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GPS-based TEC observations in comparison with IRI95 and the European TEC model NTCM2
Adv. S/me Her. Vol. 22, No. 6, pp. 803-806, 1998 0 1998 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/98 $19.00 + 0.00 PII: SO273-1177(98)00101-X
GPS-BASED TEC OBSERVATIONS IN COMPARISON WITH IR195 AND THE EUROPEAN TEC MODEL NTCM2 N. Jakowski,
E. Sardon
and S. Schltiter
DLR e. V., Femerkundungsstation
Neustrelitz, Kalkhorstweg 53, D-l 7235 Neustrelitz
ABSTRACT GPS-derived TEC data are compared with IRI95 and the European TEC model NTCM2. It is evident that IRI95 does not include the plasmaspheric content. Monthly medians differ by about (1 to 4) 10’6m-2 which fits quite well to earlier estimations of the plasmaspheric 01998 COSPAR. Published by Elsevier Science Ltd. content under low solar activity conditions. INTRODUCTION For space based radio measuring systems operating at radio frequencies less than 10 GHz ionospheric effects cause objective limitations of the achievable accuracy. Since the first-order propagation effects are proportional to the Total Electron Content (TEC) of the ionosphere, these effects can be corrected by dual frequency difference techniques or by applying ionospheric TEC models. Ionospheric models are often used to correct the measurements of satellite altimeters (e.g. ERS 1,2), to correct time transfer measurements or satellite tracking signals (e.g. Bilitza er al., 1988). Thus, accurate ionospheric models are helpful beyond ionospheric research. The aim of this paper is to compare GPS-derived TEC data with IR19.5 and with the regional TEC model. DATA BASE AND ESTIMATION
OF TEC DATA
Slant TEC data were determined from both the code- as well as the carrier phase measurements carried out at numerous GPS stations of the European network of the International GPS Service for Geodynamics - IGS (e.g. Zumberge et al., 1994). A special calibration method (Sardon et al., 1994) was applied; it includes Kalman filtering technique and yields slant TEC data as well as link related calibration constants that were found not to change appreciably over several months. The accuracy of GPS-derived TEC depends on the precise determination of cycle slips and initial ambiguities and on the adequate modelling of ionospheric effects. With data of the European IGS stations hourly TEC maps were constructed for one year from February 1995 until January 1996. The TEC maps cover the European area 32.51$170”N and -201h160”E such they may be applied by mid-European GPS users. Grid intervals are 2.5’ and 5’ in latitude $ and longitude h , respectively. Our hybrid mapping technique combines the measured vertical TEC values with a new TEC model (Jakowski and Jungstand,l994; Jakowski et al.,1996 ). Our previous TEC model NTCMl, used for this purpose in former time, is based on long-term Faraday rotation measurements carried out at Neustrelitz, Graz and Florence. Combining model and measurement data has the advantage that even when data are missing geophysically reasonable TEC data are derived. The model algorithm takes into account the significant dependence on solar activity and the geomagnetic latitude. The relevant geophysical processes are discussed elsewhere (e.g. Jakowski and Paasch, 1984). Taking into account all these dependencies, the total electron content TEC is described by the expression:
(1) i=O
j=O
kc0
1~0
803
N. Jakowski er nl.
804
where Hi denotes the diurnal variation, Yj the sasonal variation, Sk the solar activity dependence and MI the dependence on the geomagnetic latitude. The corresponding 60 coefficients were computed by least squares fitting of the measured data to the model algorithm. The overall RMS error of this fitting procedure was 0.8~1O~~rn~* corresponding to a range error of 13 cm on the Ll frequency. Since the data base originates from a year of low solar activity (F ra.7c lOO), the derived model coefficients are only valid for 60 such conditions. Z We intend to introduce a solar activity dependence with data of subsequent years. With our updated Eq. 1 regional TECmaps are constructed on a routine base. Hourly maps are available in WWW under http://www.nz.dlr.de. Figure 1 is an example showing the midlatitude trough. 10 20 30 40 -10 0 -20
e
LONGITUDE
/‘E
Fig. 1: TEC map over Europe indicating an electron density trough in the course of an ionospheric perturbation on II April 1997, 0O:OO UT (Difference between contour lines is I .5x1 0’6m-2). COMPARISON
OF OBSERVATIONS
WITH IR195 AND NTCM2 MODELS
Note that the IR195 TEC data were obtained by integrating the corresponding vertical electron density profiles up to 1000 km height. Figure 2 shows the diurnal variation of TEC at a mid-latitude location under different seasonal conditions. As it can be seen, there is a permanent difference of about (1 to 4) 1016m-2 ‘E 15 IRI 95 -between IRI95 values and ‘D NTCM2/GPS derived TEC data 0 10 \ that we attribute to the plasma5 spheric contribution. A crosschecking with independently 0 20 25 0 5 10 15 0 5 10 15 20 25 obtained vertical sounding data UNIVERSAL TIME /h UNIVERSAL TIME /h might help to estimate the accuracy and reliability of different types of TEC data. Slab thickness data derived from IR195 and TEUGPS-foF2 Juliusruh measurements shown in Figure 3 are very sensitive to electron the plasmaspheric content and probably helpful to 20 25 0 5 10 15 0 5 10 15 20 25 estimate the reliability of the UNIVERSAL TIME /h UNIVERSAL TIME /h IR195 topside electron density profile. Fig. 2: Diurnal variations of NTCM2, IRI93 and GPS-derived TEC data for different seasons at 5O”N/IS”E in 1996.
