Inside GNSS Media & Research

NOV-DEC 2017

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34 Inside GNSS N O V E M B E R / D E C E M B E R 2 0 1 7 I t is common knowledge in the GNSS community that the iono- sphere is dispersive in the L-band, meaning the refractive effects on the carrier phases are proportional to the wavelengths of the carriers, in turn causing differential variation in the measured codes and phases of the vari- ous navigation signals transmitted by the satellites. Use of multiple signals of distinct center frequency transmitted from the same GNSS satellite allows direct observation and removal of the great majority of the ionospheric delay, and gives the impression to users that the ionosphere may not be a problem for modernized receivers. While the general assumption of nearly perfect correlation between the effects mea- sured on multiple independent signals is correct in normal conditions, it does not appear to hold in the presence of ionospheric scintillation. High and Low Latitude Scintillation Effects Scintillation refers to random fluctua- tions in the received wave field strength ("signal fading"), as well as phase and group delay caused by the irregular structure of the propagation medium. Ionospheric scintillations are random rapid variations in the intensity and phase of the received signals resulting from plasma density irregularities in the ionosphere. Many of the important contributors to ionospheric scintillation are already known, such as the variation of scintil- lation activity with magnetic activity, geographic location, local time, season, and the 11-year solar cycle. e most significant and frequent scintillation activity including both phase and amplitude variations is observed in low latitude regions within about 15° of the Earth's magnetic equa- tor, particularly in the hours aer local sunset. In high latitude regions scintilla- tion is frequent but generally less severe in terms of signal tracking disruptions than that in the equatorial regions. e high-latitude environment can be divid- ed into two subregions, the polar caps (regions around the magnetic poles) and the auroral zones (approximately circu- lar regions around the two geomagnetic poles located at about 67° north and south geographic latitudes, and about 3° to 6° wide). Of these, the polar cap expe- riences both amplitude as well as phase scintillation activity, while mainly phase scintillation is observed at high latitude auroral regions. In mid-latitude regions scintillation is rarely observed, but during intense ionospheric storm conditions phenom- ena can extend into the mid-latitudes. Figures 1 and 2 show examples of ionospheric scintillation as observed on the detrended signal intensity (effectively power) and detrended carrier phase measurements at 69.5° latitude (Tromsø, Norway) and 21° latitude (Hanoi, Vietnam), respectively. In the particular event shown in Figure 2 the depth of fades reaches 43 decibels (dB) on L1CA which is severe by any metric, and is a substantial qualita- tive difference from the high-latitude phase scintillation events where only very weak fading activity is typically observed. It should be noted that iono- spheric activity is more dependent on the geomagnetic latitude of the user than the geographic latitude. While it might be clear that Tromsø station is located in the auroral region, the Hanoi station has somewhat lower geomagnetic latitude than geographic latitude and is in fact located within the equatorial zone. GNSS Solutions is a regular column featuring questions and answers about technical aspects of GNSS. Readers are invited to send their questions to the columnist, Dr. Mark Petovello , Department of Geomatics Engineering, University of Calgary, who will find experts to answer them. His e-mail address can be found with his biography below. GNSS SOLUTIONS MARK PETOVELLO is a professor (on leave) at the University of Calgary. He has been actively involved in many aspects of positioning and navigation since 1997 and has led several research and development efforts involving Global Navigation Satellite Systems (GNSS), software receivers, inertial navigation systems (INS) and other multi-sensor systems. E-mail: Do modern multi-frequency civil receivers eliminate the ionospheric effect?

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