HFU HF Underground
Technical Topics => Propagation => Topic started by: KaySeeks on November 24, 2018, 2007 UTC
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Hey Chris,
I see quite a discrepancy between the Australian SWS real-time foF2 map:
http://www.sws.bom.gov.au/HF_Systems/6/5
and the equivalent map on the HFU propagation page:
https://www.hfunderground.com/propagation/#ionosphere
For reinforcement, I got some contemporary data from North American ionosonde data and it much more closely aligns with the SWS than the HFU maps.
Since there is really only one source for real time ionosonde data, I am going to assume that map on the HFU page is predicted by the software mentioned on the map and not based upon real time measurements. Correct?
I realize that recent ionosonde data is featured here, later on the page:
https://www.hfunderground.com/propagation/#skipzone
but I would think that this could be made a bit more clear.
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Hi KaySeeks,
I'm honestly not sure of the origins of the map data, but I'm happy to add any text you think would be useful to clarify things.
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The maps show different things.
One is the Critical F2.
The other is the foF2.
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The maps show different things.
One is the Critical F2.
The other is the foF2.
OK great. However, if you go onto their web page for a description of what Critical F2 is (http://www.spacew.com/www/fof2.html (http://www.spacew.com/www/fof2.html)) , you get this:
The following image is a recent high-resolution global map of F2-layer critical frequencies. This corresponds to the maximum radio frequency that can be reflected by the F2-region of the ionosphere at vertical incidence (that is, when the signal is transmitted straight up into the ionosphere). ...
This map can be used to determine the frequencies that will always be returned to the Earth. Transmitted frequencies higher than the indicated contours (which are given in MHz) may penetrate the ionosphere, resulting in lost power to space. Frequencies lower than the indicated contours will never penetrate the ionosphere. Lower foF2 values indicate a weaker ionosphere and correspond to regions with lower Maximum Usable Frequencies (MUFs). Higher foF2 values indicate a stronger ionosphere and correspond to regions with higher MUFs.
"the maximum radio frequency that can be reflected by the F2-region of the ionosphere at vertical incidence (that is, when the signal is transmitted straight up into the ionosphere)" is the definition of foF2. Note that Spacew.com use "foF2" and "critical frequency" interchangeably in that last quoted paragraph.
When you look up the definition of foF2 elsewhere, it is defined as "the critical frequency", which I take to be "critical F2": https://www.wmo-sat.info/oscar/variables/view/64 (https://www.wmo-sat.info/oscar/variables/view/64)
So we can pretty well dispense with the thought that the two things are different.
And we are back to my original question as why these maps would be be so different. I don't know. Given that spacew.com is updating their map on their website every 5 minutes, it doesn't seem like it is is any sort of prediction of the future but rather more real time. All I can say is that spacew.com's map is a heck of a lot higher frequency than the SWS map and the ULowell databases.
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And we are back to my original question as why these maps would be be so different. I don't know. Given that spacew.com is updating their map on their website every 5 minutes, it doesn't seem like it is is any sort of prediction of the future but rather more real time. All I can say is that spacew.com's map is a heck of a lot higher frequency than the SWS map and the ULowell databases.
All of those maps are quasi-realtime, and not predictive.
The biggest difference is how each index or key is weighted in averaging, spatially, and over what timeframe, which is a matter of choice for the map programming.
The maps are based on a melding of ionosonde data from various geographically separated ionosonde sources.
The map algorithms average the differential between the ionosonde curves and that forms contours.
Also, there's the "Spread F index, or fxI, upon which the MUFs are calculated, and may be utilized for map source data.
fxI is above foF2.
Normally, on the Lowell Digisonde ionograms, you see it as a green color, scattering of points.
- 1978. Spread F index, fxI: The URSI/STP Committee, noting that a measure of the top frequency of
spread F is urgently required for CCIR purposes and also has scientific interest (http://ftp://ftp.ngdc.noaa.gov/ionosonde/documentation/UAG_23A_Searchable.pdf), and that a proposal
to introduce such an index has been widely supported by those responsible for stations, recommends
that a new ionospheric parameter denoted fxI (with computer symbol 51) be adopted for international
analysis, tabulation and normal circulation through WDCs and other publication methods, defined and
applied according to the instructions following. It is recommended that all stations at high latitudes
or subject to equatorial spread F tabulate and circulate this parameter, and that stations at
other latitudes be invited to volunteer to analyze'the parameter as a trial. Tests are particularly
important at stations where the spread of frequencies of spread F often exceeds fB/2 at certain hours.