F
805
GPS-Derived TEC Observations
5
20 15-
g
lo-
”T/RI95 “““”
_..._ .‘r”“’3 /1996 “’
r/NTCM2-
0 0
55ON/15”E
a
I.
5
10
5
““l”““.“““““” .__.. T/RI95 T/NTCM~
15-
-
15
20
TIME
: :
2o
5 10 UNIVERSAL
&
15:
NTCM
2
GPS/TEC IRI 95
lo-
= --
I-/NTCM2-
25
0
,‘h
9 /1996 55”N/
20
.
15”E
_
_i
15-
g
lo-
7 b
55”N/
15”E
_
25
““‘f”“7”’ 12 /199E . .._. 55ON/ 15”E
. . . . . . . . . . . . . ..I... 0
slab thickness
15 ‘E/ OUT Ott 1996
7/NTCM2
15 20 TIME /h
5
25
15 20 TIME /h
5 10 UNIVERSAL
“‘.“” T/RI95
0
”
Fig. 3: Diurnal variation of the vertical Juliusruh ionosonde data and IRI95.
Y
g
““’ 6 /1996
““s”““.““” T/RI95 _....
..___-.1--.__....._.--.-......~._... o.....,... 4....i....,...
,__.._..-...........--.--..-.-.--.-0 . . ..I . ..I. .a.‘.‘,’ 0
20 15-
..(....“..
UNIVERSAL
20
_k
5 10 UNIVERSAL
15 20 TIME /h
z derived from GPWEC
25
combined
with
:
:
-0 7
IO:
0 t-
_-----_ -- "".'."'......~ 0 .. .,.....a.........,. ---__
30
40 50 LATITUDE
60
ot 30
70
/ON
t ,........ 1
* 40 50 LATITUDE
60
70
/“N
7 E
20 .““’NTCM “‘I”““““““““‘,““““‘: 2 -em... : 15 ‘E/ 18UT. 15: GPS/TEC IRI 95 -Ott 1996 :
p
10:
0 w
5:
l0. 30
w II.“...’ j......., 40 50 60 70 LATITUDE /ON
5: 0 ‘.. .,.... ‘.. ......” . . . . . ..a.......... 30 40 50 60 70 LATITUDE /ON
Fig. 4: L&tudinal variations of NTCA42-, IRI95-model and GPS-derived times at 15’E in October 1996.
TEC data for different local
N. Jakowski er al.
806
TEC at Kellyville 20 September
(66.99N,309.05E) 1996
(Ap=34)
15
--IRI
c
-
OV
0
I
I
6
12
95 GPS
I 18
24
UT/h
Fig, 5: Comparison
of GPS- and IRI95-derived
(666.7’; 309.1 ‘E).
SUMMARY
TEC data for Kellyville
We attribute the rather high slab thickness values obtained by night to the smaller contribution of the ionosphere compared to that of the plasmasphere. These results may allow to improve the topside of the IRI model (Bilitza, 1994). The plasmaspheric content can be seen even by day in latitudinal TEC profiles at 15”E down to 40’N. As seen in Figure 4, the IR195 TEC data show a rather strong, almost linear increase towards lower latitudes by day, probably related to the equatorial crest. In the high-latitude ionosphere the IRI95-TEC behaves rather similarly to that of the GPS-derived TEC. During geomagnetically perturbed days, however, positive deviations by more than 100 % may occur as shown in Figure 5 for the Kellyville GPS station (50.9”W; 67.O”N).
