Principles of Secondary Radar

Question 1

List three different types of transponder modes

Answer 1

A, C and S

Question 2

What is reflection and how does it affect SSR processing?

Answer 2

Reflection of interrogator and/or transponder reply signal paths, usually from ground features or buildings close to the radar head, may result in more than one position being displayed for a single target. This causes split tracks on the radar display

Question 3

What frequencies are modes and codes

Answer 3

Modes are 1030Mhz and Codes 1090Mhz

Question 4

What is the main difference between a secondary and a primary radar system?

Answer 4

A secondary radar system differs from primary radar in that it does not rely on a reflected signal to detect an aircraft. SSR operates using a request and reply system. The ground equipment sends out a signal, which triggers a reply transmission from a transponder onboard the aircraft.

Question 5

What is garble and how does it affect SSR processing?

Answer 5

When targets are close together on secondary radar their transponder reply pulses may overlap, they are then said to be garbled. Garbling may confuse the decoding equipment resulting in the display of false codes, or both correct codes being cancelled out.

Question 6

What is the difference between Mode A and Mode C?

Answer 6

Mode A provides aircraft position and identification. Mode C provides level information

Question 7

Mode A provides aircraft position and identification. Mode C provides level information

Answer 7

Less power is required for the same or even greater range.
The response strength is independent of target size or material.
There is no unwanted ground or weather clutter.
Positive identification may be made without aircraft manoeuvres.
Special reply codes can be used to indicate the aircraft’s level
Special reply codes are used to indicate emergency, RCF or unlawful interference
SSR codes can be used to correlate an aircraft’s radar target with the aircraft’s flight plan.

Question 8

Explain in general terms what Mode S is.

Answer 8

Mode S establishes selective and addressed interrogations with aircraft within its coverage.
Such selective interrogation improves the quality and integrity of detection, identification and altitude reporting, and may permit the exchange of data with the ATM System, and more data to be encoded in aircraft transponder replies.

Question 9

Give three examples of the additional information a Mode S transponder can provide.

Answer 9

• Aircraft identification
• Air Speed (Indicated Airspeed and Mach Number)
• Ground Speed
• Magnetic Heading
• Roll Angle
• Selected Altitude (selected vertical intention)
• Track Angle Rate
• True Track Angle
• Vertical Rate
• Reporting of ACAS II RA indications by the transponder