IGS Workshop 2004 Summary Recommendations

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[IGSMAIL-4963]: Recommendations from IGS 10th Anniversary Symposium and Workshop

by way of Ruth E. Neilan John.Dow at esa.int 
Fri Jul 2 10:18:39 PDT 2004

 


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IGS Electronic Mail      02 Jul 10:18:41 PDT 2004      Message Number 4963
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Author: John M. Dow

Dear Colleagues,

Please find below the recommendations compiled by the session chairs
following the IGS 10th Anniversary Symposium and Workshop held in 
Bern, 1-5 March
2004.

Please do not hesitate to contact me if you have questions or comments.

Best regards,
John M. Dow.


==========================================================

IGS 10th Anniversary Symposium and Workshop

1-5 March 2004, Bern, Switzerland

Recommendations



1.0        Symposium Recommendation
2.0        IGS Reference Frame Maintenance
3.0        Other Reference Issues
4.0        Real Time Aspects
5.0        Network Issues
6.0        Data Transfer and Data Centers
7.0        Integrity Monitoring of IGS Products
8.0        GNSS Sessions
9.0        Precise Orbit Determination
10.0       Antenna Effects
11.0       IGS Troposphere Combination Products
12.0       IGS Ionosphere WG



==========================================================


1.0 Symposium Recommendation
----------------------------

1.1 "The International GPS Service (IGS) endorses the final report of
the United Nations Action Team on Global Navigation Satellite Systems
(GNSS) (A/AC.105/C.1/L.274). The proposed establishment of an
International GNSS Committee as a mechanism to further cooperation
among system providers, international GNSS organizations, and users
is strongly encouraged. The IGS and the International Association of
Geodesy (IAG) will support and contribute to the successful
realization of this effort."

2.0 IGS Reference Frame Maintenance
-----------------------------------

2.1 To resolve potential constraints issues, it is proposed that for
GPS weeks 1268 to 1270 (April 25 - May 15, 2004), the ACs contribute
SINEX solutions obtained without constraints on any parameters along
with their usual SINEX solution. If for any reason, any apriori
constraints (orbit, troposphere ... etc) are used on any parameters,
they must be reported along with their expected influence on SINEX
parameters.

2.2 Check/compare the effect of the weighting strategy on the
estimated transformation parameters with the current IGS SINEX
combination strategy by selecting a few GPS weeks.

2.3 Estimate and report a scale factor between IGS weekly combined
solution and the IGS realization of ITRF.

2.4 Review the combination procedures with the GNAACs, to better
explain and possibly reduce the observed differences. Ideally, in
this type of analysis, the processing noise should be kept well below
(one order of magnitude) the signal.

2.5 The modeling differences between ACs need to be compared to
understand the observed small systematic differences between the AC
station coordinates, orbits and clocks. As a starting point, a
summary of all the AC processing/modeling is being compiled. The
information available from the *.acn files is used for this
compilation. The ACs should update the file every time any
significant analysis change is made.

2.6 Generate two lists of station position discontinuities: one with
"known/certain" station position discontinuities and another one with
"suspected/probable" discontinuities. Some ACs have already
identified a number of discontinuities; their contribution is
certainly welcomed. A related activity is to recombine the
weekly/cumulative solutions to include the discontinuities.

2.7 Provide updates to the reprocessed weekly SINEX solutions. It is
suggested to keep those solutions separate from the official ones
(CDDIS), and with a distinct, but similar naming convention. Updates
should be provided when significant improvements have been made.

2.8 The ACs need to verify the stability of the RF stations before
constraining them during the generation of the (ultra) rapid
orbit/clock products. Additionally, a PPP will be applied after the
combination to check the RF station positions.

2.9 The ACs should be prepared to reprocess the IGS data. The
detailed procedure should be discussed after the absolute antenna
phase center variation models are decided (see Antenna session).

2.10 All IGS satellite clocks should be in ITRF center of network.
This is the case for the (Ultra) Rapid products and should be
realized for the Final product too. ACs should fix their shifted
station coordinates while back substituting for final clocks (use of
AC station solutions transformed into RF by Helmert transformation)
(short term)

2.11 The quality of the PPP realization of ITRF using IGS products
(Rapid and Final) will be monitored; changes in the combination have
to be prepared. For the most demanding users, the 7-parameter
transformations will be made available.



3.0 Other Reference Issues
--------------------------

3.1 Develop reinforced IGS reference frame strategy:  The IGS should
officially designate reference frame stations according to a set of
operating standards mutually accepted by all components of the
organization.  The station operators must be actively involved and
committed to this process.  The IGS needs to develop a long-range,
proactive strategy to reinforce and secure the long-term stability of
a sustainable and robust reference frame incorporating appropriate
quality assessment systems and much improved user interfaces.

