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GPS–VTEC measurements and IRI predictions in the South American sector
Advances in Space Research 34 (2004) 2035–2043 www.elsevier.com/locate/asr
GPS–VTEC measurements and IRI predictions in the South American sector R.G. Ezquer
a,b,c,*
, C. Brunini d, M. Mosert a,e, A. Meza d, R. del V. Oviedo c, E. Kiorcheff b, S.M. Radicella f
a CONICET, Buenos Aires 296, 4000 S. M. de Tucuma´n, Argentina Fac. Reg. Tucuma´n, Universidad Tecnolo´gica Nacional, Rivadavia 1050, 4000 S. M. de Tucuma´n, Argentina Lab. Ionosfera, Dto. de Fı´sica, Universidad Nacional de Tucuma´n, Independencia 1800, 4000 S. M. de Tucuma´n, Argentina d FCAG, Observatorio Astrono´mico, Paseo del Bosque 1900, Universidad Nacional de La Plata, Argentina e CASLEO, CC 467, 5400 San Juan, Argentina f ARPL, The Abdus Salam International Centre for Theoretical Physics, strada costiera 11, 34014 Trieste, Italy b
c
Received 3 February 2004; received in revised form 24 March 2004; accepted 25 March 2004
Abstract Vertical total electron content (VTEC) measurements obtained with GPS satellite signals during 1999 are used to check the validity of the international reference ionosphere (IRI) to predict this ionospheric variable in the South American sector. Measurements obtained during June solstice and September equinox at nine stations are used. The considered latitude range extends from 18.4 to 64.7 and the longitude ranges from 281.3 to 297.7. The deviation between modelled and measured values was obtained. The results show that, in general, for both seasons IRI overestimates VTEC at nighttime and sunrise hours. Better predictions were obtained in the latitude range (18.4°, 33.1°) for the period 8–22 LT. For high latitude stations, IRI does not give good predictions. Additional studies covering more stations and conditions and using ionosonde data will be useful to complete the IRI validation. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Vertical total electron content; Ionospheric model; GPS–VTEC; IRI
1. Introduction The behaviour of the ionosphere is of primary importance to HF propagation. For the frequency management and design of HF systems, ionospheric critical frequencies are required. The total electron content (TEC), defined as the number of free electrons in a column of 1 m2 crosssection extending from the ground to the top of the ionosphere, is a parameter of great importance for systems which use transionospheric radio waves. When a *
Corresponding author. Tel.: +54 381 4364093x244; fax: +54 381 4305401. E-mail address: [email protected] (R.G. Ezquer).
radio wave traverses the ionosphere, several effects are produced in it. Most of these effects are proportional, at least to the first order, to the TEC. The highest TEC values in the world occur at the equatorial anomaly (EA) peaks located at approximately 15° either side of the magnetic equator. Studies on the TEC over the Tucuma´n (26.8, 294.7; geomag. Lat.: 15.5) satellite signals receiver station, placed near the Southern peak of the equatorial anomaly, have been done in Argentina, using geosynchronous satellite signals (Ezquer and de Adler, 1989, 1995; Ezquer et al., 1998). For satellite orbit determination, the satellite position and velocity are determined with the help of radar signals transmitted between the ground station
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2004.03.015
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R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
Table 1 GPS receiver stations arranged by decreasing latitude Stations
Latitude
Longitude
Geomag. Lat.
Geomag. Lon.
