The UNB3 model for netural atmospshere signal propagation delay is a hybrid model consisting of the Saastamoinen vertical prpagation delay models, the Niell mapping functions, a look-up table of surface meteorological parameter values, and models to propagate these surface values to any arbitrary station height. The model was derived, in part, from the 1966 U.S. Standard Atmosphere
Supplements. It was validated by comparing to ray-traced radiosonde measurements of the signal delay obtained from 173 stations in North America between 1987-1996.
The model input parameters are day-of-year, elevation angle, height and latitude.
The UNB3 model was originally developed by the University of New Brunswick for the FAA-WAAS program. For the WAAS receiver firmware model, the Niell mapping functions have been replaced by the Black and Eisner mapping function with little degradation in accuracy. This version of the model has also been independently validated for use in Europe and Japan.
For a thorough discussion of tropospheric propagation delay and the models used to account for it in GPS and other space-based radiometric data, see:
o Modeling the Neutral-atmosphere Propagation Delay in Radiometric Space Techniques, by V. de Brito Mendes, a Ph.D. dissertation published as Department of Geodesy and Geomatics Engineering Technical Report No. 199, University of New Brunswick, Fredericton, New Brunswick, Canada, April
1999. Electronic supplement: http://mat.fc.ul.pt/eg/lattex/PhD_e_sup.html.
For further details about UNB3, see:
o "Tropospheric Delay: Prediction for the WAAS User" by P. Collins and R.B. Langley in GPS World, Vol. 10, No.7, July 1999, pp. 52-58.
o "Limiting Factors in Tropospheric Propagation Delay Error Modelling for GPS Airborne Navigation," by P. Collins, R. Langley, and J. LaMance, published in the Proceedings of The Institute of Navigation 52nd Annual Meeting, Cambridge, Massachusetts, June 19-21 1996, pp. 519-528.
o Minimum Operational Performance Standards for Global Positioning System / Wide Area Augmentation System Airborne Equipment, RTCA/DO-229A, prepared by Special Committee 159, RTCA, Inc., Washington, D.C., June 1998.
o A Tropospheric Delay Model for the User of the Wide Area Augmentation System, by J.P. Collins and R.B. Langley, Final contract report for Nav Canada Satellite Navigation Program Office and published as Department of Geodesy and Geomatics Engineering Technical Report No. 187, University of New Brunswick, Fredericton, New Brunswick, Canada, September 1997.