Subject: [IGSMAIL-3057]: IGS Analysis Workshop summary
From: Jim Ray (USNO 202-762-1444) <[email-masked]>
To: [email-masked]
Cc: [email-masked]
Date: Fri, 13 Oct 2000 8:23:03 EDT

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IGS Electronic Mail      13 Oct 05:23:05 PDT 2000      Message Number 3057
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Author: Jim Ray & Tim Springer

The IGS Analysis Center Workshop 2000 took place during 25-29 September at
the U.S. Naval Observatory.  This was an excellent occasion for many
interesting presentations and very fruitful discussions.  All the presenters,
their colleagues, the session conveners, and the local organizing committee
(the USNO Earth Orientation Dept.) are gratefully thanked for their efforts.

Three broad themes dominated the meeting: the IGS/BIPM Timing Pilot Project
formed a sub-workshop for the first two days; the IGS near real-time products
and their applications were the focus of the following day; potential
interactions between the IGS and various GNSS systems (GPS, Galileo, GLONASS)
were considered on another day.  The final half-day saw a number of intriguing
reports on innovations and new developments from the Analysis Centers (ACs).
A subset of the papers presented will be published as a special issue of the
journal GPS SOLUTIONS.

Theme 1.  IGS/BIPM Timing Pilot Project
-----------------------------------------------------------------------------
This two-day meeting had four segments related to the Pilot Project plan.
Jim Ray (USNO) gave an overview of data analysis issues, presenting evidence
showing an observed accuracy for GPS-based clock estimates which is larger
than the formal errors (~125 ps) by roughly a factor of 4.  There are large
variations among sites and a few are much worse, but the main error sources
(code multipath, receiver temperature variations, etc) remain uncertain.
Evidence indicates that longer analysis arcs give improved clock accuracy
due to greater code averaging.  Simulations by Rolf Dach (AIUB) support this
but also illustrate some possible dangers of accumulated systematic errors
if long-term continuity is enforced by overlaps between arcs.  Not yet
studied are alternate filtering approaches of short-arc results.  The short-
term precision of clock estimates is even less clear, though long baseline
results generally fail to find stabilities better than about 2 * 10^-15
averaged over 1 day, roughly consistent with the formal errors.  Results
from different analyses also agree at about this same level.

Tim Springer (AIUB) described the new methods he has implemented to combine
satellite and receiver clock estimates from the IGS ACs.  These have been
running in a demonstration mode since the beginning of this year, with 
comparison results published in the regular Final and Rapid combination
reports.  The new "clock RINEX" format is used for the exchange of estimates
and for the distribution of results.  The new clock combination adds
considerable robustness and outlier detection, which requires far less manual
intervention, as well as the new receiver clock products.  It was proposed
that these become official on 5 November 2000 (start of GPS week 1087).

Unfortunately, the usefulness of the current IGS clock products is limited
by the underlying time scale, which is based on a linear alignment to
broadcast GPS time for each day separately.  Large day-to-day discontinuities
in time and frequency are seen.  Ken Senior (USNO) presented a Kalman filter
method of generating an internal time scale from an integration of combined
IGS frequency standards.  He proposed to implement this approach within the
IGS clock combination by the end of this year.  The longer-term steering of
such a time scale remains unclear until calibrated links to UTC become
available in near real-time.

Jim Zumberge (JPL) and Ray discussed the prospects for predicted satellite
clocks in the post-SA era.  Over 24 hour spans, the IGS already does about as
well as the broadcast GPS clocks (~10 ns RMS).  With more frequent updates,
such as the 12-hour interval of the IGS Ultra-rapid products, the IGS could
perform better raising the possibility of providing global access to time at
the ~2 ns level.  The instability of GPS time (to which IGS clock products
are currently aligned) would then be the major error source, although the
older satellite clocks also cause problems.

Felicitas Arias (BIPM), representing Gerard Petit (BIPM), described the IGS
products already being used to improve intercontinental time transfers for
UTC/TAI.  These chiefly involve ionosphere maps to correct single-frequency
common view receiver data and precise GPS orbits.  She also reported work in
progress at the BIPM Time Section to automate the process of computing
UTC/TAI, which will facilitate moves to a possible real-time realization of
UTC.  Petit has already started studying this subject.  The BIPM expects to
begin a pilot experiment in the near future.

