Expertise on Radio Science
The Radio science group of NPL is the only unique group in India that has the expertise in characterizing tropospheric and ionized media of the Earths atmosphere especially with reference to radio wave propagation for applications to communication, strategic applications, fixing, navigation, etc. apart from basic scientific research. In the past the group interacted with various user organizations like three wings of defence, videsh sanchar nigam limited, research group of All India Radio, C-DOT, private FM operators, special protection group etc. and has provided consultancy services to various user organizations in the area of fixed and mobile communications. The group through its Regional Warning Centre has also been providing space weather alerts and solar activity predictions to agencies dependent on radio communications for critical operations. The scientific research pursued by this group can provide inputs to design future communication systems over Indian
regions and also helps in improving the performance assessment of existing systems.
Radio channel measurements and modeling for fixed and mobile communications
In the last few years, our country has witnessed tremendous growth in the area of wireless communication especially in the sector of mobile communication with the introduction of GSM and CDMA technologies. Various studies have indicated that economic progress of any country is highly correlated with the telecom density. Propagation/channel modeling is an active area of research and important component of system design of mobile/fixed communication systems. It helps to avoid surprises when the actual service begins.
Radio channel measurements and modeling for fixed and mobile communications
- New data sets in GSM bands of 900/1800/2000MHz were raised by interacting with various user organizations depending on the links of opportunity to study the propagation impairments over various regions of India.
- Multipath fading and precipitation effects affecting the performance of ine-of-sight links at 2,7,8 and 13 GHz were extensively studied and remedial measures were explored.
- Effects of water vapour, and cloud on microwave have been studied.
- Various statistical, empirical models were tested by us using the data collected in cellular outdoor communication scenario consisting of urban, suburban, rural, tunnel and vegetation type environments.
- Numerical electromagnetic codes like AWAS were utilized to explain the observed results at 900 MHz and 1800 MHz for cellular base stations in urban, suburban and rural zones.
- It has been shown that in the near field regions of base stations there is severe fading and signal stabilizes in far field zones. Path loss exponent in the intermediate zones is found to be 3 which has been modeled using electromagnetic theory.
Numerical electromagnetic codes like AWAS(analysis of wire antennas and scatter) to explain the observed results of 1800 MHz base stations in the urban and suburban regions of Delhi.
Fig 1. Comparison of observed results of path loss of University Area base station with various models at 900 MHz.
Fig 2. Comparison of observed results of path loss of base station with various models at 1800 M
Fig 3. Variation of path loss exponent for UA base station at 900MHz
In ionosphere most of the critical applications are dependent on an accurate knowledge of the Total Electron Content (TEC) along the path of any radio signal at the time the propagation is actually taking place. For applications such as aircraft landing operations or missile guidance this information is needed for onboard computations in near real-time. International Reference Ionosphere (IRI) models are primarily used for this along with regional corrections to the model derived from local measurements. Recently to study the polar ionosphere Ionosonde has been set up at Antarctica. Analysis of this data can give insight into the linkages between the low latitude Indian region and high latitude ionospheric polar regions which should help in global ionospheric modeling studies. These along with the existing Digital Ionosonde of NPL and the country's GPS network already in place will enable serious effort in this direction. It is essential to expand this network by adding more Ionosondes and tomographic receiver sites so as to be able to realize near real time ionospheric corrections and modeling studies needed for position fixing accuracies to a better than a metre. NPL has a network of Digital Ionosonde Systems (Delhi, Bhopal and Antarctica), dual frequency GPS receivers (Delhi, Trivandrum, Antarctica), VLF receiver (Antarctica) for having extensive measurements of F layer critical frequencies and heights (foF2, hmF2 etc) and Vertical Total electron content ( VTEC). There is a large ionospheric data bank from 1975 to 1993 from number of stations across the Indian latitudes. These data are used to develop regional ionsospheric models. Modelling of Airglow emission along with space weather studies has been the new initiative undertaken to understand the chemistry, thermo-ionospheric coupling and upper atmospheric dynamics.
During the last decade the following activity has been carried out using ionospheric monitoring instruments like Ionosonde and GPS
- Response of upper ionosphere to X-ray and EUV fluxes during Solar flare.
- To study the response of low-latitude ionosphere to the geomagnetic storms.
- To study the long term variation of ionospheric parameters.
- To study the variability of Equatorial Ionization Anomaly phenomenon.
- Additional stratification between F1 and F2 layer over Delhi
- To study the ionospheric parameters and their comparison with International Reference Ionosphere Model.
2. GPS as a Global Ionospheric Scintillation and Total Electron Content Monitoring System:
- Ground Based Technique : Pulse Sounding Technique – Ionosonde SystemThe pulse sounding technique is the most powerful technique, which is used for measurement of different parameters of bottom side of earth’s ionosphere. In a typical mode of operation, the sounder is swept from 1-25 MHz in 15s, using a pulse repetition frequency of about 60 s-1 and a peak power of up to 20 kW. In a conventional recording system, the echo received from the ionosphere is used to modulate the intensity of a spot of light on an electronic time base. Distance along the time base represents the “time of flight” of the radio pulse, which, if divided by the free-space velocity, gives the equivalent path length, this being twice the “virtual height” h’ of the reflection point in the ionosphere. In the recorder, a photographic film is moved at right angles to the time base as the frequency is varied, so that the spot of light traces a graph of virtual height h’ against radio frequency. Range and frequency calibration markers are usually inserted automatically. These recording are known as “ionograms” or h’ (f) curves. Now a days the most advanced type of ionosondes are used in different part of the world. The modern digital ionosonde is complete hardware and software system, which acquires high-resolution ionospheric data for real time HF frequency management. With pulsed and frequency modulated continuous wave transmissions, a combination of vertical and oblique sounding capabilities and comprehensive post-processing software.
A Novatel make GSV-4004B GISTM receiver is installed at National Physical Laboratory, New Delhi for real time monitoring of ionospheric total electron content (ITEC) and Ionospheric disturbances over Low-mid latitude ionospheric region. The GSV-4004B GISTM receiver is dual-frequency 12 channel GPS receiver and specifically configured to measure amplitude and phase scintillation from L1 frequency along with ITEC from the L1 & L2 frequencies. The receiver has 25 Hz raw signal intensity noise bandwidth and a 15 Hz phase noise bandwidth insure that all spectral components of both amplitude and phase scintillations are measured with a sampling rate of 50 Hz. Single frequency (L1) carrier phase is compared against a stable ovenized crystal oscillator (OCXO) to insure that all phase scintillation effects are recorded. GISTM system data now is in use to study the day-to-day and seasonal variation of Ionospheric Total Electron Content (ITEC) and occurrences characteristics of ionospheric scintillation overn Delhi, Antarctica normally and during space weather events.
3. Very Low Frequency (VLF) Receiver:
An indigenous made Very low frequency (VLF) receiver was installed during 27th InSEA. The precisely tuned loop antenna is used to receive 19.8 KHz frequency, which is transmitted from BIPM Australia. The strength of the said frequency is continuously received which is then converted into digital one by using a Analog to Digital convertor card procured from National Instrumentation and the output is onscreen displayed as well as stored in the system with the help of indigenous developed software in LabView.
NPL has contributed to ISRO by contributing space borne payloads for the study of ionospheric dynamics related studies in the satellite missions of ISRO. Presently the group isparticipating in Sense project of ISRO by providing space borne payloads. It is also participating in balloon studies of atmospheric electric field and conductivity.
The RPA experiment onboard SROSS-C2: