IGS Workshop 2002 Summary Recommendations


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[IGSMAIL-3845]: Summary of 2002 IGS Workshop

IGS Electronic Mail      25 Apr 12:53:11 PDT 2002      Message Number 3845

Author: NRCan

Summary of 2002 IGS Network, Data and Analysis Center Workshop, Ottawa,
April  8-11, 2002.

The workshop, re-scheduled from October 2001, was held at the Courtyard
Marriott hotel in the heart of Ottawa's popular Byward market area. The
3.5 day workshop regrouped more than 100 participants and was organized
around the central theme "Towards Real Time".  The technical program
included the following eleven sessions:

1. Opening
2. Real-time applications and products
3. Real-time data/products exchange
4. Data center issues
5. Network issues
6. Poster session
7. Reference frame
8. Receiver and satellite antenna calibrations
9. Ground based GPS ionospheric estimation
10. Low Earth Orbiters (LEO)
11. Review of IGS Products

Altogether nine position papers corresponding to the above sessions were
presented and all but one were available at the workshop www site
(http://www2.geod.nrcan.gc.ca/~pierre/igs_workshop.html) prior to the
start of the workshop. The following contains brief summaries of each
session, followed by a draft of workshop recommendations, which were
compiled at the end of the workshop by the Network and Analysis Center
Coordinators, A. Moore and R. Weber, along with the session chairs.

1. Opening Session

The workshop participants were welcomed by the Assistant Deputy Minister
(ADM), Dr. Irwin Itzkovitch, of the Department of Natural Resources
Canada (NRCan) who outlined Canada's approach to the national reference
frame delivery and maintenance, which are inherently based on NRCan's
Geodetic Survey Division (GSD) GPS and VLBI programs.  NRCan's
participation to IGS is viewed as crucial and mutually beneficial to
both parties. After the ADM's introduction, there was a short welcoming
address by Norman Beck, Chief of the Active Control Section. Professor
Gerhard Beutler, the first chair of the IGS Governing Board (GB) and
currently the first vice president of IAG, followed with a brief
presentation reminding the workshop participants that it was in Ottawa,
almost nine years ago, that many IGS initiatives and products were
initiated.  However, his main message was about the role the IGS and
other IAG services will play within a new, reorganized, and increasingly
multidisciplinary IAG and its IGGOS Pilot Project. He reminded us, in
his usual witty way, that the workshop theme "Towards Real-Time" implied
that there must also be some unreal-times. Professor Christoph Reigber,
the current chair of the IGS GB, addressed the workshop on behalf of the
board and summarized the recent strategic planning (SP) exercise. He
confirmed that real time IGS data and products were likely to play an
increasingly stronger role to satisfy various and important projects,
like LEO missions, atmospheric monitoring, etc. The enhanced role of IGS
Central Bureau (CB) was also envisioned in the IGS SP, which is
currently available from IGS CB. The SP report was, for the first time,
distributed during the workshop.

Following the opening remarks, three working group (WG) and pilot
project (PP) status reports were presented since they were not assigned
individual sessions during the workshop; namely the Tropospheric WG,
Precise Time Transfer PP and the International GLONASS (IGLOS) PP. The
Tropospheric WG, chaired by G. Gendt of GeoForschungsZentrum (GFZ), has
been producing official IGS products of combined tropospheric zenith
path delays for several years, but is now concentrating on near
real-time (NRT) tropospheric products in conjunction with accelerated
IGU orbit production and delivery.  J. Ray of the United States Naval
Observatory (USNO) who co-chairs the Precise Time Transfer PP, presented
a status report (posted on the workshop www site). The objective of
realizing an IGS time scale and aligning the IGS clock products to UTC
has been reached and other goals such as timing receiver calibration and
increased participation of timing labs in PP are progressing well.  The
plan calls for formal adoption of the IGS time scale and the
correspondingly aligned IGS combined clock products by the end of 2002.
The new improved alignment has helped to identify daily clock
discontinuities at some stations, which represents a valuable and
efficient quality check and provides an impetus for further
investigation of some station installations. Finally, J. Slater of NIMA
gave the status of the IGLOS PP.  Currently, there are only 6-7
operational GLONASS satellites, and only 3 Analysis Centres (AC), are
routinely computing the satellite orbits, which are combined by the AC
Coordinator (Robert Weber) with a delay of one or two months. This is
hardly acceptable, more GLONASS satellites, ACs and faster GLONASS
combinations are needed. Ultimately, IGLOS should be integrated into the
routine IGS operation and products.

