Ionospheric Scintillation Effects on Single and Dual Frequency GPS Positioning

The low latitude ionosphere poses a challenge to both GPS users and Satellite-Based Augmentation System (SBAS) providers. Single and dual frequency GPS receivers used in low-latitude regions can suffer from rapid amplitude and phase fluctuations known as scintillation. Scintillation occurs when the GPS or SBAS satellite signal travels through small-scale irregularities in electron density in the ionosphere, typically in the evening and nighttime in equatorial regions. Frequent scintillation and high rates of change in Total Electron Content (TEC) can cause loss of lock to dual frequency and even single frequency receivers. At these times, GPS users in low latitudes can experience decreased levels of accuracy and confidence in stand-alone positioning. We present observations of scintillation and its effects on stand-alone positioning at a station in Rio de Janeiro, Brazil, during a two-week span in February of 2002, a post solar maximum year. A GPS Silicon Valley Ionospheric Scintillation Monitor (ISM) is colocated with a dual frequency MiLLennium receiver. The ISM provides measurements of amplitude scintillation at L1 once per minute via the S4 index of normalized standard deviation of signal intensity. Multipath can inflate S4, falsely indicating ionospheric scintillation activity, and is removed with the use of a multipath/scintillation discriminating technique. Positioning is performed post-process by iteratively solving a linearized model of the range equations. For single frequency measurements, the ionosphere is estimated using the GPS broadcast ionospheric model. Dual frequency positioning has the advantage of exploiting direct measurements of TEC. We use carrier phase leveled measurements of TEC, when available, to estimate position accuracy available to a dual frequency user experiencing scintillation. For these TEC measurements the satellite and receiver inter-frequency biases have been estimated. We present nighttime observations of TEC in Brazil and the relationship between high rates of TEC and amplitude scintillation. The effects of scintillation on both a single frequency receiver and dual frequency receiver’s available constellation for positioning are shown. In addition, a dual frequency user’s L2 availability is shown, as this impacts one of the main advantages of a dual frequency receiver: the ability to directly measure and remove the error due to the ionosphere. We aim to illustrate what kinds of effects the single frequency user and the dual frequency user can expect to experience, and what kind of result this can have on the accuracy and confidence in their position solutions.