The Effects of the Ionospheric Scintillations - Term Paper Example

ENGINEERING (Satellite Communication Scintillations) 30TH July, 2008 TABLE OF CONTENTS 1. Executive Summary………………………………………………………... 3.0 2. Introduction………………………………………………………… ……... 3.0 3. Earth Station Information………………. ……………………………….... 4.0 4. Satellite Information……………………..………………………………… 5.0 5. Time Details………………...……………………………………………….6.0 6. Frequency of Measurement………………………………………………….7.0 7. Spans Sunspot Cycle Number……………………………………………… 8.0 8. Conclusion……………………………………………………..………….. 10.0 9. Bibliography…………………………………………………….……….. 11.0 EXECUTIVE SUMMARY Studies on the effects of the Ionospheric scintillations have been conducted from time to time in the past decades. Use of different methods of measurement such GPS. DGPS, EGNOS etc have been applied for the measurement of the Ionospheric activities. Emphasizing on the techniques of measurement, the following paper deals with the information related to the Earth station and the Satellites, the time details including the frequency of measurements and the relevant data to support facts and factual. INTRODUCTION All over the world, the concept of scintillations has received a tremendous attention especially through the deployment of the recent solar cycle, in UK. It is been found out from the past researches that Ionospheric scintillation is most likely to occur in the equatorial and auroral regions. There appearance is generally seen after the sunset which may prolong for few hours and there occurrence lies in relation with the solar activity and the corresponding season. A major problem in calculation of the scintillations is the improper tracing of the signals which result in the negative or inaccurate results affecting the GNSS users and the applications as well. Radio wave scintillations are frequent fluctuations in amplitude and phase of signals waiving through the atmosphere, resulting in complete loss of lock. The random fluctuations of the medium play a very important role in contributing towards the scintillation effect; however these fluctuations are being related to inhomogeneities inside the medium such as the spectral density, correlation length, height of irregularities and the velocity and direction of displacement (Be´niguel Y, 2002). EARTH STATION INFORMATION With the help of the GPS scintillation monitors, it is been measured that high latitudes high phase scintillations have been measured for low intensity scintillations (Béniguel Y, Forte B, Radicella M S, Strangeways J H, Gherm V, Zernov N Z, 2004). At high latitudes, scintillations at 1.7 GHz have been noticed, for latitudes below 50 degrees, the occurrences of scintillations are restricted near the equinoxes only. As per the measurements performed in Alaska and Norway, the scintillation index is quite low down rarely exceeding 0.2. Taur has conducted an advanced research, whereby the results of measurements at 22 different locations have been note with different latitudinal range varying from 20˚S to 53˚N (Taur R R, 1973). This whole process took a fifties months for completion. Scintillations at higher frequencies are produced by the ionospheric irregularities with an intense electron density fluctuation which has an augmenting affect on the scintillations at lower frequencies as well. The dependence of scintillations on the geomagnetic latitude in unclear, more measurements are required to define the correlation between the solar activity and the equatorial scintillation regime. One of the most important criteria to represent scintillation activity is the scintillation index S4 which is defined as the normalized standard deviation signal intensity. It is related to the peak- to – peak fluctuations of the intensity; however the exact relationship depends on the distribution of the intensity. GPS phase scintillations at high latitudes has a daily pattern observed in the aurora oval, during magnetic quiet periods, the aurora oval has the largest latitudinal extension, with the corresponding maximization of the phase scintillation activity around magnetic midnight. A comparison between the scintillation activity and the Earth’s brightness temperature variation over the Indian Ocean was done to find out the dynamical coupling between the ionospheric and tropospheric layers over the equator. The research resulted in the conclusion that significant correlation between the scintillation occurrences and tropospheric disturbances at 80˚ -95˚E longitudes or 5˚ - 20˚ west of Kototabang existed (Tadahiko Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Akinori Saito and Michi Nishioka, 2006). SATELLITE INFORMATION Scintillations do not affect all the visible satellites, if four or more satellites exist then navigation information is needed by the standard receiver. For a geostationary satellite, the fading rate of auroral region scintillation ranges from 2-6 fades per minute, in turn affecting the motion of the ionospheric irregularities. LBRA (the Ballistic and Aerodynamic Research Laboratory) is the DGA technical center using the information received from the Global navigation satellite System as GPS; however the ionospheric disruptions induce different error for the codes and phased. The number of satellites and their positions affect the positioning precision and accuracy. However, Satellites with PLL tracking errors above 15˚ are considered irrelevant for the DOP calculations. All other sources with the visible satellites are used to calculate a mean UERE combination due to scintillations (Béniguel Y, Geiswiller J, Adam J, Sarrou C, Sajous C, Maeyer De T, 2006). Scintillations levels over a certain limit will lead to loss of lock on satellites and may prove very crucial during periods when four or five satellites are actually visible. Even the SBAS reference stations may be severely affected during such circumstances due to the dependence on the GPS receivers