Satellite Radio Beacon Studies at Delhi: Some Special Features

152-160At the National Physical Laboratory, New Delhi, satellite radio beacon studies have been actively pursued since early sixties when transmissions from Russian satellite COSMOS-V were first recorded in 1962-63. Some of the special results of these studies such as determination of mean field height, explanation of time-lag in the occurrence of quasi-transverse (Q-T) point at spaced frequencies, and variation of electron content, are described. Some space applications such as evaluation of refraction errors in satellite ranging and position fixing, and monitoring of tropospheric events using beacon technique are also mentioned. Suggestions are also made for future satellite beacon studies, with particular emphasis on Indian subcontinent conditions

Latitudinal Variation of Electron Content &amp; <i>f_o</i>F2 in Subauroral Zone

105-109Hartmann [Hartmann, G. K., Kleinheubacher Berichte, 17 (1974),237] made a simultaneous study of latitudinal variation of the total electron content (I) and foF2 in the auroral zone and showed during evening and morning, for about 1-2 hr, the presence of opposite latitudinal gradients in I and foF2 beyond about 60 ᵒN. I and foF2 were theoretically calculated up to about 60 oN using Stubbe's model [Stubbe, P., J. atmos. terr. Phys., 32 (1970), 865; Scient. Rep. No. 418, The Pennsylvania State University, 1973]. Beyond 60 oN, the ion- and heat-productions due to precipitated electrons were added. It is shown that it is possible to reproduce both the formation of the trough and the gradient reversal at the beginning and ending of trough formation using appropriate precipitating flux with the modified euv model valid for auroral zone

Ionospheric Amplitude & Phase Scintillations at Multiple Frequencies

385-392Simultaneous scintillation observations for both amplitude and phase from the ATS-6 beacon experiments for signals at 40 and 140 MHz, recorded at Lindau (51'6°N; 10'1°E) have been analyzed. It has been found that a combination of 1 sec sampling period and a value of about 200 for the filter-coefficient for a digital high pass filter give reasonable value of scintillation index S4. The average value of spectral index n varies from 1'4 to 1'5 both in case of weak as well as strong scintillation cases. For a power-law irregularity spectrum of the form K-P, the value of p derived from actual spectra varies from 3'5 to 4'9 for 40 MHz and from 3'2 to 4'2 for 140 MHz against the value of 3'6 predicted by n based on weak scintillation theory. The autocorrelation functions behave quite well in accordance with the expectations of the theory. For strong scintillation cases the data show broadening of spectrum and a relative difference in the behaviour of autocorrelation functions. The high frequency roll-off rate is sometimes smaller at 140 MHz than at 40 MHz which is contrary to the results published in literature. In the case of phase scintillations the slope of the spectrum varies from 2'1 to 2'6 and does not show any systematic trend as we pass from weak scintillations to strong ones

A Semi-empirical Model of Ionospheric Electron Content at Low Latitudes for Solar Minimum

63-68An attempt has been made to develop a semi-empirical model based on a harmonic analysis approach for calculating the electron content at three locations in and around the northern equatorial anomaly crest for solar minimum conditions using the data collected during the ATS-6 period (Oct 1975-July 1976) in India. The harmonic coefficients show seasonal dependence at all the locations. The ratio of the first and second harmonic was found to be higher compared to those reported from midlatitudes indicating thereby a smoother diurnal variation at low latitudes. A set of 25 coefficients comprising the mean, first and second harmonic coefficients was found to be sufficient for calculating the electron content within ± 15%ofthe observed values under worst case conditions. The present model which requires only the precalculated set of coefficients as inputs and eliminates the necessity to integrate electron density profiles will enable the user to apply first order corrections to time delay (or refraction errors) with little demand on the computer

140 MHz ATS-6 Radio Receiving System

55-57A two-channel single frequency, double frequency conversion, phase-lock radio receiver to receive ATS-6 signals is described. The equipment contains an automatic lock acquisition for both channels with monitoring outputs for earphones, lock-in indication by LEOs, displayed search range and universal outputs for chart recorders (both amplitude and phase) and digital recordings (phase only). The system is capable of measuring Faraday rotation effects and amplitude scintillations. A short back-fire dish with crossed dipoles is proposed as antenna for this system. All modules are built up of standard electronic components to enable easy replicability. The receiving system has been designed and tested at the electronic laboratory of the Max Planck Institute fur Aeronomie

Some geographic and geophysical aspects of electron content measurements from satellite radio beacon transmissions

