International GPS Service (IGS) Bureau International des Poids et Mesures (BIPM)

The International GPS Service (IGS) and the Bureau International des Poids et Mesures (BIPM) established a joint Pilot Project for Time and Frequency Comparisons using GPS in early 1998. The goal of this Pilot Project was to investigate the use of GPS phase and code measurements to improve worldwide availability of accurate time and frequency. The activity was formally ended as a Pilot Project at the end of 2002, after which time the efforts are now aimed at transitioning to more operational modes.

GPS offers the opportunity for highly accurate time and frequency transfer among many remote sites. In analyses of GPS observations, there is a potential for determining clock differences with uncertainties well below one nanosecond, comparable to the best techniques available. To exploit this potential, however, important technical issues remain to be resolved, which this Pilot Project investigated.

The Pilot Project was open to all relevant groups, agencies, and individuals. These activities are not intended to limit in any way the freedom of participants to engage in independent research. The objective was to organize and coordinate efforts among groups to achieve the goals of the Project and to share resources, data, and experiences. The Pilot Project was conducted by a Working Group formed by those who responded to the Call for Participation. The Working Group was co-chaired by Felicitas Arias (BIPM) and Jim Ray (USNO).

1st Working Group Meeting -- 22-23 June 1998 at BIPM, Sèvres, France

IGS Network Systems Workshop -- 02-05 November 1998 in Annapolis, Maryland, USA

2nd Working Group Splinter Meeting at PTTI -- 30 November 1998 in Reston, Virginia, USA

30th Precise Time & Time Interval (PTTI) Meeting -- 01-03 December 1998 in Reston, Virginia, USA

Joint Meeting of the 13th European Frequency and Time Forum and 1999 IEEE International Frequency Control Symposium -- 12-16 April 1999 Besançon, France

14th Meeting of the Consultative Committee for Time and Frequency (CCTF) -- 20-22 April 1999

At its 14th meeting, held at the BIPM (Pavillon de Breteuil, Sèvres, France), the CCTF considered the interim report of the Pilot Project working group and adopted Recommendation S5 (1999) in support of the Pilot Project. Associated with this gathering, on 19 April the 4th general meeting of the CCTF Working Group on TAI was also held at BIPM.

IGS Analysis Center Workshop -- 08-10 June 1999 at Scripps Institution of Oceanography, La Jolla, California, USA

31st Precise Time & Time Interval (PTTI) Meeting -- 07-09 December 1999 in Dana Point, California, USA

14th European Frequency and Time Forum -- 14-16 March 2000 in Turin, Italy

IGS Analysis Workshop -- 25-29 September 2000

The IGS Analysis Workshop 2000 included a separate 2-day meeting on 25-26 September 2000 to focus on the IGS/BIPM Pilot Project. The workshop was held at the U.S. Naval Observatory in Washington, DC, USA. A summary of the workshop and its recommendations is given here. Electronic preprints of the proceedings are available at this site.

32nd Precise Time & Time Interval (PTTI) Meeting -- 28-30 November 2000 in Reston, Virginia, USA

15th European Frequency and Time Forum -- 06-08 March 2001 in Neuchâtel, Switzerland

A special session on "Calibration problems" was held as part of the 15th EFTF.

15th Meeting of the Consultative Committee for Time and Frequency (CCTF) -- 20-21 June 2001

At its 15th meeting, held at the BIPM (Pavillon de Breteuil, Sèvres, France), the CCTF adopted Recommendation 2 (2001) in support of the Pilot Project. Associated with this gathering, on 19 June the 5th general meeting of the CCTF Working Group on TAI was also held at BIPM. An interim report (MS/Word) of the Pilot Project working group was submitted to the CCTF together with a group proceedings paper (MS/Word) from the IGS Analysis Center Workshop held in September 2000 at USNO.

33rd Precise Time & Time Interval (PTTI) Meeting -- 27-29 November 2001 in Long Beach, California, USA

16th European Frequency and Time Forum -- 12-14 March 2002 in St. Petersburg, Russia

4th Intern. Symp. on Time Scale Algorithms -- 18-19 March 2002 at BIPM, Sèvres, France

Setting up a New IGS Station

• IGS Procedures

  Only high-quality geodetic-type receivers, capable of collecting dual-frequency pseudorange and carrier phase observations, are used by the IGS. Data are normally collected at a 30-second sampling rate and forwarded to an IGS Data Center within 48 hours (preferably within 1 hour) from the end of the UTC day on which it was collected.

