nRF51 - time of flight / distance

[travisf]

I'm interested in creating a time of flight algorithm and am trying to calculate what to expect for accuracy.

Is it possible to get an IRQ from when the sync arrives at the RX modem? If so, how deterministic is the timing of the RX modem? Can this IRQ trigger a timer? Can this timer save information using DMA?

Thanks!

[Janek Mann]

That's an interesting question (and might be best to just try it out, I'd be curious to know what you find!). However since the clocks on both devices are 16MHz it would not surprise me if you ended up with the same measured end-to-end delay each time (or needed to wait a long time for the clocks to desynchronise). In other words, in order to be able to measure the distance with higher resolution by averaging, you need the timing jitter to be >= 1 clock cycle, with a known distribution.

[Janek Mann]

Ah I didn't see your last comment before posting mine. 24 feet (7.5m) is actually very impressive, and could even be useful in some scenarios! I also suspect there are various sources of non-uniformity in the distribution (there'll be a PLL in the receiver radio locking onto the clock from the transmitter {or isn't there? The RM / PS are rather concise}, and this will interact with the address resolver in some fashion which may be difficult to predict).