HF Charts
09:45
10/03/10
The HAP chart is a guide to frequency selection for HF
communications between mobiles and a specified base station, in this case . The
recommended frequency for communications with the base at a given hour is read
off from the point in the map corresponding to the mobile's location.The
predictions are made from real time data which are updated each hour.
'dash lines' - If present, these delineate areas of unlikely communication. Poor
communication areas which can result from one or more of the following
conditions:
- the Absorption Limiting Frequency (ALF) is higher than the Optimum Working
Frequency (OWF).
- the ALF exceeds the highest frequency in the user's frequency set.
- there is no suitable frequency from the user's set that is between the OWF and
ALF.
A feature of
the ionosphere is its ability to reflect radio waves. However, only radio waves
within a certain frequency range will be reflected and this range varies with a
number of factors.
The most widely used instrument for ionospheric measurement is the ionosonde.
The ionosonde is essentially a high frequency radar which sends short pulses of
radio energy into the ionosphere. If the radio frequency is not too high, the
pulses are reflected back to earth.
The ionosonde records the time delay between transmission and reception of the
pulses. By varying the frequency of the pulses (typically 1-22MHz), a record is
obtained of the time delay at different frequencies. This record is referred to
as an ionogram.
The highest (see note) frequency which the ionosphere will reflect vertically is
called foF2. These foF2 measurements from various sites can be used to create a
map of foF2.
The above map can be used as a guide to NVIS ionospheric frequency support.
Map is updated at approximately 40 minutes past the hour.
Note: The geomagnetic field splits a radio wave in the ionosphere into two
separate components, termed the ordinary (o) and extraordinary (x) waves. It is
the o-wave which is routinely scaled from ionograms.
The data presented in this page are experimental, and are derived from the
automated interpretation of regional ionograms.
The data presented on this page is experimental, and is derived from the automated interpretation of hourly regional ionograms. The chart displays a three-hour coloured band that moves down the the chart window in the course of a day, returning to the top around the start of the day. This band gives observed communication frequencies and a prediction for the next period. It is based on real-time ionospheric information and an extrapolation in time. All times in the chart are in Universal Time (same as GMT and Zulu).
This image shows the DIFFERENCE between current OBSERVED HOURLY conditions and PREDICTED MONTHLY conditions for the Australian region. The colours blue, green, yellow, red, correspond to "enhanced", "normal", "mildly depressed" and "depressed" conditions respectively. Depressions and enhancements are with respect to the IPS predicted monthly T index for that month.
This plot shows the trend in the observed Australian regional T index over the past 30 days. The daily values can fluctuate during disturbances. The dashed blue line represents the median of these daily values and is an indicator of the quiet level of ionospheric support over the period plotted. The forecast daily T index for the current UT day is shown as the unfilled rectangle.
The following image is a recent high-resolution map of Maximum Usable Frequencies (MUFs) for 3,000 kilometer radio signal paths. It is also a map showing the current location of the auroral ovals, the sunrise/sunset terminator and the regions of the world where the sun is 12 degrees below the horizon (which estimates the gray-line corridor where HF propagation is usually enhanced).
Using this Map
This is a highly informative map that can be used by amateur and professional radio communicators to determine maximum usable frequencies for any world-wide path at the indicated UTC (Zulu) time.
RED contour lines will appear superimposed on the MUF map if x-rays reach levels capable of producing short wave fadeouts on sunlit paths. When this occurs, the red contour lines represent the highest frequency (in MHz) that may be absorbed by the enhanced solar flare x-rays.
The MUF for any 3,000 kilometer path can be determined by finding the midpoint (or half-way point) of the path and examining the MUF at that midpoint on the map by finding the labelled MUF contour value. All contours are given in MHz.
For 4,000 kilometer paths, multiply the given contoured MUF values by 1.1. The MUF for the given 4,000 km path is then determined at the midpoint of the desired path.
For longer path lengths, divide the path into equal 3,000 or 4,000 km segments and compute the MUFs corresponding to the two midpoints that are 1,500 or 2,000 km from each end of the path. Then select the lower of these two MUFs.
The map shows the radio auroral zones as green bands near the northern and southern poles. The area within the green bands is known as the auroral zone. Radio signals passing through these auroral zones will experience increased signal degradation in the form of fading, multipathing and absorption.
The radio auroral zones are typically displaced equatorward from the optical auroral zones (or the regions where visible auroral activity can be seen with the eye).
The great-circle signal path from the Eastern United States to Tokyo Japan is shown along with the distance of the path (in km) and the great-circle bearing from the U.S. to Tokyo (in degrees from north).
If this signal path crosses through the green lines indicating the position and width of the radio auroral zones, propagation will be less stable and degraded compared to if the signal never crossed through the auroral zones. Using your mouse, PROPLAB-PRO will let you plot the great-circle paths and azimuths between any two points while this display is continually updated.
The yellow Sun symbol near the equator indicates the location where the Sun is directly overhead.
The regions of the world where the Sun is exactly rising or setting is known as the Grayline and is shown as the solid gray-colored line that is closest to the Sun symbol.
The second solid gray-colored line defines the regions of the world where the Sun is exactly 12 degrees below the horizon. This line defines the end of evening twilight. Everything inside of this second line is experiencing night-time conditions.
The area between the two lines (shaded a lighter shade than the night-time sector) is known as the grayline and has special significance to radio communicators. Signals which travel inside the grayline region often experience significant improvements in propagation because of the loss of ionization in the D-region as the Sun sets. However, because the higher F-regions of the ionosphere remain strongly ionized for longer periods of time, signals with higher frequencies are able to travel to greater distances with less attenuation when they are within the grayline.
The great-circle path from the eastern U.S. to Japan is also shown with the accompanying distance (in kilometers) and bearing (clockwise from north). Notice how this path may occassionally pass into the influential auroral zones if geomagnetic activity increases or during the night-times.
