B.S. Mathur

Satellite broadcasting of time and frequency signals

The authors discuss the advantages of broadcasting time from a satellite over the more traditional ground based methods such as short wave and low frequency standard time and frequency broadcasts. The authors describe several one way satellite time services currently operational and discuss, in particular, a broadcast service using the Indian domestic satellite INSAT. The signal format, transmitting and receiving setups, and the methods for satellite position prediction are described in detail for the INSAT broadcast. Some typical results of time and frequency calibration are also discussed. >

Standard time and frequency signal broadcast via INSAT-accuracy improvements using differential mode

We describe the standard time and frequency signal (STFS) broadcast via the INSAT satellite and its use in accurate time synchronization and frequency calibration anywhere in India. Under normal operation it is possible to achieve a synchronization accuracy of better than /spl plusmn/20 /spl mu/s. For users with higher accuracy requirements we discuss the capability of the STFS broadcast to provide time synchronization with an accuracy better than 1 /spl mu/s using a mode of operation which we call differential STFS. Here we further correct the error residuals in the received STFS using data from a reference station collocated with the uplinking site. Both analytical and actual experimental data are presented.

Passive TV technique for time transfer via Indian Satellite INSAT-1B

The use of TV signals for time comparison is a very well-known technique. The TV signal can be utilized both passively and actively for the same purpose. Recently, this technique has become popular with the use of geosynchronous satellites for TV transmission. Indian Satellite INSAT-1B transmits TV signals to have a full coverage over the Indian subcontinent. So, attempts are being made to utilize these TV signals for time transfer from the National Physical Laboratory (NPL), New Delhi, to other users in India. In the terrestrial TV network, differential propagation delay remains to be a constant factor unless there is a change in the network configuration. But in the satellite TV network, differential propagation delay does not remain constant because the satellite position with respect to the earth does change around its mean value due to the non-circular satellite orbit and nonzero inclination of the plane of the satellite orbit with respect to equatorial plane. So the maximum variations of differential propagation delay about its mean value have been worked out for different locations in India based on some theoretical calculations coupled with findings of some general behavior of INSAT-1B. These observations predict that the variation of differential propagation delay from its mean value will be within 10 μs for most parts of India and half of India will remain even within the range of 5 μs. These observations tally well with those obtained with the help of the actual predictions of satellite positions. Experimental data also corroborate these findings.

An Automatic Data Recording System for time Transfer Via TV Network

The utilization of TV network for precise time-transfer is very well-known. TV telecast in India is usually in the evening hours which limits the extensive utilization of the technique, mainly because of the manning problem. National Physical Laboratory, has developed an automatic recording system to facilitate the data taking. This system automatically switches on the TV receiver at the pre-programmed time, takes the measurements, stores data in the memory cell and switches off the TV after the measurements are over. This will encourage the use of time transfer via TV network in India.

Simultaneous Two-Way Time Transfers between National Physical Laboratory, Space Applications Centre and Madras Earth Station Via the Symphonie Satellite

Simultaneous two-way time transfer experiments via satellite symphonie, in which National Physical Laboratory, New Delhi (NPL), Space Applications Centre (SAC), Ahmedabad and Madras Earth Station, Madras (MES) participated, are reported in this paper. The uncertainties involved in alternate one-way mode of clock synchronization reported earlier were removed and a much improved precision and accuracy was achieved. In addition, the results obtained in a Round Robin method, where one station transmits and the other two receive between NPL, SAC and MES are also presented.

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.

Broadcast of ATA Time Signals Via Satellite Symphonie

An experiment, to study the improvements in accuracy and precision in broadcasting standard time ATA format via satellite over ATA high frequency broadcast, is described. The ATA format was transmitted in both, the routine code with ticks and tones and with BCD code carrying information about time and date i.e. Year, Month, Day, Hour, Minute, Second and DUT1 correction. The signals were transmitted from Delhi Earth Station and measurements were made at Delhi Earth Station (DES), Ahmedabad Earth Station (AES), Madras Earth Station (MES) and Transportable Remote Area Communication Terminal (TRACT) stationed at Calcutta: thus covering a large cross-section of India.

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...

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.