An introduction to the important subject of instrumental calibration, prepared
by Petit, was presented by Pascale Defraigne (Royal Obs. Belgium), followed by
a report from Joe White (NRL) on their laboratory work.  An end-to-end
calibration has been made for an Ashtech Z12-T receiver and antenna, as well
as for several components.  Some issues remain to be resolved, but prospects
seem promising for an absolute calibration at the ns level, at least as good
as for current single-frequency receivers.  Zhieng Jiang (BIPM) described the
corresponding differential calibration approach comparing the Z12-T to a
calibrated NBS timing receiver.  The consistency of these two independent
methods remains to be established.  Ed Powers (USNO) reported work underway
to modify AOA receivers to accept external 1 pps synchronization, similar to
the Z12-T.

The effects of inter-code satellite biases were well covered by Yang Gao
(U Calgary) and Stefan Schaer (AIUB).  Differences between the C/A (C1) and
P1 modulations are up to +/- 1.7 ns depending on the satellite.  Gao showed
that there also exist smaller receiver-dependent differences, the cause of
which is not understood.  The CODE AC estimates the P1-C1 differences daily,
as well as P1-P2 biases needed to interpret ionospheric results, and posts
the results publicly.  Later in the week, Mike Heflin (JPL) presented
analyses on behalf of David Jefferson (JPL) demonstrating the improvements
obtained using the current IGS P1-C1 bias convention (see IGS Mail #2744).

For time transfer, the relationship between an external reference and the
internal receiver circuitry must be accurately known.  Jon Clarke (NPL)
described their evaluations of the Javad Legacy receiver, finding it to be
an intriguing possibility for the future but needing further study.  Jiang
related similar experiences with the Z12-T in Paris.

The effects of environmental changes, especially temperature, on frequency
stability have been well documented by Defraigne and her colleagues in
Brussels, as well as by the BIPM group.  Updates of these results were
presented.  The most sensitive components seem to be cables (for which very
stable types are available) and the receivers.  For timing applications,
strict control of the receiver environments is essential.  The effects of
other influences (e.g., magnetic fields and humidity) have not been studied.

Thomas Schildknecht (AIUB) offerred several proposals to facilitate the
exchange of clock information between groups and techniques; e.g., by 
standardizing the logging of station configuration data.  In addition to aiding
comparison studies, this should simplify and improve the computation of TAI
in the future.  Reviews of the ongoing comparison campaigns between common
view, two-way satellite, and geodetic time transfer methods were presented by
Clarke (for his NPL colleagues) and by Lisa Nelson (NIST).  Interpretation of
the geodetic time transfer results is complicated by the inevitable changes
that have occurred at the stations.

The final timing session dealt with station installation issues.  Jan
Johansson (Onsala Obs.) gave an overview of geodetic aspects, including
monumentation, antenna mounts, and radomes; Clarke covered the hardware
aspects, with an emphasis on the concerns for timing.  In some ways, the two
perspectives are distinct, but they can mesh harmoniously.  Examples from
the Brussels, USNO, and Haystack Obs. groups were offered.  Defraigne has
developed software to create BIPM common view schedules from raw 1-second
Ashtech Z12-T observation files, which will simplify operations at timing labs
by eliminating the need for separate timing receivers.  Rick Hambly (CNS
Systems) showed the impressive results he and Tom Clark (NASA) have obtained,
now that SA is turned off, using a very inexpensive single-frequency timing
receiver.

The recommendations for the IGS/BIPM Timing Pilot Project are listed in the
Appendix below.  Two notable milestones were set during a small meeting held
in the USNO Library in the evening of 25 September:  1) The IGS will implement,
at least in a test mode, the new internal time/frequency scale being developed
at USNO by late 2000.  2) Prof. Leschiutta (IEN) will arrange for a special
session on "Calibration problems" during the 15th European Frequency and Time
Forum (EFTF), to be held in Neuchatel, Switzerland, during 6-8 March 2001.
Timing laboratories will be invited to present calibration results (absolute
or differential) for all available Ashtech Z12-T receivers; results for any
other receiver types are also welcomed.  Progress in these two areas will
provide the basis for an evaluation of the geodetic technique at the next
meeting of BIPM's Consultative Committee on Timing and Frequency, to be held
sometime in 2001.

