1、Rec. ITU-R TF.767-2 1 RECOMMENDATION ITU-R TF.767-2 Use of global navigation satellite systems for high-accuracy time transfer (Question ITU-R 152/7) (1992-1998-2001) The ITU Radiocommunication Assembly, considering a) levels over intercontinental distances; the need for operational time transfer an
2、d comparison at the highest possible accuracy b) that there is a growing worldwide need for inexpensive time transfers to accuracies less than 1 ps, which are fully automatic (in particular, capable of resuming synchronization after power losses without human intervention) and which possess adequate
3、 characteristics with respect to reliability, availability and cost; c) at levels of less than 100 ns; the demonstrated capabilities of satellite techniques for providing time transfer accuracies d) the advantages offered by the Global Positioning System (GPS) in terms of current availability of mul
4、tiple satellites, the system of 24 satellites providing continuous global coverage for time transfers, and the strong commitment to monitoring and control of the system. The precisely known relationship between GPS system time and coordinated universal time (UTC), the use of on-board atomic standard
5、s, and the availability of numerous commercial sources for GPS timing receivers; e) transfer over many years; the extensive favourable experience gained from the wide use of GPS satellites for time f) liaison between the GPS system and civil users; the establishment of a Civil GPS Service Interface
6、Committee to provide information and g) the technical capabilities to intentionally degrade the accuracy of the GPS signals that are available to general civil users through the use of selective availability (SA) and anti-spoofing (AS) techniques (see Annex 1); h) the demonstrated technical feasibil
7、ity of at least partially compensating for the adverse effects on time-transfer accuracy related to selective availability and anti-spoofing procedures by use of the GPS common-view technique, averaging the received timing signals over one or more days, using a weighted average across several satell
8、ites, or use of delayed precise satellite position information; j) that the Global Navigation Satellite System (GLONASS) is a comparable system to GPS; k) that Satellite Based Augmentation Systems (SBASs), such as the Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Sy
9、stem (EGNOS) and the Multi- function Transportation Satellite (MTSAT) satellite-based augmentation system are anticipated to have similar important utility, 2 recommends Rec. ITU-R TF.767-2 1 that the use of GPS, GLONASS and SBAS satellites for worldwide time transfer and comparison be seriously con
10、sidered, particularly in those applications requiring global coverage and accuracy levels of less than 1 ps; 2 that the GPS and GLONASS common-view time-transfer technique be seriously considered for applications requiring accuracies in the range of 10-100 ns; that by using this technique time stabi
11、lities and accuracies can be further enhanced by: - applying a weighted average across several satellites; - using a codeless receiver to measure and compensate for the satellite signals delay through the ionosphere; - applying corrections using precise satellite ephemeris data; - determining accura
12、te (1 m) coordinates for the receiver antennas; 3 that in an environment where selective availability andor anti-spoofing measures are activated users requiring the best possible accuracy should compensate as far as is possible for the intentional degradation by applying the following techniques: -
13、exact (within 1 s of the same start and stop times) common view to cancel SA satellite clock modulation; - averaging the received signals across all available satellites and over several days; - applying later corrections using delayed precise satellite ephemeris data. ANNEX 1 Intentional degradatio
14、n on GPS signals The GPS has been designed with the ability to intentionally degrade the GPS signal that is available to general civil users. The degradation is called “selective availability“ (SA). In addition to SA, there is also anti-spoofing (AS). A set of decryption keys is required to remove t
15、he effects of AS and SA. The activation of AS will make the GPS precise positioning service (PPS) inaccessible to those without the appropriate keys. The PPS gives the full accuracy of the GPS. The clear access (UA) 1.023 MHz code will remain available for all users and provides the standard positio
16、ning service (SPS). With SA on, the SPS provides access to GPS Time and UTC as maintained by the United States Naval Observatory (UTC (USNO) but the time modulation causes peak-to-peak uncertainty of several hundred nanoseconds. The SPS has stated (20) uncertainty in two dimensions of 100 m for posi
17、tion and 340 ns for time. With one-day averaging 100 ns uncertainty have been obtained, Rec. ITU-R TF.767-2 3 and with three-day averaging 50 ns uncertainty have been obtained. A stable reference clock is necessary to realize these accuracies. Most of the international timing centres have adopted th
18、e GPS common-view method as the best operational means for time transfer. The coordinates of the GPS antennas at timing centres are now known to be better than 1 m. The Consultative Committee for Time and Frequency (CCTF) has set up a special Task Group to study systematic effects in GPS timing equi
19、pment (mainly receivers). Various kinds of biases and temperature dependencies have been discovered; work continues in this area. Several steps have been taken to assist civil users in obtaining information about GPS. For example, a Civil GPS Service Interface Committee (CGSIC) has been established.
20、 Within the CGSIC is a timing sub-committee. This sub-committee services the Bureau International des Poids et Mesures (BIPM), international timing centres, and industry (telecommunications, power companies, etc.). The CGSIC provides: - - - To further assist GPS users, weekly or monthly bulletins ar
21、e available fi-om many organizations: for example, BIPM, the National Institute of Standards and Technology (NIST), USNO and other national metrological institutes. In addition, World Wide Web access is available at BIPM, NIST and USNO. Many organizations have personnel who can answer detailed questions that civil users may have. general and specific GPS information; an operating bulletin board serviced by the United States Coast Guard; and information regarding techniques and research and development efforts.