for the calculation of the ionospheric delay corrections for allocation to the users. Meteorological satellites are used to measure the earth’s infrared brightness temperature and the scintillation activity. Existence of some correlations between the scintillation occurrence and tropospheric disturbance over the Indian Ocean was found ((Tadahiko Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Akinori Saito and Michi Nishioka, 2006). TIME DETAILS Scintillations are most prevalent in and around the auroral zone, near the equator from 55 to 90 degrees. These specifications are bound to change along with the change in the time of the day; the different seasons of the year, the sunspot number etc. Scintillations primarily occur during the night time in the equatorial region during the 20:00 – 02:00 local time period for almost 40% of the years’ time. The seasonal variation of GHz scintillation varies at the time of vernal and autumnal equinoxes. Scintillations activity is not uniform throughout the year but reveal a considerable amount of variation with the change in the seasons. Scintillations at a specific point occur at fairly specific time of the day (Craft D H,( JR), Westerlund H L, Scintillations, 1972). High level scintillations have disturbed a number of satellites such as on November 6th, 2001 in Hammerfest, two satellites were affected for atleast 2 % of the time. From October 29th to 30th, 2003 the time increased to almost 3% (Aquino M, Rodrigues S F, Souter J, Moore T, Dodson A, Waugh S, 2006). FREQUENCY OF MEASUREMENT The frequency dependence of the ionospheric scintillation measurements is directly related to the power spectra of the electron density fluctuation, therefore to accurately forecast the frequency; information about the spectra needs to be attained. For frequencies between 1.5 and 4 GHz, the frequency dependence in the equatorial regions is directly proportional to its wave length, and for the frequencies above 4GHz, the scintillation index waivers as peer the square of the wavelength (Taur R R, 1974). The complex phase method revealed that the points of observation which were within the ionosphere layer at the frequency level of 1 GHz denoted high fluctuations at the amplitude of that field. It is seen that at specified frequencies, the ionospheric scintillations are generally not found inside the layer but is observed in the region below the Earth’s surface. The radio frequency dependence of the magnitude of the variations is extremely helpful in finding out the cause of the scintillations. SPANS SUNSPOT CYCLE NUMBER When the sun spot cycle number is high, the equatorial scintillations show an eminent occurrence. The correlations between the GHz scintillation and the sunspot activity cannot be verified during the last 11 years of measurement at Bahrain, Hong Kong and Brazil. Though initially positive relation could be market but the decline over the period of time nothing tactical could be observed. However, in the equatorial and polar cap regions with strong diurnal and seasonal variations, a positive correlation in the number of sunspots and GHz scintillations have been observed. CONCLUSION It can be concluded that many models have been put to use for the measurement of Ionospheric scintillations and few of such models like the GIM model is highly successful in defining characteristics like the time series synthesis of transmitted signal phase and intensity, the average duration of fades, the Doppler spectrum, and the probability density function. In Europe, a complete database of scintillations is maintained and many forecasting techniques have been developed over time in lieu of the topic. Scattering, diffraction and multipath transmission can be generated by either the atmospheric irregularities or the surface unevenness (Fang J D, Tseng T F, Calvit O T, Jan 1982). The worldwide growing satellite communication system from its early start in 1965 till date is moving forward for more advancement and growth. BIBLIOGRAPHY 1. Aquino M, Rodrigues S F, Souter J, Moore T, Dodson A, Waugh S, Ionospheric scintillation and impact on GNSS users in Northern Europe: Results of a 3 year study, Space Communications 20 (2005/2006) , p17–29. 2. Be´niguel Y, published on 10th May, 2002, Global Ionospheric Propagation Model (GIM): A propagation model for scintillations of transmitted signals, RADIO SCIENCE, VOL. 37, NO. 3, 1032, 10.1029/2000RS002393 3. Béniguel Y, Forte B, Radicella M S, Strangeways J H, Gherm V, Zernov N Z, 2004, Scintillations effects on satellite to Earth links for telecommunication and navigation purposes, Annals Of Geophysics, Supplement To Vol. 47, N. 2/3. 4. Béniguel Y, Geiswiller J, Adam J, Sarrou C, Sajous C, Maeyer De T, The effects of scintillations on the positioning Errors, Space Communications 20 (2005/2006) 31–39. 5. Craft D H,( JR), Westerlund H L, Scintillations At 4 and 6 GHz Caused by the Ionosphere, AIAA 10th Aerospace Sciences Meeting, AIAA Paper 72- 179, January 17-19, 1972. 6. Fang J D, Tseng T F, Calvit O T, Feb 1982 A Measurement of the MARISAT L-Band Signals at low Elevation Angles Onboard Mobil Aero, IEEE Transactions On Communications, VOL. COM-30, NO. 2. 7. Fang J D, Tseng T F, Calvit O T, Jan 1982, A Low Elevation Angle Propagation Measurement of 1.5-GHz Satellite Signals in the Gulf of Mexico, IEEE Transactions On Antennas And Propagation, Vol. Ap-30, No. 1 8. Tadahiko Ogawa, Yuichi Otsuka, Kazuo Shiokawa, Akinori Saito And Michi Nishioka, 2006, Ionospheric Disturbances Over Indonesia and Their Possible Association With Atmospheric Gravity Waves From the Troposphere, Journal of the Meteorological Society of Japan, Vol. 84A, pp. 327—342 9. Taur R R, 1973, Ionospheric scintillation at 4 and 6Gllz, Comsat Technical Review Volume 3 Number 1 10. Taur R R, modified on March 1, 1974, Ionospheric scintillation at frequencies above 1 GHz, Comsat Technical Review Volume 4 Number 2 Read More