Theme 2.  IGS Near Real-Time Products & their Applications
-----------------------------------------------------------------------------
As recognized at previous IGS Analysis Center Workshops, a growing community
of users seeks orbit products suited for high-accuracy real-time applications.
The Ultra-rapid service was initiated at the 1999 workshop at SIO and began
in a demonstration mode earlier this year.  These products are issued with
only 3 hours latency (initially) and include 24 hours of observations together
with 24 hours of predictions.  They are updated twice daily so that the mean
latency of the predictions is only 9 hours, compared to the 36 hours of the
IGS "classic" Predicted orbits.  Peng Fang (SIO) presented a position paper
reviewing developments during the past year and demonstrating the major
progress that has been achieved.  The relevance of these efforts for the
various low-Earth orbiter (LEO) missions was addressed by Tony Mannucci (JPL).
Future IGS participation in producing Ultra-rapid orbits and clocks for LEOs
was considered, particularly for GPS occultation missions that will provide
weather data.

It was agreed that the Ultra-rapid (IGU) products should be made official
beginning on 5 November 2000, and that the older Predicted (IGP) products
could be discontinued soon afterwards, with sufficient notice to users.  While
great strides have been made in the network coverage of hourly RINEX stations
and in data delivery, further improvements and greater redundancy are needed.
There is also clear room for improving the analysis quality given the much
better performance of the GFZ AC compared to the others.  For some ACs, their
IGP predictions are better than those for the IGU despite the much longer IGP
prediction span.  Gerd Gendt (GFZ) briefly outlined the methodology his group
uses, which spawned considerable discussion publicly and privately.  The ideas
exchanged will certainly lead to better IGU analysis products from all the ACs.

A series of papers from perhaps the most demanding real-time user community
followed.  Stan Benjamin and Seth Gutman (both at NOAA/FSL) provided very
comprehensive views of the data assimilation process used for NCEP's short-
range weather forecasting and the potential impact of GPS-based tropospheric
sounding data.  Results from a ground-based demonstration network in the
mid-U.S. were shown.  The even more extensive GPS met project in Japan (more
than 1000 stations nationwide) was described by Yuki Hatanaka (GSI).  These
results were complemented with several highly informative posters.  It appears
that to have a significant impact in weather prediction, GPS-based zenith
troposphere delay estimates must have biases that do not exceed 1 mm and noise
errors no more than 5 mm RMS.  Eric Calais (CNRS) showed how "on-the-fly"
integrity checking could be integrated into the data analysis needed to
extract tropospheric delay estimates.  This improves the reliability, which
would otherwise suffer from orbit prediction errors caused by the older
satellites that undergo frequent attitude manuevers.

The Ultra-rapid processing performed at ESOC (Igancio Romero) and at JPL
(Tomas Martin-Mur) were also described.  Brian Luzum (USNO) showed the direct
relationship between Earth orientation predictions and net rotational errors
in the IGS orbit predictions (IGP and IGU).  Continuing improvements in the
IERS Bulletin A EOP predictions have reduced real-time polar motion errors
below 0.7 mas (~9 cm at GPS altitude) and UT1 errors below 0.12 ms (~23 cm).
The observed half of the Ultra-rapid products offers the potential of twice-
daily Bulletin A updates to further reduce latency, although the quality of
the IGU EOP values is poorer than the current IGS Rapid products by a factor
of ~2.

