The air traffic control (ATC) system is used worldwide to direct air traffic from the ground. The ground stations can see the aircraft on their radar and the ATC system onboard the aircraft let's the air traffic controllers know the aircrafts altitude, aircraft code and depending on the ATC installed other information.

My colleague and I will do a full system test of the ATC transponder using a IFR ATC-601 testset.
ATC testset

Two seperate ATC transponders are installed but the aircraft only uses one at the time.
The flightcrew selects wich ATC transponder is active by selecting ATC #1 or ATC #2 on the ATC control panel.
For the test we will test both transponders but we will start with #1.
atc control panel
The system uses a top and bottom antenna wich are connected to the transponders by two transfer relays.
When the ATC control panel is selected to 1 the transfer relays connect both antennas to the ATC transponder #1 and vice versa.

We set a arbitrary value in the control panel wich is not used for traffic. On this airport we normally use 1234.
arbitrary code

The ATC system can also select between different air data sources. the control panel can select Air Data Computer #1 or #2.
ADC 1 selected

Now the aircraft needs to be put in the 'air mode' because this ATC system is inactive when the airplane is on the ground.
We do this by pulling the air/ground circuit breaker, fooling the aircraft to thinking it is in air. (first we pull the stby hydraulic pump circuit breakers because otherwise they turn on automatically wich can cause surfaces and thrust reversers to move if selected).
With the aircraft now in air we continue with entering some options in the testset.
heights
Notice that the bottom antenna is selected to be tested and that the range and heights (in feet) are entered in the setup menu.

Then we look at the 1030Mhz gain. This value can be read from the testset's antenna. The 1030Mhz signal is the frequency at wich the groundstations (in this case the testset) interrogates the aircraft.
Gain

Wich is 9.4 deciBells.
gain

Same goes for the 1090Mhz gain wich is 9.6 deciBells. The 1090Mhz signal is the frequency wich the aircraft use to reply to the ground stations.
The cable loss is found on the antenna cable.
cable loss

Now the setup is complete and we can start testing. My colleague points the testsets antenna at the bottom ATC antenna and we can begin.
ATC test specialist

We press the start test button.
start test

The testset sends interrogation pulses and the ATC transponder replies to those interrogation pulses. On the testset we can see that there is communication between the testset and the transponder by the flashing interr and reply lights.
interrogate and reply

The test is monitoring continuous data from the aircraft and therefore never ends unless we select stop.
We wait a little while until the ATC testset has collected the data, select stop and we can scroll through the different tests results that we have.
atc test results
The reply delay is the time it takes for the transponder to respond to the interrogation from the testset. There is a minimum time requirement for the different modes of operation. These results are all well within limits.

The reply jitter test is also in limits.
Jitters are timing errors within a system. In a communications system, the accumulation of jitter will eventually lead to data errors.
jitter test

We test the reply frequency from the ATC transponder wich should be 1090Mhz plus or minus a set limit.
reply frequency

The ATCRBS (Air Traffic Control Radio Beacon System) is an enhancement of the primary ATC ground radar. Aircraft equipped with the ATCRBS have a transponder onboard wich responds to interrogations from ATCRBS ground stations.
For example, the basic ATC ground radar is called PSR (primary surveillance radar) or skin paint radar, this is the regular radar wich sees the reflected signals from the aircraft fuselage and displayes this as a dot on a radar screen. The ATCRBS system uses a second radar system called SSR (secondary surveillance radar) wich communicates with the aircraft so that the radar operator also has altitude and ident information on his radar screen so that he or she can more accurately track the airplanes.

In ATCRBS we use 4 modes of operation, the mode 1, mode 2, mode 3/A and mode C. Mode 1 is used by sort military targets during phases of a mission. Mode 2 is used to identify military aircraft missions. Mode 3/A is used to identify each aircraft in the radar's coverage area (this is the ATC mode A, in our case we have selected 1234 on the ATC control panel, this ident code is known as 'squawk'). Mode C is the altitude reporting mode.
There are 2 more modes of operation from the ground ATC stations wich are the mode 4 and the mode S.
Mode S is a more modern system than the ATCRBS and most civil aircraft now use this. The mode S is a two way communications system between air/ground and air/air including much more data and also TCAS (Traffic Collision Avoidance System) information.
The mode 4 is also known as IFF (Identification Friend or Foe) wich is used for military use to identify aircraft as being ally or enemy.

