### VOACAP Method 25: Ionospheric Parameters and Semi-thickness

###### Q: What are the definitions of the Method 25 ionospheric parameters?

The ionospheric parameters definitions are as follows:

YE = 20.0 HE = 110.0 HS = 110.0 LAT LONG LMT UT E F1 Y1 H1 FH/2 F2Z Y2 H2 ES MED HI M3000 HPF2 RAT ZEN ZMAX MAGL 54.3N 25.6E 6.7 5.0 1.08 0.0 0.0 0.0 0.6 3.8 71.6 270.9 0.0 0.0 0.0 3.19 291.1 4.2 102.7 73.2 51.9N 45.5N 28.1E 6.9 5.0 1.70 0.0 0.0 0.0 0.6 4.8 81.1 252.9 0.0 0.0 0.0 3.32 273.2 3.8 97.6 69.7 42.9N 28.6N 31.3E 7.1 5.0 2.16 0.0 0.0 0.0 0.5 6.1 99.6 292.4 0.0 0.0 0.0 3.45 255.6 3.1 87.8 61.0 26.0N 11.7N 33.6E 7.2 5.0 2.06 0.0 0.0 0.0 0.4 6.1 99.6 292.4 0.0 0.0 0.0 3.16 294.8 2.9 78.0 52.6 9.0N 2.8N 34.6E 7.3 5.0 2.16 0.0 0.0 0.0 0.4 6.3 106.1 297.1 0.0 0.0 0.0 3.13 299.6 2.8 73.2 54.3 0.0N YE = YI(1,1) = E layer semithickness of first control point HE = HI(1,1) = E layer height of maximum ionization of first control point HS = HS(1) = height of reflection of first control point

The following refer to each of the control points (in this case 5 of them). The number of control points varies with the distance between TX & RX. "k" will be the control point.

label = variable = definition LAT = CLAT(k) = Latitude of control point LONG = CLONG(k) = Longitude of control point LMT = CLCK(k) = Local mean time UT = GMT = UT at transmitter E = FI(1,k) = E layer critical frequency F1 = FI(2,k) = F1 layer critical frequency Y1 = YI(2,k) = F1 layer semithickness H1 = HI(2,k) = F1 layer height of maximum ionization FH = GYZ(k) = Gyrofrequency (FH/2 = GYZ(k)/2) F2Z = FI(3,k) = F2 layer critical frequency Y2 = YI(3,k) = F2 layer semithickness H2 = HI(3,k) = F2 layer height of maximum ionization ES = FS(1,K) = Sporadic E critical frequency lower decile MED = FS(2,k) = Sporadic E critical frequency median decile HI = FS(3,k) = Sporadic E critical frequency upper decile M3000 = F2M3(k) = F2 layer M(3000) factor HPF2 = HPF2(k) = virtual height at 0.834 critical frequency RAT = RAT(k) = ratio of F2 layer height of maximum ionization to semithickness ZEN = ZENANG(k) = zenith angle for F1 layer ZMAX = ZENMAX(k) = maximum zenith angle for F1 layer MAGL = GLAT(k) = geomagnetic latitude

The Sporadic E values in this example are 0.0 because the FPROB variable ("0.0" for Es) removes the sporadic E layer.

[Thanks to Greg Hand for explaining these.]

###### Q: What is the term "semi-thickness"? I see it being used in the explanations above.

**George Lane:** It is an old term dating back to the 1940's.
Back then they used a set of templates which they would slide
over the ionogram based on an assumed parabolic shape to the
ionospheric layer of interest. They would find the curve which
had the best fit when the template was aligned with the height
of the maximum electron density for the layer. The parabolic
curve with the best fit was numbered with the semi-thickness
(half thickness) of the parabola. It was a term needed to
determine the actual ray path as it passed through the various
layers.

The ionogram plots show electron density as a function of height. It was recognized early on that these plots tended to show the layers as having a parabolic shape. If you consult a geometry text, you will find that there is a locus point for a parabola which is centered on the line where the tangent to the line is vertical. This is the height of the maximum electron density. Each parabola has a width factor associated with the locus point. This is where the line tangent becomes horizontal. By curve-fitting to the ionograms, the scaler was able to find the semi-thickness of the layer based on simple geometry. These parameters are needed in order to determine the amount of bending which occurs as the ray at a particular frequency penetrates the 3 layers. Newer prediction models such as PropLab actually compute the ray path for a particular electron density profile. IONCAP was the first program to attempt this in a quasi-ray trace approach.