I agree with you on the Mode S. The 24 bit ICAO address is used to selectively target aircrafts. Lets say a radar sends out a Mode S interrogation to an aircraft. But simultaneously, a radar in an airport will be continuously receiving squitters from aircrafts upto 100 miles away right? So won't these squitters get mixed up with the Mode S reply? Also what about the case of ADS-B 1090 ES where signals will be sent from and between all aircrafts on the same 1090 MHz every second? Won't this definitely cause a mix up? How can there ever be a proper reception then?

What about ADS-B IN and ADS-B OUT for each aircraft? Again, both are using the same 1090 MHz which only increases the chance of message collisions further.

@Clive The beamwidth of a mode S radar is quite narrow. It does not receive squitters from aircraft all the time. It will only receive squitters from those aircraft that are in the beam, and they are in the radar beam only for a brief time. The probability of overlapping messages within the beam is not that high. Secondly if the signal strength of the messages differs by about 3dB, then it is usually no problem to degarble the overlapping replies.

@Clive Initially ADS-B OUT increases the probability of message collision, it adds at most 6.2 120μs messages per second to the frequency occupation for each aircraft. ADS-B IN will not add messages, it is only listening. On the longer term ADS-B OUT/IN will reduce the need for TCAS interrogations as hybrid ACAS solutions will partially use of ADS-B for conflict detection. Currently most of the frequency occupation is caused by TCAS so the nett effect will be a reduced probability of message overlap.

With reference to ADS-B OUT/IN. I meant to ask that since both IN and OUT use the same frequency, won't they get mixed up (while the signals propagate through air)? The aircraft 1090 ES transmitter/receiver isn't like the radar right (in the sense of high directivity. It works that way for SSR on ground since they are rotating)? Or is it?

ADS-B receivers have in general antennae that cover 360 degrees. They have a much higher change of receiving overlapping messages. But as long as the signal strength difference between the overlapping messages is more than 3dB, they can usually be untangled. If not, the message will just get lost which is not a problem, there will be another one soon enough. A terminal area radar will rotate once every 5 seconds. ADS-B transmits position 10 times in that period. To have a radar like update rate from ADS-B, you can loose about 90% of the messages.

Thankyou DeltaLima for your answers. One more question. The normal transpoder responds in the direction of the received radar beam. So how does the transponder antenna do that? Won't it have to rotate to face the required direction? Also, is the ADS-B transmitter different from the transponder antenna? 1090 ES is actually an extension of MODE S transponder right? Do both use the same antenna? Can an antenna have both functions ie of broadcast antenna and of directional antenna?

@Clive The transponder does not reply directionally, the reply is in all directions. Larger aircraft have two antennae, one on top, one on the bottom. The antenna that received the strongest signal from the interrogation will be used for the reply. Usually ADS-B transmissions are done by the Mode S transponder and go out the same antennae, alternating between top and bottom. The TCAS receiver uses a direction finding antenna for reception, but transmits omnidirectional.

"The transponder detects incoming radar signals and broadcasts an amplified, encoded radio signal in the direction of the detected radar wave." Source : science.howstuffworks.com/transport/flight/modern/…. (Probably misleading?)

@Clive Yes, definitely misleading. There are surveillance systems that do not have co-located interrogators and receivers. These multilateration systems work because the reply is in all directions.

Thanks. Also, do you know which kind of modulation is used for transmission of signals from Radar-Aircraft and vice versa? Yes, I know that the interrogation signal uses DPSK and the reply is PPM (Shouldn't it be Manchester Coding?? I think thats more appropriate.). But digital signals can't travel through space, can it? So it should use some form of analog modulation like frequency or amplitude modulation (like the LDACS use OFDM modulation. ADS-B is operating in the L band too, so is it OFDM here too)?

@Clive For the downlink, the 1090MHz carrier frequency is pulse position modulation. I.e. it switches on and off rapidly.

I guess you could see it as a form of amplitude modulation with the amplitude being either 0 or 1.

Are you referring to the data bits of Mode S reply?

Yes.

The preamble starts with a number of framing pulses, basically telling when the first bit will start.

Each bit is in us long

If there is a pulse of 0.5us in the first half of the bit frame, it is regarded a 0. If the pulse is in the second half, it is a 1.

Or the other way around, I don't remember

Although it is stated everywhere as Pulse Position Modulation (which is analog modulation isn't it? i.e. original data is analog), Manchester Coding matches your description perfectly (and its for digital data).

It is pulse position modulation in the analog world. In the RF frontend of the receiver it is typically converted to a digital manchester encoding

In the analogue world, the envelope outlined by the Manchester coded part of your drawing is filled with a 1090MHz signal. So a block of 0.5us will see about 545 periodic waves of constant amplitude

correction : ppm is digital modulation though used in analog world

So you mean amplitude modulation is used for SSR interrogations and replies? I read all ATC radio comms are Amplitude modulated.

If you want to build your own receiver you can look for the miniadsb project. It uses a series of amplifiers and AD8313 logamp to convert it to a machester coded signal. The difference between the two is that the machester is voltage based, the ppm is RF power based.

That is VHF voice comm

I am presenting a seminar on ADS-B. That is why I ask a lot of question. :P

Who is your audience?

(in College)

Just students and a few professors.

I am heavily involved in the development and standardisation of ADS-B in Europe and worldwide, so I guess you have found the right person ;-)

Yes I am indeed quite lucky. I did read your profile.

Does the ATC ground radar have two antennas? Or just one (duplexing)?

for reception and transmission

There are several antennas in most ground radars. However, the main beam is usually duplexed

The problem is that a narrow beam causes side lobes as well.

Aircraft in these sidelobes are interrogated as well, and reply

That is compensated in the MODES by using P2 pulse in the interrogation isn't it

The sidelobes have lower power than the main lobe, so a radar uses a second omnidirectional antenna to indicate the power of the side lobes.

Indeed

If the P2 pulse from the omni is stronger than the other pulses, the aircraft will not reply

Is there any encryption being used for ADS-B? I haven't come across any sources that say so.

No there is no encryption

So ADS-B is prone to attacks?

It certainly has its weaknesses, but there are many ways to detect and prevent attacks.

forbes.com/sites/andygreenberg/2012/07/25/…

In general all forms of surveillance have their security weakness, the trick is to make these weaknesses complementary so that the total system is resilient against attacks.

how are they detected?

Typically multiple surveillance techniques are used and crosschecked

That would be almost impossible in the case of ground station flood denial wouldn't it?

True, but that is a weakness of all radio technoligies isn't it?

Yes. But if encryption was put in place, maybe it wouldn't be so prone.

But then it would be quite useless as well.

The idea of ADS-B is to broadcast your presence so interesting parties can take action not to crash into you.

Yes you're right

Some data could be encrypted, but then still with the current modulation scheme a flood denial would do damage.

Thanks for your help. I will get in touch later if further questions arise. Got to read some reports on ADS-B.

A system could be developed that is less prone to flood attacks whilst also providing encrypted connections for some data. The problem is that due to all regulations the aviation industry is very conservative, and very expensive.

by the way, if some form of multiplexing was used don't you think message collision probability could be reduced?

Your welcome, just ping me on this chat if you need more info.

Ok :)

Yes, a lot could be done. But you have to realize that the system is inherited from the 1950's and that changing things in aviation takes a very long time.

If you like to work on modern radio technology, avoid aviation :-)

Are you studying electronics engineering?

Yes. I will be graduating next year. Hope to do masters in something...maybe communications related (any suggestions?). This is the first time I'm reading up a lot on aviation related stuff. Its quite interesting.