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Atmospheric Model for the GNSS

Map of the zenith wet delay, derived from the Global Forecast System (31.05.2013, 18 UTC)
Lupe

The Atmospheric Model for the GNSS (the component which describes the propagation of the radio signals in the atmosphere) consists of a tropospheric and ionospheric part. 

Troposphere: the pressure, temperature and humidity fields are provided from a global Numerical Weather Model (NWM); the Global Forecast System (GFS) of the National Centers for Environmental Prediction (NCEP) (www.ncep.noaa.gov). We use a ray-trace algorithm [1] and compute station-specific zenith hydrostatic (wet) delays, derive the three hydrostatic (wet) mapping function coefficients [2] and the horizontal delay gradient components [3]. In post-processing mode we use the NWM analysis whereas in real-time mode we use NWM short-range forecasts. As an example the animated figures available at ftp://ftp.gfz-potsdam.de/pub/home/kg/zusflo/TRO/MOVIEs/, show the time evolution of the tropospheric delay parameters for some arbitrary day. The tropospheric delay parameters are used as a reference in validation studies [4] and they help to improve precise point positioning [5].

Ionosphere: the underlying electron density field is provided from a climatological model, the latest version of the International Reference Ionosphere (IRI) (http://iri.gsfc.nasa.gov/),, and Earth's magnetic field is provided from the 12th generation of the International Geomagnetic Reference Field (IGRF) (http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html). We use a ray-trace algorithm [6] to derive ionospheric delay parameters which can then be used to perform higher-order ionospheric corrections in precise applications [7]. As an example the animated figures available at ftp://ftp.gfz-potsdam.de/pub/home/kg/zusflo/ION/MOVIEs/ show the (simulated) impact of higher-order ionospheric corrections on estimated station coordinates, clocks and tropospheric delays parameters in precise point positioning.

Tropospheric- and ionospheric delay parameters derived for specific stations and/or a global grid are available upon request. 

References:

[1] Zus, F., Dick, G., Dousa, J., Heise, S., and Wickert, J.: The rapid and precise computation of GPS slant total delays and mapping factors utilizing a numerical weather model, Radio Sci., 49, 207–216, doi:10.1002/2013RS005280, 2014.

[2] Zus, F., Dick, G., Dousa, J., and Wickert, J.: Systematic errors of mapping functions which are based on the VMF1 concept, GPS Solut., 19, 277–286, 2015a.

[3] Zus, F., Dick, G., Heise, S., and Wickert, J.: A forward operator and its adjoint for GPS slant total delays, Radio Sci., 50, 393–405, doi:10.1002/2014RS005584, 2015b.

[4] Li, X., F. Zus, C. Lu, T. Ning, G. Dick, M. Ge, J. Wickert, and H. Schuh.: Retrieving high-resolution tropospheric gradients from multiconstellation GNSS observations. Geophys. Res. Lett., 42, 4173–4181. doi: 10.1002/2015GL063856, 2015.

[5] Lu, C., Li, X., Zus, F., Heinkelmann, R., Dick, G., Ge, M., Wickert, J., and Schuh, H.: Improving BeiDou real-time precise point positioning with numerical weather models, Journal of Geodesy, 91, 1019-1029, doi:10.1007/s00190-017-1005-2, 2017.

[6] Zus, F., Z. Deng, S. Heise, and J. Wickert: Ionospheric mapping functions based on electron density fields, GPS Solut., 21, 873–885, doi:10.1007/s10291-016-0574-5, 2017a.

[7] Zus, F., Z. Deng, and J. Wickert: The impact of higher-order ionospheric effects on estimated tropospheric parameters in Precise Point Positioning, Radio Sci., 52, doi:10.1002/2017RS006254, 2017b.

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