This scientific report presents the results of a dedicated experiment that was conducted within the framework of the Low-latitude ionospheric Sensor Network (LISN) observatory to measure the characteristics of medium-scale (hundreds of km) Traveling Ionospheric Disturbances (TIDs) as they transit through the low-latitude ionosphere. A small array of 3 GPS receivers separated by 4-5？km placed in a triangular configuration was installed near Huancayo in Peru possessing several characteristics of a radio-interferometer. During the campaign days, 17–30 July 2008, TIDs were observed daily. On July 20, 2008 between 22 and 24？UT several TIDs moved across the small array of GPS receivers with a velocity near 130？m/s, were directed northward and had wavelengths close to 450？km. Other GPS receivers that were operating hundreds of km away from Huancayo show also similar TEC traces and provide a phase velocity equal to 150？m/s. This value was measured using the GPS at Piura, Cuzco and Huancayo. Based on this positive result, we conclude that small and/or regional arrays of GPS receivers can be used at low latitudes to study the role that gravity waves may have on seeding plasma bubbles. 1. Introduction Traveling ionospheric disturbances (TIDs) are the ionospheric manifestation of neutral density oscillations called atmospheric gravity waves (AGWs or GWs). AGWs originate in the troposphere or stratosphere, and often propagate to F-region heights, where they induce oscillations in the plasma density via ion-neutral coupling. When AGW winds drive plasma along magnetic field lines via ion-neutral collisions, “classical” TIDs result [1–3]. However, at night the electrodynamics are favorable to the production of “nonclassical” or “electrodynamic” TIDs, such that vertical movement of the F-layer results [4–6]. TIDs that reach F-region altitudes are divided into two classes: medium-scale (MSTIDs) and large-scale (LSTIDs). MSTIDs primarily originate from tropospheric weather systems and mountain turbulence; depending on background wind conditions and other factors, these may propagate in any direction. LSTIDs are produced by auroral energy injection during geomagnetic storms. They typically propagate meridionally from polar regions towards the equator. AGWs have a wide range of observed wavelengths and propagation velocities, frequently having horizontal wavelengths of up to hundreds of kilometers and speeds of hundreds of meters per second. As they propagate from lower altitudes, their amplitudes grow exponentially. At mid-latitudes, AGWs have been observed to have
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