# How is the distance between Geneva detector and the Gran Sasso one measured?

Does anyone know how is the distance between detectors in the now famous neutrino experiment measured? Also, how was the time of flight measured?

• We really should convince the US military to bring in one of their GPS receviers, the ones that can use the high-precision encrypted signals. They seem to be able to hit targets at distances over 700 km – MSalters Sep 29 '11 at 14:28
• I'd like to know this too. Was it just done with GPS or did they send anything between the two sites? – Martin Ueding Oct 15 '11 at 11:34
• Note that there isn't a "detector" at the accelerator site in the same sense as there is one at Gran Sasso. They're using the pick-off beam current monitors. – dmckee --- ex-moderator kitten Mar 21 '12 at 19:24

Integrating GPS is used to find the precise position and orientation of a survey point at each site to much better than the usual resolution limits (itegration averages out the sources of random error), then a theodolite survey is done to find the position of the ultimate detector elements at that site (OPERA at the receiving end and the pick-off magnets at the sending end) relative those hard-points.

The survey at Gran Sasso was particularly difficult because it must be made along a four-lane wide road for kilometers before getting to the laboratory complex, and worse the tunnel is such an important artery that it was not possible to stop traffic entirely for the survey.

GPS sattelites are not in sync in the Einstein way because

Synchronisation around the circumference of a rotating disk gives a non vanishing time difference that depends on the direction used.

As each sat must be in sync with the one at left side and the one at right side and all with all they share a common time (universal pre-calculated). The GPS system appears to be like an 'instant observer'. The speed of light that is in use in the GPS system is the mean speed of light in a closed path. For a non-Einstein referential (a different synchronization) a complete study of the properties of the light/observer system must be used as was done here and the 'one-way' light speed must be calculated and used. This way the errors will vanish.

The error between using a mean speed or the 'one-way' speed to calculate distances is irrelevant in usual circunstances but not in this stringent situation.