Correct the group index

An OTDR turns time into distance with the fibre’s group index (IOR). If the instrument was set to a wrong value, every distance in the file is off by the same proportion, and events land in the wrong place on the route. You can tell ruumica the right value for one file; the file itself is never changed.

Before you start

• The trace is ingested, or ready to be (Ingest OTDR traces).
• You have the group index from the cable manufacturer’s data sheet for this wavelength: a value between 1.40 and 1.60, with at most five decimals.

Steps

1. See which value the file uses. It is shown as n = …:

   $ asbuilt sor inspect fixtures/projects/ee-demo/traces/c2_f05_1310_ba.sor
   …
   supplier asbuilt / SYNTH-OTDR  wavelength 1310.0 nm  pulse 30 ns  n = 1.468
   …
   

2. Ingest the trace again with --ior and the data sheet’s value:

   $ asbuilt sor ingest --project ee-demo --cable FO-12-002 --fiber 5 --direction B_TO_A --from-node CAB-3 --to-node SC-1 --ior 1.4700 fixtures/projects/ee-demo/traces/c2_f05_1310_ba.sor
   927a492ce30d7f78  PROVISIONAL  end 152.201 m  factor 1.0199
     Warning OPTICAL_UNIDIRECTIONAL: one direction only: the verdict stays PROVISIONAL until the opposite direction is paired
     Info IOR_OVERRIDDEN: group index 1.47 used instead of the file's 1.468; distances scaled by 1.468/1.47
   

   Every distance is recomputed: here the fibre end moves from 152.408 m to 152.201 m.

3. To undo it, ingest the trace again without --ior. It returns to the file’s own value:

   $ asbuilt sor ingest --project ee-demo --cable FO-12-002 --fiber 5 --direction B_TO_A --from-node CAB-3 --to-node SC-1 fixtures/projects/ee-demo/traces/c2_f05_1310_ba.sor
   927a492ce30d7f78  PROVISIONAL  end 152.408 m  factor 1.0215
     Warning OPTICAL_UNIDIRECTIONAL: one direction only: the verdict stays PROVISIONAL until the opposite direction is paired
   

For a pair, give each file its own value: --ior applies only to the file it is given with. With asbuilt sor pair, use --ior-ab and --ior-ba.

Check

The output shows IOR_OVERRIDDEN with both values. The closeout book and package.json (ior_override) record the value used, so a reader can see that distances were corrected.

What can go wrong

You see	Meaning	Do
is outside 1.40–1.60	The value is not a plausible group index for silica fibre.	Check the data sheet.
has more than 5 decimals	ruumica stores five decimals.	Round to five decimals.