Y.B. Acharya

Compact light-emitting-diode sun photometer for atmospheric optical depth measurements

A new compact light-emitting diode (LED) sun photometer, in which a LED is used as a spectrally selective photodetector as well as a nonlinear feedback element in the operational amplifier, has been developed. The output voltage that is proportional to the logarithm of the incident solar intensity permits the direct measurement of atmospheric optical depths in selected spectral bands. Measurements made over Ahmedabad, India, show good agreement, within a few percent, of optical depths derived with a LED as a photodetector in a linear mode and with a LED as both a photodetector and a feedback element in an operational amplifier in log mode. The optical depths are also found to compare well with those obtained simultaneously with a conventional filter photometer.

A low-current logarithmic LED electrometer

A new circuit for measuring low dc currents using an LED-based logarithmic electrometer is presented. The electrometer is designed to measure currents from 1 pA to 0.1 mA. Temperature compensation is achieved using the ratio technique in the temperature range -20/spl deg/C-70/spl deg/C. Variations in the scale factor as a function of the input current and the device constant n as a function of temperature are also discussed.

ND:YAG BACKSCATTER LIDAR AT AHMEDABAD (23 N, 72.5 E) FOR TROPICAL MIDDLE ATMOSPHERIC STUDIES

A backscatter Nd:YAG laser radar, the first of this type in India to our knowledge, was developed at the Physical Research Laboratory, Ahmedabad (23° N, 72.5° E), for middle atmospheric studies. The system has been operational since April 1992. The system details and sample results on the Mt. Pinatubo volcanic aerosol layer and its decay as observed over this tropical site are presented and discussed. The future augmentation of the system and planned activities are also outlined.

The vertical distribution of aerosol concentration and size distribution function over the tropics and their role in radiation transfer

By measuring the atmospheric attenuation of the direct solar radiation and the angular distribution of the scattered radiation at different altitudes, the altitude profile of the aerosol concentration and size distribution function can be obtained. This principle has been employed to study the vertical distribution of aerosols over the tropical site, Thumba (8.5°N, 76.9°E), using rocket-borne photometers upto an altitude of 22km in February 1980. Later, a suntracking multichannel photometer was developed and flown on a balloon platform from Hyderabad (17.5°N, 78.6°E) on April 18, 1984 and aerosol measurements were made upto an altitude of 32km. Comparison of the two sets of data shows that the aerosols measured over Thumba are influenced by the near equatorial eruption of Sierra Negra volcano which occurred three months earlier. Mie scattering coefficients are computed for different wavelengths for the aerosol concentrations and size distribution functions measured over Thumba and Hyderabad and the spectral variations are compared.

Mode radius and asymmetry factor of Mt. Pinatubo volcanic aerosols from balloon‐borne optical measurements over Hyderabad during October 1991

Using Sun-tracking photometers onboard balloon, the Pinatubo volcanic aerosol layer has been studied over Hyderabad (17.5°N) in October 1991, about 4 months after the eruption. From the aerosol extinction coefficients the mode radius and from the angular distribution of the scattered radiation intensity measurements the asymmetry factor g are determined. Mode radii are found to be in the range of 0.22±0.05 µm within the aerosol layer while g values are found to be 0.83±0.04, indicating the presence of larger particles. The mass determination reveals that the mass of the Pinatubo layer is about 0.053 gm−2 which is 3.75 times higher than the earlier reported value for El Chichon layer, about 4 months after the eruptions.

The ozonesonde intercomparison experiment at Thumba

An Indo-Soviet collaborative experiment on Ozonesonde Intercomparison was conducted at TERLS in March 1983. Thirteen rocket ozonesondes, eleven balloon ozonesondes and seven meteorological rockets were launched from Thumba. The rocket and balloon soundings were supported by on site Dobson spectrophotometric observations, surface ozone measurements as well as measurements with a Volz type filter photometer. The programme has yielded data on ozone vertical profiles from eleven rocketsondes, seven balloon-sondes and four sets of Umkehr observations. The data is studied with a view to intercompare the various sensors.

