Question
What is a Root Allan Variance (RAV) curve? What information can I get from it?
Answer
Root Allan Variance, or Allan Variance method was developed by David Allan, in the 1960s, for the purpose of monitoring stability in atomic clocks,but the method works very well for studying gyroscope bias as well. IEEE-STD-952-1997, Appendix B, provides a lot of detailed information on thismethod. Various useful pieces of information can be gleaned from this plot:The Angle Random Walk (ARW) can be derived from the RAV plot at an integration time of 1 second.The RAV value at this integration time also equals the noise density of the gyroscope. The ARW corresponds to the region with slope of -0.5 in theRAV chart.In-run Bias or Bias Stability is often pulled from the minimum value observable on the RAV chart. While this value is often given as directlyoff the chart, it really should be corrected by dividing by 0.66 (B in IEEE standards) to correct the RAV variance for actual performance variation(variance of averages is what is shown on the RAV).Rate Ramp corresponds to the region with slope of +1 in the RAV chart. This is a representation of long term drift in the gyroscope output.