Mode S Question

I was always under the impression that our Mode S “address” (hex, octal, or binary), was pretty much arbitrary. In that the FAA match an address to a registration, in no logical assignment method.

Fast-forward to now,
Just swapped out my KY76A for a KT74 Mode S transponder.

In the setup, you enter the aircraft ID (registration).
The hex address comes next, and I was armed with that info, ready to plug it in.

To my surprise, after entering my N-number and moving to the next screen for Hex Address, the pre-entered value equaled my unique address.

(I did not supply my N-number to the supplier of the transponder so they did not pre-enter that info. And besides, the N-Number field was blank at the beginning of the setup)

A couple of google searches for a “formula or algorithm” that converts N-numbers to Hex came with mixed results.

Some hits say there is (for certain countries), other hits say no.

I can say this, unless my new KT74 is possessed with some ability to read minds, there has to be an algorithm and the transponder is programmed to decode (or encode, depending on your perspective)

Not sure what I’m getting at with this post, just thought it was interesting.

Two years ago, I installed a Stratus ESG and it knew the correct code once N number was entered.

You're correct. In the US (not sure about elsewhere) your hexadecimal "international" address is linked to your N number by a straightforward formula (straightforward, that is, to those who can count by 16's!), and some ADSB devices will calculate your hexadecimal address from the N number, some the other way around, some want you to enter both.
Edit: you can find internet sites that will decode either an N number, or a hexadecimal one, into the other.

BobTurner said:

You're correct. In the US (not sure about elsewhere) your hexadecimal "international" address is linked to your N number by a straightforward formula (straightforward, that is, to those who can count by 16's!), and some ADSB devices will calculate your hexadecimal address from the N number, some the other way around, some want you to enter both.
Edit: you can find internet sites that will decode either an N number, or a hexadecimal one, into the other.

Click to expand...

I knew about the conversion sites.
But I always thought it just pointed to a “master list”.

I ran a very crude and rough test,,
Look up the Hex code for N1
Then look up N2

The hex addresses are not incremented by one.
So there is some real voodoo math going on in the background.

I know, I know,,
Not a very good experiment.

And I am not a programmer, nor did I stay in a Holiday Inn Express last night.

Anyway, it surprised me.
I learned something.

Can definitely say in New Zealand the algorithm does not work. I used one of the “ find you hex code “ websites but it turned out the value it came up with was not what NZCAA allocated to the registration.

StephenC said:

Can definitely say in New Zealand the algorithm does not work. I used one of the “ find you hex code “ websites but it turned out the value it came up with was not what NZCAA allocated to the registration.

Click to expand...

Since you are down under, did you try turning the algorithm upside down?