SSR

Question 1

What is radar cross section?

Answer 1

The extent to which a solid object reflects or scatters radio waves.

The small (relatively) the radio wave is to the object, the better the reflection.

Question 2

How does a SSR work?

Answer 2

A ground based unit on the ground transmits coded interrogated pulses. A/C equipped with a transponder and are within range will respond with their own coded pulse giving info like:

A/C ident
Height
Other parameters (optionally)

Transponder also used in A/C’s TCAS system.

Question 3

What are the advan of SSR over PSR?

Answer 3

• its doesn’t rely on reflections and so is clutter free and has greater range
• its independent of A/C radar cross section
• it indents an A/C by attaching a data tag showing ident, altitude and speed.
• by use of special codes, can give ATC immediate notification of an emergency, unlawful interference or radio failure.

Question 4

What must an A/C have in order for an SSR to locate it?

Answer 4

A working transponder that has been switched on.

The PSR can still pick up on an A/C without a transponder but there is no A/C ident.

So an SSR site with no primary radar, and A/C with no transponder, to these sites an A/C is invisible to it.

Question 5

What frequencies do SSR use?

Answer 5

Interrogation: 1030mHz

Transponders: 1090mHz

Basic interrogation signal is transmitted as pp with width o.85µS

Question 6

What are the SSR operating modes?

Answer 6

Mode:
A 8µS: Transmission of A/C transponder code
B Alternate to A
C 21µS: Transmission of A/C px altitude
D Not in use
S (3.5µS) Multiple pulses: A/C selection and transmission of flight data for ground surveillance

Ref table page 349.

Question 7

How many combinations can a squawk code have?

Answer 7

Squawk code is a 4 digit A/C ident, and can have 4096 combinations.

Question 8

How does a SSR interrogator ID the different modes?

Answer 8

By the pulse spacing used by the different modes.

Mode A and C transponders will respond after P3, but with Mode S transponders, this equipment will only respond after P4, and will respond with their unique 24-bit A/C address code.

Question 9

What are the advan of SSR over PSR?

Answer 9

• Does not rely on echoes - removes clutter and
independent of A/C cross-section

• The controller is provided with additional info about
the A/C e.g. Squawk code, flight no, flight ID, selected altitude

• Provides controllers with notification of distress/emergency
(7700), radio failure (7500) or hjhack (7600).

Question 10

What are the 3 squawk codes?

Answer 10

• 7500 - hijack/unlawful interference
• 7600 - radio failure
• 7700 - emergency/distress

Question 11

Apart from squawk code, what other additional info is transmitted in mode C?

Answer 11

A/C px altitude in 100ft increments (Flight levels)

Question 12

The interrogator transmits in a series of pulses. What is the purpose of pulse P2?

Answer 12

P2 is a control pulse, transmitted after P1 to ensure that no response is sent in response to side lobes of the interrogating antenna.

Question 13

What additional info, apart from mode A and C info, does mode S ELS (56bit) send to ATC?

Answer 13

• A/C ID (ICAO 24 bit address)
• Flight ID - registration/flight no
• Altitude - 25ft increments

ELS aka basic functionality
ELS is required by EU IFR airspace

Question 14

What additional info, apart from mode A and C info, does mode S EHS (80bit) send to ATC?

Answer 14

• ˚M HDG
• IAS/MN
• Vertical rate
• Roll angle
• Track angle rate
• ˚T track angle
• GS
• Selected altitude.

EHS more advanced mode S facility.
Required for all A/C planning on flying IFR in EU > 57000kgs or >250kts TAS cruise speed.

Question 15

What info is sent back to ATC in mode A?

Answer 15

Transmits 4 digit squawk code (4096 combo of unique no.s)

Pulse pair spacing (PPS) of 21 µS

Question 16

What info is sent back to ATC in mode C?

Answer 16

• Squawk code
• Px altitude (100ft increments) from A/C own altimeter set to 1013hPa
• PPS 21 µS (pulse pair spacing)

Question 17

Modes S also has P4 which is double the length at 1.6µS. Mode A and C can also send out P4 but because they are shorter this is ignored by the transponder.

Answer 17

The longer P4 signal makes the A/C reply with its 24 bit address. This is called “all call”.

After receiving the interrogator can block out the A/C and only call it up when needed, this is “selective call”

Question 18

What are the 2 disadvantages of SSR?

Answer 18

• Code garbling - caused by 2 A/C being less than
1.7NM apart and answer at the same time to the
same ground station

• Fruiting - reception of replies caused by interrogation from other
stations being too close in proximity to each other

Question 19

All A/C are allocated an ICAO A/C address…

Answer 19

This address is hard coded into the mode S transponder.

Question 20

A mode S interrogation contains:

Answer 20

• an A/C address
• an all call address
• a broadcast address

Question 21

When changing the squawk code on a transponder…

Answer 21

The transponder should be set to STBY before changing the squawk code.

Question 22

What is SPI?

Answer 22

Special position ident aka squawk code.

This is what ATC can ID an A/C on their screen.

Question 23

What is fruiting?

Answer 23

When 2 or more ground stations are close together and interrogation (response) may only be detected by the ‘wrong’ ATC unit.

These unwanted replies are called fruit, and trying to defruit is the attempt to remove this interference but fruiting remains a prob in areas of high traffic density.