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Improvements in target ranging by HF radar using the European ionospheric model PRIME
Phys. Gem. Eurrh (C), Vol. 24, No. 4, pp. 305-310, 1999 0 1999 Elsevier Science Ltd All rights reserved 1464- 19 17/99/$ - see front matter
Pergamon
PII: S1464-1917(99)00003-3
Improvements in Target Ranging by HF Radar using the European Ionospheric Model PRIME A. Bourdillon’,
G. De Franceschi2,
B. Zolesi2 and Y. Le Roux3
‘Structures Rayonnantes, UPRES-A 6075, UniversitC de Rennes, France 21stituto Nazionale di Geofisica, Vigna Murata 605, 00143 Rome, Italy 3DMR/TSI, France-TCltcom, CNET, Lannion, France Received 20 May 1998; revised 21 August 1998; accepted 23 October 1998
km. Instantaneous values of the ionospheric parameters can be also determined when measurements are available for a minimum of five separated locations inside the considered region. These instantaneous values are then used to obtain instantaneous vertical electron density profiles. PRIME and the well-known IRI (Bilitza, 1990) have been used to model the ionosphere under median and instantaneous conditions over a restricted mid-latitude area in the European longitudinal sector (40”N + 50”N; 6”W + 8’E) containing the Toulon (43.1°N, 6.O”E) - Lannion (488’N, 356.7”E) path along which oblique soundings (OS) have been performed by the SCIPION I-IF system. The modelled values of electron densities, N,, calculated from 55 km up to 500 km with a height step of 2 km under median and instantaneous conditions, have then been used in the Jones and Stephenson ray-tracing program (1975), simulating the radio wave trajectories along the path. Comparison between experimental and simulated oblique soundings (OS) as well as the range error results by raytracing application are discussed in sections 2 and 3 and using the conditions i) neglecting the effect of collisions, ii) considering only the ordinary mode of propagation, iii) varying the elevation angle between 4” and 70” (with a step of 0. 2”), and iiii) considering radio frequencies in the interval 5-19 MHz (with a step of 0.5 MHz).
Abstract. Target ranging by I-IF radar can be done by correcting the group path of target echoes for the delay due to the propagation through the ionosphere. One possibility is to simulate the propagation by ray-tracing but in this case the final accuracy obtained in target registration depends mainly on the quality of the ionospheric model used to raytrace. In this study, the PRIME ionospheric model has been used to perform ray-tracing on a 950 km path, between Toulon and Lannion, for which oblique soundings data from the SCIPION sounder are available for December 1991. The ray-tracings were performed by using the vertical electron density profiles at five points along the path calculated by PRIME for monthly median and/or instantaneous conditions. Vertical electron density profiles from the IRI model have also been considered for comparison. The results of this investigation are presented and the accuracy in target ranging is discussed as obtained for the conditions of the simulations. 0 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The primary objective of PRIME (Prediction and Retrospective Ionospheric Mapping over Europe, 1995) was the development of techniques for generating retrospective and predicted maps and models of the ionosphere over Europe (35”-55”N, lO”W-35”E) with improved accuracy in respect to the current internationally adopted procedures. These techniques were embodied in a software program (PRIME-l, 1995) able to give monthly median estimates of the standard ionospheric characteristics, Total Electron Content (TEC) values up to the user-specified height (less than or equal to 1000 km) and vertical electron density profiles up to a height of 1000
2. Simulations and Comparison Results Figure 1 shows the results of the ray-tracing simulations obtained making use of IRI and PRIME for median and instantaneous conditions for 10 December 1991 at 10 UT, respectively. Large discrepancies between simulated (crosses) and experimental OS (black squares) are observed on the time delay and on the maximum usable frequency for the F layer propagation mode. The corresponding electron
Correspondence to : A. Bourdillon 305
A. Bourdillon et al.: Target Ranging by HF Radar
306
n/91
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_._: _ ...,_.i
.,__~._ . ... . .
_.. .._. _..___-.___;__
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, _I____.
,
,_
I 20 Fripency
(MHz)
10/12/91 1OUT
2
i0 Frfquency
(MHz)
10/12/91 1OUT
Fig. 1. SCIPION and simulated gmup time delay vs frequency for 10 December 1991 at 10.00 UT by using a) PRIME median pfofiles (top), b) PRIME instantaneous pmfiks (middle), c) IRI pmfiks (bottom).
307
A. Bourdillon et al.: Target Ranging by HF Radar
10/U/91 10UT POITIJZRS
10/12/9110UT POITIERS I
.^
1..