2. Real-Time Applications and Products

Tropospheric and LEO applications have the most demanding requirements
for IGS data and products within 1 to 3 hours.  Initially, the session
position paper called for IGS Ultra rapid orbit solutions with delays of
no more then 1 hour. However, from the discussions following the
position paper, it became clear that the proposed 1 hour delay was
unrealistic, given the status of the current data delivery schedule as
well as AC workload. ACs quickly compromised on a 3 hour-delay.  A
presentation on Ultra rapid processing given by N. Romero of ESA
initiated discussion on the omission, by most ACs, of a relatively high
number of satellites from the ultra rapid solution. This goes directly
against the standing IGS policy adopted in 1998 specifying that the IGS
orbit products should be as complete as possible and include even
marginal satellites with appropriate accuracy codes. R. Muellerschoen of
JPL gave an impressive presentation on the JPL real time orbit/clock
solutions supporting NASA's wide area  (worldwide) DGPS. The JPL/NASA
system is designed to support all NASA navigation needs at or below 10
cm. The NASA system can also be utilized for sub ns time transfers,
provided that calibrated receivers are utilized as outlined in a
presentation by E. Powers of USNO.  Two European presentations on NRT
tropospheric solutions, relying on IGU products, within the scope of the
COST716 project (http://www.knmi.nl/onderzk/index.html), were given by
R. Pacione and H. van der Marel. The real time needs for ionospheric TEC
information were described by B. Wilson of JPL. P. Heroux of GSD
outlined the Canadian approach to national reference system
delivery/maintenance, which is also moving towards real time and relies
on IGS and VLBI.  Real time and near real time requirements for natural
hazard monitoring were presented by Y. Bar-Sever of JPL. His
presentation clearly showed the complementarily nature of seismic
monitoring and continuous GPS precise positioning. Namely, GPS provides
monitoring at low frequencies with periods of hours to days where
seismology becomes biased or completely fails.

3. Real-Time Data/Products Exchange

This was the first opportunity for the newly created IGS Real-Time
Working Group (RTWG) to share its initial findings and formulate future
plans. The results of an initial investigation, which are summarized in
the position paper, were presented by M. Caissy on behalf of RTWG.  They
suggested that IGS adopt UDP (User Datagram Protocol) for Internet real
time GPS data streaming since it has been tested and successfully used
by both JPL and NRCan. The UDP (unicast or multicast) protocols, unlike
the more standard TCP/IP, do not require point to point connections, and
consequently are more efficient than TCP/IP (no opening/closing
connections, smaller packet size overhead etc.).  This is true, in
particular, for smaller intermittent packets typically seen in GPS real
time data stream. On the downside UDP, unlike TCP/IP, does not include
receipt acknowledgements and/or retransmission, which may be needed for
product delivery.  G. Weber of BKG and G. Hedling of Lantmateriverket
demonstrated the use of TCP/IP in their presentation of an operational
RT DGPS system in Europe.  K. MacLeod of GSD described UDP operational
tests at NRCan. Following these presentations, security issues and
firewall problems were discussed quite extensively in this and the
following DC session. In the end, there seemed to be general agreement
that both the UDP and TCP/IP protocols for RT data exchange should be
researched and prototypes developed by RTWG for IGS. A solid interest
was shown by both IGS DCs and ACs since this development could lead to
timely, robust and redundant near RT or RT data streaming directly to
the interested DCs and ACs.

4. Data Center Issues (DC)

Overall DC statistics, performance and current issues, and the GSFC DC
in particular, were summarized by C. Noll of GSFC. For example, most of
the recent LEO (GPS) data are now available at the GSFC DC. The need for
the establishment of a DC WG was also raised. (It was subsequently
formalized by the IGS GB at its April 11 meeting, and will be chaired by
C. Noll). Firewalls and security issues were discussed extensively as
well as the somewhat confusing data management at DC (found to be
inadequate in particular for new, uninitiated users). This was shown in
a presentation by E. Gaulue of IGN. H. Habrich and M. Scharber
summarized status and new developments at the BKG and SIO DCs,
respectively. The GPS Seamless Archive Center (GSAC) developments at SIO
are maturing and look promising.  It was also pointed out that the
GLONASS/GPS data streams will soon be integrated at the RINEX level as
previously planned and announced.

5. Network Issues

Following the DC session, the invited vendors, who also sponsored Monday
night's ice-breaker reception, made short presentations on their latest
GPS receiver and related hardware developments. Some of the current or
new receivers are, or will be capable of stand-alone station monitoring
and include internet (IP) communication software. Also discussed was the
current IGS standard P1/P2 pseudorange pair, which poses some additional
complications for some receivers, which observe C1 rather than P1
pseudoranges. As pointed out by some vendors, P1 observable would make
their receiver more expensive. Perhaps, in the future, IGS should
cooperate more closely with manufacturers when adopting standards such
as the P1/ P2 pseudoranges.  The P1-C1 differences, when used
inconsistently with the fixed IGS orbit/clock product in undifferenced
processing, will result in significant position and receiver clock

The network issues session was introduced by the IGS CB Network
Coordinator, A. Moore. Solid progress has been made on data
availability, timeliness and integrity, though some improvements are
still possible. With the continuous addition of tracking stations in
dense regional networks, the concept of associated IGS network was put
forward. This proposition, paralleling AACs, would allow inclusion of
new stations in regions already saturated with IGS stations.  It would
ensure that IGS becomes open to new participation and provide a model
for manageable growth of IGS networks. L. Combrinck gave an update on
the situation in Africa.  He reminded participants that, with the
exception of very few nations (like South Africa), the African
situation, particularly the communication/internet infrastructure, is
very fragile and not likely to improve in the near future. P. Fang of
SIO, on behalf of Chinese colleagues, gave an update on the situation in
China. While there are many suitable stations and considerable interest
in China, reflecting on all IGS components, the situation remains
difficult, primarily stemming from political decisions/willingness to
release/exchange appropriate data and information to IGS. R. Neilan,
Director of IGS CB, summarized the status of the GPS modernization,
including the new L2CS and L5 frequencies, which could potentially
revolutionize GPS. Currently, there are 12 Block IIR with L2CS and 6
Block IIF satellites (with the additional L5 frequency) planned to be
launched. The GPS modernization and the new GPS III benefit from healthy
competition by GALILEO.  J. Dow of ESA presented a status on GALILEO.
Phase I, covering all system development and initial tests has been
approved by EC. The system is quite complementary and designed to be
interoperable with GPS as far as the broadcast frequencies are
concerned, yet offers a satellite constellation different from GPS (e.g.
27+3 satellites with 14h22m orbits). The full operation is envisaged for
2008. IGS input into GALILEO design is ensured and considered important.
L. Estey of UNAVCO, indicated in his presentation that RINEX is ready
and flexible enough (thanks to the recent revisions), to accommodate the
GPS modernization and its new frequencies. At the end of this session,
participants were once again reminded that the promised integration of
GPS/GLONASS observations for GPS/GLONASS stations would take effect
shortly at all DCs. No integration of GPS/GLONASS orbits into a single
sp3 file is being considered at this time, according to R. Weber, the
current AC Coordinator.

6. Poster Session

The poster session was intended for ACs to exchange their recent
experience and compare their approaches. Most ACs presented at least one
poster, usually highlighting significant changes implemented since the
last AC workshop, held in September, 2000 in Washington DC. In addition
to the AC posters, there were also poster presentations on regional and
global station solutions, near real time orbit and tropospheric
processing, LEO (CHAMP) orbit comparisons as well as the latest
information on the recently launched GRACE mission. During the poster
presentation, which was held in the AGU poster tradition, i.e. with cold
(Canadian) beer, there was also a demonstration of GPS.C - the Canadian
RT DGPS system.

7. Reference Frame (RF)

The current IGS RF Coordinator, R. Ferland of GSD, gave a progress
report on Reference Frame (RF) WG activity. The production of the IGS
station combined products (both weekly and cumulative) is proceeding
smoothly, though some AC solutions had to be excluded from geocenter
monitoring due to problems with the removal of constraints from their
SINEX submissions.  This identified problem should be corrected as soon
as possible. Nevertheless, the IGS geocenter solutions agree quite well
with the independent SLR solutions. The recent switch (on December 02,
2001) to ITRF2000 and its IGS00 realization has been quite smooth.
Currently, the IGS combination already includes about 200 stations and
satisfies the target IGS Station Polyhedron of about 200-250 stations.
The size of this globally distributed polyhedron network was derived
independently and allows for precise relative determinations over
baselines of about 2000 km. For these reasons, additional back
substitutions to derive polyhedron stations, as originally envisaged, is
no longer necessary.  This goal, in fact, is already being met in a
single combination step. The RFWG, on behalf of IGS, also took part in
the IERS PP aimed at testing of EOP/ITRF alignment. Since the IGS orbit,
clock, station and ERP combinations were designed to be consistent with
the ERP/ITRF, it was not surprising to see a high degree of agreement in
submitted solutions, with the IGS adopted minimum constrain approach
showing one of the best consistency/results. S. Y. Zhu of GFZ looked
into the apparent difference of a few ppb between GPS and ITRF scales.
He concluded that it cannot be due to GM or a common satellite antenna
offset, as in both cases the resulting scale effect is negligible in GPS
global analyses. This has confirmed earlier tests done by various ACs.
In terms of regional station integration, an unresolved and open
question remains "what should be the recommended approach for
continental AAC to integrate RNAAC solutions into the IGS realization of
ITRF (i.e. the IGS station polyhedron)? R. Ferland addressed this
problem in his second presentation. The two main options considered
were: constraining (consistently with the IGS variance-covariance matrix
or even much higher constraints) and minimum constraints suitable for
non-global networks, e.g. Blaha's inner constraining type (presented
here by Z. Altamimi of IGN). The RFWG will need to perform some
additional testing before consensus is reached and a specific method is
recommended for integration of regional solutions into the IGS
polyhedron. In many respects, EUREF's approach presented by its
Director, C. Bruyninx of ROB, should serve as a model for such
IGS/ITRF densification. The outline of the newly emerging NAREF
initiative, given by M. Craymer of GSD was encouraging, in particular
the clear commitment to commence regular submissions to IGS. Finally, M.
Rothacher of TUM, the current IERS Analysis Center Coordinator,
highlighted the result of the discussions related to the new SINEX
version of 2.0.

8. Receiver and Satellite Antenna Calibrations

M. Rothacher introduced the session with the position paper
presentation. Very significant progress has been made since the 1999 and
2000 AC workshops. Namely, at the 1999 AC workshop, satellite antenna
patterns were first identified as a potential cause for the mysterious
and unexplained scale bias of about 15 ppb introduced when precise
absolute calibrations of receiver antennas were included into global GPS
solutions. The absolute (anechoic chamber) and relative antenna
calibrations have been recently confirmed by independent, ingenious, and
very precise absolute calibrations methods.  One of these methods was
developed by the Hannover group and uses a robot rotating and tilting a
GPS antenna while observing real GPS signal.  This method was well
described in a presentation by M. Schmitz of Geo++. Calibrating
satellite antennas on the ground, due to their size and electronic
complexity/adjustment proved to be quite a challenge as seen from the
presentation by G. Mader of NGS. However, solving for antenna phase
pattern with respect to absolutely calibrated receiver antenna phase
center variations (PCV), gave quite precise and repeatable results.  Two
distinct satellite antenna PCV's, namely for the Block II/IIA and Block
IIR satellite types were obtained in that fashion. A separate
presentation by R. Schmid of TUM showed that introducing the satellite
and receiver antenna PCV's  greatly diminishes the 15 ppb scale error.
It is important to realize that both receiver and satellite PCV's are
subject to an initial (and somewhat arbitrary) datum height (scale)
offset, which is commensurate with a common antenna offset, or the scale
of GPS. Consequently, the ITRF/VLBI scale can be used to solve/fix this
antenna offset convention/datum problem, i.e. consistently with ITRF
scale. Another approach to this antenna-offset/datum problem can be
imposing the condition that a (weighted) sum of PCV variations, over a
certain range of azimuths and elevation angles, be equal to zero. (NOTE:
This is in fact how the current (DM) antenna offsets, used as a
convention by IGS, have been derived, i.e. it is based on an early
antenna chamber calibration, which assumed no elevation and azimuth
variations and which gave the current conventional height L1/L2 offsets.
It is quite remarkable that this old and crude antenna measurement, in
fact, implies a scale that is correct within only 2-3 ppb of the ITRF
scale convention.) Another, quite independent approach to antenna PCV
calibration, presented by P. Elosegui of Harvard-Smithsonian/Center for
Astrophysics, is based on comparing the precise phase observations
obtained by the multi-directional GPS antenna under calibration with
respect to a directional parabolic one. Since the reference parabolic
antenna is virtually immune (once properly calibrated) to PCV and
multipath, while common effects are canceled out through procedure (e.g.
the short baseline, delays), the remaining difference is due to
multipath as well as to antenna PCV  (note any satellite antenna PCV is
also eliminated here). The advantage of this method is that it is
station specific, unlike the previous approaches, and thus also includes
multipath. Potentially, it can also be used for mobile calibrations of
IGS sites w/o any interruption or changes to the IGS operations (subject
only to the requirement of an antenna splitter at each station). 

As a first step towards a significant improvement over the current
convention (which uses PCV relative to the DM antenna type), it was
proposed that IGS adopt, by January 2003, a new IGS antenna convention
(receiver, satellite antenna PCV's and the corresponding height
offsets), subject to prior evaluation and testing by ACs.  Also
discussed during this as well as the Network session was an urgent need
to develop clear IGS guidelines on equipment and antenna changes in
particular, which should provide sufficient overlaps to ensure a long
term continuities of the IGS times series. This is also imperative in
view of equipment improvements and the future GPS modernization

9. Ground Based GPS Ionospheric Estimation

The ionospheric group, headed by J. Feltens of ESA is a very active
group that had already met earlier this year in Darmstadt, Germany.
Consequently the agenda and recommendations were well focused. The most
important goal, as outlined in the position paper, is an official
production of ionospheric combined product, which has been long in
preparation, and which has been maturing quickly as indicated by
evaluations utilizing TOPEX TEC data.  Currently, the yet unofficial IGS
ionospheric combinations are as precise, reliable and complete as the
best AAC contributions. There is a clear commitment, after some
fine-tuning (e.g. global weighting) to launch this official IGS
ionospheric combined product. S. Schaer of CODE AC presented an
impressive (and real) ionospheric video. He has also compared station
DCB's, furthermore he is also responsible for the official IGS P1-C1
bias estimations. He has also stressed the importance of minimizing
discontinuities between daily ionospheric grid maps.  An interesting
presentation on validation using TOPEX TEC data as well as the
assimilation of TOPEX ionospheric profiles into numerical ionospheric
models was given by B. Wilson of JPL. He also reviewed past (and
published) research on the significance of the neglected (3rd and
higher) ionospheric and magnetic terms in two frequency GPS positioning.
The effect, under extreme ionospheric conditions, could reach up to 2-cm
phase range errors in precise GPS positioning. P. Heroux of GSD also
presented a simple but effective approach for ionospheric map quality
evaluation utilizing observed and computed P1-P2 delays at a selected
set of IGS stations, preferably not included at the ionospheric grid map
generation. During subsequent discussions and with regards to daily
discontinuities, it was pointed out that it would also be equally
beneficial to include the last epoch of each day (24:00) in all IGS
daily orbit/clock files as well. This would also enable the
detection/mitigation of any orbit/clock daily discontinuities.

