Ionogram inversion F 1-layer treatment effect in raytracing

This paper shows the importance of the F1-layer shape in the electron density profiles obtained from ionograms with different inversion techniques when the profiles are used in ray tracing. This layer often controls the propagation on the path with ranges less than about 2000 km, particularly for spring and summer periods. Ionograms from two different stations, Hainan (19.4N, 109E) and El Arenosillo (37.1N, –6.7E), obtained during the month of July 2002 (average sunspot number: 99.6) during geomagnetic quiet conditions (Ap-index between 9 and 15) are analyzed. The profiles obtained with two different inversion techniques with different options are used together with the ray tracing program of the Proplab-Pro software. This program calculates the features of the received signal as angle of arrival, path length, height of reflection and range for each given profile assumed to define a spherically symmetric ionosphere in the region along the path. For each ionospheric condition (location, day, hour) the difference between range values obtained with Proplab-Pro program using profiles from the two techniques and the different options (POLAN no valley, POLAN valley, POLAN1-layer and NHPC) are considered. Mailing address: Dr. Gloria Miró Amarante, The Abdus Salam International Centre for Theoretical Physics (ICTP), Strada Costiera 11, 34014 Trieste, Italy; e-mail: amarante@ictp.it


Introduction
The aim of this paper is to show how different ionogram inversion techniques, POLAN (Titheridge, 1985(Titheridge, , 1988(Titheridge, , 1990) ) and NHPC (Huang and Reinisch, 1996), can affect ray tracing results used to characterize a radio propagation link in particular with the propagation parameters: range, group path and height of reflection (apogee).
It is shown in different studies (Sprague, 1994) that the F1-layer often controls the prop-agation path for ranges less than about 2000 km, particularly for spring and summer periods.
In order to analyze the problem of F1-layer effect on propagation a ray tracing program of Proplab-Pro software has been used to calculate how the signals are propagated through the ionosphere in the presence of the F1-layer looking at features of the received signal, such as its angle of arrival, path length, height of reflection and range.
The results of improving the propagation model by means of ray tracing techniques could be applied in several communications problems including those related to Over-The-Horizon (OTH) radar operation.

Methodology
The software used to calculate the behavior of the radio signals (3-30 MHz) as they travel through the ionosphere is known as Proplab-Pro (Solar Terrestrial Dispatch, Canada).This program (http://www.spacew.com/Docs/propman.pdf)simulates the path between transmitter and receiver taking into account a realistic ionosphere by using ray tracing techniques.
For the computation of the signals, the ray tracing technique called comprehensive is chosen as a Proplab-Pro's option.It includes the effects of the Earth's magnetic field and electron collisions with neutral particles by using the Appleton-Hartree formulation (Ratcliffe, 1959).Although Proplab-Pro program can use the International Reference Ionosphere (IRI) to model profiles of the ionosphere, this paper shows the possibility to consider electron density profiles inverted from ionograms.These ionograms are measured by digisondes designed by the University of Massachusetts Lowell and located at Hainan (19.4N, 109E) and El Arenosillo (37.1N, -6.7E).To avoid possible problems derived from automatic scaling of the ionograms all the characteristics are edited using the software Sao-Explorer.Proplab-Pro considers these electron density profiles (which can be interpreted as refractive index profile for a ray tracing technique) as the state of the ionosphere at the mid-path but extended as spherically stratified over the geographical region seen by the projected ray over the earth surface.
In each case, a link is defined by means of the transmitter and receiver coordinates (table I).
The short distance between transmitter and receiver in both links allows us to consider one single hop condition.
Data used correspond to July 2002 with a sunspot number of 99.6.Four geomagnetically quiet days are chosen with Ap-index between 9 and 15.
The ray paths under each ionospheric condition (location, day, hour) given by the elec-tron density profiles obtained with the different inversion procedures are traced sweeping elevation angles (15-85 with a step of 5) and operation frequencies (3-14 MHz with a step of 1MHz).Proplab-Pro calculates for each propagating mode parameters as the ground range, the latitude/longitude of the ray, geometrical path distance (that is, the total distance travelled by the ray), group path, etc.
One of the most common techniques of ionogram inversion is the Polynomial Analysis or POLAN, developed by Titheridge (1985Titheridge ( , 1988Titheridge ( , 1990)).This technique solves the inversion problem by breaking up the profile into simpler sections for which physically expectable solutions can be found and using extrapolation and interpolation for the remaining part of the profile.
On the other hand, modern digisondes use NHPC algorithm developed by Huang and Reinisch (1996) to define a profile from the base of the E-layer to hmF2 implemented automatically into the digisonde operation.
In order to define the different electron density profiles used in the ray tracing procedure these two techniques have been used including different options for POLAN (POLAN no valley,POLAN valley,. POLAN1 layer considers the whole profile as a «single layer» by not inputting explicitly the critical frequency of F1-layer when giving the input ( f, hv) array.This means treating F region as having only a F1 cusp without specifying foF1.Whereas POLAN no valley and POLAN valley consider the F region as consisting of two layers (F1+F2) by inputting explicitly the critical frequency foF1 when giving the input ( f, hv) array.However, in order to re-create the true-height-profile with valley, the ionisation in this region must be estimated as the information on the ionisation between the layers cannot be measured by an ionosonde.The electron concentration there is less than the critical frequency of the lower layer, and any wave which has a frequency sufficient to penetrate the first layer will not be reflected back by these lower density electrons.A possible estimation for the true-height of the upper layer can be made by assuming that the ionisation between the layers is the same as the peak density of the lower layer, so that there is no valley but a sim- ple cusp to merge the F1-and F2-layers.POLAN no valley is based on this estimation.On the other hand, POLAN can take into account the variation in height and depth of the valley with time of day, date and latitude.This is done automatically in POLAN valley option (see Titheridge 1985Titheridge , 1988Titheridge , 1990)).
In this context, previous studies have shown the difficulties introduced by the ionogram inversion techniques in the presence of F1-layer.Bamford (2000) shows the differences found by comparing experimental oblique ionograms with the one reconstructed with ray tracing and electron density profile inverted from vertical ionogram.

