Frequently Asked Questions (under construction!)
GPS Time Transfer
How do you know the GPS receivers in Gran Sasso and CERN are not introducing any systematic error?
Metrology labs around the world use GPS time transfer techniques to exchange observations of the delays of their local clocks with respect to GPS clocks. These observations are then merged by the Bureau International des Poids et Mesures (BIPM) in Paris to produce Coordinated Universal Time (UTC). In addition to these techniques, metrology labs also exchange observations using other independent means such as Two-Way Satellite Time Transfer (TWSTT). In this setup, a certain bandwidth is rented from a geostationary satellite operator to establish a two-way link, which is completely independent of GPS. Travelling atomic clocks (after accounting for relativity and other effects) are a third way which is used to ascertain that GPS time transfer is unbiased. The satellite receivers in Gran Sasso and CERN are the same brand and model. So are the antennae and the antenna cables. They have been characterized by two independent metrology labs (METAS and PTB) using different methods, which agreed to within 2 ns even if conducted with a 2-year separation in time.
Have you taken the antenna cable delay into account?
Calibrations conducted by METAS and PTB both included the final cables and antennae. This is a very important concept: inclusive calibration. It is also used to calibrate other things like fiber links. You have to try to include as much as possible of your final setup in the calibration procedure. So at METAS, for example, both GPS receivers were set up with their final cables and antennae, and their PPS outputs were measured against the METAS reference PPS. This provided, in addition to the relative calibration of interest, an absolute calibration with respect to UTC.
Does this system suffer from any kind of Sagnac effect, related to the rotation of the Earth?
Was the PTB calibration done with a traveling cesium clock? If so, did you apply relativistic corrections to its time base?
No it wasn't. The PTB calibration was done with a traveling GPS used for time comparison between metrology labs. This unit consists of a GPS receiver GR50, a SR60 Time interval Counter (TIC), a GPS antenna, and ~50m of HELIAX foam coaxial cable.
What is the impact of the Ionosphere on the accuracy of the time transfer?
Why the GPS receivers are installed on the surface?
It is very convenient to keep an antenna cable length of ~40m, in such a way that the ensemble can be dismounted and sent to calibration by another lab (PolarRX2e sent to METAS) or another lab can bring a traveling GPS to our installations (PTB calibration at CERN and LNGS).
Fiber length calibration
Why haven't you used fiber reflectometry to measure fiber lengths?
We avoided using reflectometry to calibrate our delays for the following considerations:
- Commercial reflectometers are known to be accurate at the ~10ns level, whereas we know that our internal delay chain is stable at the ~1ns level.
- Lab tests show that swapping the transmitter and receiver over a ~5Km optical fibber roll induces a change in the delay at the order of 0.1ns. Sagnac effects over an extended 10Km fibber would add an additional asymmetry of only ~0.1ns on the earth equator. This means that a simple two-way scheme over an spare fibber has the potential to do a time-transfer between two points separated by 10Km with ~1ns accuracy.
- Delay paths should include as many components as possible in order to reduce any systematic error.
These fibers are not length-compensated online. What is the length change throughout the year?
Beam Current Transformer cable calibration
What does the BCT measure?
The BCT is made of a toroidal magnetic core with a winding around it that delivers a voltage when the beam goes through the center of the toroid. It's a bit like a conventional transformer, with the beam playing the role of the primary winding and its effect being observed in the secondary winding.
Why is the BCT cabling delay so large?
There are no electronics near the beam pipe for radiation reasons. The original mission of that BCT was to make sure the beam had the required characteristics for CERN Neutrinos to Gran Sasso (CNGS) operation. Using it as a means to provide a time-stamped acquisition for neutrino time of flight experiments was an afterthought. In any case, the cable length and the amplifier combined with the BCT still provide a bandwidth of several hundreds of MHz so there is no appreciable distortion in the waveform.
You have calibrated using an LHC beam. How do those measurements apply to the CNGS beam?
Lab measurements indicate that there is a variation in beam-to-output delay with respect to transverse beam position. We measured an effect of almost 10 ns for a 10 cm transverse displacement. But the different in transverse position between an LHC beam and a CNGS beam is less than 2 mm. So this effect is not a problem. Also, for the purpose of measuring BCT delay, the actual time structure of the beam is irrelevant. This is why we chose the LHC beam, with its properly separated bunches that make analysis of the delay much simpler.