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

ecords in both stations <strong>the</strong> contribution <strong>of</strong> <strong>the</strong> satellite clock vanishes, not biasing with its errors<strong>the</strong> time comparison. At present, GPS C/A-code common-views allow to obtain 2-10 ns level intime synchronization. The common-view method reduces some uncertainties <strong>of</strong> physical originand, a factor which was relevant until <strong>the</strong> middle <strong>of</strong> <strong>the</strong> year 2000, it cancelled <strong>the</strong> deliberatedegradation introduced in GPS time (Selective Availability), with this degradation beingeliminated on 2 May 2000.263. GPS observations are dependent on <strong>the</strong> effects <strong>of</strong> <strong>the</strong> propagation medium on <strong>the</strong> signal.One channel GPS C/A-code time receivers track one satellite at a time. As <strong>the</strong> C/A-code istransmitted in only one frequency, <strong>the</strong>re is no possibility to determine <strong>the</strong> delay introduced by <strong>the</strong>propagation <strong>of</strong> <strong>the</strong> signal along <strong>the</strong> ionosphere. Much progress has been done by using <strong>the</strong>International GPS Service (IGS) products to correct GPS data. All <strong>the</strong> GPS common viewsobtained with single-frequency receivers in TAI are corrected by using <strong>the</strong> IGS total electroncontent (TEC) maps. All GPS links in TAI are corrected using IGS precise satellite ephemeredes.264. During 2003, GPS multi-channel dual-frequency receivers had been incorporated into <strong>the</strong>network <strong>of</strong> time links for TAI: BIPM had four <strong>of</strong> <strong>the</strong>se links <strong>of</strong>ficially in <strong>the</strong> calculation in June2004, with uncertainties <strong>of</strong> 1 ns or less.265. The BIPM establishes and distributes twice a year schedules for <strong>the</strong> observation <strong>of</strong> GPSsatellites. These schedules are necessary to organize <strong>the</strong> common views that are <strong>the</strong> basis <strong>of</strong> <strong>the</strong>clock links.266. The development <strong>of</strong> multi-channel multi-code receivers permits to increase in an order<strong>of</strong> magnitude <strong>the</strong> number <strong>of</strong> observations with respect to <strong>the</strong> single-channel mode. Multi-channelC/A-code receivers observe all GPS satellites in view using 13-minute tracks every 16 minutes at<strong>the</strong> standard hours. No GPS schedule is necessary to programme <strong>the</strong> observations since <strong>the</strong>receiver automatically tracks all satellites in view. Data are stored in a single file with astandardized format. Under this constraint, <strong>the</strong> number <strong>of</strong> satellite tracks increases by a factor <strong>of</strong>10 with respect to <strong>the</strong> number <strong>of</strong> single-channel common-view tracks. This implies animprovement in <strong>the</strong> quality <strong>of</strong> time and frequency transfer. However, multi-channel observationsmay be subject to systematic variations. Environmental conditions affect receivers by imposingsystematic effects. Temperature stabilized antennas (TSA) have been developed at <strong>the</strong> BIPM andare currently commercially available.267. TWSTFT is a technique that allows <strong>the</strong> comparison <strong>of</strong> two distant clocks by using astransponder a geostationary telecommunications satellite. The <strong>the</strong>oretical precision expected intime transfer <strong>of</strong> several hundred picoseconds has been reached in <strong>the</strong> first semester <strong>of</strong> 2004, when<strong>the</strong> time laboratories operating TWSTFT stations in Europe and North America startedscheduling daily and sub-daily observations.268. The network <strong>of</strong> clock comparisons at <strong>the</strong> BIPM is at present supported by common viewson GPS satellites and by links using TWSTFT.Future prospects269. The stability <strong>of</strong> <strong>the</strong> atomic time scale has been improved due to <strong>the</strong> replacement in mostlaboratories <strong>of</strong> <strong>the</strong> old caesium clocks by <strong>the</strong> more high-performance new-type ones. Thealgorithm used in <strong>the</strong> calculation <strong>of</strong> TAI assigns relative weights to participating clocks, thusassuring that <strong>the</strong> stability is highly dominated by <strong>the</strong> best clocks. Primary frequency standards48