Ionograms are an image of frequency versus time delay (virtual height) of HF echoes from the ionosphere recorded by an ionosonde. An ionosonde is a swept frequency HF pulsed radar used to monitor the ionosphere. SWS WDC has archived ionogram data from 26 stations, 12 of them are still active at present. An IPS 5D ionosonde has been sounding since 01/01/2004 at Learmonth. The 5D vertical sounding ionosondes normally sweep in frequency from about 1 to 21.5 MHz. Frequency step resolution for the 5D is 5kHz, with up to 4095 steps available. The step interval is variable, becoming more coarse at higher frequencies as ionograms are generally displayed on a logarithmic scale. Every five minutes, a Ionogram data file is recorded, cleaned and the cleaned ionogram is transfered to SWS Sydney head office for autoscaling and further analysis. The raw ionogram file is recorded in a DVD monthly, which will be posted to Sydney SWS head office every month. The IPS 5D ionosonde was stopped in 29/04/2019 and replaced by an IPS 5F ionosonde since 02/05/2019. SWS WDC archives all raw ionogram files and cleaned ionogram files. During 25/11/1994-28/01/2010, Learmonth also had an Lowell ionosonde. The Learmonth digital clean ionogram data is available since 01/01/2004, and digital raw ionogram data since 25/11/1994. The Learmonth scaled hourly ionospheric data include parameters of foF2, foF1, FoE, foEs, fbEs, fmin, fxl, f'scaling F/s, M(3000)F2, h'F2, h'F, h'E, h'Es, h'Scaling R/S and Type Es. They are avalable since 01/01/2004. The Learmonth median data of foF2 and M(3000)F2 are available since 01/2004.

Frequencies used to sonde ionopshere

The apparent height of an ionospheric layer deduced from the time delay of a reflected radio pulse upon the assumption that it travelled at the speed of light over its entire path. However, the radio wave actually slows down as it is refracted so that the virtual height is greater than the true height of the refracting layer.