OTDR Event Distance and Index of Refraction

Converts OTDR round-trip time to fiber distance with the entered and true group indices, then removes loose-tube excess fiber and recorded splice slack to estimate ground-route distance. A 0.0018 index error moves a 5.1 km event about 6.3 m and a 40 km event about 49 m; cable construction and storage loops can move the dig location farther still. The cable datasheet, route as-built, and applicable test standard govern.

d = c t / (2 n); corrected distance uses the fiber's true group index; cable distance removes entered excess fiber, and route distance removes recorded storage slack.

OTDR time-of-flight distance and loose-tube excess-fiber conventions. Group index and excess length are ENTERED from the cable data at the test wavelength.

A time-of-flight conversion and 2 entered length corrections.

Estimate. AHJ and licensed professional govern.

Field names used by the API: round_trip_time_us, entered_group_index, true_group_index, excess_fiber_pct, slack_per_splice_ft, splice_points, comparison_span_km, entered_distance_m, corrected_distance_m, index_error_m, comparison_error_m, route_distance_m

• Group index is entered specific to fiber type and test wavelengththe cable manufacturer's data
• Slack is known all storage loops on the path are countedthe route as-built
• Trace interpretation excluded pulse width, dead zones, gainers and event identification are not modeledthe applicable test procedure