoids “synchronous garble”

Review Q &A Chapter 14 Transponder 14. 1 A transponder receives an ____ from a ground station and transmits a ____. Answer: Interrogation, reply. 14. 2 The transponder operates in a system known as “SSR” What do the letters mean? Answer: Secondary Surveillance Radar. 14. 3 How is an airline transponder labeled? Answer: By a 24 -bit code 14. 4 How many digits are in a transponder ID code? Answer: 4. 14. 5 What are 2 main advantages of transponder signals over primary radar, or “skin” return? Answer: Stronger signals, reduced garble. 14. 6 The first secondary surveillance system is known as ____. The improved system is called ____. Answer: ATCRBS, Mode S 14. 7 What is the main benefit of Mode S? Answer: Discrete calling. 14. 8 What transponder information is carried by Mode A? Mode C? Answer: Mode A is ID, Mode C is altitude. 14. 9 When a radar interrogator wants every aircraft in range to reply (ATCRBS and Mode S), it transmits____. Answer: All Call.