  General instructions have been prepared by the Central Bureau for adding a new station to the IGS network and for organizing operations:

  Procedures for Becoming an IGS Station

  Standards for IGS Stations and Operational Centers

  Monumentation Design and Implementation Recommendations

  Other sources of IGS general information, especially helpful to new users, can be found at:

  IGS FAQ page

  IGS Publications

  IGS Mail messages

  IGS Reports

  To subscribe/unsubscribe to IGS Mail messages or IGS Reports, you can send e-mail to the IGS Central Bureau at Majordomo@igscb2b.jpl.nasa.gov with the following command in the body of your message:

      subscribe igsmail [your e-mail address]
  

  or

      subscribe igsreports [your e-mail address]
  

• Station Names

  In choosing the name for a new station, the following conventions should be followed:

  A 4-character acronym for each receiver should be proposed to and approved by the IGS Central Bureau (igscb@igscb.jpl.nasa.gov).

  For each geodetic reference point, a DOMES number (a globally unambiguous marker number used by ITRF) must be requested from the ITRF Section of the IERS Central Bureau (see http://lareg.ensg.ign.fr/ITRF/domesreq.html; e-mail contact: domes@ensg.ign.fr).

• IGS Log Files

  A log file should be completed and sent to the IGS Central Bureau documenting the configuration of each IGS station. A blank log file may be obtained at ftp://igscb.jpl.nasa.gov/igscb/station/general/blank.log. Procedures for submitting or updating logs are described at http://igscb.jpl.nasa.gov/network/sitelog-submissions.html.

  Maintaining up-to-date IGS site log files is critical. Operators are encouraged to include "Additional Information" in the log files for such items as the model type and environmental control for the frequency standard, and cable types, lengths, and stability specifications.

  Station operators are urged to report any change in the status of the frequency standard (when an external standard is used) or any other operational aspect that could affect the timing performance of your station. Changes of general interest (such as a new frequency standard) should be reported as timely IGS Mail messages. Notification of other changes expected to affect timing users only can be sent to Jim Ray to post at this site. Such reports are especially important for the "fiducial" clock stations.

Hardware Options

• Guidelines for Manufacturers of GNSS Receivers used for Timing

  The Consultative Committee for Time and Frequency (CCTF) has asked (Recommendation S5, 2001) that manufacturers of receivers used for timing with global navigation satellite systems (GNSS) implement the technical quidelines for receiver hardware compiled by the CCTF Group on GNSS Time Transfer Standards (CGGTTS). These guidelines aim to achieve a system that can transfer time with an accuracy of 1 ns or better. The guidelines are available here in MS/Word format (34.8 kB).

• UNAVCO Advice & Assistance

  The University NAVSTAR Consortium (UNAVCO), which supports geoscience research projects, has a very informative Web site containing much useful advice and test reports dealing with many aspects of continuously operating GPS stations.

  UNAVCO also maintains a range of helpful software tools. In particular, the teqc toolset provides many useful functions for handling and examining raw GPS data, including RINEX file translation, data editing, and quality checking. For further information, please refer to L.H. Estey & C.M. Meertens, GPS Solutions, 3(1), 42-49, 1999.

• Report of IAG Special Study Group 1.158 -- GPS Antenna and Site Effects

  Special Study Group 1.158, under Section I of the International Association of Geodesy, presented this report on its findings during the quadrennium 1995-1999. The report addresses the problem of site-specific errors and presents some recommendations on how to handle these errors.

• Report of IAG Special Study Group 1.182 -- Multipath Mitigation

  Special Study Group 1.182, under Section I of the International Association of Geodesy, presented this report on its progress for the IAG Travaux 1999-2001. The report addresses detection and mitigation of multipath with the goal of improving geodetic peformance. The Special Study Group also maintains a helpful website.

• GPS Receiver & Antenna Vendors

  For information on GPS receivers which allow input of reference signals from external frequency standards, please refer to the following web sites:

  • Allen Osborne Associates, Inc. (AOA)

    A discussion of problems with TurboRogue receiver tracking at low elevation angles, especially under conditions of enhanced ionospheric activity, is given here. A firmware upgrade (version 3.2.32.11) has been released by AOA to correct this problem; for details, please see IGS Mail #3758.

    AOA is offerring to retrofit their older TurboRogue models (SNR-8000, SNR-8100, and SNR-12) with their new ACT tracking technology, which is also available in their newer Benchmark models.

