Friday, October 17, 2008

VOACAP reliability, the end of the myth ?

Sorry, I have been away from the blog for quite some time... Just that I was blogging elsewhere ! Following is a series of articles recently published on the site, which question the actual reliability of VOACAP ionospheric simulations. Just as a reminder WSPR is a bidirectionnal (TX/RX) digital beacon mode invented by K1JT, which provides a SNR measurement and automatically uploads it to a Worlwide database. The sensitivy is extreme (-30 dB in 2500Hz bandwidth, equivalent to +4dB in 1Hz bandwidth). Here is the first article (slightly modified).

In April 2008, I published an article on my personnal blog showing some offset between the levels reported by WSPR and the VOACAP models. This article was based on reports provided by W1BW on my 30m WSPR signal. I am now working on the data collected during the 40m daily tests with ZL2TLD. The first results are even more surprising...
Without entering now into details, I find some 50dB offset between the WSPR reports and the
VOACAP models...

There are a 3 hypothesis:
1- I did a big mistake somewhere... (to get 50dB difference it has to be a very big mistake !)

2- VOACAP is not properly calibrated for >10,000kms pathes, or for vy low SSN's, such as those we are currently facing.
3- We all took for granted that VOACAP was the ultimate ionospheric prediction tool, without seeing that it could be completly "out of range" in a certain number of cases.

To convince you that there is a problem, have a look at the attached. It has been obtained using the
popular VE3NEA's HAMCAP (free on his site). With this interface to VOACAP the risks of mistake are minimal (you just setup, the TX power, and the type of antennas at both sides). The solar data is obtained automaticaly through IONOPROBE (or manualy through the usual means).

- First is the setup screen (you may click to enlarge), showing that I configured Hamcap for 100W TX power (for antennas I used a 1/4GP on TX side and a dipole at 55 feet on RX side).
- Following is the chart screen, showing an expected level of -25dB, for the "long path" (had put the mouse in the middle of the yellow rectangle, on the 7Mhz line, to get the 0530z SNR displayed at the bottom)

- Finaly is a plot of the long path WSPR-reports at 1W normalised TX power (the offset between the 2 series of reports, can easily be explained by a higher noise level at my location)

Now just a few maths: - I used 100W as TX power in hamcap, thus 20dB above the 1W normalised power used for the data plot
- VOACAP or HAMCAP are using SNR's in 1Hz bandwidth; WSPR in 2500Hz bandwidth... the bandwidth relation translated in dB's is 34. Which means that you need +4dB in 1 Hz bandwidth to get WSPR to report -30dB in 2500Hz.
Here the expected level is -25dB, thus lacking some 29dB to reach the required +4dB SNR in 1Hz bandwidth.

In short VOACAP tells you that the path is impossible, lacking some 49dB of antenna or power gain... Even 55 if you consider that the average bilateral levels peak at -24dB and even more if you consider that I used a dipole at 55 feet in the model and that Glenn is using a windom at 15 feet !!!

The attached VOACAP plot, is for the Required Power and Antenna Gain, vs time. You can see that according to VOACAP 51.47dB are missing to get the required +4dB in 1Hz BW (for 33% reliablility) - moreless the same value provided by HAMCAP in the above-example.
For this one I used "method 21" (long path forced model), but the classical "Method 30" gives exactly the same value.
While the minimum RPWRG is reached at 0600z, which matches exactly the observed maximum, the "required Power and antenna gain" is completly out of range... According to this graph, I should run some 400kW to have 33% probablity to get a -25 WSPR report from ZL2TLD, while it works every day at 2 W (even at .5W) !
Serious offset isn't it ?

Now; If you believe that I did a big mistake somewhere, I invite you to access the database, and work on some of the huge data available there.

More to follow...

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