The Earth
The Solar System & Time
Using Aeronautical Charts
Basics of Navigation
Distance, Speed & Time
Vertical Navigation
Fuel Planning
Practical Navigation Techniques
Radio Navigation
Practice Exam


Primary Radar

Primary Surveillance Radar sends a signal out and anything that reflects the signal back shows up on the radar display.

An aircraft must be in line of sight of the radar antenna to be displayed, so the radar range is usually increased by installing the antenna at higher elevations above all obstacles.

Secondary Surveillance Radar (SSR)

Secondary Surveillance Radar (SSR) incorporates a ground based antenna that transmits coded pulses which are received by aircraft transponders. The aircraft transponder sends a response back to the ground unit with various bits of information.

The information sent by the transponder depends on the transponder mode selected by the pilot:

Mode A (‘ON’) transmits:
The four digit squawk code
The aircraft’s position
Mode C (‘ALT’) transmits:
The four digit squawk code
The aircraft’s position
The aircraft’s pressure altitude
An older style transponder that can only transmit Mode A (‘ON’) or Mode C (‘ALT’)
Mode S transmits:
The four digit squawk code
The aircraft’s position
The aircraft’s pressure altitude
Additional data (e.g. aircraft registration, information for traffic collision avoidance, and much more)
A modern transponder that will transmit Mode S information when the pilot selects ‘ALT’ (Mode C)

Transponder Codes

The term ‘squawk’ is used to describe the 4 digit code selected on a transponder. For example, air traffic control may instruct you to “Squawk 4551”.

There are some transponder codes that are used in specific circumstances:

7700 – Emergency
Used to indicate an emergency condition
7600 – Radio Failure
Used to indicate a radio failure
7500 – Unlawful Interference
Used to indicate unlawful interference with the planned operation of a flight (i.e. hijack)
7000 – VFR Conspicuity
A conspicuity squawk code is used when no other specific squawk is appropriate (e.g. in most uncontrolled airspace)
7010 – Aerodrome Traffic Pattern Conspicuity
At some aerodromes, if you are staying in the traffic pattern (the circuit) the ATS Unit or the local operating instructions may instruct you to squawk 7010
Used when entering United Kingdom airspace from an adjacent region where the operation of transponders has not been required
You must remember the meanings of these 4 transponder squawk codes:

7700 – Emergency
7600 – Radio Failure
7500 – Unlawful Interference
7000 – VFR Conspicuity

Transponder Rules

It is mandatory to have an SSR transponder in certain circumstances, such as in Transponder Mandatory Zones (TMZs), in some controlled airspace, and when flying at or above FL100.

If your aircraft is fitted with a serviceable Mode C transponder, it is mandatory for you to continuously operate it unless otherwise dictated by ATC.

The TMZ is marked on the chart by red coloured semi-circles and labeled with its altitude limits. In this case, the TMZ is from the surface to 1500 feet (SFC-1500′)