.- :---
: ____,
Plasma Frequency YMHx)
-.
“2
.ziz a--_ MMED.
:iL-
Plasma Frequency &Hz)
Fig. 2. Ekctron density height profiles for 10 December 1991 at 10.00 UT and for Poitiers modelled by PRIME for median (dotted line) and instantaneous conditions (continuous line), and by IRI (dashed line). POLAN pmtile is uko plottedfor comparison(crosses).
Fig. 3. Adjustedby compression PRIME(continuous and dotted lines) and IRI (dashed line) electron density height profiles for 10 December 1991 at 10.00 UT. POLANprofile is also plottedfor comparison (crosses).
density profiles calculated for Poitiers (about at the middle point along the path) are plotted in Figure 2 together to the profile obtained by POLAN inversion (Titheridge, 1985) of the related vertical ionogram. It can be noted that PRIME and IRI profiles do not fit the measured profiles, especially in the E-F region below the F layer peak. We therefore investigated the effect on the ray paths of an “artificial adjustment” of the modelled profiles. For this purpose, raytracing has been performed making use of the same profile but compressing the height scale above 55 km by a factor 0.8. This is equivalent to compress the whole ionosphere downward but the shape of the profile is conserved in this process (Figure 3). The ray-tracing results obtained with these new profiles are shown in Figures 4a, 4b and 4c, indicating a significant improvement in the simulated/ experimental comparison in the time delay and in the maximum usable frequency of the F layer mode. No clear improvement, however, is obtained by using the instantaneous condition with respect to the median one, nor by using IRI with respect to PRIME, particularly near the junction between the E layer and F layer propagation mode and near the maximum usable frequency of the F layer
mode. For the same day it has been observed on several examples (not shown here) that agreement between measurements and simulations is better when using a “compressed” profiles but the compression factor is not always the same, in particular in the morning hours. Finally, a HF radar application of this study can be done simply by assuming the OS receiver at Lannion as the detected target. Figure 5 shows an example of ray-tracing simulated group path (km) behaviour vs range (km) obtained on the basis of the PRIME median profile for December 1991 at 13.00 UT for the working frequency of 14 MHz. At the given epoch and frequency, the dashed line indicates the experimental group-path deduced by SCIPION data and the continuous line fixes the Toulon-Lannion ground-range (i.e. about 950 km). The range error magnitude (km) is essentially determined by the distance between the group-path curve/dashed line intersection and the continuous/dashed lines intersection, being in this case about 100 km. Figure 6 shows the same group-path curve but obtained using the compressed vertical profile. As it can be noted, the range error decreases to about a few km. The results of this analysis are summarised in Figs 7,8 where the
308
A. Bowdillon et al.: Target Ranging by HF Radar
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Fig. 4. SCIPION and simulated group time delay vs fmquency for 10 December 1991 at 10.00 UT by using compressed a) PRIME median protiles (top), b) PRIME instantaneous profiles (middle), c) IRI profiles (bottom).
A.
Month. med.
1OUT F= 14.0 MHZ
12191
IO
1000
Range
10
10112191
12
1500
Range (km)
Fig. 6. Simulated group path vs range for 10 December 1991 at 10.00 UT and for 14 MHz using PRIME compressed median profiles.
10/12/91
09 UT
14
1OUT F= 14.0 MHZ
1000
1500
Fig. 5. Simulated group path vs range for 10 December 1991 at 10.00 UT and for 14 MHz using PRIME normal median profiles.
8
Month. med.
(km)
1 O/l 2/91
309
Bourdillon ef al.: Target Ranging by HF Radar
16
Frequency
8
10
12
14
13 UT
16
17
18
Frequency
Fig. 7. Variations of range error (km) vs working frequency (MHz) for 10 December 1991 at 09.00 UT in the case of normal and compressed RIME median profiles.
