Current Oblique Ionograms
SWS operates a Barry Chirpsounder HF receiver. The image below is an oblique ionogram from this instrument for the circuit Auckland-Sydney. This image is updated hourly. An oblique ionogram shows HF commincators what frequencies that are currently propagating over the circuit. In the oblique ionogram below you may see sequence of horizontal "v" shaped curves, each higher than the other. The "height" is time delay (y axis), as the different curves represent different paths, or number of bounces between the ionosphere and the ground between the transmitter and receiver. The more bounces the greater the time delay as the path is longer. These paths are referred to as "hops" or "modes". Diffrent paths have different angles of arrival and number of "hops" between transmitter and receiver, and hence have different frequencies of support. Sporadic "E" can appear as a low long line in the display.
Note that SWS does not have any control over the operation of the Barry Chirpsounder HF transimitter. This is operated by the Royal New Zealand Defence Force. If the transmitter is off for maintenance then the ionograms below will be blank, or cease updating.
Using the x axis Select a horizontal "v" shaped curve (mode) that spans a frequency from your freqency set. This then tells you that this frequency should enable communication across the Tasman Sea from Sydney to Auckland, for this hour. Note that depending on the number of frequencies in your allocated frequency set, more than one frequency may be usable via different "modes" or "hops" (represented as curves), for the same hour.
You can compare the above oblique ionogram with the SWS Realtime Auckland HAP chart, by first noting the observed frequency of the "nose" (junction frequency) of the first mode (right most "horizontal v shape") in the above oblique ionogram. Now go to the Auckland Hourly HAP chart and note the colour contour over Sydney in the chart. To get the predicted frequency for that hour, you then look up the colour in the colour to frequency legend at the the top of the HAP. The predicted frequency should be approximately 80 to 90% of the first mode MUF. The prediction may lag the observed value around local dawn (approximately 20UT). The predicted value is made less than the observed MUF (ie a conservative estimate) to allow for any inaccuracy in automatic ionogram interpretation. Sporadic E can confuse the simple auto MUF detector in these ionograms.