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IRI slant electron content predictions
Adv. Space Rex Vol. 27, No. 1, pp. 65-70,200l 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/O]$20.00 + 0.00 PII: SO273-1177(00)00152-6
Pergamon www.elsevier.nl/locate/asr
IRI
SLANT ELECTRON CONTENT PREDICTIONS
R. G. EzquerlW2,C. A. Jade?, S. M. Radicella4, E. Kiorcheff 2, R. de1 V. O&do’, J. R. Manzano’, and L. A. Scidb’
1Laboratorio de Ion&$era, Dep. de Fisica, UNT, Indepe~ncia 1800, 4000 Tucumdn, Argentina. 2 CONKET - F&dad Regional Tucumdn, Universidd Tecnol@ica National, Argentina. 3 Universihd National de salta, Argentina. 4 l’k Abdks Salam International Centre for Theoretical Physics, Treste, Ita&
ABSTRACT The International Reference Ionosphere model is used to calculate the slant electron content (SEC) encountered by a radio signal emitted from a satellite and received at a middle latitude station. Two cases were considered, namely: a) The satellite and the ground station are in the same meridian, b) the satellite and ground station have different longitudes. For the first case only modelled values for different solar activities, hours and seasons are considered. For the second case a comparison with measurements obtained with a synchronous satellite radio signal during a low solar activity period is done. The results show the solar control and the dependence with the zenithal angle of the satellite. The comparison with measurements shows very good agreement between predictions and measurements for some periods. But, in general, the model underestimates SEC. These results suggest that additional studies covering more stations and conditions are required in order to co&-m the need of improvements in IRI model 0 2001COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION The ionosphere produces diierent effects on the signals that traverse it and causes errors in the systems which use transionospheric radiowaves. An important effect on a transionospheric signal is the group delay (2). If t is the time of travel of a radio signal on a distance S in vacuum, and t’ is the time of travel through the ionosphere for the same S, then: z =t’-t
(I)
which can be expressed as: z = (40.3 IN ds ) / (c f 2,
(2)
where N is the electron density in mm3,f is the signal’s frequency in Hz, c is the velocity of light in vacuum and z is in seconds. The integration is over the ray path. For a vertical path: 2=(40.3
jNdh)/(cf’)
(3)
65
R. G. Ezquer et al.
66
The integral ( N dh is called total electron content of the ionosphere (TEC), and is the number of free electrons present in a unit cross section vertical column which extends from ground up to the top of the ionosphere. So, for ionospheric corrections TEC measurements are needed, or ionospheric model predictions can be an useful tool. Different ionospheric mo.dels have been developed to predict the N distribution in height, which is called N-profile (Chiu, 1975; Anderson et al, 1987), including the International Reference Ionosphere (IRI) (Bilitza, 1990). Nevertheless, most of the signal paths are slant paths. In this work the electron content over slant paths (SEC) are calculated using IRI model. METHOD Two cases were considered, namely: a) the satellite and the ground station are in the same meridian b) the satellite and the station have different longitudes. For these calculations the length of the slant path is divided in segments of 20 km. The verticals which pass through the ends of these segments intersect the Earth’s sur%ce in different points called auxiliary stations. The coordinates of these stations are determined. With IRI model the electron densities at the points where the slant path intersects the verticals of the auxiliary stations are calculated and from them the slant N-profile is obtained. With this slant N-profile the SEC up to 2000 km altitude is calculated. The IRI code provides two distinct peak density models, in this paper the CCIR model has been used. A x&ware was developed to calculate SEC with this method. It works in the following way: 1. It determines the coordinates of the auxiliary stations 2. Automatically runs the IRI software to obtain N-profile over these stations. 3. Then builds the slant profile and calculates SEC=jNds
(4)
For case a), the software requires the following inputs: + Zenithal angle of satellite (x) 4 To know if the satellite is to the North or to the South of the ground receiver station + Geodetic latitude and longitude of the ground receiver station + RI2 + Month and day + Universal time and for the case b) the inputs are: Geodetic latitude and longitude of the ground station Geodetic latitude and longitude of the satellite Altitude of satellite R12 Month and day Universal time DATA AND RESULTS For case a) the considered station is Rome (41.9“ N, 12.5“E). Figure 1 shows the calculated SEC over this station for different months, high (R12 = loo), middle (R12 = 55) and low (R12 = 10) solar activity at 05
61
Slant Electron Content r
1. -
12 ?