PUR3 MARA BOGT RIOP AREQ TUCU SANT RIOG PALM
18.4 10.7 4.6 1.6 16.4 26.8 33.1 53.8 64.7
292.9 288.3 285.9 281.3 288.5 294.7 289.3 292.2 295.9
24.8 22.1 16.5 9.7 5.0 15.5 21.7 42.4 53.3
3.0 358.9 355.5 351.0 358.4 4.1 359.2 1.6 4.1
and the satellite. The ionospheric corrections that have to be applied to determine the satellite position accurately are proportional to the TEC along the radarsatellite path (Hartmann and Leitinger, 1984). For ionospheric corrections, TEC measurements or TEC predictions from ionospheric models can be a useful tool. Different models have been developed to predict ionospheric variables (Anderson et al., 1987; Bilitza, 1990 and others). One of the most widely used empirical models is the international reference ionosphere (IRI) (Bilitza, 2001). Ezquer et al. (1998) checked the validity of IRI in predicting the vertical total electron content (VTEC) over Tucuma´n, using geosynchronous satellite signals received during 1982. Their results showed that the model overestimates VTEC around the daily minimum and underestimates it the rest of the day. In a previous work, Ezquer et al. (2002) checked the validity of IRI in predicting VTEC over AREQ (16.5, 289.0, mag. Lat: 5.1), using measurements obtained with GPS signals received during 2000. They found that the model overestimates VTEC at nighttime and during sunrise and sunset. For some cases, good predictions have been observed for hours of maximum ionisation. In order to extend this study to other latitudes, in the present work we compare the IRI predictions with the GPS–VTEC measurements obtained at nine stations in the South American sector.
2. Data The data correspond to June 1999 (Rz12: 93.0) and September 1999 (Rzl2: 102.0) The considered stations are given in Table 1 and shown in Fig. 1. The La Plata ionospheric model, described in detail by Brunini et al. (2001), is used in this paper to obtain VTEC from single-station GPS observations. Briefly, VTEC is obtained using the so-called geometry-free linear combination of both LI and L2 GPS carrier phase observations, /1 /2 ¼ aSTEC þ sr þ ss þ t;
Fig. 1. GPS receiver stations used to obtain VTEC.
where /1 and /2 are the observations, STEC is the line of sight slant total electron content, sr and ss are the L1–L2 inter-frequency electronic delays produced in the hardware of the receiver and the satellite (expressed in linear units), a = 0.105 m/TECU is a constant to convert linear into TEC units and v is the L1–L2 combined measurement error. TECU is 1016 m2. ‘‘Levelling’’ the ambiguous carrier phase observation to the less precise but unambiguous Pcode observations, allow us to reduce the effect of carrier phase ambiguities in the geometry-free linear combination. The ionosphere is approximated by a single shell of infinitesimal thickness with equivalent VTEC, located at 450 km above the Earth surface. An obliquity factor equal to 1/cos z 0 , z 0 being the zenithal distance of the slant path at the piercing point of the signal on the shell, is used to convert vertical into slant TEC,
STEC ¼
1 VTEC: cos z0
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043 Table 2 VTEC (1016 m2) – 1999 June – measurements, IRI predictions and deviations
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R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
Table 3 VTEC (1016 m2) – 1999 June – measurements, IRI predictions and deviations
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
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Table 4 VTEC (1016 m2) – 1999 September – measurements, IRI predictions and deviations
The spatial and temporal variability of the VTEC is modeled by an expansion in terms of the geographic longitude (k) and latitude (u) of the piercing point and the local time (t), VTECðk; u; tÞ ¼ AðtÞ þ BðtÞðu u0 Þ þ CðtÞðk k0 Þ;
where k0 and u are the geographic coordinates of the GPS station. The time dependent coefficients, A(t), B(t) and C(t), are modeled as trigonometric polynomials. The computation of the VTEC is performed in two consecutive steps:
2040
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
Table 5 VTEC (1016 m2) – 1999 September – measurements, IRI predictions and deviations
(1) Daily solutions are computed to estimate the coefficients of the trigonometric polynomials of the VTEC expansion and the inter-frequency electronic
delays for each satellite and the receiver. All these parameters are jointly estimated by least squares, using the GPS observations of each station. The
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
2041
Table 6 VTEC (1016 m2) – 1999 June and September – measurements, IRI predictions and deviations
sampling rate of the observation was 30 s and the elevation cutoff mask was set to 20°. For those stations that have been processed by the international GPS service (IGS) and for all satellites, the interfrequency electronic delay estimates were constrained, within a given a-priori standard deviation, to be equal to the values provided by the IGS (Herna´ndez-Pajares, 2003).
