A. Sen Gupta

A Time Transfer System using Pseudo Random Noise (PRN) Biphase Modulated Carrier

The standard time and frequency signal broadcast by NPL via one of the narrowband channels of INSAT has been regularly operational since last five years and has been successfully used by several users in different parts of India. The present paper investigates a possibility of introducing a new modulation scheme namely, a Pseudo Random Noise (PRN) biphase modulation. The PRN sequence used has been truncated by about 2% to make it easier to generate and handle. It has however been shown that such truncation does not significantly affect the autocorrelation properties of the sequence. An attempt has been made to keep the data transmission structure similar to that in the present broadcast as far as possible. The complete time transfer system consisting of the transmitter and the receiver have been developed. The schematics have been discussed in detail in this paper. Although the present system has not been tried out on an actual satellite channel it has been tested extensively on the bench.

Ata and Its Users

The National Phyical Laboratory (NPL), in collaboration with Overseas Communications Service (OCS), has been giving HF time broadcast tinder the call sign ATA since 1959. These signals are being used by a large number of scientific users in India and the neighbouring countries as a reliable source of standard time. Since its inception, the transmitting system of ATA has undergone various stages of development leading to its present state. This paper reviews these stages of development and describes the present set-up in details. The individual responsibilities of NPL and OCS for ATA broadcast are explained. Some future plans for development have also been discussed.Recently, a questionnaire on the usage of ATA was circulated in India. The varied response of the users to this have also been described.

Precise T & F Intercomparison via VLF Phase Measurements

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, accurac...

Comparison of Caesium fountain clocks in Europe and Asia

A remote comparison campaign of Caesium fountain clocks from four National Metrology Institutes in Europe and Asia was carried out in May 2013. Six fountains at Physikalisch-Technische Bundesanstalt (PTB) Germany, Scientific Research Institute of Physical-Technical and Radio Technical Measurements (VNIIFTRI-SU) Russia, National Physical Laboratory of India (NPLI) India and National Institute of Metrology (NIM) China were compared by Two-Way Satellite Time and Frequency Transfer (TWSTFT) and GPS Carrier Phase (GPS CP) techniques. The frequency differences and the comparison uncertainties between the compared fountain pairs were evaluated. The comparison uncertainty (uc) results from combining the uncertainties of the two compared fountain clocks and the uncertainty introduced by the comparison link. All the frequency differences between the fountains agreed within the 1-sigma uncertainty in the low 10-15 level. We present in this paper the measurements and the evaluation of the campaign in details.

A novel approach towards analysis of 2-dimensional spatial intensity profile of laser beams

Using a 1-D photodiode array in place of a conventional 2-D array for capturing the spatial intensity profile of a laser beam yields a 2-D profile instead of a highly detailed 3-D profile; nonetheless it also reduces the cost and computational requirements substantially. We have developed one such low cost prototype hardware-software package and demonstrated that many features, such as ultra portability, can be included if a highly detailed 3-D profile is not absolutely necessary. In this paper, we present an elementary description of the instrument developed and discuss some of its experimental outputs.

Results of differential mode of operation of the Standard Time and Frequency Signal (STFS) broadcast via INSAT

We describe the operation of the Standard Time & Frequency Signal (STFS) broadcast via INSAT satellite in the so-called online differential mode. The transmission setup at the Delhi Earth station and the recent version of the STFS decoder are briefly described. Regular data recording carried out at NPL, New Delhi, NRL, Trombay, field station at Mt Abu and ERTL(E), Calcutta have been used in this paper to draw conclusions regarding improvement in time transfer accuracy.

Microsecond Time Synchronisation between GMRT, Pune and NPL, New Delhi using INSAT Standard Time and Frequency Broadcast in a Differential Mode

We demonstrate the capability of the Standard Time and Frequency Signal (STFS) broadcast via INSAT to provide time synchronisation with the Indian Standard Time to a 1–2 μsec accuracy in a differential mode. The data used in this study comprise of (a) those recorded at the Giant Metrewave Radio Telescope (GMRT) site at Khodad, Pune and (b) those simultaneously recorded at NPL. GPS receivers have been used at Khodad and at NPL to provide the time reference for measurements of the residual errors of the received STFS time. The residual time error in the received STFS is primarily due to error in the predicted satellite position and is similar at both stations. The common mode error largely cancels in the differential STFS leading to the present high time transfer accuracy.

Digital Phase Lock Loop for Tracking Very Low Frequencies

The present paper describes the design of a Digital Phase Lock Loop (DPLL) for tracking the phase of an input signal of 5 kHz. The DPLL uses 5 MHz external reference frequency sources which is divided down to 5 kHz for phase comparison with the input signal. The phase error depending on the polarity is converted into add or subtract correction pulses which are applied to the reference 2 MHz pulses train. The corrections thus applied, shift the phase of the reference 5 kHz in discrete steps of 0.2 μs in proper direction till phase lock is achieved.The DPLL circuit has been tested both on a simulated signal as well as using received 5 kHz time signals from INSAT. It has been found that an input signal in excess of 200 mV is required for proper locking and the locking range is ±1 parts in 106.

Time Transfer Experiments with Indian Experimental Satellite “Apple”

The availability of Indian Experimental Satellite “APPLE” gave an opportunity to try out various techniques and to firm up the specifications of the hardware and software required to be developed for precise time transfer, especially in relevance to a time dissemination service via Indian Operational Satellite INSAT-1. Some of the time transfer techniques were earlier experimented with French-German Satellite “Symphonie”. Time dissemination techniques such as TV, both passive and active and transmission of time code likely to be used with satellite INSAT-1B were tried out on APPLE on an experimental basis. The techniques of transmitting one pulse per second (1 PPS) to the satellite was used to range the satellite and it was compared with the conventional tone range technique. A simple, satellite orbit determination programme using the range data was developed and tested. This paper gives the results of the experiments carried out via “APPLE”.

ATA Field Strength Computation over the Indian Subcontinent

An attempt has been made to compute the received field strengths of ATA transmissions over the Indian Subcontinent. This short communication describes some illustrative results of these computations. These results are expected to serve as useful guide to the ATA users.