Geographic distribution of the ionospheric electron content has been studied from the Faraday rotation of satellite radio transmissions recorded at several stations in the Indian zone. The effect of large scale ionospheric irregularities on the Faraday rotation rate and rotation angle has been demonstrated. The integrated electron production rate has been obtained as a function of solar activity from the rate of change of electron content around sunrise for the period October (1964)–September (1968) (the rising half of the present solar cycle) and compared with other estimates, both theoretical and experimental. There are days of unusually high electron content apparently unconnected with any observed geophysical event; these days also show high 30 MHz cosmic radio noise absorption and the two are linearly related

Diurnal Variations of Annual & Semi-annual Periodicities in-IEC at Lunping

17-20The diurnal variations of the annual and semi-annual periodicities in the F-region of the ionosphere over Lunping are investigated by analyzing the monthly mean values of ionospheric electron content (IEC) for a period of three consecutive years representing low, medium and high sunspot activity. Significant solar cycle variations in both amplitude and phase have been observed. The diurnal maximum in IEC occurs progressively later with increase in solar activity. The phase of the annual component shifts from winter in solar minimum to late autumn in solar maximum, whereas the phase of the semi-annual maxima is only in equinox conforming to the global behaviour. The variations are mostly explained in terms of the E × 8 drifts and changes in the atmospheric composition

Ionospheric Electron Content in Subtropical Regions

62-71The observational studies on the ionospheric electron content in the subtropical regions are reviewed. It is found that the gross features of the equatorial anomaly in electron content, viz. its structure and diurnal development, are consistent, in all longitude sectors, with the anomaly in electron density in the topside and the bottom side. The general features of the diurnal and seasonal variations of the electron content as a function of latitude and solar activity are discussed. The electron content variations at Delhi and Hawaii, which have almost the same geomagnetic latitude and dip, but differ in geographic location by 7° of latitude, are compared. It is further noted that the annual variation in the electron content is pronounced whenever the seasonal anomaly is absent and the semiannual component dominates when the seasonal anomaly is present. The electron content distributions in northern and southern hemispheres are also compared. Electron content contours have been generated and presented for use as ionospheric inputs for estimating errors in satellite navigation systems

Characteristics of Large Scale Ionospheric Irregularities over Delhi

224-226The Faraday fading records obtained at Delhi for 40 MHz transmissions from satellite BE-B for the period October 1964 to March 1969 and from satellite BE-C for the period May 1965 to March 1969 have been utilized for studying the characteristics of large scale ionospheric irregularities. To estimate the size of the irregularities, the irregularity height was assumed to be around 350 km based on a previous study, in which Faraday fading records taken simultaneously at Delhi and Kurukshetra were utilized. The present study reveals that: (i) during low solar activity, the frequency of occurrence is small during daytime and high during night-time, while during high solar activity the frequency of occurrence shows no preference; (ii) during low solar activity the horizontal sizes vary from 50 to 700 km, the most common size being about 175 km, while during high solar activity, the sizes vary from 5 to 300 km, the most common size being about 25 km; and (iii) the electron content of the irregularities varies from 1.0 x 1013 to 1.3 x 1016 m-2 being 0.1 to 35% of the background electron content during low solar activity and from 4 x 1014 to 1.2 x 1016 m-2, being 0,03 to 12% of the background content during high solar activity

Total Electron Content Measurements at Gauhati Using 140 MHz ATS-6 Transmissions

241-244The amplitude and phase of 140 MHz transmissions from geostationary satellite ATS-6 (0oN, 35°E) have been recorded at Gauhati (26·15°N; 91·75°E) for the period Nov. 1975 to Aug. 1976. The calibration technique for geostationary satellite Faraday rotation measurements suggested by Checcacci and Giorgeo [Checcacci, P. F. &amp; Giorgeo, M, T., J. almos.terr. Phys. 37 (1976),119] has been improved and generalized, It is then utilized to remove the nπ ambiguity in Faraday rotation values and also to estimate the correct value of diurnal minimum Faraday rotation angle so that the recorded Faraday rotation values can be converted into electron content values. Typical diurnal variation curves have been obtained for all the months except for equinoxes when the satellite transmission was off very often due to ecliptic conditions and hence no complete diurnal curves could be obtained for these months, It has been found that though December and January behave like typical winter months, the month of November is more like an equinoctial month. Similarly, though June and July behave like typical summer months the month of May is more or less similar to equinoxes. The characteristic features of the diurnal and seasonal behaviour of the electron content have been discussed