  • Ashtech Precision Products (Thales Navigation)

    At the IGS Network Systems Workshop in November 1998, it was announced that modifications for time transfer applications are now available as the "Metronome" option. This option is currently available only for the rack-mounted Z12 receiver model. The setup requirements for the Metronome-equipped, Z12-T receiver are described here.

    Note that the standard, unmodified Ashtech Z12 receiver can also accept an external frequency standard input. However, to ensure that the internal clock is faithfully locked to the external standard, care must be taken to select the proper operational options. The setup procedures have been provided by Rober Snow (Magellan Corp.) for the firmware versions RD00 and CD00; for the Ashtech Z-Surveyor, the setup procedures are given for firmware versions ZB00.

    New prices and new receiver models are available in 2001.

    For suggestions on signal multipliers to use to synthesize a 20 MHz input frequency from a 5 MHz source, please check here.

  • Javad Navigation Systems

    All Javad receivers accept input 5, 10 and 20 MHz from external frequency standards to be used as the main oscillator of the receiver.

    A Javad JPS Legacy receiver, together with single-depth antenna, was loaned to the NOAA Geosciences Research Division for tests and evaluation. For part of the loan period, it was deployed at USNO near to the current IGS station "USNO", which uses an AOA TurboRogue receiver. The USNO deployment was for 16 days, from 18 March 1999 through 02 April 1999. The Javad station was named "USNJ" and ran from the same external 5 MHz H-maser signal supplied to the USNO receiver. The Javad test data are available at ftp://maia.usno.navy.mil/pub/javad/. An evaluation of the timing performance is available in this report.

    Another test for carrier phase time transfers has been organized by Jon Clarke at NPL using a JPS EuroCard receiver. For comparison, RINEX data from their main geodetic receiver (NPLB) and conventional common-view data from a R100 GPS/GLONASS receiver are also available via anonymous ftp. For further information, please refer to this message.

  • NovAtel

  • Trimble

    Trimble has announced that its Acutime(TM) 2000 Synchronization Kit is available at trimblestore.com/timing. The Acutime 2000 Synchronization Kit is a high-performance, cost-effective reference time source that precisely synchronizes computers, servers and Internet applications to UTC within 50 ns (1-sigma) using GPS.

• Effects of Mixed Receiver Types & (P1-C1) Satellite Biases

  Codeless receivers operate by different distinct technologies. As the IGS network transitions from a core of Rogue/TurboRogue (TR) receivers to newer codeless tracking architectures, differences in the pseudorange observables must be considered. The new receivers provide pseudorange observables which can be biased compared with TRs. This is of no significance for data analysts who process only carrier phase observables. For analysts who use the pseudorange observations, their estimates for satellite clocks can depend on the receiver model and the RINEX translation process. To avoid mixing data with different satellite biases, which will degrade the IGS satellite clock products (and precise point positioning using them), recommendations for RINEXing and analysis procedures have been developed to maintain compatibility. For background on this topic, please refer to IGS Mail #2320 and for the change in adopted IGS bias convention, in effect starting 02 April 2000, please refer to IGS Mail #2744. A new class of receiver types was recognized in IGS Mail #3737 (13 Feb 2002). The current (P1-C1) bias values, as well as previous values, are archived in this file.

• Antenna Phase Calibration

  The beam patterns of GPS antennas generally deviate from the perfectly hemispherical ideal. Effectively, this means that the phase center of the antenna, and hence the geodetic reference point, will depend on the direction of the signal from a particular satellite. Azimuthal variations can usually be ignored and only the elevation angle dependence considered. Neglecting these effects can cause significant errors in station height determinations, up to ~10 cm.

  The IGS maintains a table of elevation-dependent phase center corrections for a wide variety of antenna models; see this file. These give the differential corrections relative to the AOA Dorne Margolin T antenna with chokerings (AOAD/M_T). For further information on this topic, please refer to G.L. Mader, GPS Solutions, 3(1), 50-58, 1999.

  The antenna phase pattern for the TSA-100 temperature-stabilized antenna from 3S-Navigation were measured by the NGS group and are reported in this message.

• RF Cables

  For consideration in choosing antenna RF cables, Eric Burt and Ed Powers have assembled a table of specifications giving minimum and maximum delay variations due to temperature changes, as well as attenuation losses at L1 and L2, for a number of Heliax coaxial cable types from Andrew Corp. In addition, Dave Stowers offers some experience at JPL with LMR-400 cables from Times Microwave Systems.

