report of the action team on global navigation satellite systems (gnss)

255. Methods to compare distant clocks are necessary to calculate <strong>the</strong> clock differences whichare <strong>the</strong> base <strong>of</strong> <strong>the</strong> computation <strong>of</strong> TAI. In <strong>the</strong> last decade, <strong>the</strong> time community has largelyexploited <strong>the</strong> Global Positioning System (GPS) satellites for time transfer using receivers adaptedfor time comparisons. In 1981, <strong>the</strong> first GPS time links were introduced in TAI. The technique,still in use today, was <strong>the</strong> GPS common-view single channel C/A-code. It allows time comparisonwith an uncertainty <strong>of</strong> one part in 10 14 for average times <strong>of</strong> a few days. The stability reached byTAI for periods <strong>of</strong> a month is <strong>of</strong> two parts in 10 15 .256. In <strong>the</strong> last years, multi-channel GPS and GPS-GLONASS receivers have beencommercially developed. The stability <strong>of</strong> time transfer increases in a factor <strong>of</strong> about three whenmulti-channel satellite observations are used. GLONASS P-code signal has always been availableto civil users; it has been demonstrated that it reduces <strong>the</strong> noise level by a factor <strong>of</strong> about fiverelative to GPS C/A-code time comparisons.257. The GPS carrier-phase technique allows frequency comparison at a level <strong>of</strong> one part in10 15 , and it will be a useful tool in accurate time transfer after some remaining problems beinvestigated and solved.258. The technique <strong>of</strong> two way time and frequency transfer by telecommunication satellite(TWSTFT) has been largely investigated: in optimal conditions (daily and sub-dailyobservations) it has a better performance than <strong>the</strong> GPS C/A single/multi-channel single frequencytime transfer techniques used in TAI. About ten TWSTFT links are used at present in <strong>the</strong>calculation <strong>of</strong> TAI.Present organization <strong>of</strong> time links259. The establishment <strong>of</strong> TAI and UTC is based on atomic clock data coming fromworldwide distributed laboratories, and <strong>the</strong>refore it is strongly dependent on <strong>the</strong> means <strong>of</strong> timecomparison. The international time scales computed at <strong>the</strong> BIPM are based on data <strong>of</strong> some 220clocks located in about 50 time laboratories distributed all over <strong>the</strong> world. The quality <strong>of</strong> clocksparticipating in TAI has evolved in <strong>the</strong> last decade, with <strong>the</strong> development <strong>of</strong> more precisecaesium commercial clocks. Caesium clocks <strong>of</strong> <strong>the</strong> new type are stable in about one part in 10 14over periods <strong>of</strong> a day. Very accurate methods <strong>of</strong> time transfer are necessary so that <strong>the</strong>comparison does not degrade <strong>the</strong> accuracy <strong>of</strong> <strong>the</strong> atomic clocks.260. The present organization <strong>of</strong> <strong>the</strong> international time links at <strong>the</strong> BIPM is based on two-timecomparison techniques: common views on GPS satellites and TWSTFT.261. The introduction <strong>of</strong> GPS into time transfer in <strong>the</strong> 1990’s led to an improvement <strong>of</strong> oneorder or more <strong>of</strong> magnitude with respect to o<strong>the</strong>r methods used at that moment for clocksynchronisation. Clock comparison plays a key role in <strong>the</strong> elaboration <strong>of</strong> <strong>the</strong> international timescales. Clocks situated at distant laboratories, spread all over <strong>the</strong> world, can be compared with anuncertainty <strong>of</strong> several nanoseconds by using GPS. About 80% <strong>of</strong> <strong>the</strong> clock comparisons for TAIare exclusively done by using <strong>the</strong> clocks on board GPS satellites, with <strong>the</strong> 20% remnant beingprovided by ano<strong>the</strong>r technique and <strong>the</strong> corresponding time link by GPS as a backup.262. The GPS common-view method is used in <strong>the</strong> calculation <strong>of</strong> time links at <strong>the</strong> BIPM. In<strong>the</strong> common-view technique, two stations simultaneously observe <strong>the</strong> same GPS satellite. Bothstations record <strong>the</strong> difference between each local clock and GPS time at <strong>the</strong> same instant andusing <strong>the</strong> same clock on board <strong>the</strong> satellite in common view. When making <strong>the</strong> difference <strong>of</strong>47