Theme 3.  Interactions Between the IGS & GNSS Systems (Galileo, GPS, GLONASS)
-----------------------------------------------------------------------------
Updates on the GalileoSat project of ESA (John Dow), and technical
presentations on the design of the constellation (Miguel Romay-Merino, GMV)
and of the future broadcast ephemerides and clock generation (Alvaro Mozo-
Garcia, GMV) opened the session.  Significantly, the Galileo orbits will be
somewhat higher than GPS to avoid 12-hour resonances and eliminate the need
for station-keeping manuevers.  A discussion followed on the position paper
recommendations, presented by Pascal Willis (IGN & CNES).  It was agreed to
initiate an early involvment of IGS in Galileo, to be fostered by creating a
new IGS Working Group.  Willis will propose a charter at the next IGS
Governing Board meeting in December, including broad international membership
and an official participation in the future Galileo Users Needs Definition
Forums.  The scope of this group will also include closer interactions with
GPS operations and GPS-III definition, as well as the Galileo project and
related overlay systems.  Furthermore, a specifications document will be
drafted as a general guide for future GNSS system architecture decisions,
stressing the requirements of the high-end user community represented by the
IGS.  Finally, possible field campaigns of observations, using EGNOS and WAAS
in a first step and for Galileo in a second step, will also be investigated.
This IGS Working Group will have a close relationship with the IAG/CSTG sub-
commission on "Precise Microwave Satellite Systems."

The session on closer interactions with GPS operations began with two brief
remarks.  Henry Fliegel (Aerospace Corp) noted work being done together with
USNO astronomers in Flagstaff, Arizona to monitor photometric properties of
the GPS satellites.  These show some deficiencies in the existing solar force
models and indicate that aging effects are significant.  Fliegel proposed
that such observations could be combined with the very precise empirical
formulations used by the IGS ACs to produce a new generation of physical force
models.  Arthur Dorsey (Lockheed-Martin) pointed out the accuracy limitations
of the current GPS navigation message, which could not fully convey the
quality of the IGS Ultra-rapid orbit products, for example.  The subsequent
discussions were a spirited and very useful exchange between IGS "insiders"
and several representatives with ties to GPS operations, contractors, and
other governmental agencies.  Suggestions included active overtures by the
IGS (coordinated by the Central Bureau) to the Interagency GPS Executive
Board (IGEB), the Joint Program Office (JPO), DOD's high-level Independent
Review Team (IRT), NIMA, and GPS contractors.  Yoaz Bar-Sever (JPL) urged that
the IGS offer itself as the recognized conduit between the GPS system and the
high-end user community.  A briefing to the IGEB, JPO, and/or IRT should be
made to present such a proposal.  Larry Hothem (USGS) offerred to include a
joint JPO-IGS session at the ION GPS-2001 meeting to be held in Salt Lake City
during 11-14 September 2001.  It was agreed that the IGS technical interface
and requirements discussions should be led by the new Working Group proposed
by Willis (see above).

The final GNSS operations session reviewed the status of the IGS's
International GLONASS Service (IGLOS) Pilot Project.  Jim Slater (NIMA) gave
an overview of the project since the IGEX Workshop held in September 1999.
The progress of the analysis groups (now 3 to 4 with varying delays) and the
orbit combination effort were summarized by Robert Weber (TU Wien),
complemented with analysis reports from ESOC (Ignacio Romero) and BKG (Heinz
Habrich).  Results have reached the decimeter level, despite very large
geographical gaps in the network coverage, by relying on multi-day arcs.  In
some ways, the GLONASS orbits appear easier to model than GPS satellites.
New tracking stations are expected soon, which will improve the global
coverage.  The greatest uncertainty remains the fate of the constellation.
Rumors of replenishment launches abound.  These are badly needed to sustain
the constellation; most of the current 7 already exceed their expected
lifetimes.  Sufficient user interest remains to continue this project
provided that the constellation remains viable (at least 6 satellites).

Theme 4.  Innovations & Other Topics
-----------------------------------------------------------------------------
Gerry Mader (NOAA/NGS) and Markus Rothacher (TU Munich) both presented new
results on absolute antenna phase measurements from the Institut fuer
Erdmessung (U Hannover) and Geo++ collaboration (see IGS Mail #2880).  The new
results rely on an ingenious robotic device that rotates a test antenna while
maintaining the position of its center within 1 mm.  Mader showed that the
absolute calibrations are consistent with the IGS set of differential phase
patterns (adopting the Dorne Margolin T chokering antenna as a reference),
although a couple of antenna models need verification.  Rothacher compared the
new data to previous anechoic chamber measurements of absolute patterns and
also found generally good agreement.  The consensus is that these latest
results definitively establish the absolute phase patterns for most geodetic
antennas.  The difficulty then arises that use of these patterns in GPS data
analyses causes the terrestrial reference frame to shift scale by 15 ppb
(a change of nearly 10 cm in station heights globally).  This is implausible
in view of the very accurate and consistent frames determined by SLR and VLBI.
Rothacher argued that the GPS scale can be reconciled with SLR and VLBI if
non-uniform phase patterns for the satellite transmitter arrays are considered
together with the highly uncertain offsets between the transmitter phase
centers and the spacecraft centers-of-mass.  It was noted that the new
International VLBI Service (IVS) has formed a joint working group with the IGS
and the International Laser Ranging Service (ILRS) to explore the use of
interferometric techniques to image the GPS L-band emissions.  This is a
challenging technical undertaking which will probably take some time to 
evaluate.  In the meantime, Mader suggested pressing the DOD to make range
measurements using Block IIR satellites in storage.  Rothacher felt that the
IGS should begin to prepare to eventually switch from relative to absolute
phase patterns, which would include analysis software changes to handle
satellite patterns as well as file format mods to add azimuthal variations to
the existing elevation-dependent information.