• Distribution of Reference Frequency Signals

  Distribution amplifiers are sometimes needed to split the reference signal from an atomic frequency standard for use by several pieces of equipment. For a distribution amplifier with very high isolation, exceptionally low phase noise, and a very low temperature sensitivity, please check the suggestion here. For suggestions on signal multipliers to use to synthesize a 20 MHz input frequency from a 5 MHz source, please check here.

IGS Stations Colocated at BIPM Timing Laboratories

The IGS tracking network currently consists of nearly 300 permanent, continuously operating, geodetic stations globally distributed. Of these, almost 80 are equipped with external frequency standards, the remainder using internal crystal oscillators. The external standards in use are: ~40 with H-masers, ~25 with cesiums, and ~15 with rubidiums. The IGS stations listed below are located at timing laboratories.
IGS stations located at BIPM timing laboratories (as of March 2002)

IGS Site	Time Lab	GPS Receiver	Freq. Std.	City
AMC2	AMC *	AOA SNR-12 ACT	H-maser	Colorado Springs, CO, USA
BOR1	AOS	AOA TurboRogue	cesium	Borowiec, Poland
BRUS	ORB	Ashtech Z-XII3T	H-maser	Brussels, Belgium
KGN0	CRL *	Ashtech Z-XII3	cesium	Koganei, Japan
MDVO	IMVP	Trimble 4000SSE	H-maser	Mendeleevo, Russia
MIZU	NAO	AOA Benchmark	cesium	Mizusawa, Japan
NPLD	NPL *	Ashtech Z-XII3T	H-maser	Teddington, UK
NRC1	NRC *	AOA SNR-12 ACT	H-maser	Ottawa, Canada
NRC2	NRC *	AOA SNR-8100 ACT	H-maser	Ottawa, Canada
OBE2	DLR	AOA SNR-8000 ACT	rubidium	Oberpfaffenhofen, Germany
OPMT	OP	Ashtech Z-XII3T	H-maser	Paris, France
PENC	SGO	Trimble 4000SSE	rubidium	Penc, Hungary
PTBB	PTB *	AOA TurboRogue	H-maser	Braunschweig, Germany
SFER	ROA *	Trimble 4000SSI	cesium	San Fernando, Spain
SPT0	SP	JPS Legacy	cesium	Boras, Sweden
TLSE	CNES	AOA TurboRogue	cesium	Toulouse, France
TWTF	TL *	Ashtech Z-XII3T	cesium	Taoyuan, Taiwan
USNO	USNO *	AOA SNR-12 ACT	H-maser	Washington, DC, USA
WTZA	IFAG	Ashtech Z-XII3T	H-maser	Wettzell, Germany
WTZR	IFAG	AOA SNR-8000 ACT	H-maser	Wettzell, Germany

* participates in two-way satellite time transfer (TWSTT) operations

The former timing lab at TUG (GRAZ receiver) ended operations in 2000. In addition to these, several time labs are equipped with geodetic receivers which are not yet incorporated into the IGS network but for which data are publicly available. Some of these are listed below.

Additional BIPM timing laboratories equipped with geodetic receivers (as of May 2001)

Site Name	Time Lab	GPS Receiver	Freq. Std.	City
LPTF	BNM-LPTF	Ashtech Z-XIIT	H-maser	Paris, France
NPLF	NPL *	Javad Legacy	H-maser	Teddington, UK
ROAH	ROA	AOA TTR4-P	cesium	San Fernando, Spain

* participates in two-way satellite time transfer (TWSTT) operations
• Common-View Files from RINEX Data

  Pascale Defraigne (ORB) and Gérard Petit (BIPM) have written a "Proposal to use geodetic-type receivers for time transfer using the CGGTTS format," which describes a procedure to form standard CGGTTS-format common-view observation files from RINEX data obtained from geodetic GPS receivers. Your comments on this proposal are welcome. This proposed procedure aims to permit time links using calibrated geodetic GPS receivers to be introduced into the TAI calculation. The proposal is available here as a MS/Word document.

• BIPM Time Transfer Receivers

  A summary of the timing receiver types used at each timing laboratory contributing to TAI are listed here in pdf format (as of April 2000).

Frequency Standards and Other Equipment Deployed at IGS Stations

• History of Clock & Other Deployments

  A summary file of the deployment history for GPS receiver, antenna, frequency standards, and other equipment at IGS stations is given here. This information is extracted automatically from the IGS site logs maintained at http://igscb.jpl.nasa.gov/igscb/station/log/ and is updated daily. It is not entirely reliable, however, due to incomplete or inaccurate log information.

