The oblique ionograms for various radio path lengths are derived from vertical incidence ionograms that are measured by Digisonde stations. With these oblique ionograms the behaviour of the ionosphere on those radio paths can be investigated. This will help to understand what kind of radio propagation conditions may be expected on those radio paths.
The example oblique ionogram in figure 1 shows a number of traces, representing various ionospheric layers, including a number of multi-hop components. Theoretically these traces and components should be available. On real life radio links, however, some of them may appear to be quite weak, depending on the electron densities in these ionospheric layers.
Since all ionograms are based on measurements on a small area of the ionosphere above the Digisonde station, the oblique ionograms only apply to radio paths where the Digisonde station is at the midpoint of the radio path.
At Digisonde vertical incidence ionograms a MUF value is indicated. Note that this MUF is the Classical MUF, which is calculated from the Critical Frequency, i.e. the highest frequency, of the F2 layer o-mode wave at vertical incidence. For real radio communication links, however, this Classical MUF has little value. As figure 2 shows, the actual Operational MUF is determined by the highest frequency of the F2 layer x-mode wave at oblique incidence for the radio path concerned. Therefore the actual Operational MUF is indicated at all oblique ionograms. Note that the Operational MUF is always (much) higher than the Classical MUF.
Sometimes the MUFx values for Sporadic E (Es) may be higher than the F2 MUFx values. During the Es season the Es MUFx may extend far into the VHF spectrum. In the oblique ionogram plots, however, the Es traces are not plotted for frequencies higher than 40 MHz. The highest Es MUFx values for 1 hop and 2 hop Es links for any of the 10 radio paths are shown in a table, also for values higher than 40 MHz.
For some actual oblique ionogram measurements see this page.