A payload for the study of electric fields and electron density in the equatorial region

A rocket borne payload for simultaneous measurement of the electric field along and perpendicular to the rocket spin axis and the electron density in the medium was developed and flown from Thumba (8° 31′N, 0° 47′S dip) onboard two Centaure rockets for the study of plasma dynamcis in the equatorial E-region. The arrangement of sensors in this payload allows near continuous measurements of some of these parameters to be made.

Radio Frequency Modelling for Future Wireless Sensor Network on Surface of the Moon

In order to study lunar regolith properties, wireless sensor network is planned to be deployed on surface of the Moon. This network can be deployed having few wireless sensor nodes capable of measuring soil properties and communicating results, as and when ready. Communication scenario on lunar surface is quite different as compared to that on the Earth, as there is no atmosphere and also there are lots of craters as well as various terrain topologies. Since the deployment of sensors on the Moon is a challenging and difficult task, it is advisable to predict the behaviour of communication channel on lunar surface. However, communication models like Irregular Terrain Model used for terrestrial communication networks are not directly applicable for Unattended Ground Sensor type sensor networks and need modifications according to lunar surface conditions and lunar environment. Efforts have been put to devise a model of radio frequency environment on the Moon using basic equations governing various physical phenomena occurring during radio propagation. The model uses Digital Elevation Model of four sites of the Moon, measured by Terrain Mapping Camera on board Chandrayan-1, a recent Indian mission to the Moon. Results presented in this paper can provide understanding of percentage area coverage for given minimum received signal strength, potential sites for sensor deployment assuring wireless communication, decision whether a given sensor node can work and can provide suggestion for possible path of rover with cluster head to remain in contact with the nodes. Digital Elevation Model based results presented here can provide more insight in to the communication scenario on the Moon and can be very useful to mission planners.

A new technique for measuring the leakage current in Silicon Drift Detector based X-ray spectrometer—implications for on-board calibration

In this work, we report a new technique of measuring the leakage current in Silicon Drift Detectors (SDD) and propose to use this technique as a tool for on-board estimation of the radiation damage to the SDD employed in space-borne X-ray spectrometers. The leakage current of a silicon based detector varies with the detector operating temperature and increases with the radiation dose encountered by the detector in the space environment. The proposed technique to measure detector leakage current involves measurement of the reset frequency of the reset type charge sensitive pre-amplifier when the feedback capacitor is charged only due to the detector leakage current. Using this technique, the leakage current is measured for large samples of SDDs having two different active areas of 40 mm2 and 109 mm2 with 450 micron thick silicon. These measurements are carried out in the temperature range of -50°C to 20°C. At each step energy resolution is measured for all SDDs using Fe-55 X-ray source and shown that the energy resolution varies systematically with the leakage current irrespective of the difference among the detectors of the same as well as different sizes. Thus by measuring the leakage current on-board, it would be possible to estimate the time dependent performance degradation of the SDD based X-ray spectrometer. This can be particularly useful in case where large numbers of SDD are used.

Development of Impedance-Based Miniaturized Wireless Water Ice Sensor for Future Planetary Applications

Keeping in view the future in situ space applications, to study planetary surface science, we have designed and developed an impedance-based extremely compact wireless sensor node to infer the presence of water/ice in a soil sample and reported in this paper. Although reflectometry-based soil moisture sensors are available commercially, they have slightly longer probe, needing more volume, from the payload point of view. The reported wireless impedance sensor reproduced in space-qualified form can be useful to measure the permittivity of lunar regolith and infer the presence of water ice. It works in the frequency domain with capability to sweep the frequency in a given measurement range. The sensor does not necessarily have long probe, and it needs only electrical contact with the soil surface, thus eliminating extra energy for insertion by the rover on the lunar surface, in one of its possible space applications. The sensor has been made to operate in wireless mode, which is a basic need in any space applications. The data are transmitted at 2.4 GHz to the aggregator. We have tested sand-type terrestrial soil and lunar soil simulant JSC-1A. To verify the performance of the sensor, we have also tested Milli-Q water. All these experimental results are reported in this paper. Results of Milli-Q water, whose permittivity is known, agree with those reported in the literature, verifying excellent performance of the sensor. Thus, the sensor can be very useful for space applications, where size and weight are critical issues. A wireless impedance sensor can also be quite useful for terrestrial applications, for example, in agriculture, and also in other impedance-based applications.