Hope this "nominal" thing will be resolved... Just a thought: the first things Inmarsat published were "arcs" which seemed based on angles. Perhaps this angle was found using arctangent from satellite altitude and BTO "converted" to distance (after taking away some time constant)?
Another remark. Just looking at the data, there is a strange similitude between the last data and the ones of 18:25. And they show the two highest BTO values:
07/03/2014 18:25:27,421 IOR-R600-0-36E1 8 R-Channel RX 0x10 - Log-on Request (ISU)/Log-on Flight Information (SSU) 142[BFO] 17120[BTO]
07/03/2014 18:25:28,852 IOR-P600-0-36FC 10 P-Channel TX 0x11 - Log-on Confirm
07/03/2014 18:25:29,572 IOR-P600-0-36FC 10 P-Channel TX 0x40 - P-/R-Channel Control (ISU)
07/03/2014 18:25:29,572 IOR-P600-0-36FC 10 P-Channel TX Subsequent Signalling Unit
07/03/2014 18:25:30,213 IOR-P600-0-36FC 10 P-Channel TX 0x41 - T-Channel Control (ISU)
07/03/2014 18:25:30,213 IOR-P600-0-36FC 10 P-Channel TX Subsequent Signalling Unit
07/03/2014 18:25:34,461 IOR-R1200-0-36ED 4 R-Channel RX 0x15 - Log-on/Log-off Acknowledge 273[BFO] 51700[BTO]
07/03/2014 18:25:35,408 IOR-P10500-0-386B 10 P-Channel TX 0x15 - Log-on/Log-off Acknowledge
and
08/03/2014 00:19:29,416 IOR-R600-0-36F8 10 R-Channel RX 0x10 - Log-on Request (ISU)/Log-on Flight Information (SSU) 182[BFO] 23000[BTO]
08/03/2014 00:19:31,572 IOR-P600-0-36FC 10 P-Channel TX 0x11 - Log-on Confirm
08/03/2014 00:19:32,212 IOR-P600-0-36FC 10 P-Channel TX 0x40 - P-/R-Channel Control (ISU)
08/03/2014 00:19:32,212 IOR-P600-0-36FC 10 P-Channel TX Subsequent Signalling Unit
08/03/2014 00:19:32,852 IOR-P600-0-36FC 10 P-Channel TX 0x41 - T-Channel Control (ISU)
08/03/2014 00:19:32,852 IOR-P600-0-36FC 10 P-Channel TX Subsequent Signalling Unit
08/03/2014 00:19:37,443 IOR-R1200-0-36F6 10 R-Channel RX 0x15 - Log-on/Log-off Acknowledge -2[BFO] 49660[BTO]
08/03/2014 00:19:38,407 IOR-P10500-0-386B 10 P-Channel TX 0x15 - Log-on/Log-off Acknowledge