Atmospheric Model for the GNSS

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

© F. Zus (GFZ)

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.