A new technique that has the potential to revolutionize clock frequency comparisons using GPS signals has been developed by the BIPM Time Department in collaboration with colleagues from the Centre National d'Études Spatiales (CNES) and the Collecte Localisation Satellites (CLS).
The Global Positioning System (GPS) has been used to compare clocks at a distance for decades. The present technique of choice is Precise Point Positioning (PPP), which provides the majority of the links used for UTC. Its performance for frequency comparisons is of the order a few parts in 1016 in 10 days, which is adequate for all commercial clocks, but insufficient to compare the best Cs or Rb fountains which claim an accuracy below 2 × 10−16, not to mention the forthcoming ion clocks or atom lattice clocks.
One way to overcome the current limitations of GPS for clock comparisons is to base the results only on the phase of the transmitted signals and to explicitly account for the integer-cycle nature of the phase ambiguities that need to be resolved between the different arcs and satellites that are successively observed during the comparison. Developing this technique, called IPPP (PPP with integer ambiguity resolution), has been a topic of collaboration with colleagues at the CNES and the CLS over the last few years. The CNES and CLS teams constitute one analysis centre of the International GNSS Service (IGS), and they are developing specific IGS products needed for IPPP. In the framework of her 2013-2014 post-doctoral position, jointly funded by the BIPM and the CNES, Amale Kanj developed operational procedures for IPPP that allowed several long-term tests to be carried out (comparisons over weeks to months). A recent joint publication with colleagues at the CNES and CLS describes the work (Petit et al. Metrologia, 2015).
The most significant result compares IPPP to a 420 km optical-fibre time link in Poland, which is in continuous operation and which reports to the BIPM. As a result of the fibre link's better accuracy, it was possible to demonstrate that the IPPP technique reaches, in about 5 days, a performance of 1 × 10−16 in comparing the frequency of two clocks and that the achieved accuracy continues to improve with longer averaging times. This represents a significant improvement over classical PPP (Fig. 1 compares the IPPP in blue with the classical PPP in red) and is the best published performance for a GPS frequency comparison.
|Fig. 1. Comparison between the optical fibre link between AOS and GUM and the results obtained with IPPP (blue) and PPP (red) over 41 days. Left: Plot of the time differences with arbitrary offset; right: stability analysis.|
Because IPPP can compare two clocks whatever their locations on Earth and can be operated immediately with existing equipment, it will be a significant step towards the comparison of ultra-accurate clocks. Optical fibre links will progressively build up into networks that cover continental areas, however this will take time. For world-wide frequency comparisons at the sub-10−16 level, no operational techniques are on the horizon except during the Atomic Clock Ensemble in Space (ACES) mission, which is expected to fly onboard the International Space Station in 2016-2018.
Petit G., Kanj A., Loyer S., Delporte J., Mercier F., Perosanz F., 1 × 1016 frequency transfer by GPS PPP with integer ambiguity resolution, Metrologia, 2015, 52, 301-309.