AND CONCLUSIONS
Subtracting the contribution of the plasmasphere, our comparison between GPS-derived TEC, NTCM2 and IR195 shows rather good agreement. The difference between NTCMUGPS data and IR195 corresponds to a plasmaspheric contribution of about (1 to 4) 10’6m-2 which agrees with earlier estimates of the plasmaspheric electron content under low solar activity conditions (Kersley and Klobuchar, 1978). At latitudes less than 40’N the IR195 TEC data have been found to be essentially greater than corresponding NTCM2 values. This discrepancy needs further discussion in comparison with a more extended GPS data base or independent data sets. Generally speaking, the GPS derived data provide a useful pool for further improvements in the IRI model especially on the topside. ACKNOWLEDGEMENTS The authors thank the IGS community and the staff of the ionosonde station Juliusruh for making available their high quality GPS data sets and ionosonde data. We are grateful to Prof. K. Rawer for critical reading the manuscript. REFERENCES Bilitza, D., Topside models: Status and Future Improvements, Adv. Space Res., 14, (12)17-( 12)26 (1994). Bilitza D., Rawer K., Pallaschke S., Study of ionospheric models for satellite orbit determination, Radio Science, 23,223-232 (1988). Jakowski, N. and E. Paasch, Report on the observations of the total electron content of the ionosphere in NeustrelitzJGDR from 1976 to 1980, Ann. Geophys., 2,501-504 (1984). Jakowski N. and A. Jungstand, Modelling the Regional Ionosphere by Using GPS Observations, ProcLInt. Beacon Sat. Symp. (Ed.: L. Kersley), Aberystwyth, UK, 11-15 July 1994,366-369 (1994). Jakowski, N., Sardon, E., Engler, E., Jungstand, A., and D. Kliihn, Relationships between GPS-signal propagation errors and EISCAT observations, Ann. Geophysicue 14,1429- 1436 (1996). Kersley L. and J.A. Klobuchar, Comparison of protonospheric electron content measurements from the American and European sectors, Geophys. Rex Lett., 5, 123- 125 ( 1978). Sardon. E., Rius, A., and N. Zarraoa, Estimation of the receiver differential biases and the ionospheric total electron content from Global Positioning System observations, Radio Science, 29,577-586 (1994). Zumberge, J., Neilan, R., Beutler, G. and W. Gurtner, The International GPS-Service for Geodynamics-Benefits to Users., Proc.ION GPS-94, Salt Lake City, September 20-23 (1994).
GPS-BASED TEC OBSERVATIONS IN COMPARISON WITH IR195 AND THE EUROPEAN TEC MODEL NTCM2 N. Jakowski,
E. Sardon
and S. Schltiter
DLR e. V., Femerkundungsstation
Neustrelitz, Kalkhorstweg 53, D-l 7235 Neustrelitz
ABSTRACT GPS-derived TEC data are compared with IRI95 and the European TEC model NTCM2. It is evident that IRI95 does not include the plasmaspheric content. Monthly medians differ by about (1 to 4) 10’6m-2 which fits quite well to earlier estimations of the plasmaspheric 01998 COSPAR. Published by Elsevier Science Ltd. content under low solar activity conditions. INTRODUCTION For space based radio measuring systems operating at radio frequencies less than 10 GHz ionospheric effects cause objective limitations of the achievable accuracy. Since the first-order propagation effects are proportional to the Total Electron Content (TEC) of the ionosphere, these effects can be corrected by dual frequency difference techniques or by applying ionospheric TEC models. Ionospheric models are often used to correct the measurements of satellite altimeters (e.g. ERS 1,2), to correct time transfer measurements or satellite tracking signals (e.g. Bilitza er al., 1988). Thus, accurate ionospheric models are helpful beyond ionospheric research. The aim of this paper is to compare GPS-derived TEC data with IR19.5 and with the regional TEC model. DATA BASE AND ESTIMATION
OF TEC DATA
Slant TEC data were determined from both the code- as well as the carrier phase measurements carried out at numerous GPS stations of the European network of the International GPS Service for Geodynamics - IGS (e.g. Zumberge et al., 1994). A special calibration method (Sardon et al., 1994) was applied; it includes Kalman filtering technique and yields slant TEC data as well as link related calibration constants that were found not to change appreciably over several months. The accuracy of GPS-derived TEC depends on the precise determination of cycle slips and initial ambiguities and on the adequate modelling of ionospheric effects. With data of the European IGS stations hourly TEC maps were constructed for one year from February 1995 until January 1996. The TEC maps cover the European area 32.51$170”N and -201h160”E such they may be applied by mid-European GPS users. Grid intervals are 2.5’ and 5’ in latitude $ and longitude h , respectively. Our hybrid mapping technique combines the measured vertical TEC values with a new TEC model (Jakowski and Jungstand,l994; Jakowski et al.,1996 ). Our previous TEC model NTCMl, used for this purpose in former time, is based on long-term Faraday rotation measurements carried out at Neustrelitz, Graz and Florence. Combining model and measurement data has the advantage that even when data are missing geophysically reasonable TEC data are derived. The model algorithm takes into account the significant dependence on solar activity and the geomagnetic latitude. The relevant geophysical processes are discussed elsewhere (e.g. Jakowski and Paasch, 1984). Taking into account all these dependencies, the total electron content TEC is described by the expression:
(1) i=O
j=O
kc0
1~0
803
N. Jakowski er nl.
804
where Hi denotes the diurnal variation, Yj the sasonal variation, Sk the solar activity dependence and MI the dependence on the geomagnetic latitude. The corresponding 60 coefficients were computed by least squares fitting of the measured data to the model algorithm. The overall RMS error of this fitting procedure was 0.8~1O~~rn~* corresponding to a range error of 13 cm on the Ll frequency. Since the data base originates from a year of low solar activity (F ra.7c lOO), the derived model coefficients are only valid for 60 such conditions. Z We intend to introduce a solar activity dependence with data of subsequent years. With our updated Eq. 1 regional TECmaps are constructed on a routine base. Hourly maps are available in WWW under http://www.nz.dlr.de. Figure 1 is an example showing the midlatitude trough. 10 20 30 40 -10 0 -20
e
LONGITUDE
/‘E
Fig. 1: TEC map over Europe indicating an electron density trough in the course of an ionospheric perturbation on II April 1997, 0O:OO UT (Difference between contour lines is I .5x1 0’6m-2). COMPARISON
OF OBSERVATIONS
WITH IR195 AND NTCM2 MODELS
Note that the IR195 TEC data were obtained by integrating the corresponding vertical electron density profiles up to 1000 km height. Figure 2 shows the diurnal variation of TEC at a mid-latitude location under different seasonal conditions. As it can be seen, there is a permanent difference of about (1 to 4) 1016m-2 ‘E 15 IRI 95 -between IRI95 values and ‘D NTCM2/GPS derived TEC data 0 10 \ that we attribute to the plasma5 spheric contribution. A crosschecking with independently 0 20 25 0 5 10 15 0 5 10 15 20 25 obtained vertical sounding data UNIVERSAL TIME /h UNIVERSAL TIME /h might help to estimate the accuracy and reliability of different types of TEC data. Slab thickness data derived from IR195 and TEUGPS-foF2 Juliusruh measurements shown in Figure 3 are very sensitive to electron the plasmaspheric content and probably helpful to 20 25 0 5 10 15 0 5 10 15 20 25 estimate the reliability of the UNIVERSAL TIME /h UNIVERSAL TIME /h IR195 topside electron density profile. Fig. 2: Diurnal variations of NTCM2, IRI93 and GPS-derived TEC data for different seasons at 5O”N/IS”E in 1996.
F
805
GPS-Derived TEC Observations
5
20 15-
g
lo-
”T/RI95 “““”
_..._ .‘r”“’3 /1996 “’
r/NTCM2-
0 0
55ON/15”E
a
I.
5
10
5
““l”““.“““““” .__.. T/RI95 T/NTCM~
15-
-
15
20
TIME
: :
2o
5 10 UNIVERSAL
&
15:
NTCM
2
GPS/TEC IRI 95
lo-
= --
I-/NTCM2-
25
0
,‘h
9 /1996 55”N/
20
.
15”E
_
_i
15-
g
lo-
7 b
55”N/
15”E
_
25
““‘f”“7”’ 12 /199E . .._. 55ON/ 15”E
. . . . . . . . . . . . . ..I... 0
slab thickness
15 ‘E/ OUT Ott 1996
7/NTCM2
15 20 TIME /h
5
25
15 20 TIME /h
5 10 UNIVERSAL
“‘.“” T/RI95
0
”
Fig. 3: Diurnal variation of the vertical Juliusruh ionosonde data and IRI95.
Y
g
““’ 6 /1996
““s”““.““” T/RI95 _....
..___-.1--.__....._.--.-......~._... o.....,... 4....i....,...
,__.._..-...........--.--..-.-.--.-0 . . ..I . ..I. .a.‘.‘,’ 0
20 15-
..(....“..