Fig. 8. Variations of range error (km) YSworking frequency (MHz) for 10 December 1991 at 13.00 UT in the case of normal and compressed PRIME median profiles.
range error vs working frequencies are plotted for 10 December 1991 at 09.00 and 13.00 UT showing the improvements that are obtained with the artificial
modification of the modelled ionosphere tracing.
used in the ray-
310
A. Bourdillon er al.: Target Ranging by HF Radar
3. Concludiug Remarks Good agreement can be obtained between the SCIPION measurements and the ray-tracing simulations by using height modified electron density profiles with respect to the PRIME and IRI model outputs (Figs 4a, 4b, 4~). The question is: is it artificial or real ? Notwithstanding further investigations, some considerations can be done: 1) the comparison of the model outputs with the measured profile shows good agreement after compression, i.e. after an adjustment of the modelled profiles to the real ionospheric situation represented by the measured profiles; 2) the comparison between the initial and modified profiles (Figs 2,3) indicates that the compression of the profile is equivalent to decrease the height of the maximum electron density of the F layer; 3) December 1991 was a period of high solar activity for which the modelled value of the maximum height of the F layer (300 km) is reasonable but it is an overestimation with respect to the experimental value (260 km); 4) contrary to expectation, only a weak improvement was found by using instantaneous profiles instead of median ones. This is probably due to the fact that the instantaneous mapping procedure consists mainly to
adjust the maximum plasma frequency of the F layer but the height of the maximum electron density is weakly modified. Finally, for HF radar applications, the difference between the true range (Toulon-Lannion, 9.50 km) and the radar deduced range has been computed for different working frequencies using PRIME vertical profiles. It has been found that, by using modified compressed profiles, the range error decreases to reasonable values of about 5-10 km (Figs 7,8).
4. References Bi1itm.D.. International Reference ionosphere, NSSDC 90-22 ( World Data Center A Rockett and SateUites),Greenbelt. USA, 15.5, 1990. Jones, R. M., and J.J. Stephenson, A versatile Three Dimensional Ray Tracing Computer Program for Radio Waves in the Ionosphere, OT Rep. 75-76. 185 p., Nat. T&n. Inform. Serv., US Departement of Commerce, Springfield. VA, 1975. PRME, Rnal Report (Advanced Issue), ECSC-BEC-EAEC, Bmssel, 1995. PRIME-I User’s Handbook, Fmnce Telecom CNET, Lannion, 1995. Titheridge, J.E., Ionogram analysis with the generalized program Report UAG-93 World Data Center A for STP, NOAA, Boulder, POW, USA. 1985.
Pergamon
PII: S1464-1917(99)00003-3
Improvements in Target Ranging by HF Radar using the European Ionospheric Model PRIME A. Bourdillon’,
G. De Franceschi2,
B. Zolesi2 and Y. Le Roux3
‘Structures Rayonnantes, UPRES-A 6075, UniversitC de Rennes, France 21stituto Nazionale di Geofisica, Vigna Murata 605, 00143 Rome, Italy 3DMR/TSI, France-TCltcom, CNET, Lannion, France Received 20 May 1998; revised 21 August 1998; accepted 23 October 1998
km. Instantaneous values of the ionospheric parameters can be also determined when measurements are available for a minimum of five separated locations inside the considered region. These instantaneous values are then used to obtain instantaneous vertical electron density profiles. PRIME and the well-known IRI (Bilitza, 1990) have been used to model the ionosphere under median and instantaneous conditions over a restricted mid-latitude area in the European longitudinal sector (40”N + 50”N; 6”W + 8’E) containing the Toulon (43.1°N, 6.O”E) - Lannion (488’N, 356.7”E) path along which oblique soundings (OS) have been performed by the SCIPION I-IF system. The modelled values of electron densities, N,, calculated from 55 km up to 500 km with a height step of 2 km under median and instantaneous conditions, have then been used in the Jones and Stephenson ray-tracing program (1975), simulating the radio wave trajectories along the path. Comparison between experimental and simulated oblique soundings (OS) as well as the range error results by raytracing application are discussed in sections 2 and 3 and using the conditions i) neglecting the effect of collisions, ii) considering only the ordinary mode of propagation, iii) varying the elevation angle between 4” and 70” (with a step of 0. 2”), and iiii) considering radio frequencies in the interval 5-19 MHz (with a step of 0.5 MHz).
