One ion trap (IT) and one retarding potential analyzer (RPA) onboard the Taiwan Sounding Rocket V (SR-V) were launched to measure ionospheric plasma irregularities on 18 January 2006. After the fairing separated, voltage readings (V G1) of the first grid (G1) in the IT indicated abnormally high negative voltages appeared at the upleg between 83.7 and 120.1 km altitude for 19.7 seconds. It is postulated Gl had temporarily shorted out with the other two grids. Such the anomaly in the V G1 brought out the expansion of a plasma sheath around opening of the IT. More ions were attracted into the collector. Remarkable ion currents detected by the IT led to malfunctions of the RPA simultaneously. In this article, laboratory simulations and the International Reference Ionosphere model are performed to evaluate scale factors for the IT to the anomaly. The calibrated total ion concentration profile at the upleg indicates a peak density of the E 5 layer at 93.0 km altitude of about 6.9 x 10 3 # cm -3 with a thickness of 3.4 km. It is very similar to that at the downleg. It implies that the SR-V might encounter the same E 5 layer twice in a distance of 150 km away.