• Current Clock & Other Deployments

  A similar summary file of the current IGS equipment configuration is given here. This file is also updated automatically each day.

• Anomalies & Events Reports

  Reports which describe events or anomalies that could affect the performance of the frequency standards used at or clock results from IGS Stations are archived here. This information may not be complete.

Time Laboratories Equipped with GLONASS

The time laboratories equipped with GLONASS or GPS+GLONASS receivers (and also participating in the International GLONASS Experiment, IGEX) are listed here. This summary has been prepared by Wlodek Lewandowski (BIPM).

Information on the health and status of the GLONASS constellation is available at http://satnav.atc.ll.mit.edu/.

Selective Availability Discontinued

By decision of the U.S. President, the intentional degradation of the GPS clock signals by Selective Availability (SA) was discontinued at midnight EDT on 01 May 2000 (04:00 UTC on 02 May). This change now allows civilian GPS users to obtain positioning and timing determinations that are more accurate than before by about a factor of up to ten.

From Richard Langley, to the Canadian Space Geodesy Forum on 02 May 2000:

"Attached is a plot from the GPS Support Center Web site showing the transition of SA to zero. The data were captured using a Trimble SV6 receiver and the plot was prepared by Rob Conley of Overlook Systems for U.S. Space Command." (Note that CEP = circular error probable, the 50% probability limit for true horizontal coordinates, and that SEP = spherical error probable, which includes the height component.)

This postscript plot from Tim Springer shows the dramatic improvement in satellite clock stability that occurred on 02 May when SA was turned off. Tim also computed the Allan deviations for all satellites clocks on 06 May 2000, which are shown in this postscript plot.

The benefits of removing SA for using IGS orbit and clock files to perform precise point navigation are discussed by Jan Kouba in IGS Mail #2824.

Reports & Messages Related to Satellite Clocks

E-mail reports describing events, anomalies, or other aspects related to GPS satellite frequency standards are given here.

Satellite Antenna Phase Center Offsets -- Adopted IGS Values

The vector offsets between the phase centers of the GPS satellite transmit antenna arrays and the satellite centers of mass are not well known. It is the dynamical motion of the centers of mass which must be modelled in the data analysis even though the signals originate from the transmit arrays. Any errors in the radial component of the vector offsets will manifest themselves almost entirely as biases in the estimated values for the satellite clocks. Therefore, in order to ensure that satellite clock determinations from the various IGS Analysis Centers can be compared and combined consistently, the IGS has adopted a common set of values for the antenna phase center offset vectors. The rationale and method for implementing these values is documented here. This set of offset values was officially implemented by the IGS starting 29 November 1998.

In the usual satellite-fixed coordinate system (where the z-axis is directed from the satellite center of mass towards the Earth center), the IGS offset values are:

   Block II & IIA    dx = 0.279 m   dy = 0.000 m   dz = 1.023 m
   Block IIR         dx = 0.000 m   dy = 0.000 m   dz = 0.000 m

Note in particular that no offset is used for Block IIR satellites, of which the first was launched in 1997. These values should not be confused with the offsets used by the GPS Operational Control Segment (see following section).

If a user wishes to compare the IGS satellite clock values with other results using different antenna offsets, then corrections must be applied according to:

   Csat_i(user) = Csat_i(IGS) - {[ dzi(user) - dzi(IGS) ] / c}

where

   dzi(IGS)  = IGS value for dz for satellite PRNi
   dzi(user) = user's value for dz for satellite PRNi
   Csat_i(IGS)  = IGS value for clock of satellite PRNi
   Csat_i(user) = user's value for clock of satellite PRNi
   c  = speed of light = 299792458 m/s

Satellite Antenna Phase Center Offsets -- Adopted GPS Operations Values

The GPS Operational Control Segment (OCS) uses a different set of values from the IGS for the offsets from the satellite center of mass to the antenna phase centers. The OCS offset values are listed below, in the usual satellite-fixed coordinate system (where the z-axis is directed from the satellite center of mass towards the Earth center):

   Block II & IIA    dx = 0.2794 m   dy = 0.0000 m   dz = 0.9519 m
   Block IIR         dx = 0.0000 m   dy = 0.0000 m   dz = 1.1725 m (nominal)