U can see in this next picture the 'CODE' is 1234, this is the Mode 3/A (or mode A) reply from the aircraft, the ALT is 0 FT, this is the Mode C reply from the aircraft meaning that the aircraft is at 0 feet at 1013mb (sea level at standard day).
air traffic control radio beacon system

The ATCRBS interrogation consists of pulses to wich the transponder replies, different pulses and signal strengths decide what kind of interrogation is made but more about that later.
The mode A signal contains the aircraft code in octal format so the 4 numbers range from 0 to 8. The mode C reply contains altitude data in gray code (wich is similar to octal code but less bit changes if the numbers increase or decrease steadily) and the SPI (Special Identification Pulse) is used to identify the aircraft, this last option can be used to identify a specific aircraft on a radar screen by requesting the flight crew to press the 'IDENT' switch on the control panel.
On this next picture u can see the framing pulse spacing between the first and the last framing pulse. Not to be confused with the interrogation pulses, the framing pulses are used to confirm legitimate replies from the aircraft (these pulses are a part of the reply message NOT the interrogation pulses). The correct spacing of these pulses means that the data can be seen as a correct reception of the ATCRBS reply data.
There are limits set for how far these pulses may differ from 20.30 micro seconds and also for the pulse width of both pulses.
pulses
The normal reply signal format would be 15 time slots F1 (framing pulse 1), then 12 slots for the reply data, then F2 (framing pulse 2) and then the SPI discrete. In mode A these 12 slots reply data is the aircraft squawk code set on the ATC control panel (in our case 1234) and in a mode C these same 12 slots reply data is the altitude code in the same octal format as the ident code.

Then my colleague presses the IDENT button the ATC control panel.
ident switch

U can see the IDENT discrete (ID) in front of the 4 digit code, this IDENT function is the SPI discrete in the reply data.
IDENT
Air traffic controllers can request a specific aircraft to press the ident switch so that it would light up on their radar screen as the one that has pressed ident.

Now, the newer mode S system uses the same frequencies as the ATCRBS system but we don't want a ATCRBS equipped aircraft to reply to a mode S interrogation, the mode S system onboard aircraft only reply to specifically adressed interrogations or mode S all call's. Mode S equipped aircraft also respond to mode A and mode C calls.
The ATCRBS system uses a directional antenna to interrogate aircraft, a inherent design feature of this directional antenna is the SLS (side lobe suppression).
The SLS was a enhancement on the directional antenna.
A directional antenna is never fully directional, aircraft flying in the general area of the pulse receive the pulse too but slightly weaker than the aircraft wich is directly in line of the antenna.
directional antenna

Note that the red aircraft would receive the signal much stronger than the green aircraft would.
Right after the directional antenna sends its first pulse it sends a second pulse with a omnidirectional antenna (in all directions).
omnidirectional pulse

The received interrogation signal received by the red aircraft would look like this. The first pulse is received much stronger than the second pulse
side lobe suppression

And the interrogation signal received by the green aircraft would look like this.
side lope supression

Because now the first pulse is equal or smaller than the second pulse the ATCRBS onboard the green aircraft will ignore this interrogation pulse because it assumes it was not directed to it. (side lobe suppression system)
The red aircraft received the first pulse much stronger than the second pulse so this aircraft would reply.
The ATCRBS transponder should be suppressed for 35 + or - 10 microseconds after a SLS pulsepair is received. As I have explained, a SLS pulse pair is when the P1 pulse is equal or smaller than the P2 pulse.