UNIVERSAL
20
_k
5 10 UNIVERSAL
15 20 TIME /h
z derived from GPWEC
25
combined
with
:
:
-0 7
IO:
0 t-
_-----_ -- "".'."'......~ 0 .. .,.....a.........,. ---__
30
40 50 LATITUDE
60
ot 30
70
/ON
t ,........ 1
* 40 50 LATITUDE
60
70
/“N
7 E
20 .““’NTCM “‘I”““““““““‘,““““‘: 2 -em... : 15 ‘E/ 18UT. 15: GPS/TEC IRI 95 -Ott 1996 :
p
10:
0 w
5:
l0. 30
w II.“...’ j......., 40 50 60 70 LATITUDE /ON
5: 0 ‘.. .,.... ‘.. ......” . . . . . ..a.......... 30 40 50 60 70 LATITUDE /ON
Fig. 4: L&tudinal variations of NTCA42-, IRI95-model and GPS-derived times at 15’E in October 1996.
TEC data for different local
N. Jakowski er al.
806
TEC at Kellyville 20 September
(66.99N,309.05E) 1996
(Ap=34)
15
--IRI
c
-
OV
0
I
I
6
12
95 GPS
I 18
24
UT/h
Fig, 5: Comparison
of GPS- and IRI95-derived
(666.7’; 309.1 ‘E).
SUMMARY
TEC data for Kellyville
We attribute the rather high slab thickness values obtained by night to the smaller contribution of the ionosphere compared to that of the plasmasphere. These results may allow to improve the topside of the IRI model (Bilitza, 1994). The plasmaspheric content can be seen even by day in latitudinal TEC profiles at 15”E down to 40’N. As seen in Figure 4, the IR195 TEC data show a rather strong, almost linear increase towards lower latitudes by day, probably related to the equatorial crest. In the high-latitude ionosphere the IRI95-TEC behaves rather similarly to that of the GPS-derived TEC. During geomagnetically perturbed days, however, positive deviations by more than 100 % may occur as shown in Figure 5 for the Kellyville GPS station (50.9”W; 67.O”N).
AND CONCLUSIONS
Subtracting the contribution of the plasmasphere, our comparison between GPS-derived TEC, NTCM2 and IR195 shows rather good agreement. The difference between NTCMUGPS data and IR195 corresponds to a plasmaspheric contribution of about (1 to 4) 10’6m-2 which agrees with earlier estimates of the plasmaspheric electron content under low solar activity conditions (Kersley and Klobuchar, 1978). At latitudes less than 40’N the IR195 TEC data have been found to be essentially greater than corresponding NTCM2 values. This discrepancy needs further discussion in comparison with a more extended GPS data base or independent data sets. Generally speaking, the GPS derived data provide a useful pool for further improvements in the IRI model especially on the topside. ACKNOWLEDGEMENTS The authors thank the IGS community and the staff of the ionosonde station Juliusruh for making available their high quality GPS data sets and ionosonde data. We are grateful to Prof. K. Rawer for critical reading the manuscript. REFERENCES Bilitza, D., Topside models: Status and Future Improvements, Adv. Space Res., 14, (12)17-( 12)26 (1994). Bilitza D., Rawer K., Pallaschke S., Study of ionospheric models for satellite orbit determination, Radio Science, 23,223-232 (1988). Jakowski, N. and E. Paasch, Report on the observations of the total electron content of the ionosphere in NeustrelitzJGDR from 1976 to 1980, Ann. Geophys., 2,501-504 (1984). Jakowski N. and A. Jungstand, Modelling the Regional Ionosphere by Using GPS Observations, ProcLInt. Beacon Sat. Symp. (Ed.: L. Kersley), Aberystwyth, UK, 11-15 July 1994,366-369 (1994). Jakowski, N., Sardon, E., Engler, E., Jungstand, A., and D. Kliihn, Relationships between GPS-signal propagation errors and EISCAT observations, Ann. Geophysicue 14,1429- 1436 (1996). Kersley L. and J.A. Klobuchar, Comparison of protonospheric electron content measurements from the American and European sectors, Geophys. Rex Lett., 5, 123- 125 ( 1978). Sardon. E., Rius, A., and N. Zarraoa, Estimation of the receiver differential biases and the ionospheric total electron content from Global Positioning System observations, Radio Science, 29,577-586 (1994). Zumberge, J., Neilan, R., Beutler, G. and W. Gurtner, The International GPS-Service for Geodynamics-Benefits to Users., Proc.ION GPS-94, Salt Lake City, September 20-23 (1994).