FDC
Data Latency
Dave Jesse on December 5, 2012
1

One important characteristic of flight data is that, as we discussed previously, the parameters are sampled different rates and there may be different latency (the time from the measurement being taken to being stored) and so before we can do any computations we need to look at the timing issues in some detail.

When computing the sample point for a parameter with no latency, the time from the start of the data frame is given by T = n.dt where n is the number of samples from the start of the frame and dt is the sample interval. For example, when there are 64 samples taken in each second, the third sample will be 3/64ths of a second from the beginning of the data frame.

This changes for a parameter with latency L, as the time of the measurement with respect to the start of the data frame is given by

T = n.dt – L

For a sample with 100mS latency and stored in word 50 of our 64 word frame, the value of T is 681.25 mS (I’ll leave you to check the maths).

For the really switched on readers, you will have spotted that parameters with large latency sampled close to the start of the frame can have negative T values. This means that the parameter was actually sampled during the preceding frame.

This issue of data latency needs to be catered for before any calculations can be performed as described in my previous blog.

So my next blog subject is in your hands. Would you like to read about how to calculate heading or how to measure outside air temperature? Place your votes below!

TTFN

Dave