It's a [substitution cipher][1]:  A=1, B=2, ..., Z=26.

Decoded, it reads *(station 675-525)*:

>! I WILL RETURN BATMAN

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There are also two other messages that you do no get until later in the game.  One of them is *(station 500-900)*:

> 5 15 9 7 21 18 18 14 3 5 24 15 12 7 22 3 10 5 15 9 22 3 8 25 26 15 16 25 10 15 17 25

This one is still a mono-alphabetic substitution cipher, but with a slightly different code:  A=3, B=2, C=1, D=26, E=25, ..., Y=5, Z=4.

When finally decoded, it reads

>! YOU WILL PAY FOR WHAT YOU HAVE DONE TO ME

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The last one is *(station 200-500 - you have to beat the game first??)*

> 3 20 26 18 26 16 24 1 11 4 24 9 3 8 5 2 12 18 6 16 7 11 3 10 17 5 13 4 21 8

Unfortunately, this one doesn't seem to be a mono-alphabetic substitution cipher - no matter what substitutions we use, we get gibberish:

> CTZRZPXAKDXICHEBLRFPGKCJQEMDUH

There are rumors circulating (originating, it appears, with IGN) that when decoded, this reads  *"I am your darkest nightmare dark night.  Gotham city waits for his savior."*  However, this is clearly wrong - there are way too many letters!

Fortunately, not too long ago [one of the super-sleuths on Batman: Arkham City forums decoded it][3]:

>! If you treat it as a [Vigenère cipher][4] using the key SCARECROW, the message comes out to:  
>! FEAR WILL TEAR GOTHAM CITY TO SHREDS  
>! Therefore, it is pretty obvious **the messages are being sent by Scarecrow**.


  [1]: http://en.wikipedia.org/wiki/Substitution_cipher
  [3]: http://community.batmanarkhamcity.com/forums/showthread.php/6627-HIdden-Cryptographic-Sequencer-Message/page4
  [4]: http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher