D34    3 month test dataset.

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This is part of work-package 6100, the purposes of which are slant delay retrievals

Objectives:  Develop and test methods tocalculate unbiased slant delays and/or intermediate products for use in NWP assimilation, and provide a 3-month dataset for validation and assimilation tests.

Methodology/Work Description: The derivation of zenith and slant GPS delays from GPS observations involves several assumptions about the atmospheric structure. In particular, assumptions about the atmospheric homogeneity and receiver multipath when observing satellites at low elevation angles (close to horizon) influence the results. The multipath must be carefully modelled as a function of receiver environment while the atmospheric model used for the mapping must be carefully chosen in cases of atmospheric inhomogeneities. Even when estimating only slant delays, mapping functions are still needed in order to separate receiver clock errors from atmospheric delays. Traditionally, mapping functions are empirical functions derived from multi-year averages of radiosonde data. A new approach is to derive the mapping function directly from NWP models. This could result in a significant improvement of IWV for low elevations. Pre-processing of raw slant delays before assimilation will be investigated, using additional input from NWP analysis. This will help to discriminate site dependent effects (multipath, antenna phase centre variations) and receiver clock errors from atmospheric delays, but it can also be used to derive intermediate quantities such as ZTD, horizontal gradients, scale height and or timing information, which could be used as an alternative to assimilating slant delays.  

* TUD will modify currently used software, if necessary, and develop additional modules to estimate slant delays and model multipath 
* TUD will carry out the processing of raw GPS data and compile a 3-month dataset for a small but dense network for assimilation and validation purposes
* TUD will compile a dataset for a 2 week test period corresponding to a period of interest because of dynamical storm system activity
* KNMI will develop a procedure to compute direct mapping function from HIRLAM
* TUD and KNMI will test the direct mapping function approach, and develop and test the pre-processing approach

The test dataset has been made by TUD and is being used at KNMI and FMI in the construction of slant delay operators for numerical weather prediction model HIRLAM. For the method used for estimation of GPS slant delay data from GPS observations see the report D33. For the data contact Hans van der Marel at H.vanderMarel@tudelft.nl.