GPS measurements on the London Millennium Bridge

Trials conducted on the London Millennium Footbridge in 2000 monitored movements using three GPS receivers. After initial processing, the results gave plausible vertical and lateral (sideways) displacements, but also described implausible longitudinal movements along the axis of the bridge. Digital signal processing techniques applied to the data are reported. Due to the satellite constellation in the UK, a large void exists from the zenith to the horizon in a northerly direction. This lack of GPS satellites results in poor satellite geometry in the north-south direction, and hence poor north-south precision, explaining the unsatisfactory element of the Millennium Bridge results. More importantly a technique to overcome the issues is described. Notwithstanding this, the lateral vibration frequencies obtained from the GPS data agree extremely well with those found in the modal surveys carried out by Arup, even when displacement amplitudes are small.