Using GPS to measure the deflections and frequency responses of the London Millennium Bridge

Millennium Footbridge in 2000 monitored movements using three GPS receivers. GPS data were obtained over a total period exceeding 11 hours and were post-processed.
The London Millennium Bridge is well documented; it runs approximately north-south from St Paul’s on the north bank towards the Tate Modern on the south bank of the Thames.
After initial processing, the results gave plausible vertical and lateral (sideways)
displacements, but also described implausible longitudinal movements along the axis of the bridge. DSP (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, and so this geometric phenomenon explains the unsatisfactory element of the Millennium Bridge results. More importantly a technique to overcome the issues is described. A software based GPS data simulator, developed at the IESSG, was then used to re-create the actual Millennium Bridge data. Subsequently pseudolite data was simulated, and showed that if a pseudolite had been used on the day of the field trials more reliable results would have been obtained.
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.