Ionodata

This project is a app for determining the possibility of Near Vertical Incidence Skywave (NVIS) propagation. Although, it is not a prediction tool and it currently only covers the area around southern Victoria, Australia, it is showing its potential as a useful model. First a quick refresher of the ionosphere and its role in supporting different types of radio wave propagation, See our Amateur Radio HF Communications activity for more details:

Ionospheric Layers

Circling the Earth, at an altitude of between 50 and 650 kilometres is a region of our atmosphere know as the ionosphere. Here, ultraviolet radiation from the sun ionizes molecules, liberating free electrons. Free electrons have the possibility of affecting shortwave radio communications. In some instances, they absorb the RF energy, in others they re-radiate the RF energy, sometimes causing refraction (bending) of the signals back to Earth. The concentration of ionization varies throughout the region according to some broadly-defined layers called the D, E, F1 and F2 layers. The altitude of these layers varies constantly due to the changing angle of the sun. At night, all layers start to disappear due to the lack of radiation from the sun and the recombination of the ions and electrons. The concentration of free electrons in these layers, coupled with the density of the atmosphere at different altitudes and the rate at which recombination proceeds at night, gives each of these layers different characteristics. Here is a general summary:

Characteristics of Ionospheric Layers:

C-Layer: Very thin layer which hardly affects radio communications at all.
D-Layer: 50-80km. Tends to absorb, not refract, RF energy travelling through this layer, in both directions, due to higher atmospheric pressure in this region.
E-Layer: 100-125km. Little effect on radio communications, except when ionised by sporadic meteor showers.
F1-Layer: 200-300km. Significant effect on radio communications due to lower atmospheric pressure and slower rate of recombination at night.
F2-Layer: 300-400km. Same as F1 layer. Often merges with the F1 layer at night.

Ionospheric propagation

Ground Waves: Signals from a transmitter travel only a short distance by ground wave. They can curve around the Earth and to some extent over the horizon.
Sky Waves: Signals from a transmitter travel into the sky. The can be absorbed, refracted or pass through the ionosphere into space.
Absorption: Signals are absorbed in the D-Layer and are not refracted back to the Earth.
Single-Hop: Signals are refracted by the ionsophere, once, but are then attenuated by the ground or the D-Layer.
Multi-Hop: Signals are refracted by the ionosophere, then reflected back into the ionosphere by the ground, where they are refracted again back to Earth. This can happen multiple times. Signals can travel all around the globe in all directions, even meeting themselves back at the source.
Sporadic-E-Layer: Signals are refracted in the E-layer due to temporary ionization caused by a meteor shower.
Ducting: Signals are refracted between layers in the ionosphere, before returning to the Earth.
Near Vertical Incidence Skywave: Signals are virtually reflected by the ionosphere, returning to the Earth a short distance away.

Types of Ionospheric Propagation

History

When we run SARCnet, during the day on the 40 m band with a horizontal loop antenna, we are predominantly experiencing NVIS propagation. We wanted to know, from time to time, if NVIS propagation was currently supported by the ionosphere or not. It occurred to us that virtual incidence sounders, or ionosondes, run by the Australian Bureau of Meteorology work in exactly the same way and could provide us the information we needed.

Ionograms

There is a world-wide network of ionosondes, that provide us with very useful information about the ionosphere in the form of ionograms. They work a bit like a radar and show reflections from ionospheric layers as artificially-coloured traces, representing the strength of the reflection received on a graph of virtual height vs frequency. The presence of the ionospheric layers, so called E, Sporadic E, F1 and F2 are easy to see on the ionogram. We have previously described our attempts to explain NVIS propagation by creating ionogram videos here. However the technique described here is more promising.

Interpreting Ionograms

Also called scaling ionograms, interpreting ionograms is an extraordinarily complex subject. The World Data Center for Solar-Terrestrial Physics publishes a free Handbook of Ionogram Interpretation and reduction. A copy of which can be found here.

Recalling that ionograms are coloured graphs of soundings, or traces, plotted as virtual height vs frequency. On the ionogram below, at the bottom left, is the more or less horizontal trace of the E layer (symbol E) from 1.8 to 3.6 MHz with a virtual height from 80 to 110 km. Next comes a short Sporadic E layer trace (symbol Es) from 3.8 to 4.2 MHz with a virtual height from 110 to 120 km. Next is the fish-hook trace of the F1 layer (symbol F1) from 4.2 to 5.2 MHz with a virtual height of 190 to 350 km. Next is a second fish-hook trace of the F2 layer (symbol F2) from 4.2 MHz to 7.5 MHz with a virtual height of 320 to over 700 km. Also seen from time to time on ionograms are multiple traces representing reflections of the same layer (stacked vertically), and extraordinary traces of the same layer to the right. The highest frequency of a trace is taken as its critical frequency (symbol fo). The minimum virtual height of a trace is taken as its lowest height on the graph (symbol h'). The process of scaling the ionogram means determining the values of h'E, h'Es, h'F1 h'F2, foE, foEs, foF1 and foF2. This is properly done by experts in the field, but it can be estimated by software applications called ionoScaler, Autoscala, ARTIST, Interobl and newer deep-learning models like IASGAN.