   Block IIR (actual, until 21:00 on 01 February 2001):
     SVN43/PRN13     dx = 0.00310 m   dy = -.00122 m   dz = 1.17254 m
     SVN46/PRN11     dx = 0.01634 m   dy = 0.01527 m   dz = 1.51500 m
     SVN51/PRN20     dx = 0.00216 m   dy = 0.00137 m   dz = 1.61400 m
     SVN44/PRN28     dx = 0.00185 m   dy = 0.00066 m   dz = 1.51300 m
     SVN41/PRN14     dx = -.00183 m   dy = -.00018 m   dz = 0.86710 m

   Block IIR (actual, since 21:00 on 01 February 2001):
     SVN43/PRN13     dx = 0.00236 m   dy = 0.00249 m   dz = 1.61400 m
     SVN46/PRN11     dx = 0.00185 m   dy = 0.00107 m   dz = 1.51406 m
     SVN51/PRN20     dx = 0.00203 m   dy = 0.00244 m   dz = 1.61400 m
     SVN44/PRN28     dx = 0.00185 m   dy = 0.00066 m   dz = 1.51312 m
     SVN41/PRN14     dx = 0.00183 m   dy = 0.00018 m   dz = 1.61366 m
     SVN54/PRN18     dx = -.00980 m   dy = 0.00599 m   dz = 1.59229 m

Note that these offsets should not be confused with the values adopted as conventional by the IGS (see section above).

Satellite Antenna Phase Center Offsets -- Measured Values

Gerry Mader and his colleagues at NOAA/NGS have made measurements of a Block IIA satellite and determined the z-offset to be 1.66 m (for the ionosphere-corrected LC center offset). For further information, see the site at www.ngs.noaa.gov/ANTCAL.

Effects of Mixed Receiver Types & (P1-C1) Satellite Biases

Codeless receivers operate by different distinct technologies. As the IGS network transitions from a core of Rogue/TurboRogue (TR) receivers to newer codeless tracking architectures, differences in the pseudorange observables must be considered. The new receivers provide pseudorange observables which can be biased compared with TRs. This is of no significance for data analysts who process only carrier phase observables. For analysts who use the pseudorange observations, their estimates for satellite clocks can depend on the receiver model and the RINEX translation process. To avoid mixing data with different satellite biases, which will degrade the IGS satellite clock products (and precise point positioning using them), recommendations for RINEXing and analysis procedures have been developed to maintain compatibility. For background on this topic, please refer to IGS Mail #2320 and for the change in adopted IGS bias convention, in effect starting 02 April 2000, please refer to this IGS Mail #2744. A new class of receiver types was recognized in IGS Mail #3737 (13 Feb 2002). The current (P1-C1) bias values, as well as previous values, are archived in this file.

The CODE Analysis Center provides direct estimates of the satellite differential code bias (DCB) parameters -- refer to http://www.aiub.unibe.ch/ionosphere.html. Starting with GPS week 1057, they now provide (P1-C1) DCB results, in addition to the previous (P1-P2) DCB estimates. For further information, please refer to IGS Mail #2827 by Stefan Schaer.

Starting with data collected 14 January 2001, the set of (P1-C1) bias values recommended for use with official IGS products are those reported at Stefan Schaer's web site; see IGS Mail #3160 (05 January 2001).

Broadcast values for (L1 - L2) biases

JPL now provides estimates of the Tgd values (the L1 - L2 instrumental bias for each GPS satellite) to 2 SOPS every quarter and also monitors the values daily to identify any abrupt changes in the Tgd values due to configuration changes on the satellites. The uploads of the first complete set of biases were completed on 29 April 1999. For a description of the new Tgd values and the changes made please refer to this message.

Monitoring of GPS satellite clocks

• The U.S. Naval Observatory (USNO) monitors and publishes timing information for the GPS constellation, as well as distributing "Notice Advisories to NAVSTAR Users" (NANUs). This service is available at http://tycho.usno.navy.mil/gps_datafiles.html

• The National Physical Laboratory (UK) monitors and publishes the timing of the GPS constellation. This service is available at http://www.npl.co.uk/npl/ctm/gps_bulletin.html and is intended primarily to aid UK stand-alone users of GPS time in obtaining traceability from their receiver to the UK national time standard, UTC(NPL), and to provide independent integrity monitoring of GPS time.

GPS & GLONASS Observational Data

GPS and GLONASS observational data are transmitted using the RINEX exchange format; see ftp://igscb.jpl.nasa.gov/igscb/data/format/rinex210.txt. This document also describes navigation message and meteorological data formats. RINEX possesses an option to remove an a priori station clock offset (to the 1 ns level), which is described in a test report.