Results
For each ionospheric condition (location, day, hour) the difference between range values obtained with Proplab-Pro program using profiles from the two techniques and the different options are calculated.These radio propagation values are filtered in order to obtain radio fre-quencies and elevation angle in such a way that reflection heights are located in the F1 region and the lower part of F2-layer (150-250 km).
It is seen that there are important differences between results obtained with the two techniques and the different options, reflecting the fact that the F1 mode trace depends strongly on the electron density height profile between the E-and F1-layers (Krasheninnikov et al., 1996).
The option POLAN1-layer is chosen as the reference for these differences taking into account that this option is the only one that gives an electron density profile that is always continuous in the values and in the first derivative dN/dh.This point has been verified in some cases where dN/dh presented discontinuities and the results produced by the ray tracing technique Proplab-Pro showed problems for frequencies and angle values with height of reflection close to this region.
Maximum values of these differences in range are shown in tables II and III for El Arenosillo and Hainan link respectively.In general, the results show that the differences are lower in the case of the no valley POLAN op-   tion and can reach large values with the NHPC technique.
Figures 1-4 show as examples the comparison between electron density profiles and the first derivatives obtained by different inversion techniques and the worst cases for the range values differences during different ionospheric conditions.
These plots show that cases where electron density profiles are the same in comparison to those obtained with the reference option POLAN1-layer but not their corresponding first    III).On the other hand, table III shows for the same period a maximum range difference of 291 with no valley and valley options.However, figure 1 shows that the corresponding profiles and dN/dh look similar up to the apogee region (170-175 km).
Another result is that no relationship is found between plasma frequency changes and range reached as seen in figs.3 and 4.There is no defined trend between these differences and frequency and angle values.Frequencies and angles of arrival obtained cover from 6 to 14 MHz and from 15 to 70 degrees respectively.
It is interesting to note that the same results are found for the two links considered that are at very different geographical longitude.

Conclusions
The main results obtained in this paper can be summarized as follows: -For operation conditions the height of reflection and range obtained with ray tracing depends on the inversion technique chosen mainly with a reflection height in the F1 region.
-The differences in range reach largest values using the NHPC and valley options.These differences were also found in cases where electron density profiles look similar.
-In general, the lowest range differences were found for no valley option because this option gives a profile very similar to the reference one.However, there are cases with similar profiles and first derivatives that show differences in range of about 300 km.
-No dependence was found between differences in range and frequency, angle or geographical longitude.
Further studies will be done to check these results during different ionospheric conditions.

Fig. 1 .
Fig. 1.Electron density profiles and first derivatives dN/dh with different inversion techniques for day 197 and hour 04:15 UT corresponding to Hainan link.

Fig. 2 .
Fig. 2. Worst cases for the range values differences during the ionospheric conditions described by fig. 1.

Fig. 3 .
Fig. 3. Electron density profiles and first derivatives dN/dh with different inversion techniques for day 182 and hour 14 UT corresponding to El Arenosillo link.

Fig. 4 .
Fig. 4. Worst cases for the range values differences during the ionospheric conditions described by fig. 3.

Table I .
Transmitter and receiver geographic coordinates.

Table II .
Maximum range differences for El Arenosillo link.Modes (NH =NHPC, 1L=POLAN1-layer, NV=POLAN no valley, V=POLAN valley).Manually scaled ionograms are indicated with the word 'edited'.APOG1 and APOG2 are the apogee heights obtained with first and second option in column Modes and RANG1 and RANG2 their ranges.

Table III .
Maximum range differences for Hainan link.Modes (NH=NHPC, 1L=POLAN1-layer, NV=POLAN no valley, V=POLAN valley).Manually scaled ionograms are indicated with the word 'edited'.APOG1 and APOG2 are the apogee heights obtained with first and second option in column Modes and RANG1 and RANG2 their ranges.