Introduction: Monitoring of GPS inter-frequency Biases

The Global Positioning System (GPS) observables are affected by the ionosphere. The dispersive nature of the medium and the use of two frequencies in the observations make possible to measure the total electron content of the ionosphere (TEC).
One major source of error in the TEC estimation from GPS observables, is the effect of the differential satellite and receiver instrumental biases, which can reach up to several nanoseconds (1 ns= 2.86 x 1016 e/m2).

The additional estimation of the instrumental biases may constitute an insurmountable burden in some practical applications such as real-time estimation of TEC , or the estimation may be difficult or correlated to the ionospheric parameters, particularly in situations where the ionospheric behavior may be difficult to model: equatorial or auroral regions, ionospheric storms etc..
A priori values of the instrumental biases, estimated under good conditions or with a global network, can solve those problems.

The DLR Neustrelitz remote sensing ground station is processing GPS receiver and satellite inter-frequency biases on base of a selected number of IGS stations by an automatic daily routine. The stations are:

Id City (Location) Lon(°E) Lat(°N) Ht(m) Agency
ankr Ankara (Turkey) 32.7586 39.8875 974.7680 IfAG
bor1 Borowiec Near Kornik & Poznan(Poland)
(Our current reference station)
 17.0668 52.1002 124.0000 SRC-PAS
brus Brussels (Belgium) 4.3592 50.7978 149.6700 ROB
gras Caussols (France) 6.9206 43.7547 1320.3182 CNES
kiru Kiruna (Sweden) 20.9684 67.8573 391.0600 ESA
kit3 Kitab (Uzbekistan) 66.8800 39.1400 643.0000 GFZ
mate Matera (Italy) 16.7044 40.6491 535.6500 ASI
mets Kirkkonummi (Finland) 24.3953 60.2175 94.5680 FGI
noto Noto (Italy) 14.9898 36.8761 126.2300 ASI
nzmo Neustrelitz (Germany) 13.0421 53.1946 116.853 DLR
onsa Onsala (Sweden) 11.9255 57.3953 46.5789 OSO
penc Penc (Hungary) 19.2815 47.7896 291.7900 FOMI
sfer San Fernando (Spain)
-6.2056 36.4643 85.8300 ROA
vill Villafranca (Spain) -3.9520 40.4436 647.4900 ESA
wtzr Koetzting (Germany) 12.8789 49.1442 666.0240 IfAG
zwen Zwenigorod (Russia) 36.7600 55.7000 270.0000 GFZ

As we are not using any internal calibration of GPS receivers, it is not possible to separate the satellite bias from the receiver bias. There is always a reference station for which the bias is set to 0.0. In order to remove the effect of the reference station we compute the GPS satellite and receiver biases relative to the mean of all satellite biases.

We will very much appreciate hearing from people working with our TEC maps and GPS instrumental biases about any problems. This could be very helpful for us in improving our methods, and we may also be able to provide additional information to help in the interpretation of the results.

Literature
Coco et al., IEEE Trans. Aerosp. Electron. Syst, 27(6), 931-938, 1991
Sardon et al., Radio Science, 29(3), 577-586, 1994.
Sardon, E. and Zarraoa, N., Radio Science, 32(5), 1899-1910,sept-Oct 1997

Questions and comments are welcome!
Just mail to Stefan.Schlueter@dlr.de