Fine calibration: one of many SSS improvements

SSS itself implemented, I’m now trying to process a back-log of improvements that were made since the algorithm’s initial publication in the mid-2000s. There are four or five of these modifications, some of which will boost noise rejection by an order of magnitude or more. The first set of three improvements goes under the umbrella term “fine calibration.”

The SSS algorithm depends heavily on the location and geometry of the MEG sensors. Therefore, it’s not surprising that any small error in the believed location or behavior these sensors will introduce a significant error in the filter’s output. Fine calibration consists of three modifications to correct for these sensor inconsistencies. For all of these improvements, we record empty room data and construct a “fine calibration” file. The first fix updates the orientation of each sensor coil. Because the sensor coils pickup the magnetic flux through their coil loops, a more accurate estimate of the true orientation will yield more accurate representation in the multipolar moment space. The second fix concerns the gradiometers only. Again, there are small imperfections in the MEG coils, and gradiometers measure small signal differences between pairs of loops. If one gradiometer loop has any physical differences from its twin, a substantial error will be introduced into the recorded signal. Therefore, we simulate small point-like magnetometers at the center of each gradiometer to account for this gradiometer “imbalance.” The third and final fix is concerned with imperfections in the magnetometers. Again, we’re dealing with physical devices, so we measure if any of these sensors have readings that are slightly too high or low in amplitude and correct for this with a calibration coefficient from that same fine calibration file. This final improvement has a relatively small effect compared to the first two.

I’ve finished the code for the fine calibration implementation, but the filtered results aren’t a close enough match with the proprietary code just yet.  On the bright side, the undiscovered bug not causing the filter to completely fail. Once I find the issue, I’ll be on a sprint to implement temporal SSS before the end of summer!