The organization of the IGS data flow and procedures for accessing the data are described in the memo here.

Clock Data

RINEX format extensions have been adopted for the exchange of a variety of clock data types. The current format version is described here.

Common-View Files from RINEX Data

Pascale Defraigne (ORB) has written software to form standard CGGTTS-format common-view observation files from RINEX data obtained from geodetic GPS receivers. The goal of this tool is to permit time links using calibrated geodetic GPS receivers to be introduced into the TAI calculation. The software is available at ftp://omaftp.oma.be/dist/astro/time/RINEX-CCTF/.

Satellite Ephemeris Data

The sp3 format used to exchange satellite orbit and clock information is described in ftp://igscb.jpl.nasa.gov/igscb/data/format/sp3c.txt. The orbits are expressed in an Earth crust-fixed (rotating) frame.

Note that the time tags used in all these files (except the common-view CGGTTS files) are based on GPS time, not UTC.

Clock Analysis Activities

• IGS Analysis Center Coordinator web site
• IGS clock analysis plan
• IGS "fiducial clock" stations
• IGS Analysis Center clock solutions
  The satellite and station clock solutions from individual IGS Analysis Centers are now distributed as standard products. They are available for the following ACs: CODE, EMR, ESA, GFZ, and JPL. (In addition, USNO contributes clock solutions for use in the IGS Rapid and Ultra-rapid combinations.) The files are posted for each 1-day period and are organized by GPS week. The file format is described in this document.
• IGS clock combination results (preliminary)
  The new, preliminary clock combination uses the clock rinex files produced by the IGS Analysis Centers rather than the standard method of using only the satellite clock information in the orbit sp3 files. Summary reports and combined clock files are available starting 02 January 2000.
• new IGS clock combination implemented
  As agreed upon during the IGS Analysis Center workshop, held from 25-29 September 2000 at the U.S. Naval Observatory, the new IGS clock combination became official starting with the products for 05 November 2000.
• new IGS time scale available
  As agreed during the IGS 2000 Analysis Center workshop, a new internally realized time scale has been implemented to improve the stability of the IGS clock products. This work has been done by Ken Senior (USNO) and is described in this preprint. The re-referenced .clk files and plots (time domain and Allan deviation) are available for inspection at this website starting with week 1086 (29 Oct 2000). Note that quadratic trends are removed for plotting purposes and only every 3rd point is shown. The clock labelled "GPST" is the realization of the current IGS time scale compared to the new scale; it is not an observed clock. The GPST values have been removed from the current IGS clocks to produce the new re-referenced clocks, while fully preserving the original consistency of the IGS clock and orbit products.
  

  Refinements to the new time scale were implemented on 28 November 2001 and new a realigned time scale was released for the IGS Rapid clocks (in addition to the previous Final products).
  

  Further changes were implemented on 15 February 2002 to refine the clock weights and the Allan deviation confidence limits.

• plot of (UTC - GPS Time) from BIPM's Circular T
  This plot shows daily deviations of GPS Time from UTC as published in the BIPM's Circular T. Note that IGS clock products are aligned to GPS Time by linear, daily fits. Hence the stability of the IGS products on time scales of about a day and longer is limited by the instability of GPS Time itself.
• Discontinuities in clock estimates at day boundaries
  • Multipath and discontinuities at day boundaries
  • Use to assess accuracy of clock estimates
• Use of predicted GPS orbits
• Prediction of satellite clocks for real-time operations
  • USNO Ultra-rapid clock predictions
  • GFZ Ultra-rapid clock predictions
  • Status of IGS Ultra-rapid clock predictions

GPS Satellite (P1-C1) Biases

• Effects of mixed receiver types -- background information
• Change in IGS (P1-C1) bias convention, 02 April 2000
• Change in source of IGS (P1-C1) values, 14 January 2001
• New class of receiver types recognized, 13 February 2002
• Stefan Schaer's ionosphere web site
  The CODE Analysis Center estimates directly the (P1-C1) biases from a mixed network of cross-correlation style receivers and modern receivers. A set of bias values is posted derived from the moving 30-day average of daily estimates. In addition, plots of the bias values and an archive of past estimates are maintained.
• Current and archived IGS (P1-C1) bias values
• Converter utility (cc2noncc.f)
  For analysts to use to transform C1 and P2' observables from AOA TurboRogue, AOA ICS-4000Z, and Trimble 4000 RINEX files to be compatible with modern non-cross-correlation style receivers. Also transforms C1 from TRIMBLE 5700, LEICA CRS1000, and LEICA SR9600 receivers to be compatible with P1.