10. Low Earth Orbiters (LEO)

Recently, the LEO WG has become very active with the availability of
data from several LEO's already in orbit. Steady progress has been
realized in CHAMP precise orbit determination (POD) such that the best
CHAMP POD test results, based on reduced dynamic methods, are well below
the 10 cm precision level. The best kinematic POD (independently
determined epoch positions) is approaching the 10 cm precision level.
This was shown in the LEO position paper presentation made by H.
Boomkamp of ESA and in separate presentations on different LEO POD
approaches by B. Schutz of CSR, M. Rothacher of TUM and U. Hugentobler
of CODE. This is the first and necessary step of LEO WG. The subsequent
goal/aim (as specified in its charter) is the investigation of possible
improvements to the IGS core products (orbits/clocks) through
simultaneous LEO and IGS data processing. An initial attempt to answer
this difficult question was already presented by S. Y. Zhu of GFZ.
Clearly, the LEO POD must have the highest possible precision before any
meaningful contribution to the current IGS orbit/clock products is made.
This is why it was suggested that the WG should concentrate, for the
time being, only on one specific LEO satellite. Also benefit/effects and
timeliness of the IGS Rapid versus IGS Final orbit/clock combination
products were discussed. The question was also raised if IGS Final
orbit/clock combination delays should be reduced from the current 2
weeks down to perhaps one week.

11. Review of IGS Products

Previous sessions and this session's position paper reviewed the IGS
products in view of increasing demands on timeliness to satisfy the
requirements of near RT applications for LEO POD, troposphere and
precise positioning. The IGS core products of combined orbits/clocks and
station coordinates aspire to be consistent and conform to current IERS
standards. That is the reason for the adoption of the new ITRF2000 and
that the official IGS ERP accumulated series (igs00p02.erp), which spans
the ITRF94, ITRF96, ITRF97 and ITRF2000, has been transformed into the
ITRF2000 realizations.  Thanks to the minimum constraint approach
adopted for IGS products since 1998, the ITRF96/97 and ITRF97/2000
transformations are nearly exact. A similar transformation of the IGS
combined orbit products into the current ITRF realization was also
proposed in this session's position paper.  Apart from the already
proposed 3-hour production cycle for IGU and tropospheric solutions, the
main question that remains is the effect of the proposed IERS2000
Conventions on IGS products.  J. Kouba of GSD, in his presentation,
tested the new subdaily ERP model, which is already available from the
IERS2000 Conventions www site, by using independent pole rate solutions.
He concluded that the IERS 2000 subdaily ERP model could be adopted
since it fits the IGS data as well, or perhaps slightly better, than the
IERS96 subdaily ERP model. A detailed report on this testing is
available from the workshop website. J. Ray of USNO reviewed the
proposal for the new celestial pole (CP) definition which is to be
included in the new IERS 2000 Conventions. Since there still are some
clarifications needed, and since IGS analyses are relatively insensitive
to CP, as long as consistent transformations are used going to and from
CP, he recommended a "wait and see" approach. There was also a question
raised about the need for IGS satellite clock products at a time
interval shorter than the current 5-min sampling. Linear interpolation
of 5-min satellite clocks is precise at the 0.1 ns rms level, allowing
precise kinematic positioning (at any interval) with about 5-10 cm (rms)
precision. This represents only a slight precision decrease from the
positioning obtained with the original, not interpolated, satellite
clocks. Some participants felt that for the most precise LEO
applications, this 5-10 cm precision level may not be acceptable.
However, the significant effort and additional computational burden
involved with clock determinations at higher rates need to be
considered. The IGS time scale and the newly aligned IGS clock
(including the sp3) products have reached a mature stage and are ready
for adoption by IGS, as clearly and convincingly demonstrated by K.
Senior of USNO, the developer of the IGS time scale. R. Weber of TU
Vienna /AIUB, the current AC Coordinator summarized all three IGS
orbit/clock combinations and also stressed the importance for ACs to
include ALL satellites, including the marginal ones, but with proper
accuracy codes. This is in particularly important for the IGU combined
product, where a number of satellites are routinely excluded by ACs due
to a lower accuracy. These missing satellites are badly needed by RT and
near RT applications as it was indicated in several presentations during
this workshop. T. Yunck of JPL gave an entertaining, but somewhat
controversial presentation on the GPS scale and consistency. Finally, S.
Hilla of NGS, who took on the challenge to update the SP3 orbit/clock
format, presented his SP3 format update proposals. The latest variant,
which allows accuracy codes for both orbits and clock at each epoch and
is largely backward compatible, seemed to have received the widest
acceptance. Though concerns were expressed that the proposed accuracy
code exponential base of 1.25 is not compatible with the base of 2 used
for the header accuracy codes, and that it did not allow for any x, y, z
orbit correlations.

