It turns out that Dr. Holland of the DSTG provides an extended discussion of the 18:25:27 logon event in his paper "The Use of Burst Frequency Offsets in the Search for MH370". A link to that paper is provided below.
Holland Paper
This narrative will use screen shot cut-pastes from the Holland paper.
The first screen shot below shows the BFO data logged at and following the 18:25:27 event.
When one looks at Figure 7 above it is tempting to characterize it as an "overshoot" commonly associated with control system transient behavior. That is the conclusion reached by the IG and independent blogger DrB. However, when one looks at all the other logon events presented in the Holland paper, no other logon BFO history has an overshoot signature. Several such events are shown below (screen shot from Figure 9 of the Holland paper).
Only logon 7 (the MH370 18:25:27 logon) displays what resembles an overshoot. Holland himself does not attempt to explain the 18:25:27 BFO data as overshoot behavior. Instead he elects to discard the 18:25:27 BFO as an outlier.
A casual reader might find the outlier explanation acceptable. Unfortunately other data does not support this conclusion. In particular the radar data logged at the time of the 18:25:27 event is as presented by the DSTG below.
Aircraft Speed: 510 knots
Aircraft Track: 296 degrees @6.8N 95.9E
When one computes the BFO associated with the above radar data, one finds a value of 142Hz which just happens to be the value logged by Inmarsat at 18:25:27, and discarded by Holland as an outlier. The IG/DrB do not discard the first logged value, but insist it is simply part of the overshoot behavior associated with the OCXO warmup after turn-on, and it is serendipitous that it happens to agree perfectly with the expected BFO value. The IG/DrB are also apparently not disturbed by the fact that none of the other logon events presented by Holland exhibit any sign of overshoot behavior.
And so it goes... Draw your own conclusions.