(2) The (previously) estimated inter-frequency electronic delays are used to correct the GPS observations and to obtain the VTEC for every observed piercing point, using the following expression: VTEC ¼
cos z0 ½ð/1 /2 Þ ðsr þ ss Þ: a
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R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
To reduce errors due to the obliquity factor, only observations with a zenithal distance lower than 25° were considered in this second step. Using this method, we processed, day by day, all the available data for June and September 1999, and then we computed an hourly median VTEC for every station. A few hourly intervals were not computed because of a lack of data. The error in the VTEC measurements obtained with this method is 2 TECU (Brunini et al., 2001) The deviation between IRI prediction and GPS– VTEC measurement was calculated by Dev ¼ ½ðprediction measurementÞ=measurement 100 3. Results Table 2 shows the results for June solstice, period 0–11 LT. White, light grey and grey cells indicate cases where the absolute value of deviation between predictions and measurements (jDevj) is lower than 30%, greater than or equal to 30%, lower than or equal to 50%, and greater than 50%, respectively. It can be seen that, in general, the model overestimates VTEC for the period of minimum ionisation. For hours of maximum VTEC IRI underestimates this ionospheric variable from AREQ to the North and overestimates it from TUCU to the South. jDevj can reach 100% or more. Predictions with jDevj lower than 30% are obtained for MARA. For RIOP, AREQ, TUCU and SANT deviations greater than 50% are observed during the last hours of the night. For the period 7–11 LT, it can be seen that jDevj is lower than 30% for PUR3, MARA, BOOT, RIOP and AREQ, and those corresponding to high latitude are greater than 50%. Moreover, the results show that the difference between prediction and measurement is greater then the error in the GPS–VTEC measurements. Table 3 shows the results for June solstice, period 12– 23 LT. Good predictions are observed for MARA, BOGT, RIOP and AREQ. The worst predictions correspond to RIOG and PALM. The fact that the VTEC measurements for TUCU at hours of maximum ionisation are lower than those corresponding to AREQ, suggest that the southern peak of the equatorial anomaly was not over TUCU. Table 4 shows the results for September equinox, period 0–11 LT. Good predictions are observed for PUR3 and MARA. Deviations greater than 50% are observed around the daily minimum from BOGT to SANT. For RIOG and PALM IRI overestimates VTEC in more than 50% of the daylight hours. Table 5 shows the results for September equinox, period 12–23 LT. In general good predictions are observed except for RIOG, PALM and SANT during the last hours of the day.
Table 6 shows the deviations between predictions and measurements for both seasons. It can be seen that there are two sectors in this table. One of them extends from 0 to 7 LT, and the other from 8 to 23 LT. In the first sector high deviations are observed from BOGT to SANT. In the second sector, in general, good predictions are observed from PUR3 to SANT, and the worst predicted values correspond to high latitude stations.
4. Conclusions For both seasons, in general, IRI overestimates VTEC for the period 0–7 LT. Few cases showed jDevj lower than 30%. For June solstice, period 8–22 LT: IRI gives good predictions for MARA, BOGT, RIOP and AREQ; the degree of accuracy of the predictions decreases for the other stations, especially for those of high latitude. For September equinox, period 10–23 LT: IRI gives good predictions for PUR3, MARA and AREQ. Good predictions are obtained for TUCU during hours of greatest ionisation. For RIOG and Palm IRI does not give good predictions. The observed disagreements between predicted and measured values could arise because peak characteristic or the shape of the electron density profile, or both, are not well predicted. Additional studies covering more stations and conditions and using ionosonde data will be useful to complete the IRI validation.