Urs Hugentobler previewed work underway in the AIUB group to develop a new
orbit analysis package that is equally adept at handling satellites of a wide
variety of types and altitudes.  Tom Johnson (USNO) showed that the nodal
rotations of the GPS orbits from the IGS can be used to detect low-order
variations in the Earth's gravity field associated with motions of geophysical
fluids, analogous to similar studies for LAGEOS I and II.  Mike Heflin (JPL)
assessed his group's GPS contribution for ITRF2000, which is currently in
preparation.  Their empirical error models for GPS site velocities are:
(2.91/T), (4.28/T), and (8.58/T) mm/yr for uncertainties in the north, east,
and vertical components, respectively, where T is the observation span in
years.  Remi Ferland (GSD/NRCanada) presented similar results for the IGS
combined contribution to ITRF.  In addition, he showed some preliminary work
on new projects to incorporate regional densification networks into the IGS
SINEX combination, and to reanalyze and consistently combine older SINEX
results prior to the official start of the current combination.  Both these
efforts will significantly enhance the value of the IGS combined terrestrial
frame.  Finally, Mike Craymer (GSD/NRCanada) gave a progress report on the
IAG Commission X effort to revitalize the Subcommission for North American
and form a NAREF regional reference frame, similar to the very successful
EUREF.  It is hoped that the NAREF working group will become an active IGS
Regional Network Associate Analysis Center in the near future.

At the end of the discussions on 28 September, Tim Springer reviewed the
recommendations from the 1999 Workshop at SIO (see IGS Mail #2359).  Rather
remarkably, most of the recommendations have been largely fulfilled or are
ongoing.  The most important actions have received the most attention, such
as starting the Ultra-rapid products, reconciling standardized site names and
related SINEX information, and developing combined satellite and receiver
clock products.  These achievements attest to the hard work and cooperative
spirit of every component of the IGS, and should be a source of pride for
all of us.

Appendix.  Recommendations & Actions 
-----------------------------------------------------------------------------

IGS/BIPM Timing Pilot Project -- Network
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TN1. Integration with IGS network.  All timing labs are encouraged to install
  suitable dual-frequency geodetic receivers and integrate these into the
  IGS network.  The continued support of the IGS Central Bureau is urged, of
  offering advice, suggesting experts to contact, and displaying example
  sites and monument on the IGS website.

TN2.  Hardware configuration recommendations.  Participants are encouraged to:
  * investigate the suitability of new hardware as it becomes available;
  * investigate and report the performance of the Javad Legacy receivers;
  * ensure that actions needed for IGS use of the TSA-100 antenna are
    completed;
  * investigate the possibility to temperature-stabilize cables and receivers
    (especially);
  * continue work to improve continuity for timing receivers across power
    cycles;
  * support the development of software for IGS-compliant receivers to output
    the BIPM common view format.

TN3.  Geodetic control.
  * Participants are encouraged to monitor the stability of the positions for
    new timing lab stations to ensure that these are adequate for use by the
    geodetic community.
  * Timing labs are encouraged to establish geodetic control points in order
    to monitor the stability of the site and provide a reference when
    equipment changes occur at the site.
  * The Central Bureau is asked to identify experts in the problem of rooftop
    mounts so that their experience may be shared.