An ionogram showing the E, Sporadic E, F1 and F2 ionospheric layers

Observations

We have been using ionograms to explain current NVIS propagation conditions on 40m over many months. We present our, non-expert, observations here:

Since we operate on a frequency around 7 MHz we expect that the F2 layer is the main factor in propagating our radio signals. We are operating well above the critical frequency of the E and F1 layer. However we noticed that sometimes the sporadic E layer becomes a factor as well.
The most important parameter for our measurements appears to be the critical frequency of the F2 layer, called foF2. Theoretically, NVIS signals with a frequency above foF2 will pass straight through the ionosphere and not be refracted back to Earth. However, we have found by extensive observations that NVIS refractions can be observed about 500 kHz below this frequency. Transmitting each day on 7.045 MHz LSB using FreeDV, which accurately displays the SNR of received signals, we repeatably get NVIS propagation occurring when the foF2 is above 6.5 MHz.
Now the foF2 typically follows a diurnal cycle. Beginning at local sunrise, it increases from a minimum of 4 MHz or 5 MHz and usually peaks around 8 pm. The peak can be 10 MHz or more. There is sometimes another peak at 3 pm.
However, on some days there can be a total blackout NVIS propagation on 40 m. This is potentially caused by major solar events such as a Coronal Mass Ejections (CME) causing solar winds to hit the Earth a few days later. We often check the Australian Bureau of Meteorology (BOM) Australian Space Weather Forecasting Centre (ASWFC) for information about geomagnetic storms and also the K-index.
We noticed that a downturn in MUF 100km between 7 am and 9 am does not bode well for the rest of the day.
As well as observing the foF2 we calculate the Maximum Usable Frequency for 100 km, 300 km and 600 km.
The Maximum Usable Frequency between two stations is calculated using a simple flat-Earth geometrical model is as follows:
MUF = foF2 * sqrt(1 + (d / (2 * h'F2)^2))
Where:
foF2 = The critical frequency for the F2 layer
h'F2 = The minimum virtual height for the F2 layer
d = The Great-Circle skip distance between two stations
The MUF 100 km and 300 km correlates quite well with our NVIS observation too. However, the MUF 600 km seems more effected by path-loss and multi-path fading.
We have noticed that from time to time the foF2 and h'F2 information is not available. This is due to "blanketing" of the F1 and F2 layers by the sporadic E layer (Es). The following ionograms show the critical frequency of the sporadic E layer (foEs) increasing, first past foF1, then past foF2. When this occurs there are multiples of the Es layer stacked vertically.
During periods of Es blanketing we observe very good close up (< 50 km) NVIS, but poorer NVIS further afield. Communications beyond 300 Km do not appear to be affected.

An ionogram showing the sporadic E layer blanketing of the F1 layer

An ionogram showing the sporadic E layer blanketing of both the F1 and F2 layers

An ionogram showing the presence of a very rare F3 Layer

Data reduction

We obtain scaled ionogram data from the Lowell Global Ionosphere Radio Observatory Data Centre (LGDC) database, which uses ARTIST software to scale ionograms from ionosondes all around the world. By averaging the Canberra and Hobart ionosonde data, specifically the critical frequencies and the minimum virtual heights, we obtain a pseudo-plot for Melbourne (where there is no ionosonde). We then factor in the oblique angle of communications with the F2 layer to obtain the Maximum Usable Frequency (MUF) over distances of 100km, 300km and 600km.

F2 Layer and MUF

The ionodata application plots a snapshot of the data over the last 24 hours. You can use it to see if NVIS ionospheric support is available now, or likely to be in the next hour or so.

We have added an NVIS threshold line at 6.5 MHz. This is an experimental feature based on recent local contacts on 7.045 MHz (@10W using FreeDV), which commenced only after the MUF 100km reached 6.5 MHz. While we would expect NVIS to only occur when the critical frequency is above the operating frequency, it seems we get about half a megahertz head start. We don't know why, yet, but it is quite repeatable...

Note: You can refresh the data by using a refresh button, which appears after 20 mins. This feature was designed to reduce the number of hits on the LGDC database. In any case, new ionogram data is only available every 20 minutes. Please ignore "HTTPError: 503 Server Error: Service Temporarily Unavailable". Just try again later.

Ionodata app

Sporadic E Layer

From Version 1.8 of the Ionodata app we have include the sporadic E layer data in the same but lighter colours for Canberra, Melbourne and Hobart. We see, as predicted, that at some times foEs exceeds foF2.

Ionodata app showing foEs for Canberra, Melbourne and Hobart

Acknowledgments

Australian Bureau of Meteorology (BOM) Australian Space Weather Forecasting Centre (ASWFC), used with written permission, and the Lowell Global Ionosphere Radio Observatory (GIRO) Data Centre (LGDC)