The mode S system uses this design feature. A mode S interrogation is started with two equal strength pulses and then the mode S data block.
Because the two pulses are received equally strong by all aircraft ATCRBS equipped aircraft will not reply because they assume that they are in a side lobe of the directional antenna but the mode S equipped aircraft will see this signal as a mode S interrogation and will reply accordingly (if the aircraft is adressed).
If a ATCRBS interrogation is sent and a mode S equipped aircraft picks up the 2 pulses it will wait for the data word to be received but a ATCRBS interroagtion does not have a data word so the mode S transponder would not reply to a ATCRBS interrogation.
The data word is required for mode S because this system only replies to interrogations directed especially to it (mode S means mode Selective).

We test the SLS (side lobe suppression) for the correct signal strength triggering, u can see that when the P1 and P2 are equal, no reply is given by the transponder and when the P2 pulse is 9 dB lower than the P1 pulse the reply is given.
SLS

We also test the MTL (minimum triggering level in dBm's, dBm indicates power measurement relative to 1 milliwatt) at wich the transponder accepts the interrogations.
Minimum Triggering Level
This screen also shows the ERP (effective radiated power) wich is the strength of the transponders transmitted signal.
The top average applies to average of all replies from the top antenna during the power test sequence, the bot average applies to all the replies from the bottom antenna during the power test sequence and the instant applies to the average of the last 100 replies.

We also test the difference between the MTL (minimum triggering level) between the mode A and the mode C systems, they may not be more than 1 dB apart.
mtl difference

It is nice to know that when a mode A interrogation is made the reply consists of only the squawk code (no altitude) and when a mode C interrogation is made the reply consist of altitude only (no squawk code).
Whether a interrogation signal is a mode A or a mode C is decided by the time between the P1 pulse and the P3 pulse (the P1 and P3 pulses are send by the directional antenna, the P2 pulse is send by the omnidirectional antenna for the side lobe suppression).
signals

The ATCRBS PSR antenna with the SSR antenna mounted on top, the omnidirectional antenna is close to this rotating station usually mounted high on a pole. These ATC stations usually send ATCRBS, mode S and IFF modes all together.
ATCRBS

The ATCRBS system is also able to perform a ALL-CALL to all aircraft around the station.
It would send out 2 pulses with the omnidirectional antenna of wich the first one stronger than the second to ensure that the ATCRBS equipped aircraft would not see the interrogation as a side lobe.
Ground stations wich perform a ALL-CALL nowadays will need mode S transponders to reply to it also so now in a ALL-CALL signal a 1.6 micro second 4th pulse is added wich the mode S system will recognise as a ALL-CALL and will respond. The ATCRBS will ignore the 4th pulse and will just respond to the ALL-CALL anyway.
If the P4 pulse is 0.6 micro seconds in length then only ATCRBS systems will respond.
signals

The mode S all call reply consists of a four pulse preamble and contains a data block with either 56 or 112 bits of information. The four pulse preamble is what identifies this as a Mode S reply.
mode S reply

The mode S reply includes the aircraft registration code wich is decoded by the testset.
mode S all call reply

We also need to test the altitude reporting function of the mode C so we connect a pitot static testset to both the Air Data Computers.
pitotstatic

We set the altitude to 21.000 feet with a barometric pressure of 1013 mb (the QNH the ATC will use for altitude reporting).
The testset picks up the four digit octal altitude code and decodes and displays the aircrafts altitude on the testset.
U can find a list of all the altitudes with their corresponding codes here.
altitude

The code 7220 in brackets behind the altitude is the 4 digit octal code wich corresponds to 21.000 feet of altitude.
altitude code

The mode S system also communicates with TCAS (Traffic Collision Avoidance System) to steer clear of intruder aircraft in the vertical plane.
TCAS

Still want to be avionics? ;-)
schematic

Now we need to test the top antenna.
The testset responds to the antenna with the strongest signal strength so we block the bottom antenna off with a antenna shield.
signal guard

I use the manlift to bring me in line of sight with the top antenna.
manlift

I select the top antenna in the setup screen and I run all the tests again for the top antenna.
top antenna selected

There are lots more test pages on the testset but offcourse u need to refer to the maintenance manual as to wich test u should run. Not all tests need to pass in the auto test depending on configurations and settings in the aircraft and testset.
If u want more information about the IFR 601 testset, here is the instructions manual for it.
manual of the IFR 601 testset