TN4.  Environmental stability control.  Participants are encouraged to:
  * investigate optimal trade-offs to mitigate temperature effects;
  * develop and investigate new solutions to mitigate temperature effects;
  * investigate alternative cable types for temperature coefficients and 
    suitability;
  * log additional environmental data, such as humidity and EM-fields;
  * investigate environmental effects other than temperature to ensure
    that their significance has not been overlooked;
  * investigate further the effects of temperature on splitters.

TN5.  Multipath mitigation.  Timing labs are encouraged to assess the
  significance of the multipath effects at their stations in order to
  establish whether mitigation is a productive strategy.  The UNAVCO toolbox
  "teqc" can be useful in doing so.

TN6.  Operational Data Center for time labs.  To ease the operational burdens
  on individual timing labs, one or more Operational Data Centers for the
  geodetic receivers at timing laboratories should be considered.  Such an
  Operational Center could serve as a helpful interface between the IGS and
  the timing labs.

IGS/BIPM Timing Pilot Project -- Analysis
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TA1.  Publish clock accuracy and precision report.  A group report should be
  prepared and published (via the BIPM) to document the current state of
  understanding of the accuracy of geodetic clock estimates, their precision,
  and the dominant error sources.  Analysis methods to minimize the effects
  of systematic and random errors should be identified.  It is suggested that
  this report be prepared by C. Bruyninx, R. Dach, J. Kouba, K. Larson,
  G. Petit, J. Ray, Th. Schildknecht, K. Senior, T. Springer, and others.

TA2.  Official adoption of new IGS clock products.  The new clock
  combination scheme should become official, replacing the old combination
  scheme for the IGS Rapid and Final products, on 5 November 2000.  This
  method should also be implemented in the IGS Ultra-rapid combination as
  soon as feasible.

TA3. IGS time scale.  The IGS should implement an algorithm (being developed
  by K. Senior) to synthesize an internal time scale for its clock products
  to be less reliant on GPS time, which currently limits stability at
  intervals of about a day and longer.  This should be done before January
  2001, at least in a test mode.  Ideally, the long-term steering should be
  to a real-time realization of UTC, when this can be achieved, which requires
  calibrated IGS receivers at timing laboratories.

TA4.  P1/C1 biases.  All data analysts who process undifferenced pseudorange
  data are urged to adopt the IGS bias conventions and values (see IGS Mail
  #2744).  The JPL AC is recognized as the source of the IGS official set of
  P1-C1 bias values, which must be updated occasionally as the satellite
  constellation evolves.  Since there is good evidence that different
  receiver models respond to the satellite-dependent P1-C1 biases in slightly
  different ways, further research is encouraged to clarify these receiver-
  dependent effects and to devise methods to minimize their impact on
  geodetic clock estimates.

TA5.  Real-time UTC realization.  The BIPM is encouraged to pursue
  investigations into the possibilities for providing a predicted form of
  UTC.  The time labs participating in TAI and equipped with good clocks are
  asked to cooperate in this effort.  A short-term pilot experiment might be
  envisioned once the algorithm and process of calculation have been fixed
  and tested using old data.

IGS/BIPM Timing Pilot Project -- Calibration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TC1.  Receiver calibrations.  Groups are strongly encouraged to develop
  practical absolute and differential hardware calibration methods suitable
  for geodetic GPS receiver systems.  These are needed most urgently for
  deployments at timing laboratories and other facilities equipped with
  stable frequency standards.  The methods used for the calibration should be
  well documented.  A special session will be convened at the 2001 European
  Time and Frequency Forum (EFTF) to present calibration results, ideally for
  all available Ashtech Z12-T receivers (at least).  BIPM will coordinate
  activities in preparation for this meeting.

TC2.  Receiver manufacture.  Receiver manufacturers are urged to implement
  internal clock circuits which are not subject to discontinuities (e.g., due
  to power interruptions, etc), to improve temperature-dependent stability,
  and to enhance the general usefulness for timing applications.  The IGS and
  BIPM will develop suitable specifications by January 2001 that can be used
  by manufacturers and others to better understand timing requirements.  
  J. Clarke and Th. Schildknecht will lead this effort.

