Mithlesh Saxena

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.

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.

A Study on Time Link Between National Physical Laboratory and Sikandarabad Earth Station via INSAT-TV Network

Utilising TV signal directly from the satellite INSAT-1 in a common view mode, an experiment has been carried out to compare the clock at the Sikandarabad earth station with respect to the clock at National Physical Laboratory, New Delhi. These data have been analysed and an accuracy of 500 nanoseconds could be achieved. The propagation delay has been calculated with the help of the coordinates of INSAT-1 predicted from satellite orbital parameters. The impact of the error in the prediction of coordinates on the accuracy of time comparison has also been discussed.

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”.

Precise Time Transfer Using Direct Television Broadcast by the Satellite Symphonie

Precise time transfers were made and the two atomic caesium clocks at Delhi Earth Station (DES) and Ahmedabad Earth Station (AES) were synchronized to submicrosecond precision, using direct television broadcast of Delhi Doordarshan from DES to AES by the satellite symphonie. A passive technique, where a line-10 of the odd field of TV format was identified and using as the transfer pulse or time marker between participating stations, was used. The limitations and future prospects of this technique, in view of Indian Space Programme, are discussed.

Precise T & F Intercomparison Between NPL, India and PTB, Federal Republic of Germany via Satellite Symphonie-1

In this paper, we report a T and F intercomparison experiment between the National Physical Laboratory (NPL), New Delhi, India and Physikalisch-Technische Bundesanstalt (PTB), Federal Republic of Germany, carried out from May 16 to June 29, 1979. The participating earth stations were New Delhi, India and Raisting, FRG. The NPL clock was placed at New Delhi Earth Station and the Raisting Clock was calibrated with PTB/Primary standard via LORAN-C and travelling clocks. The random uncertainty of time comparisons, represented by two sample Allan Variance σ(30 seconds), was less than 10 nanoseconds. The relative frequency difference between the NPL and Raisting Clocks, SNPL, RAIS, as measured over the 44 days period was found to be-15.7 × 10-13. The relative frequency difference between PTB Primary Standard and Raisting Clock, SPTB, RAIS during this period, was measured to be–22.8 × 10-13. The relative frequency difference between NPL clock and PTB Primary Standard, SNPL, PTB, thus, is—7.1 × 10-13.The clock ra...

Controlled Counter for a Calendar Clock

For the inclusion of date in the digital clock, it is necessary to have counter whose division number should change according to month number of the year as the total number of days is different in every month. The special type of counter for this purpose has been designed and used in the fabrication of a calendar clock. This paper explains the design concept of such a counter in detail.