P. Tetreault, P. Heroux and J. Kouba
Ottawa, April 25 2002

Draft Workshop Recommendations / Compiled by A. Moore and R. Weber

Real-Time Products and Applications (J. Dow and Y. Bar-Sever)

1. A variety of applications including weather prediction, ionospheric
weather monitoring, satellite and terrestrial navigation, earthquake and
volcano monitoring, positioning of structures, surveying, timing and
earth orientation would benefit from the availability of Real-Time (RT)
GPS raw data products and from RT or Near Real-Time (NRT) products
computed from them. In view of this trend towards real time, the
relevant IGS elements should press ahead with the development of:

  a. the infrastructure needed to transfer in real time raw GPS data
  from a sub-set of the stations of the global network to servers
  located at those IGS Analysis Centres interested in participating in
  this new activity;

  b. the software needed to generate, quality check and disseminate RT
  and NRT orbit, clock and other products (Global Ionospheric Map (GIM),
  Total Zenith Delay (TZD), ...) 

2. An appropriate project structure should be set up within the IGS to
coordinate and execute this work (for example the Real-Time Working
Group (RTWG)).

3. As an interim measure, the AC's and the AC Coordinator are encouraged

  a. review the current latencies of the classical orbit and clock
  products (ultra-rapid, rapid, final) and assess whether it is
  appropriate to modify these in view of the increased availability of
  hourly stations and whether it is still necessary to maintain separate
  rapid and final products.
  b. reduce as soon as feasible the latency of the ultra-rapid products
  from the current 12 hours to 3 hours.

Real-Time Data/Products Exchange (M. Caissy and  R. Muellerschoen)

It is recommended that the IGS community guided by the RTWG move forward
on two fronts with the goal of completing phase 1 of the RTWG's charter. 
  1) Involve the broadest membership as possible from within the IGS
  2) Move forward on the development of a prototype for data and product
  exchange incorporating the design presented in the position paper with
  the following additional recommendations based on discussions at the

  It is recommended that the RTWG investigate the impact of using
  Transmission Control Protocol (TCP) in place of User Datagram Protocol
  (UDP) in order to assess the value of using TCP in the post-prototype
  phase of IGS real-time processes.

  It is recommended that the RTWG investigate the impact that the choice
  of UDP may have on our ability to traverse firewall implementations at
  institutes where the use of UDP is discouraged or denied.

  It is recommended that due to the demonstrated interest from global
  data centres, they be involved on a voluntary basis, at the prototype
  stage, in a demonstration of the concept of distributed data centres.