References Anderson, D.N., Mendillo, M., Herniter, B. A semi-empirical low latitude ionospheric model. Radio Sci. 22, 292, 1987. Bilitza, D. International Reference Ionosphere, Rep. NSSDC/ DC-R&S 90–22, National Space Science Data Center/World Data A for rockets and satellites, Greenbelt, Maryland, USA, 1990. Bilitza, D. International reference ionosphere 2000. Radio Sci. 36 (2), 261–275, 2001. Brunini, C., Meza, A., Diaz, A. 2001. Regional vertical total electron content using GPS observation, in: Proceedings of the 2001 IAG Scientific Assembly (CD edition), 2–7 September 2001, Budapest, Hungary, 2001. Ezquer, R.G., de Adler, N.O. Electron content over Tucuma´n. J. Geophys. Res. 94 (A7), 9029, 1989. Ezquer, R.G., de Adler, N.O., Radicella, S.M., Mosert de Gonzalez, M., Manzano, J.R. IRI and BPM total electron content predictions for Tucuma´n. Adv. Space Res. 15 (2), 121–124, 1995. Ezquer, R.G., Jadur, C.A., Mosert de Gonzalez, M. IRI–95 TEC predictions for the South American peak of the equatorial anomaly. Adv. Space Res. 22 (6), 811–814, 1998.
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043 Ezquer, R.G., Mosert, M., Brunini, C., Meza, A., Cabrera, M., Ara´oz, L., Radicella, S. GPS–VTEC near the magnetic equator during a high solar activity year: observations and IRI predictions, in: Proceedings of IRI Task Force Activity 2001, the Abdus Salam International Centre for Theoretical Physics, Report IC/IR/2002/ 23, Trieste, Italy, August 2002.
2043
Hartmann, G.K., Leitinger, R. Range error due to ionospheric and troposheric effects for signal frequencies above 100 MHz. Bull. Ge´od. 58, 109–136, 1984. Herna´ndez-Pajares, M. 2003. IGS IONO WG Report Appendix: Performance of IGS Ionosphere TEC Maps, presented at the 22nd IGS Governing Board, Nice, April 6, 2003.
GPS–VTEC measurements and IRI predictions in the South American sector R.G. Ezquer
a,b,c,*
, C. Brunini d, M. Mosert a,e, A. Meza d, R. del V. Oviedo c, E. Kiorcheff b, S.M. Radicella f
a CONICET, Buenos Aires 296, 4000 S. M. de Tucuma´n, Argentina Fac. Reg. Tucuma´n, Universidad Tecnolo´gica Nacional, Rivadavia 1050, 4000 S. M. de Tucuma´n, Argentina Lab. Ionosfera, Dto. de Fı´sica, Universidad Nacional de Tucuma´n, Independencia 1800, 4000 S. M. de Tucuma´n, Argentina d FCAG, Observatorio Astrono´mico, Paseo del Bosque 1900, Universidad Nacional de La Plata, Argentina e CASLEO, CC 467, 5400 San Juan, Argentina f ARPL, The Abdus Salam International Centre for Theoretical Physics, strada costiera 11, 34014 Trieste, Italy b
c
Received 3 February 2004; received in revised form 24 March 2004; accepted 25 March 2004
Abstract Vertical total electron content (VTEC) measurements obtained with GPS satellite signals during 1999 are used to check the validity of the international reference ionosphere (IRI) to predict this ionospheric variable in the South American sector. Measurements obtained during June solstice and September equinox at nine stations are used. The considered latitude range extends from 18.4 to 64.7 and the longitude ranges from 281.3 to 297.7. The deviation between modelled and measured values was obtained. The results show that, in general, for both seasons IRI overestimates VTEC at nighttime and sunrise hours. Better predictions were obtained in the latitude range (18.4°, 33.1°) for the period 8–22 LT. For high latitude stations, IRI does not give good predictions. Additional studies covering more stations and conditions and using ionosonde data will be useful to complete the IRI validation. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Vertical total electron content; Ionospheric model; GPS–VTEC; IRI
1. Introduction The behaviour of the ionosphere is of primary importance to HF propagation. For the frequency management and design of HF systems, ionospheric critical frequencies are required. The total electron content (TEC), defined as the number of free electrons in a column of 1 m2 crosssection extending from the ground to the top of the ionosphere, is a parameter of great importance for systems which use transionospheric radio waves. When a *
Corresponding author. Tel.: +54 381 4364093x244; fax: +54 381 4305401. E-mail address: [email protected] (R.G. Ezquer).
