The aim of the MACDA project was to produce an assimilated, consistent dataset from the observations made by the MGS/TES instrument. The project was undertaken at Oxford University, UK. On Earth, data assimilation is used in routine operational weather forecasting, primarily to produce accurate initial conditions for the forecasts. A similar technique has been applied to another planet in the Solar System: Mars. This was the goal of the Mars Analysis Correction Data Assimilation (MACDA) project. Observations of atmospheric temperature and dust opacity from the Thermal Emission Spectrometer on board NASA's Mars Global Surveyor spacecraft have been assimilated for three complete Martian years into the UK Mars Global Circulation Model. This four-dimensional 'reanalysis' allows us to study the evolution of the atmospheric state on a model grid while remaining consistent with the observations, the model, and our physical understanding. It also allows us to access variables that cannot be directly observed, such as winds and surface pressure, as the model balances the unobserved variables against the observed variables as the assimilation progresses.