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Transponders have registers for Mode S data and configuration. These registers are updated periodically to provide Mode S services such as ADS-B Airborne Position squitter.

DO-260B and similar documents have Register Allocation Tables that states maximum update intervals for each register(if applicable).

But there is also a register timeout mechanism that states "if a register can not be updated within twice the maximum update interval, it shall be zeroized".

So, what happens if certain register is updated at every (1.5 * maxUpdateInterval) ? Are these maximum update times mere suggestions or is there another rule that forces us to update registers at most maximum update interval?

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If you update the register, the values will be set to the values provided. From that moment, a timer starts running. When the timer reaches 2 * maxUpdateInterval, the register will be set to all zero.

If a radar extracts the register before the 2 * maxUpdateInterval is reached, it will get the values provided at the update. If it will extract the register after 2 * maxUpdateInterval is reached, it will get all zeros in the reply.

For ADS-B, transmission of the register is inhibited (IIRC) when the register is set to all zeros.

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  • $\begingroup$ I am quoting the DO-260B exactly below( appendix A, page A-53). "The ADS-B transmitting subsystem will clear all 56 bits of the Airborne Velocity message if no data is received within 2.6 seconds of the previous input data update." The maximum update interval table says 1.3 s is the m.u.i. of Airborne Velocity register. $\endgroup$ – Mehmet Çağrı Köse Mar 29 '17 at 10:39
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The minimum update interval and the timeout is there to avoid broadcasting "stale" data which can be misleading. You also have to consider that the update rate is primarily driven by the source of the data.

Most older nav systems (TSO C-129 GPS or FMS) output velocity data once per second, so the standard has to accommodate these systems. The 1.3 sec m.u.i. is there to allow for jitter in the data rate. The 2.6 sec timeout allows you to keep broadcasting in the case where a single data word is lost or corrupted in transmission.

If the update interval goes beyond the 2.6 sec, the register is cleared to avoid broadcasting the "stale" data. The reality is you wouldn't see a system that regularly updates at 1.5 x m.u.i.

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  • $\begingroup$ Do you mean i shall send last valid data between 1.3 and 2.6 second time mark( in case data starts to come as invalid). $\endgroup$ – Mehmet Çağrı Köse Mar 30 '17 at 6:13
  • $\begingroup$ Populating the GICB registers and transmitting data out of the registers are independent functions. The population function fills each register and starts a timer. The timer will reset every time the data updates. IF the timer gets to the timeout (2.6 sec in this case), it zeroizes the register. The transmit side doesn't care what the population side is doing. When a GICB request comes in or the ADS-B squitter is triggered, they grab whatever is in the register. The 1.3 sec is a normal design function. Nothing happens if you go past it until you get to the 2.6 sec limit. $\endgroup$ – Gerry Mar 30 '17 at 12:49

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