Abstract
Extensive research in studying the influence of the propagation media viz troposphere, ionosphere, plasmasphere on GPS signals led to effective use of GPS for atmospheric research and thus contributing to global space weather monitoring. Ionosphere is the ionized part of the upper atmosphere between 70 and 1000 km altitude and the Ionospheric electron content introduces delay in GPS signal propagation. Irregularities in the ionosphere due to space weather events caused by solar flares and coronal mass ejection can scatter trans - ionospheric radio signals producing fluctuations in both amplitude and phase and GPS cycle slips disrupting satellite communications and navigation. This paper discusses some space weather effects on GPS signal propagation, Total Electron Content (TEC) variation and ionospheric scintillations, based on GPS data collected at Indian low latitude region during severe magnetic storms.
Introducing GPS technology
The Global Positioning System (GPS) is a space based navigation system, consisting of a constellation of 24 satellites, in six orbital planes with 55° inclination to the equator. The satellites are placed at a height of about 20,200 km with 12 hours orbital period and operated by the United States Department of Defense (DOD) for accurate determination of position, velocity and time. All the GPS satellites are controlled by system tracking stations, ground antennae and the master control station.
In each satellite two rubidium and two cesium atomic clocks with stability 10
13 to 10
14 are used to derive the fundamental frequency f
o = 10.23 MHz. The GPS signals are transmitted at two L-Band coherent frequencies, designated L
1 (154 f
o = 1575.42 MHz) and L
2 (120 f
o =1227.6 MHz), which are derived from the fundamental frequency (f
o). Two codes are used, one of which is called C/A (coarse acquisition code, f
o /10) and the other is called P (precise, f
o) code. As the rate of P code is 10 times the rate of C/A code, its precision is 10 times better than C/A code. The L
1 and L
2 are modulated by Pseudo Random Noise (PRN) code, (each satellite is identified by this code) and transmitted after biphase modulation with the carrier.
The distance to GPS satellite is estimated by measuring the time a radio signal takes to reach us from the satellite. This is accomplished by cross -correlation of pseudo-random code generated by the satellite and the receiver. The distances from receiver to satellite measured in this way are called code pseudo ranges. Minimum four satellites are required for estimating the coordinates of a point on the Earth's surface. The position accuracy that can be estimated this way depends on our ability to account for various error sources (Reddy, 2001). The textbooks, such as Seeber (1993), Hofmann et al. (1994), Leick (1995), Parkinson and Spilker (1996) provide very good reference on this subject.
While the use of the GPS is extensive in defense, navigation and surveying applications, it is being used in geo-science, ionospheric & atmospheric studies, global climate changes, observing polar motion & earth rotation rate, mapping the gravity field, detecting seismo ionospheirc effects, transport and communications, environment management, for accurate time and frequency etc.
