New Chung-Li Ionosonde in Taiwan: System Description and Preliminary Results

In spite of being interrupted several times in its long history of operation since 1950, the routine observation of the ionosphere with various ionosondes installed at the Chung-Li ionosphere station in Taiwan has been achieved successively for more than seven decades. In this article, the system characteristics of the latest Chung-Li ionosonde and algorithm developed by National Central University for ionogram scaling and true height analysis, which started to routinely operate in 2020, are introduced. The new Chung-Li ionosonde is a pulse radar that transmits a train of short pulses with respective carrier frequencies between 2 and 30 MHz at a frequency separation of 50 kHz. The duration of an entire frequency sweep is 294.13 s, which is divided into 561 frequency channels. The 16-bit complementary code is employed to increase the signal-to-noise of the reflected echoes. The observational range is from 70 to 1221 km with a range resolution of 3.84 km. We developed an algorithm for the Chung-Li ionosonde to automatically scale the ionogram such that the true height profile of the ionospheric electron density can be retrieved. The observed traces of the ordinary wave (O-wave) and extraordinary wave (X-wave) displayed on the ionogram were first identified and separated by using 2-dimensional autocorrelation analysis combined with the image projection method. The true height analysis used stepwise regression. With the help of the International Reference Ionosphere (IRI) model and Quasi-Parabolic Segment (QPS) model, we carried out true height analysis to retrieve the ionospheric electron density profile based on the O-wave trace. An examination showed that the ionospheric parameters (i.e., foF2, h’F2) retrieved from the automatic scaling algorithm were essentially in good agreement with those obtained from manual scaling. The ionosonde-measured foF2 and hmF2 were also compared with the FORMOSAT-7 measurements made with the GPS radio occultation technique. The results show that the correlation coefficient, root mean squared deviation, and mean difference were, respectively, in ranges from 0.878 to 0.93, 0.73 to 1.06 MHz, and −0.43 to −0.26 MHz for foF2 and in ranges from 0.701 to 0.8, 22.39 to 28.45 km, and −9.28 to 11.06 km for hmF2.