!
b :
m-
$ ‘;
d ‘”
4-
lo
L
3 I
odo.a-7044muuu-00
04
1
z&mrFN.Am.oi
,,I,,,,,
4o-Bo-loQdDI4o-l)*m ‘/lWOTAL AX.
a
j
/
I,,,
0
10
OF SAlElJ.lllC
I,,
lo
*
40
a
so
70
o@
CDWREE)
Fig. 1. Calculated SEC over Rome for low (R12=10), middle (Rl245) and high (Rl2=100) solar activity, 11 UT and 05 UT. The considered months are March (white triangle), June (black square) September (black triangle) and December (white square). Positive values of zenital angle indicate that satellite is to the North of the ground station, and negative values indicate that satellite is to the South.
68
R. G. Ezquer et af.
1-lS74(RlZzM..?J 1
Fig. 2. Predicted (dashed line) and measured (solid line) slant electron content at ATS6 - Boulder path
69
Slant Electron Content
J5mlu7 1875 (Rrr
25.8)
:1I
Yw8h 1
Fig. 2. (continued)
1475 (Rf.t
MS)
.’
Apm m75 (RlZ
98.8)
70
R. G. Ezquer et al
UTandllUT. .It can be seen that for equinox, 11 UT and high solar activity, the highest values of SEC are obtained. When the zenithal angle of satellite is 80’ to the South of the station, the calculated SEC reach values about 12.10” mm’.The lowest values correspond to low solar activity at 05 UT. There is a strong dependence of SEC with the zenithal angle of satellite for R12 = 100 and 11 UT. A smooth variation of SEC with x for low and middle solar activity and 5 TU is observed. For low solar activity and daytime conditions, the SEC values calculated for diierent seasons are similar. For high x the SEC values calculated when the satellite is to the South of the ground station are greater than those obtained when to the satellite is to the North. Fritz (1976) have measured the slant electron content between Boulder (40.13” N, 254.76“ E) and the ATS 6 synchronous satellite (266“ E) for the period July/74 - May/75. They measured ionospheric, total and protonospheric electron content. In this work we use the ionospheric slant electron content measurements obtained with Faraday technique. Figure 2 shows the obtained results. It can be seen a very good agreement among predictions and measurements at hours of maximum ionisation for July and August, and also during the night for December. In general, the model underestimates the ionospheric SEC at all hours of the day. The IRI underestimation at hours of maximum ionisation, is lower than 20 % except for the period from December solstice to March equinox. CONCLUSIONS Using the IRI model the SEC over two middle latitude stations has been calculated. The used method has been described. The dependence with the solar activity the zenithal angle of satellite was shown. The results suggest that the SEC over Rome can reach a value about 12. 1017mm2for x = 80’ during high solar activity. The comparison with Faraday measurements obtained at Boulder for a low solar activity period show cases with a very good agreement between predictions and measurements. But, in general, the model underestimates the SEC. These results suggest that additional studies considering the URSI code and covering more. stations and conditions are required in order to confirm the need of improvements in IRI model. REFERENCES Anderson, D. N., M. Mendillo; and B. Hertniter, A Semiempirical Low Latitude Ionospheric Model, Radio Sci., 22,292, (1987). Bilitza, D., International Reference Ionosphere 1990, National Space Science Data Center 90-92, WDC-A R&S, Geenbelt, Maryland., USA (1990). Chiu, Y.T., An Improved Phenomenological Model of the Ionospheric Density, J. Atmos. Ten-. Phys, 37, 1563 (1975) Fritz, R.B., ATS 6 Radio Beacon Electron Content Measurements at Boulder, July 1974 - May 1975, World Data Center A for Solar-Terrestrial Physics. Report UAG - 58, Boulder, Colorado, USA (1976).
