As the FAA mentions, the ADS-B messages like Identification, Position and Velocity are transmitted randomly uniquely between $ 04 \pm 02 $ sec.
How does the transmitter define that randomization? Is it standard or fixed or else who decides that?
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$\begingroup$ Presumably it uses a form of a Random Number Generator, like any other computer would use to generate random numbers. I'm also not sure that "standard/fixed randomization" is a concept that exists. $\endgroup$– zymhanCommented Feb 7, 2020 at 13:50
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$\begingroup$ The random delay is introduced by a function called General Formatter / Manager, it is uniform and its unit of time must be less than 15 ms. Non cryptographic uniform random sequences are usually obtained using Linear-feedback shift registers but I don't know what is used for ADS-B Out.. $\endgroup$– minsCommented Jun 22, 2021 at 23:05
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The standard DO-260B dictates that ADS-B devices must generate uniformly distributed random numbers. They do that by using a PNRG algorithm.
Note sure where the number $ 04 \pm 02 $ comes from, indeed the broadcast randomization specifications are the following (source§7.3.2):
| Message | Condition | Interval |
|---|---|---|
| Airborne position | - | $ 0.5 \pm 0.1 $ sec |
| Airborne velocity | - | $ 0.5 \pm 0.1 $ sec |
| Surface position | Moving | $ 0.5 \pm 0.1 $ sec |
| Surface position | Stationary on ground | $ 5 \pm 0.2 $ sec |
| Aircraft idententification | Moving or Airbone | $ 5 \pm 0.2 $ sec |
| Aircraft idententification | Stationary on ground | $ 10 \pm 0.2 $ sec |
| TSS | Airbone and valid state | $ 1.25 \pm 0.05 $ sec |
| Aircraft Operational Status | Various conditions | Complex (see source) |
