Frequency diversity techniques applied to GNSS under adverse ionospheric conditions

One of the major deterrents to successful implementation of SBAS may be linked to sharp latitudinal gradients of ionization occurring during the daytime and intense Space Weather events in the post sunset hours, affecting transionospheric satellite links particularly in the equatorial region. These phenomena have the potential to cause serious damage to the technological infrastructure on which society relies. GPS modernization program is focused on addition of a new navigation signal L5 (1176.45MHz) to the GPS constellation. The L5 is exclusively reserved for aviation navigation services and is designed with a protected spectrum, higher power, and greater bandwidth to support life-critical and high performance applications. Overall robustness of this dual-frequency mechanism to ionospheric scintillations could be ascertained through a study of correlated scintillations. Understanding the correlation of signal fades across two frequencies is important to assess their collective mitigation effectiveness. If signal fades at two frequencies are highly correlated, the actual aim of the frequency diversity scheme would be defeated. The Indian Regional Navigational Satellite System (IRNSS) will operate at L1 (1575.42 MHz), L5 (1176.45 MHz) and S-band (2492.42 MHz) frequencies. It has been observed through studies conducted earlier [1, 2] that the detrimental effects of the sharp latitudinal gradients of ionization occurring in the equatorial region may be limited if sufficient number of satellite links are available at high elevation angles in excess of 60°. As GPS-only constellation was not possible to address this issue, it will be very important to check the availability of increased number of ionospheric pierce points when multi-constellation receivers are operational at a station like Calcutta situated near the northern crest of the EIA. As GNSS satellites will broadcast three frequencies enabling more advanced three frequency correction schemes, understandi- g the correlation between different frequency pairs under scintillation conditions is extremely important [3]. Previously, the assessment of the contribution of diffraction for range errors in the dual-frequency regime using a hybrid model have been addressed [4, 5]. Lack of correlation between pairs of GNSS frequencies has been suggested to adversely affect positioning accuracy.