Pergamon www.elsevier.nl/locate/asr
IRI
SLANT ELECTRON CONTENT PREDICTIONS
R. G. EzquerlW2,C. A. Jade?, S. M. Radicella4, E. Kiorcheff 2, R. de1 V. O&do’, J. R. Manzano’, and L. A. Scidb’
1Laboratorio de Ion&$era, Dep. de Fisica, UNT, Indepe~ncia 1800, 4000 Tucumdn, Argentina. 2 CONKET - F&dad Regional Tucumdn, Universidd Tecnol@ica National, Argentina. 3 Universihd National de salta, Argentina. 4 l’k Abdks Salam International Centre for Theoretical Physics, Treste, Ita&
ABSTRACT The International Reference Ionosphere model is used to calculate the slant electron content (SEC) encountered by a radio signal emitted from a satellite and received at a middle latitude station. Two cases were considered, namely: a) The satellite and the ground station are in the same meridian, b) the satellite and ground station have different longitudes. For the first case only modelled values for different solar activities, hours and seasons are considered. For the second case a comparison with measurements obtained with a synchronous satellite radio signal during a low solar activity period is done. The results show the solar control and the dependence with the zenithal angle of the satellite. The comparison with measurements shows very good agreement between predictions and measurements for some periods. But, in general, the model underestimates SEC. These results suggest that additional studies covering more stations and conditions are required in order to co&-m the need of improvements in IRI model 0 2001COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION The ionosphere produces diierent effects on the signals that traverse it and causes errors in the systems which use transionospheric radiowaves. An important effect on a transionospheric signal is the group delay (2). If t is the time of travel of a radio signal on a distance S in vacuum, and t’ is the time of travel through the ionosphere for the same S, then: z =t’-t
(I)
which can be expressed as: z = (40.3 IN ds ) / (c f 2,
(2)
where N is the electron density in mm3,f is the signal’s frequency in Hz, c is the velocity of light in vacuum and z is in seconds. The integration is over the ray path. For a vertical path: 2=(40.3
jNdh)/(cf’)
(3)
65
R. G. Ezquer et al.
66
The integral ( N dh is called total electron content of the ionosphere (TEC), and is the number of free electrons present in a unit cross section vertical column which extends from ground up to the top of the ionosphere. So, for ionospheric corrections TEC measurements are needed, or ionospheric model predictions can be an useful tool. Different ionospheric mo.dels have been developed to predict the N distribution in height, which is called N-profile (Chiu, 1975; Anderson et al, 1987), including the International Reference Ionosphere (IRI) (Bilitza, 1990). Nevertheless, most of the signal paths are slant paths. In this work the electron content over slant paths (SEC) are calculated using IRI model. METHOD Two cases were considered, namely: a) the satellite and the ground station are in the same meridian b) the satellite and the station have different longitudes. For these calculations the length of the slant path is divided in segments of 20 km. The verticals which pass through the ends of these segments intersect the Earth’s sur%ce in different points called auxiliary stations. The coordinates of these stations are determined. With IRI model the electron densities at the points where the slant path intersects the verticals of the auxiliary stations are calculated and from them the slant N-profile is obtained. With this slant N-profile the SEC up to 2000 km altitude is calculated. The IRI code provides two distinct peak density models, in this paper the CCIR model has been used. A x&ware was developed to calculate SEC with this method. It works in the following way: 1. It determines the coordinates of the auxiliary stations 2. Automatically runs the IRI software to obtain N-profile over these stations. 3. Then builds the slant profile and calculates SEC=jNds
(4)
For case a), the software requires the following inputs: + Zenithal angle of satellite (x) 4 To know if the satellite is to the North or to the South of the ground receiver station + Geodetic latitude and longitude of the ground receiver station + RI2 + Month and day + Universal time and for the case b) the inputs are: Geodetic latitude and longitude of the ground station Geodetic latitude and longitude of the satellite Altitude of satellite R12 Month and day Universal time DATA AND RESULTS For case a) the considered station is Rome (41.9“ N, 12.5“E). Figure 1 shows the calculated SEC over this station for different months, high (R12 = loo), middle (R12 = 55) and low (R12 = 10) solar activity at 05
61
Slant Electron Content r
1. -
12 ?
