Using GPS signals to deduce the ionospheric TEC as a tool to study the ionospheric physical processes has been widely conducted. For a duel frequency GPS receiver, both pseudo-range and phase measurements are used. Phase measurement is not sensitive to so-called multi-path effect so it is less affected by scattered or reflected signals and is more useful in determining the exact TEC value, including the signals' scintillation due to the fast fluctuations of electron density. In this paper, an observational phenomenon is introduced to show that the pseudo-range fluctuates asymmetrically day and night. During the nighttime, especially, one to three hours after the local sunset, fluctuations of pseudo-range values often appear and last for several hours. The magnitudes of these fluctuations are much larger than that observed at daytime. Some examples are given and their observational features are analyzed. Both examinations of the environments around the receiver and the asymmetrical features of the phenomenon itself show that these fluctuations can not be produced by any ground objects, but instead, may be due to some ionospheric structures. By comparing with the theoretical and observational characteristics of ionospheric Spread-F at mid- and low latitudes, particularly with its occurring time after local sunset, it is suggested that the nighttime fluctuations of GPS pseudo-range measurements is a kind of multi-path effects, which is produced due to the scattering of GPS signals by large scaled ionospheric structures such as Spread-F. Thus, GPS may provide a way to study the ionospheric irregularities.
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