Ray tracing tool

Highlights
IONOspheric Ray Tracing (IONORT) is an applicative software tool package for calculating a three-dimensional ray tracing of high frequency (HF) radio waves in the ionospheric medium.

IONORT_Fig1
Fig. 1 – The three dimensional (3-D) electron density profile grid, where i and j vary with the latitude and longitude, and k with the altitude, respectively. Column on the left composed by k cells represents the vertical electron density profile Vij. The ray path from a transmitter (TX) point to a receiver (RX) point is represented by a dotted curve. Because of the involved distances, the 3-D matrix has a spherical shell shape.

Abstract
IONORT runs under Windows operating systems. The corresponding software is coded in MATLAB for the input/output routines, while the integration algorithm is coded in FORTRAN. MATLAB graphical user interface (GUI), friendly managing the input needed to the integration algorithm and the corresponding numerical and graphical output, facilitates noticeably the numerical data input entry managed by the user and at the same time performs an useful two/three dimensional (2-D/3-D) visualization of the ray path. From a numerical point of view, in order to calculate the coordinates of the ray and the three components of the wave vector as dependent variables along the path, the core of the program solves a system of at least six first order differential equations, with Hamiltonian formalism, which are computed into a spherical coordinate geocentric system, where the group path is the independent variable of integration. IONORT uses a 3-D electron density representation of the ionosphere, as well as geomagnetic field and electron-neutral particle collision frequency models having validity in the area of interest. An analytical standard Chapman’s modelled ionosphere, useful mainly for testing purpose, completes the whole applicative software tool package.

IONORT_Tab1
Table 1. – First 25 components of IONORT (IONOspheric Ray Tracing) input vector W.

 

Fig.2. - Graphical user interface (GUI) of IONORT program. The 2-D and 3-D visualizations of the ray paths are shown at the bottom and right respectively, considering a TX point at Rome, Italy, and an azimuth angle of transmission equal to 121.6° (in direction of Chania, Greece), on 26 June 2011 at 01:00 UT. To realize the full potential of IONORT, composite simulations, based on the International Reference Ionosphere (IRI) global model, are performed not only inside but also outside the central Mediterranean region, by taking the geomagnetic field and the electron collisions into account, with single or multiple ionospheric reflections (1 – 3 hop paths): for a fixed elevation angle of 18° with a 3 MHz frequency-step iterative procedure from 3 MHz to 30 MHz; and for a fixed frequency of 15 MHz with a 5° elevation-step iterative procedure from 0° to 60°.
Fig.2. – Graphical user interface (GUI) of IONORT program. The 2-D and 3-D visualizations of the ray paths are shown at the bottom and right respectively, considering a TX point at Rome, Italy, and an azimuth angle of transmission equal to 121.6° (in direction of Chania, Greece), on 26 June 2011 at 01:00 UT. To realize the full potential of IONORT, composite simulations, based on the International Reference Ionosphere (IRI) global model, are performed not only inside but also outside the central Mediterranean region, by taking the geomagnetic field and the electron collisions into account, with single or multiple ionospheric reflections (1 – 3 hop paths): for a fixed elevation angle of 18° with a 3 MHz frequency-step iterative procedure from 3 MHz to 30 MHz; and for a fixed frequency of 15 MHz with a 5° elevation-step iterative procedure from 0° to 60°.

In-depth analysis
IONORT-ISP system (IONOspheric Ray-Tracing – IRI-SIRMUP-Profiles) was developed and tested by comparing the recorded oblique ionograms over the radio link between Rome (41.89ºN, 12.48ºE), Italy, and Chania (35.51ºN, 24.02ºE), Greece, with IONORT-ISP synthesized oblique ionograms. As upgrade of the system: a) electron-neutral particle collisions have been included by using a collision frequency model, which consists of a double exponential profile; b) ISP model of 3-D electron density profile grid has been extended down to the altitudes of the D layer; c) the resolution in latitude and longitude of ISP model for 3-D electron density profile grid has been increased from 2°x2° to 1°x1°. Based on these updates, a new applicative software tool package, named IONORT-ISP-WC system (WC means with collisions) was developed, and a database of 33 IONORT-ISP-WC synthesized oblique ionograms was calculated for single and multiple ionospheric reflections (1 – 3 hop paths). IONORT-ISP-WC synthesized oblique ionograms were compared with both IONORT-IRI-WC synthesized oblique ionograms, generated by applying IONORT in conjunction with the 3-D International Reference Ionosphere (IRI) electron density profile grid, and the recorded oblique ionograms over the aforementioned radio link. The results obtained show that: (1) during daytime, for the lower ionospheric layers, the traces of the synthesized oblique ionograms are cut away at lower HFs because of the noteworthy HF absorption; (2) during night-time, for the higher ionospheric layers, the traces of the synthesized oblique ionograms are not cut off at lower HFs because of thr negligible HF absorption; (3) IONORT-ISP-WC MUF values are more accurate than IONORT-IRI-WC MUF values.

IONORT_Fig 3
Fig.3 – Overall flowchart of IONORT-ISP system (IONOspheric Ray-Tracing – IRI-SIRMUP-Profiles).
IONORT_Fig 4
Fig.4 – Map of the central Mediterranean region, which is considered by IONORT. Red coloured stars represent the ionospheric stations used as input from ISP model. In blue colour, the radio link between the TX (Rome, Italy) and the RX (Chania, Greece) points used to test the effectiveness of IONORT-ISP system.
IONORT_Fig 5
Fig.5 – A comparison between the oblique ionogram recorded over the Rome-Chania radio link on 07 July 2011 at 15:00 UT (top panel), and the corresponding ionograms oblique synthesized by IONORT-IRI-WC (middle panel) and IONORT-ISP-WC (bottom panel) systems (WC means with collisions). The red and purple coloured vertical lines indicate the Maximum Usable Frequency (MUF) values for the single and multiple ionospheric reflections (1 – 3 hop paths) respectively. The Lower Observable Frequency (LOF) values are also indicated for the E and F layers. For the sake of clarity, only the ordinary trace computed taking the geomagnetic field into account is shown. With both 1 – 3 hop paths, a nested loop cycle was iterated with azimuth angles from 121° to 122° of step 0.2°. The elevation angle step was set to 0.2° and the range accuracy of RX point to 0.1 %. The ionograms calculated without applying the collisional model, labelled with NC (no collisions), are also shown for a further comparison.
IONORT_Fig 6
Fig.6 – Comparisons between the differences (IONORT-IRI-WC MUF – measured MUF) (green coloured squares) and (IONORT-ISP-WC MUF – measured MUF) (red coloured squares) for testing the whole database. The arrows indicate the cases for which IONORT-ISP-WC system mostly underestimates/overestimates the MUF values. The symbol * marks the 6 cases for which IONORT-IRI-WC system works better than IONORT-ISP-WC system.