!
b :
m-
$ ‘;
d ‘”
4-
lo
L
3 I
odo.a-7044muuu-00
04
1
z&mrFN.Am.oi
,,I,,,,,
4o-Bo-loQdDI4o-l)*m ‘/lWOTAL AX.
a
j
/
I,,,
0
10
OF SAlElJ.lllC
I,,
lo
*
40
a
so
70
o@
CDWREE)
Fig. 1. Calculated SEC over Rome for low (R12=10), middle (Rl245) and high (Rl2=100) solar activity, 11 UT and 05 UT. The considered months are March (white triangle), June (black square) September (black triangle) and December (white square). Positive values of zenital angle indicate that satellite is to the North of the ground station, and negative values indicate that satellite is to the South.
68
R. G. Ezquer et af.
1-lS74(RlZzM..?J 1
Fig. 2. Predicted (dashed line) and measured (solid line) slant electron content at ATS6 - Boulder path
69
Slant Electron Content
J5mlu7 1875 (Rrr
25.8)
:1I
Yw8h 1
Fig. 2. (continued)
1475 (Rf.t
MS)
.’
Apm m75 (RlZ
98.8)
70
R. G. Ezquer et al
UTandllUT. .It can be seen that for equinox, 11 UT and high solar activity, the highest values of SEC are obtained. When the zenithal angle of satellite is 80’ to the South of the station, the calculated SEC reach values about 12.10” mm’.The lowest values correspond to low solar activity at 05 UT. There is a strong dependence of SEC with the zenithal angle of satellite for R12 = 100 and 11 UT. A smooth variation of SEC with x for low and middle solar activity and 5 TU is observed. For low solar activity and daytime conditions, the SEC values calculated for diierent seasons are similar. For high x the SEC values calculated when the satellite is to the South of the ground station are greater than those obtained when to the satellite is to the North. Fritz (1976) have measured the slant electron content between Boulder (40.13” N, 254.76“ E) and the ATS 6 synchronous satellite (266“ E) for the period July/74 - May/75. They measured ionospheric, total and protonospheric electron content. In this work we use the ionospheric slant electron content measurements obtained with Faraday technique. Figure 2 shows the obtained results. It can be seen a very good agreement among predictions and measurements at hours of maximum ionisation for July and August, and also during the night for December. In general, the model underestimates the ionospheric SEC at all hours of the day. The IRI underestimation at hours of maximum ionisation, is lower than 20 % except for the period from December solstice to March equinox. CONCLUSIONS Using the IRI model the SEC over two middle latitude stations has been calculated. The used method has been described. The dependence with the solar activity the zenithal angle of satellite was shown. The results suggest that the SEC over Rome can reach a value about 12. 1017mm2for x = 80’ during high solar activity. The comparison with Faraday measurements obtained at Boulder for a low solar activity period show cases with a very good agreement between predictions and measurements. But, in general, the model underestimates the SEC. These results suggest that additional studies considering the URSI code and covering more. stations and conditions are required in order to confirm the need of improvements in IRI model. REFERENCES Anderson, D. N., M. Mendillo; and B. Hertniter, A Semiempirical Low Latitude Ionospheric Model, Radio Sci., 22,292, (1987). Bilitza, D., International Reference Ionosphere 1990, National Space Science Data Center 90-92, WDC-A R&S, Geenbelt, Maryland., USA (1990). Chiu, Y.T., An Improved Phenomenological Model of the Ionospheric Density, J. Atmos. Ten-. Phys, 37, 1563 (1975) Fritz, R.B., ATS 6 Radio Beacon Electron Content Measurements at Boulder, July 1974 - May 1975, World Data Center A for Solar-Terrestrial Physics. Report UAG - 58, Boulder, Colorado, USA (1976).