Abstract. Target ranging by I-IF radar can be done by correcting the group path of target echoes for the delay due to the propagation through the ionosphere. One possibility is to simulate the propagation by ray-tracing but in this case the final accuracy obtained in target registration depends mainly on the quality of the ionospheric model used to raytrace. In this study, the PRIME ionospheric model has been used to perform ray-tracing on a 950 km path, between Toulon and Lannion, for which oblique soundings data from the SCIPION sounder are available for December 1991. The ray-tracings were performed by using the vertical electron density profiles at five points along the path calculated by PRIME for monthly median and/or instantaneous conditions. Vertical electron density profiles from the IRI model have also been considered for comparison. The results of this investigation are presented and the accuracy in target ranging is discussed as obtained for the conditions of the simulations. 0 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The primary objective of PRIME (Prediction and Retrospective Ionospheric Mapping over Europe, 1995) was the development of techniques for generating retrospective and predicted maps and models of the ionosphere over Europe (35”-55”N, lO”W-35”E) with improved accuracy in respect to the current internationally adopted procedures. These techniques were embodied in a software program (PRIME-l, 1995) able to give monthly median estimates of the standard ionospheric characteristics, Total Electron Content (TEC) values up to the user-specified height (less than or equal to 1000 km) and vertical electron density profiles up to a height of 1000
2. Simulations and Comparison Results Figure 1 shows the results of the ray-tracing simulations obtained making use of IRI and PRIME for median and instantaneous conditions for 10 December 1991 at 10 UT, respectively. Large discrepancies between simulated (crosses) and experimental OS (black squares) are observed on the time delay and on the maximum usable frequency for the F layer propagation mode. The corresponding electron
Correspondence to : A. Bourdillon 305
A. Bourdillon et al.: Target Ranging by HF Radar
306
n/91
._ .._, _
3. 2
i . . . . __
j
:.._.__.*_.
. ..!-_-_.:_
1OUT
.
_._: _ ...,_.i
.,__~._ . ... . .
_.. .._. _..___-.___;__
. . .
.__i _ _....
, _I____.
,
,_
I 20 Fripency
(MHz)
10/12/91 1OUT
2
i0 Frfquency
(MHz)
10/12/91 1OUT
Fig. 1. SCIPION and simulated gmup time delay vs frequency for 10 December 1991 at 10.00 UT by using a) PRIME median pfofiles (top), b) PRIME instantaneous pmfiks (middle), c) IRI pmfiks (bottom).
307
A. Bourdillon et al.: Target Ranging by HF Radar
10/U/91 10UT POITIJZRS
10/12/9110UT POITIERS I
.^
1..
.- :---
: ____,
Plasma Frequency YMHx)
-.
“2
.ziz a--_ MMED.
:iL-
Plasma Frequency &Hz)
Fig. 2. Ekctron density height profiles for 10 December 1991 at 10.00 UT and for Poitiers modelled by PRIME for median (dotted line) and instantaneous conditions (continuous line), and by IRI (dashed line). POLAN pmtile is uko plottedfor comparison(crosses).
Fig. 3. Adjustedby compression PRIME(continuous and dotted lines) and IRI (dashed line) electron density height profiles for 10 December 1991 at 10.00 UT. POLANprofile is also plottedfor comparison (crosses).
density profiles calculated for Poitiers (about at the middle point along the path) are plotted in Figure 2 together to the profile obtained by POLAN inversion (Titheridge, 1985) of the related vertical ionogram. It can be noted that PRIME and IRI profiles do not fit the measured profiles, especially in the E-F region below the F layer peak. We therefore investigated the effect on the ray paths of an “artificial adjustment” of the modelled profiles. For this purpose, raytracing has been performed making use of the same profile but compressing the height scale above 55 km by a factor 0.8. This is equivalent to compress the whole ionosphere downward but the shape of the profile is conserved in this process (Figure 3). The ray-tracing results obtained with these new profiles are shown in Figures 4a, 4b and 4c, indicating a significant improvement in the simulated/ experimental comparison in the time delay and in the maximum usable frequency of the F layer mode. No clear improvement, however, is obtained by using the instantaneous condition with respect to the median one, nor by using IRI with respect to PRIME, particularly near the junction between the E layer and F layer propagation mode and near the maximum usable frequency of the F layer
mode. For the same day it has been observed on several examples (not shown here) that agreement between measurements and simulations is better when using a “compressed” profiles but the compression factor is not always the same, in particular in the morning hours. Finally, a HF radar application of this study can be done simply by assuming the OS receiver at Lannion as the detected target. Figure 5 shows an example of ray-tracing simulated group path (km) behaviour vs range (km) obtained on the basis of the PRIME median profile for December 1991 at 13.00 UT for the working frequency of 14 MHz. At the given epoch and frequency, the dashed line indicates the experimental group-path deduced by SCIPION data and the continuous line fixes the Toulon-Lannion ground-range (i.e. about 950 km). The range error magnitude (km) is essentially determined by the distance between the group-path curve/dashed line intersection and the continuous/dashed lines intersection, being in this case about 100 km. Figure 6 shows the same group-path curve but obtained using the compressed vertical profile. As it can be noted, the range error decreases to about a few km. The results of this analysis are summarised in Figs 7,8 where the
308
A. Bowdillon et al.: Target Ranging by HF Radar
lo/W91
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..X_.&
,----_-. .-.,.--. .,^__ ,... _,-_ .,--__--i _ ___.....-.__..i. X.