Data Center Issues (C. Noll and L. Daniel)

DAT1: A Subset of Data Centres (DC)s should participate in RTWG's
prototype efforts
DAT 2: Establish a DC Working Group
  -to evaluate metadata storage and exchange as well as monitoring and
  bug tracking
  -to create and maintain the
    (a) topology of data flow up to Global Data Centres (GDC) 
    (b) DC requirements and guidelines

DAT 3: GDCs and Regional Data Centres (RDC)s should participate in GPS
Seamless Archive Centre (GSAC) effort
DAT 4:  Integrate GPS/Glonass data flow into IGS paths

Network Issues (A. Moore and M. Schmidt)

NET 1: Form an IGS industry panel with representation from manufacturers
of equipment used in IGS
NET2: Implement associate regional networks and associate applications
NET3: ACs and Pilot Project (PP) / Working Groups (WG)s should
communicate to the Network Coordinator (NC) recommended equipment
guidelines (e.g. radome types, non DM antennas,...) and recommended
degree of enforcement
NET 4: Reference Frame Working Group (RFWG) should recommend conventions
for reporting time series discontinuities
NET5: IGS should keep abreast of and prepare for GPS modernization; When
equipment and signal availability schedule is clear, a phased adoption
at IGS sites should be coordinated

Reference Frame (R. Ferland and Z. Altamimi)

Test in detail the various proposed combination/constraining approaches
(fixing- minimum constraints-combined) to align regional solutions to
the ITRF. Use various regions and time spans. Agree on one proposition
to be recommended for all regions.

Antenna Calibration: (M. Rothacher and G. Mader)

Review and adopt the new IGS Phase Centre Variation (PCV)-format for
receiver and satellite antenna phase centre corrections. (envisaged date
for adoption Jan, 1st, 2003).
Adopt absolute antenna PCV for receiver antennas and new satellite
antenna offsets and patterns after conducting a thorough test campaign
(envisaged date for adoption Jan, 1st, 2003).
Set up a so-called 'Antenna WG' to keep track of antenna issues in
general and to organise the transition to absolute phase centre
variations in particular.

Avoid to the extent possible any change in the antenna setup at IGS
permanent sites. Whenever possible the same antenna type should be
installed in case of replacement due to malfunctioning of older

Ground-Based Ionospheric Estimation (J. Feltens and B. Wilson)

ION 1: 
Start with the delivery of a combined IGS Ionosphere Product (asap /
ION 2:  
Combined IGS Total Electron Content (TEC) maps should be produced with
an overlap of one day to decrease jumps at the day boundaries.
ION 3:
Global Ionospheric Associate Analysis Centres (IAAC)s TEC maps should
cover all parts of the world.
ION 4:
Explore the use of ENVISAT and JASON satellites for validation of IGS
Ionosphere Products.
ION 5:
In view of NRT Monitoring of the Ionosphere the distribution of ground
stations as well as the data flow (latency) has to be improved. 

IGS-LEO (H. Boomkamp)

Explore in detail the impact of GPS-LEO data on the classical IGS
products in combination solutions.
Explore the latency requirements for tracking data availability as well
as delivery of current IGS products to support LEO data processing (e.g.
for atmosphere sounding).

IGS-Products (J. Ray and R. Weber)

PROD 1 (Time Scale)
Adopt a new time scale for IGS Final and Rapid Products to achieve
continuity at day boundaries and allow for a direct link to UTC.
(envisaged date of adoption: July 1,2002)

PROD 2 (New SP3 format)
Review and adopt a new version of the SP3 format. To serve the user
community keep both the old and the new format in parallel for a period
of  at least 1 year. (envisaged date of adoption: July 1,2002)

PROD 3: (GLONASS data processing)
Intensify the ability to process data from combined GPS/GLONASS tracking
sites. ACs and AACs are encouraged to provide orbit and clock
submissions in order to ensure a reliable combined IGS GLONASS orbit and
clock product.
PROD 4: (IGU products)
In view of upcoming NRT- needs explore and implement a more frequent
update of the IGU -Ultra Rapid Products. An update cycle of 3 hours
(currently 12 hours) for IGU products is envisaged. Investigate the
option of different update cycles for orbits (6 hours) and clocks (3
hours). In addition explore the possibility of decreasing the latency of
IGU products from currently 3 hours to 2.5 hours as well as the
submission of 5-minutes rinex-clock files. 

Geodetic Survey of Canada
Ottawa, Ontario, Canada
Internet: pierre @ geod.nrcan.gc.ca

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