IGS/BIPM Timing Pilot Project -- Intercomparisons
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TI1.  IGS/BIPM mailing list.  The current IGS/BIPM e-mail exploder will be
  reestablished as an automated mail server at the IGS Central Bureau, with a
  mirror at BIPM.

TI2.  Exchange of timing information.  An ad hoc working group should identify
  the most appropriate format and route for exchanging the information needed
  for comparing the clocks at different stations, and inter-comparing
  different time transfer techniques.  This will include: reporting offset
  discontinuities, calibration values, hardware configurations, ties to
  UTC(lab), analysis results, and related quantities.  Th. Schildknecht and
  J. Clarke will coordinate the formation of such a working group with the
  BIPM and representatives of the other time transfer techniques.

IGS Near Real-Time Products & their Applications
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
U1.  Improve the hourly network coverage and reliability.  All ACs recognize
  and appreciate the great efforts made by many agencies, especially the IGS
  Central Bureau, the Operations Centers, and CDDIS/IGN, in expanding the
  network and providing the data service.  Yet some extra effort is still
  needed to improve global coverage and reliability.  Even though data
  interruptions are infrequent, they severely limit the AC's ability to
  generate timely high-quality products.  Improved backup services are needed
  for better redundancy and to avoid single points of failure.

U2.  Reduce the quality gap of the Ultra-rapid orbits produced by individual
  ACs.  Currently, the accuracy of Ultra-rapid contributions from individual
  ACs differs by a factor of ~2 between the best performing AC (GFZ) and
  others.  The ACs other than GFZ are strongly encouraged to make special
  efforts to improve their contributions.  GFZ is asked to provide advice and
  share their experiences with other ACs.

U3.  Declare the current experimental Ultra-rapid (IGU) products as official
  IGS products starting 5 November 2000 (GPS week 1087).

U4.  Terminate the current Predicted (IGP), i.e. the classical 48-hour
  prediction products, after the IGU becomes official and after sufficient
  notice to users.

U5.  Provide IGU clock predictions.  All ACs are encouraged to develop clock
  prediction strategies based on extrapolations of observed clock states and
  to include these in their IGU submissions.  The IGS Central Bureau is asked
  to assess user requirements and to evaluate the utility of ns-level clock
  predictions for various real-time user segments.  The possibility of adding
  satellite clock accuracy codes, similar to the existing orbit accuracy
  codes, should be investigated.

U6.  Find a solution to put accuracy codes for IGU clocks and orbits separated
  into estimated and predicted parts.

U7.  Compile detailed descriptions of the data analysis strategies used by the
  ACs for their Ultra-rapid submissions.

Interactions Between the IGS & GNSS Systems (Galileo, GPS, GLONASS)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
G1.  Creation of a new IGS Working Group on GNSS Systems will be proposed at
  the December meeting of the IGS Governing Board by the proposed chairman
  (P. Willis).  The WG will have broad international membership, with a
  mandate to include an official participation in the future Galileo Users
  Needs Definition Forums.  The scope will also include closer interactions
  with GPS operations and GPS-III definition, as well as the Galileo project
  and related overlay systems.  Furthermore, a specifications document will
  be drafted as a general guide for future GNSS system architecture decisions,
  stressing the requirements of the high-end user community represented by the
  IGS.  Finally, possible field campaigns of observations, using EGNOS and
  WAAS in a first step and for Galileo in a second step, will also be
  investigated.  This IGS Working Group will have a close relationship with
  the IAG/CSTG sub-commission on "Precise Microwave Satellite Systems".

G2.  Continue the International GLONASS Service (IGLOS) Pilot Project.  Due to
  significant ongoing interest by IGEX and IGLOS participants, as well as the
  potential and demonstrated contributions that GLONASS can make in a number
  of areas, the IGLOS PP should be continued as long as at least six
  satellites are available.  GLONASS observations should be integrated into
  routine IGS operations.  Specifically:  1) All dual-frequency GPS/GLONASS
  receivers should be formally incorporated into the IGS network.  2) Site
  logs, and observation and auxillary files for GLONASS should be integrated
  with IGS information at the IGS Central Bureau and the Data Centers.  3)
  Antenna phase centers need to be determined for combined GPS/GLONASS
  antennas.  4) ACs should process GLONASS and GPS data together, if possible.