radio wave traverses the ionosphere, several effects are produced in it. Most of these effects are proportional, at least to the first order, to the TEC. The highest TEC values in the world occur at the equatorial anomaly (EA) peaks located at approximately 15° either side of the magnetic equator. Studies on the TEC over the Tucuma´n (26.8, 294.7; geomag. Lat.: 15.5) satellite signals receiver station, placed near the Southern peak of the equatorial anomaly, have been done in Argentina, using geosynchronous satellite signals (Ezquer and de Adler, 1989, 1995; Ezquer et al., 1998). For satellite orbit determination, the satellite position and velocity are determined with the help of radar signals transmitted between the ground station
0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2004.03.015
2036
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
Table 1 GPS receiver stations arranged by decreasing latitude Stations
Latitude
Longitude
Geomag. Lat.
Geomag. Lon.
PUR3 MARA BOGT RIOP AREQ TUCU SANT RIOG PALM
18.4 10.7 4.6 1.6 16.4 26.8 33.1 53.8 64.7
292.9 288.3 285.9 281.3 288.5 294.7 289.3 292.2 295.9
24.8 22.1 16.5 9.7 5.0 15.5 21.7 42.4 53.3
3.0 358.9 355.5 351.0 358.4 4.1 359.2 1.6 4.1
and the satellite. The ionospheric corrections that have to be applied to determine the satellite position accurately are proportional to the TEC along the radarsatellite path (Hartmann and Leitinger, 1984). For ionospheric corrections, TEC measurements or TEC predictions from ionospheric models can be a useful tool. Different models have been developed to predict ionospheric variables (Anderson et al., 1987; Bilitza, 1990 and others). One of the most widely used empirical models is the international reference ionosphere (IRI) (Bilitza, 2001). Ezquer et al. (1998) checked the validity of IRI in predicting the vertical total electron content (VTEC) over Tucuma´n, using geosynchronous satellite signals received during 1982. Their results showed that the model overestimates VTEC around the daily minimum and underestimates it the rest of the day. In a previous work, Ezquer et al. (2002) checked the validity of IRI in predicting VTEC over AREQ (16.5, 289.0, mag. Lat: 5.1), using measurements obtained with GPS signals received during 2000. They found that the model overestimates VTEC at nighttime and during sunrise and sunset. For some cases, good predictions have been observed for hours of maximum ionisation. In order to extend this study to other latitudes, in the present work we compare the IRI predictions with the GPS–VTEC measurements obtained at nine stations in the South American sector.
2. Data The data correspond to June 1999 (Rz12: 93.0) and September 1999 (Rzl2: 102.0) The considered stations are given in Table 1 and shown in Fig. 1. The La Plata ionospheric model, described in detail by Brunini et al. (2001), is used in this paper to obtain VTEC from single-station GPS observations. Briefly, VTEC is obtained using the so-called geometry-free linear combination of both LI and L2 GPS carrier phase observations, /1 /2 ¼ aSTEC þ sr þ ss þ t;
Fig. 1. GPS receiver stations used to obtain VTEC.
