Precise T & F Intercomparison via VLF Phase Measurements

Abstract

The Indian subcontinent does not fall in the groundwave range of any LOKAN-C transmission. As such, at present the only alternative technique in this region for convenient and routinc T & F intercomparison is via VLF phase measurements. At NPL, New Delhi, continuous phase recording of the 16-kHz transmissions from GBR (IK) is being made. In addition, the published mid-day phase data of GBR from several laboratories-NPL (UK). RGO (Uk), PTB (FRG) and USNO (USA)—are being received regularly. In the present paper, we discuss T & F intercomparisons between the local time scale, LTC (India), at NPL and those at the above-mentioned laboratories, using the VLF phase data. A major factor which limits the accuracy of long-term comparison is the seasonal variation in the YLF propagation delay over long paths. It is shown that by taking into account the seasonal propagation delay variations in a semi-empirical way, the accuracy of T & F comparisons can be considerably improved. In fact over a one-year period, accuracy of a few parts in 10¹⁴ in frequency and 1–2 μs in time have been obtained.

The relative frequency offset difference between LTC (India, and UTC (PTB) evaluated in the present work as (7.0±0.1) 10^-13 agrees very well with that obtained via the satellite experiment described in a companion paper.