Direct Measurement of the Residual in the Ionosphere-Free Linear Combination during Scintillation

The natural phenomenon that most significantly affects GNSS positioning accuracy is the ionosphere. The GNSS satellite signals interact with free electrons along the propagation path to the receiver, leading to a group delay and phase advance proportional to the Total Electron Content (TEC). These refractive effects cause errors in the satellite range measurements which, if uncorrected, degrade overall positioning accuracy. Fortunately, the ionosphere is dispersive and the refractive effects on the carrier phases are proportional to the wavelengths of the carriers, to within first order. To mitigate the effects of ionospheric refraction, GNSS satellites broadcast radio signals at multiple frequencies. Multi-frequency GNSS receivers can utilize phase differences between carrier signals at different frequencies to deduce the TEC along the propagation path and correct for the ionospheric delay. In practice, they may not estimate the TEC directly, but instead use the so-called ionosphere-free linear combination of the observed phases to cancel the first order effect due to ionospheric refraction. International Technical Meeting (ITM) of The Institute of Navigation San Diego, CA, January 28-30, 2013 585 Ionospheric scintillation is caused by both refractive and diffractive effects, however. It is well known that phase scintillations caused by diffractive effects do not scale with the wavelength of the carrier, and therefore cannot be removed by the two-frequency technique. Numerical simulations of the diffractive effects on the phase have been performed by previous authors (Gherm, Zernov, and Strangeways, Radio Sci., 2011), who predicted a residual in the ionosphere-free linear combination of up to a few centimeters when the scintillations are intense. In this paper, we present direct measurements of the residual in the ionosphere-free phase combination for L1 and L2 reaching up to 2-3 meters during scintillation events. The data consist of 50 Hz measurements of carrier phase on L1 and L2C collected with a Septentrio PolaRX Pro receiver at São José in Brazil during 2012-2013. After post-processing to repair cycle slips, the phases are detrended to remove the contribution from geometric Doppler, and then the ionosphere-free linear combination of L1 and L2 phases is computed. The resulting residual is small except when diffractive effects contribute significantly to the scintillations. To the best of our knowledge, these are the first such direct measurements of the residual in the ionosphere-free linear combination during scintillation. These results have important consequences both for dual-frequency GNSS users and also Space Based Augmentation Systems (SBAS) that operate in regions of the globe where scintillation occurs.