where /1 and /2 are the observations, STEC is the line of sight slant total electron content, sr and ss are the L1–L2 inter-frequency electronic delays produced in the hardware of the receiver and the satellite (expressed in linear units), a = 0.105 m/TECU is a constant to convert linear into TEC units and v is the L1–L2 combined measurement error. TECU is 1016 m2. ‘‘Levelling’’ the ambiguous carrier phase observation to the less precise but unambiguous Pcode observations, allow us to reduce the effect of carrier phase ambiguities in the geometry-free linear combination. The ionosphere is approximated by a single shell of infinitesimal thickness with equivalent VTEC, located at 450 km above the Earth surface. An obliquity factor equal to 1/cos z 0 , z 0 being the zenithal distance of the slant path at the piercing point of the signal on the shell, is used to convert vertical into slant TEC,
STEC ¼
1 VTEC: cos z0
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043 Table 2 VTEC (1016 m2) – 1999 June – measurements, IRI predictions and deviations
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R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
Table 3 VTEC (1016 m2) – 1999 June – measurements, IRI predictions and deviations
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
2039
Table 4 VTEC (1016 m2) – 1999 September – measurements, IRI predictions and deviations
The spatial and temporal variability of the VTEC is modeled by an expansion in terms of the geographic longitude (k) and latitude (u) of the piercing point and the local time (t), VTECðk; u; tÞ ¼ AðtÞ þ BðtÞðu u0 Þ þ CðtÞðk k0 Þ;
where k0 and u are the geographic coordinates of the GPS station. The time dependent coefficients, A(t), B(t) and C(t), are modeled as trigonometric polynomials. The computation of the VTEC is performed in two consecutive steps:
2040
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
Table 5 VTEC (1016 m2) – 1999 September – measurements, IRI predictions and deviations
(1) Daily solutions are computed to estimate the coefficients of the trigonometric polynomials of the VTEC expansion and the inter-frequency electronic
delays for each satellite and the receiver. All these parameters are jointly estimated by least squares, using the GPS observations of each station. The
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
2041
Table 6 VTEC (1016 m2) – 1999 June and September – measurements, IRI predictions and deviations
sampling rate of the observation was 30 s and the elevation cutoff mask was set to 20°. For those stations that have been processed by the international GPS service (IGS) and for all satellites, the interfrequency electronic delay estimates were constrained, within a given a-priori standard deviation, to be equal to the values provided by the IGS (Herna´ndez-Pajares, 2003).
(2) The (previously) estimated inter-frequency electronic delays are used to correct the GPS observations and to obtain the VTEC for every observed piercing point, using the following expression: VTEC ¼
cos z0 ½ð/1 /2 Þ ðsr þ ss Þ: a
2042
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043
To reduce errors due to the obliquity factor, only observations with a zenithal distance lower than 25° were considered in this second step. Using this method, we processed, day by day, all the available data for June and September 1999, and then we computed an hourly median VTEC for every station. A few hourly intervals were not computed because of a lack of data. The error in the VTEC measurements obtained with this method is 2 TECU (Brunini et al., 2001) The deviation between IRI prediction and GPS– VTEC measurement was calculated by Dev ¼ ½ðprediction measurementÞ=measurement 100 3. Results Table 2 shows the results for June solstice, period 0–11 LT. White, light grey and grey cells indicate cases where the absolute value of deviation between predictions and measurements (jDevj) is lower than 30%, greater than or equal to 30%, lower than or equal to 50%, and greater than 50%, respectively. It can be seen that, in general, the model overestimates VTEC for the period of minimum ionisation. For hours of maximum VTEC IRI underestimates this ionospheric variable from AREQ to the North and overestimates it from TUCU to the South. jDevj can reach 100% or more. Predictions with jDevj lower than 30% are obtained for MARA. For RIOP, AREQ, TUCU and SANT deviations greater than 50% are observed during the last hours of the night. For the period 7–11 LT, it can be seen that jDevj is lower than 30% for PUR3, MARA, BOOT, RIOP and AREQ, and those corresponding to high latitude are greater than 50%. Moreover, the results show that the difference between prediction and measurement is greater then the error in the GPS–VTEC measurements. Table 3 shows the results for June solstice, period 12– 23 LT. Good predictions are observed for MARA, BOGT, RIOP and AREQ. The worst predictions correspond to RIOG and PALM. The fact that the VTEC measurements for TUCU at hours of maximum ionisation are lower than those corresponding to AREQ, suggest that the southern peak of the equatorial anomaly was not over TUCU. Table 4 shows the results for September equinox, period 0–11 LT. Good predictions are observed for PUR3 and MARA. Deviations greater than 50% are observed around the daily minimum from BOGT to SANT. For RIOG and PALM IRI overestimates VTEC in more than 50% of the daylight hours. Table 5 shows the results for September equinox, period 12–23 LT. In general good predictions are observed except for RIOG, PALM and SANT during the last hours of the day.