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-.
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x... ___/ __.__.__L
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&._
__._.._::
.
.’
‘_
_
;_- ._:..
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_, ‘.I
3. 2
20
(MHz)
Fr:quency
10/12/91 1OUT
i ./ .__-._+ _... . .
: -z.~:r~:‘:l:-‘__ : _--_+_3. 2
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.
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!
/
.
i _....
. . .L
_._. _.
-‘: -. ._ . -_
20
Fr$ogueney (MHz)
10/12/91 1oUT
Fig. 4. SCIPION and simulated group time delay vs fmquency for 10 December 1991 at 10.00 UT by using compressed a) PRIME median protiles (top), b) PRIME instantaneous profiles (middle), c) IRI profiles (bottom).
A.
Month. med.
1OUT F= 14.0 MHZ
12191
IO
1000
Range
10
10112191
12
1500
Range (km)
Fig. 6. Simulated group path vs range for 10 December 1991 at 10.00 UT and for 14 MHz using PRIME compressed median profiles.
10/12/91
09 UT
14
1OUT F= 14.0 MHZ
1000
1500
Fig. 5. Simulated group path vs range for 10 December 1991 at 10.00 UT and for 14 MHz using PRIME normal median profiles.
8
Month. med.
(km)
1 O/l 2/91
309
Bourdillon ef al.: Target Ranging by HF Radar
16
Frequency
8
10
12
14
13 UT
16
17
18
Frequency
Fig. 7. Variations of range error (km) vs working frequency (MHz) for 10 December 1991 at 09.00 UT in the case of normal and compressed RIME median profiles.
Fig. 8. Variations of range error (km) YSworking frequency (MHz) for 10 December 1991 at 13.00 UT in the case of normal and compressed PRIME median profiles.
range error vs working frequencies are plotted for 10 December 1991 at 09.00 and 13.00 UT showing the improvements that are obtained with the artificial
modification of the modelled ionosphere tracing.
used in the ray-
310
A. Bourdillon er al.: Target Ranging by HF Radar
3. Concludiug Remarks Good agreement can be obtained between the SCIPION measurements and the ray-tracing simulations by using height modified electron density profiles with respect to the PRIME and IRI model outputs (Figs 4a, 4b, 4~). The question is: is it artificial or real ? Notwithstanding further investigations, some considerations can be done: 1) the comparison of the model outputs with the measured profile shows good agreement after compression, i.e. after an adjustment of the modelled profiles to the real ionospheric situation represented by the measured profiles; 2) the comparison between the initial and modified profiles (Figs 2,3) indicates that the compression of the profile is equivalent to decrease the height of the maximum electron density of the F layer; 3) December 1991 was a period of high solar activity for which the modelled value of the maximum height of the F layer (300 km) is reasonable but it is an overestimation with respect to the experimental value (260 km); 4) contrary to expectation, only a weak improvement was found by using instantaneous profiles instead of median ones. This is probably due to the fact that the instantaneous mapping procedure consists mainly to
adjust the maximum plasma frequency of the F layer but the height of the maximum electron density is weakly modified. Finally, for HF radar applications, the difference between the true range (Toulon-Lannion, 9.50 km) and the radar deduced range has been computed for different working frequencies using PRIME vertical profiles. It has been found that, by using modified compressed profiles, the range error decreases to reasonable values of about 5-10 km (Figs 7,8).
4. References Bi1itm.D.. International Reference ionosphere, NSSDC 90-22 ( World Data Center A Rockett and SateUites),Greenbelt. USA, 15.5, 1990. Jones, R. M., and J.J. Stephenson, A versatile Three Dimensional Ray Tracing Computer Program for Radio Waves in the Ionosphere, OT Rep. 75-76. 185 p., Nat. T&n. Inform. Serv., US Departement of Commerce, Springfield. VA, 1975. PRME, Rnal Report (Advanced Issue), ECSC-BEC-EAEC, Bmssel, 1995. PRIME-I User’s Handbook, Fmnce Telecom CNET, Lannion, 1995. Titheridge, J.E., Ionogram analysis with the generalized program Report UAG-93 World Data Center A for STP, NOAA, Boulder, POW, USA. 1985.