Validation and Verification of ACS Forward Modelling (and Advisory Role on ACS Development)

Validating the ACS models that provide the theoretical framework linking the fundamental GNSS observations to parameters of interest.

The Challenge

The Analysis Centre Software (ACS) forms the primary utilisation pathway for research outputs conducted within the CRCSI Positioning Program. The ACS makes use of open-source software packages for the majority of the GNSS forward models being applied. Forward models provide the theoretical framework that link the fundamental GNSS phase and pseudo range observations to parameters of interest, such as station positions, receiver and satellite clocks and satellite orbits. If these are incorrect, inaccurate or there is an issue in the implementation of the forward model this can have an adverse impact upon the parameter estimates recovered from geodetic solutions, and lead to errors which are difficult to diagnose. There was a need to verify the operational use of these forward models, to ensure that they were implemented correctly and performed as expected. The aim of this project was to ensure that not only were the components operating correctly, but the resulting software package performed at a level set out in IERS 2010 standards or above, as well as identifying the key modelling components that needed to be addressed for the second stage of implementing the ACS.


The project partners were Geoscience Australia and the Australian National University (ANU).

The Solution

This project was a sub project of the On-going Development of the Multi-GNSS Analysis Centre Software. The project aim was to ensure the forward models being applied within the ACS software package were performing as expected and were implemented correctly. The project was segmented into a series of objectives that supported this aim:

  1. Testing the veracity of the forward models
  2. Assessing the level of compliance with international standards
  3. Developing a process for on-going validation

The project outcomes were achieved and included:

  1. A review of existing ACS components, outlining the barriers to use by dependent projects.
  2. A set of verification tests for forward modelling applied in the ACS.
  3. A new set of software models where appropriate.
  4. Contributing to delivery of the Beta version of the ACS.
  5. Contributing to the user documentation for the operation of the ACS.

Based on the audit of the models, those that needed to be re-written or enhanced were prioritised and as the project finished early, the residual funds were used for this software development by the main engineer, Simon McClusky, who began a full-time role at GA.


Good forward models are indispensable for three main reasons:

  1. Orbit prediction
  2. To aid inter-comparison between different ephemerides (data files giving calculated positions of satellites)
  3. To filter data accurately and reliably

Verifying the forward models, and producing in-house versions alleviated the risk of becoming dependent upon the open-source packages previously used in the ACS. The verification process ensured that the ACS package operates within International standards.


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