Table 6 shows the deviations between predictions and measurements for both seasons. It can be seen that there are two sectors in this table. One of them extends from 0 to 7 LT, and the other from 8 to 23 LT. In the first sector high deviations are observed from BOGT to SANT. In the second sector, in general, good predictions are observed from PUR3 to SANT, and the worst predicted values correspond to high latitude stations.
4. Conclusions For both seasons, in general, IRI overestimates VTEC for the period 0–7 LT. Few cases showed jDevj lower than 30%. For June solstice, period 8–22 LT: IRI gives good predictions for MARA, BOGT, RIOP and AREQ; the degree of accuracy of the predictions decreases for the other stations, especially for those of high latitude. For September equinox, period 10–23 LT: IRI gives good predictions for PUR3, MARA and AREQ. Good predictions are obtained for TUCU during hours of greatest ionisation. For RIOG and Palm IRI does not give good predictions. The observed disagreements between predicted and measured values could arise because peak characteristic or the shape of the electron density profile, or both, are not well predicted. Additional studies covering more stations and conditions and using ionosonde data will be useful to complete the IRI validation.
References Anderson, D.N., Mendillo, M., Herniter, B. A semi-empirical low latitude ionospheric model. Radio Sci. 22, 292, 1987. Bilitza, D. International Reference Ionosphere, Rep. NSSDC/ DC-R&S 90–22, National Space Science Data Center/World Data A for rockets and satellites, Greenbelt, Maryland, USA, 1990. Bilitza, D. International reference ionosphere 2000. Radio Sci. 36 (2), 261–275, 2001. Brunini, C., Meza, A., Diaz, A. 2001. Regional vertical total electron content using GPS observation, in: Proceedings of the 2001 IAG Scientific Assembly (CD edition), 2–7 September 2001, Budapest, Hungary, 2001. Ezquer, R.G., de Adler, N.O. Electron content over Tucuma´n. J. Geophys. Res. 94 (A7), 9029, 1989. Ezquer, R.G., de Adler, N.O., Radicella, S.M., Mosert de Gonzalez, M., Manzano, J.R. IRI and BPM total electron content predictions for Tucuma´n. Adv. Space Res. 15 (2), 121–124, 1995. Ezquer, R.G., Jadur, C.A., Mosert de Gonzalez, M. IRI–95 TEC predictions for the South American peak of the equatorial anomaly. Adv. Space Res. 22 (6), 811–814, 1998.
R.G. Ezquer et al. / Advances in Space Research 34 (2004) 2035–2043 Ezquer, R.G., Mosert, M., Brunini, C., Meza, A., Cabrera, M., Ara´oz, L., Radicella, S. GPS–VTEC near the magnetic equator during a high solar activity year: observations and IRI predictions, in: Proceedings of IRI Task Force Activity 2001, the Abdus Salam International Centre for Theoretical Physics, Report IC/IR/2002/ 23, Trieste, Italy, August 2002.
2043
Hartmann, G.K., Leitinger, R. Range error due to ionospheric and troposheric effects for signal frequencies above 100 MHz. Bull. Ge´od. 58, 109–136, 1984. Herna´ndez-Pajares, M. 2003. IGS IONO WG Report Appendix: Performance of IGS Ionosphere TEC Maps, presented at the 22nd IGS Governing Board, Nice, April 6, 2003.