It is very important to measure fxI at stations where spread F causes the foF2 count to be small at
certain hours. Spread F rules are given in Section 2-8. - The parameter fxI is defined as the highest frequency on which reflections from the F region are
recorded independent of whether they are reflected overhead or at oblique incidence. Thus, fxI is
the top frequency of spread F traces including polar or equatorial spurs, but not including ground
back scatter traces. Since this parameter can be gain sensitive it should always be measured using
the normal gain ionogram. Special care is needed when foI (foI = fxI - fB/2) is near or below fB
since absorption can then hide fxI. Detailed rules are given in section 3.3.
For more information on how the foFI may affect the drawing of the ionogram or maps, see https://apps.dtic.mil/dtic/tr/fulltext/u2/a128934.pdf (https://apps.dtic.mil/dtic/tr/fulltext/u2/a128934.pdf)
The best NVIS propagation frequency usually happens between the foF2 and the fxI.
You may see it as:
(foF2+fxI) /2
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The maps are based on a melding of ionosonde data from various geographically separated ionosonde sources.
The map algorithms average the differential between the ionosonde curves and that forms contours.
Yes, of course.
Also, there's the "Spread F index, or fxI, upon which the MUFs are calculated, and may be utilized for map source data.
Of course, this is your speculation as to whether that is what the the spacew.com map is showing. I wrote to spacew.com to ask them why there is this discrepancy. I am waiting for an answer.
For entertainment I just checked the spacew.com map versus the FxI and foF2 for the three or four soundings over the past hour at three digisonde stations far apart from each other with somewhat different propagation conditions overhead (Hermanus, South Africa; Fortaleza, Brazil; Point Arguello, California, USA) and there is no agreement. The spacew.com map is showing something that is well above the reported FxI and foF2 by these digisondes. There isn't even any sort of systematic offset or correlation that I can tell. So I am not seeing any validation of your speculation.
The best NVIS propagation frequency usually happens between the foF2 and the fxI.
That's not what I am reading from Idaho ARES, who seemingly have a vested interest in getting this right:
http://www.idahoares.info/tutorial_hf_nvis_band_selection.shtml (http://www.idahoares.info/tutorial_hf_nvis_band_selection.shtml) (scroll down to the very bottom)
They (and everybody else) is saying stay below foF2. (FxI tends to run slightly above foF2.)
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The best NVIS propagation frequency usually happens between the foF2 and the fxI.
That's not what I am reading from Idaho ARES, who seemingly have a vested interest in getting this right:
http://www.idahoares.info/tutorial_hf_nvis_band_selection.shtml (http://www.idahoares.info/tutorial_hf_nvis_band_selection.shtml) (scroll down to the very bottom)
They (and everybody else) is saying stay below foF2. (FxI tends to run slightly above foF2.)
It is a common misconception that foF2 is the top or optimum NVIS frequency, so it isn't surprising that the hams at Idaho ARES have that interpretation.
Prior to around 1978, that was the conventional wisdom.
Also, foF2 is a widely available from many sources.
Usually, you can't go wrong with picking foF2 for NVIS.
But, as a general rule, foF2 isn't optimum.
You can see the effect of fxI in the suggested MUFs for 100, 200, 400 km distances (typical of NVIS), displayed in text at the bottom of most of the Lowell Digisonde ionograms.
Propagation is usually optimum just below or at the MUF.
fxI is utilized in the formula to derive the calculated MUF.
You will notice that the furthest right hand green fish hook scattering (fxI frequency) often tends to correspond with the 100 km or 200 km MUF in the Lowell Digisonde ionogram text.
(http://lgdc.uml.edu/common/ShowIonogram?mid=31335620)
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You can see the effect of fxI in the suggested MUFs for 100, 200, 400 km distances (typical of NVIS), displayed in text at the bottom of most of the Lowell Digisonde ionograms.
You will notice that the furthest right hand green fish hook scattering (fxI frequency) often tends to correspond with the 100 km or 200 km MUF in the Lowell Digisonde ionogram text.
This is something that I could never figure out when looking at those reports. The MUFs listed at the bottom for 100, 200, 400 km are usually > foF2. At those distances the angle of incidence is pretty high, i.e., "near vertical", and I could not rationalize the two competing thoughts that:
- anything above ~foF2 should just pass right through the F2 and out into space
- the maximum usable frequency at high angles/short distances (corresponding to what is generally considered NVIS) is generally > foF2.
OK. I get it now. Thanks for that.
In any case, we're still back to my original complaint about the spacew.com maps on the propagation page. I don't know what they are showing on those maps but it doesn't seem to actually be foF2 and they should not be labelled that way.
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And not to be too stupid when I say this:
This is something that I could never figure out when looking at those reports. The MUFs listed at the bottom for 100, 200, 400 km are usually > foF2.
Of course, this is something that we already know. In daytime, skip is longer at higher frequencies, for example, and clearly signals that are at frequencies > foF2 propagate. They just may not be reflected straight back down because they are > foF2.