GPS Receiver System Considerations

• Evaluation of timing performance of a Javad Legacy receiver
• Evaluation of timing performance of a Javad EuroCard receiver
• "Proposal to use geodetic-type receivers for time transfer using the CGGTTS format"
  Pascale Defraigne (ORB) and Gérard Petit (BIPM) describe a procedure to form standard CGGTTS-format common-view observation files from RINEX data obtained from geodetic GPS receivers, with the goal of introducing time links using calibrated geodetic GPS receivers into the TAI calculation.

GPS Antenna Considerations

• Temperature sensitivity of Dorne Margolin antennas
• Antenna phase pattern measurements for the 3S-Navigation TSA-100 antenna

Calibration of Geodetic Receiver Systems

• Colocated common-view of satellite clocks
• Differential calibration of Ashtech Z12-T receivers (MS/Word)
  Gérard Petit (BIPM) and colleagues have carried out the differential calibration of two Ashtech Z12-T receivers by comparing their raw pseudorange measurements with those of classical NBS-type time receivers operated side by side. They then performed a time comparison between the BNM-LPTF and the BIPM using both classical time receivers and the calibrated Ashtech Z12-T receivers, and compared the results. Their preprint, for the EFTF'2000 meeting, is prepared as a MS/Word document.
• Absolute calibration of an Ashtech Z12-T receiver (MS/Word)
  Gérard Petit (BIPM) and colleagues have also performed an absolute calibration of an Ashtech Z12-T receiver using facilities at the U.S. Naval Research Laboratory. Their preprint, for a special issue of GPS Solutions, is prepared as a MS/Word document.
• Progress in calibration of geodetic receivers (MS/Word)
  At the EFTF'2001 meeting, Gérard Petit (BIPM) and colleagues described plans and procedures for differentially calibrating Ashtech Z12-T type receivers against a BIPM unit previously calibrated absolutely and differentially. Their preprint is prepared as a MS/Word document.
• International campaign to calibrate geodetic receivers
  BIPM has initiated an international campaign to calibrate geodetic-type GPS receivers of the model Ashtech Z12-T for time transfer. Laboratories owning Z12-T receivers and interested in time transfer are invited to participate by contacting Gérard Petit (BIPM).

Time Transfer Comparisons

• TWSTFT Reports -- from J. Azoubib & W. Lewandowski (BIPM)
  The BIPM two-way satellite time & frequency tranfer monthly reports are collected here. All documents are in MS/Word format (unix compressed) or, more recently, in pdf format. Also here are reports of the meetings of the CCTF Working Group on TWSTFT:
  • 7th Meeting -- held at USNO in Washington, DC, on 13-14 December 1999
  • 8th Meeting -- held at BIPM on 05-06 October 2000
  • 9th Meeting -- held at CSAO in Xi'an, China, on 05-06 October 2001
• Time Transfer Reports -- from K. Senior (USNO)
  Monthly comparisons between two-way satellite, GPS common-view, and GPS carrier-phase time transfer results are reported for a set of stations equipped with all three techniques. For further information, please refer to http://clockdev.usno.navy.mil/TTR. All documents are in pdf format.
• Time Transfer Operations -- Information logging
  Ken Senior (USNO) has implemented an e-mail server and an operations logging tool, which is accessible at http://clockdev.usno.navy.mil/site/logtool.htm. This includes a mailing list which serves as a discussion forum for standardizing and centralizing the logging information on time transfer systems.

Web Sites & Reports of Related Work by Participating Groups

• K. Larson (University of Colorado) -- Precise Time Transfer
• U.S. Naval Observatory -- GPS Carrier Phase
• U.S. Naval Observatory -- Clock Estimates
• GeTT analysis results (ftp directory -- see ``readme'' file)
• Royal Observatory of Belgium -- Time Section
• Report from State Research Institute of Metrology, Kharkov, Ukraine
  Alexander Tkachuk (Head of the Time & Frequency Department at the State Research Institute of Metrology in Kharkov, Ukraine) has submitted a report on their investigations during 1998-1999 into the use of a Trimble 4000SSi dual-frequency geodetic receiver for time transfer. The report is prepared as a MS/Word document.
• (please send URLs to include here)

(content last updated 15 April 2002)

(archived 22 October 2003)