3.2 Verification of IGS PPP consistency:  The IGS should commission a
thorough study of the consistency of its Final orbits and clocks for
global precise point positioning relative to the associated weekly
sets of station coordinates.  In particular, the effects of possible
geocenter and scale differences should be well studied and remedies
for any defects should be developed.  Ideally an ongoing
quality-checking process should be implemented to continuously
monitor the consistency and precision of IGS products.

3.3 IGS Precise Point Positioning (PPP) service: The IGS should
institute procedures to maintain the documentation of all necessary
analysis methods, conventions, and constants so that non-specialized
users can use IGS products with maximum accuracy and minimum effort.
Ideally, a freely available, open software package and other
automated electronic tools should be provided as a service for
precise point positioning by general and expert users.  The IGS
should consider inviting agencies to provide such services
operationally, where the quality and integrity would be continuously
monitored by the IGS.

3.4 Absolute antenna patterns and the IGS scale:  When the IGS
implements absolute antenna phase patterns for the satellites and
tracking network, the effect on the average scale of the combined
products should be carefully evaluated to verify that it closely
matches ITRF2000/IGS00.

3.5 Handling geocenter motions: The IERS is encouraged to adopt an
elaborated celestial-terrestrial transformation of the form:

ICRF = P * N * R * W * [TRF + O(t)]

to explicitly account for geocenter motion. The sense of the
geocenter offset vector is from the center of the "instantaneous"
TRF(t) frame to  the ITRF origin such that [TRF + O(t)] is aligned to
ITRF.  This should be the understanding of the geocenter parameters
in the SINEX format.  Realization of geocenter offsets using a
Helmert transformation approach, as already implemented by the IGS,
is also recommended.

3.6 Conventional contributions to station displacements: Following
traditional practice in treating Earth orientation variations, the
IERS Conventions should be interpreted such that the summation of
various model effects for a priori, non-linear station displacements
includes only those which: 1) have known closed-form expressions with
high a priori accuracy; and 2) have periods of variation near 1 d or
shorter (with some exceptions).  Currently, these criteria include
diurnal and semidiurnal tidal displacements for the solid Earth,
ocean loading, and atmospheric loading, as well as the longer-period
Earth and ocean tides and the mostly longer-period pole tide. The
ocean tidal loading should account for the whole-body translation of
the solid Earth that counterbalances the motion of the ocean mass, in
contradiction to Chapter 7 of the IERS Conventions 2003.  The
"permanent" component of the solid Earth deformation is also included
in the tidal model in keeping with long-standing geodetic practice.
Currently, the IERS does not provide models for the
diurnal/semidiurnal displacements due to atmospheric loading or
geocenter motion.

3.7 Tropospheric path delay products: The IGS Troposphere Working
Group should consider measures to ensure the highest possible
accuracy, precision, and consistency of its zenith path delay
products with the IGS00 reference frame.  In particular, the station
coordinates used for troposphere products should match those of the
IGS weekly terrestrial reference, and methods to account for the
current differences in scale should be developed and applied.

3.8 Handling subdaily variations: Analysis Centers should ensure that
they are using the newest IERS models for subdaily EOP and solid
Earth tidal variations.  The Analysis Coordinator is asked to work
with the IERS to develop suitable models for the effects of
high-frequency nutation in polar motion, subdaily geocenter
variations, and subdaily atmospheric loading.  Centers should prepare
to implement these models as soon as they become available.

3.9 Handling pole tide deformations: Analysis Centers should ensure
that they remove the mean pole position from the instantaneous polar
motion before computing the pole tide effect.  The linear trend
provided in IERS Conventions 2003, Chapter 7, eqn (23a) and (23b) is
recommended for this purpose.

3.10 Nutation models: Analysis Centers should not rely on the IAU1980
nutation model alone. To do so will cause longer period polar motion
errors.  If the IAU1980 model is used, corrections from the published
IERS nutation offsets should also be applied.  Alternatively, a more
accurate nutation model (with or without observed offsets) can be
considered.

3.11 Neglected ionospheric corrections:  The IGS and Analysis Centers
should consider methods to attenuate the present level of error
caused by the neglect of higher-order ionospheric delay corrections.
See  recommendation 12.4.



4.0 Real Time Aspects
---------------------

4.1 The UDP transport protocol is preferred for real-time data and
product distribution.

4.2 Organizations operating real-time data networks are encouraged to
reformat a subset of their data into the format proposed by the RTWG
and permit easy access to these real-time data streams.   RTWG will
provide information to make the mechanism for access clear.

4.3 Together with the DCWG, the RTWG will assess long-term archival
and provision of the data in the RT streams.

4.4 The RTWG and DCWG will together map a strategy to provide
assessments of the RT data streams.

4.5 Quality monitoring of the predicted portion of the IGS Ultra
Rapid orbits is an initial RT product goal (joint with Integrity
Monitoring session).

4.6 More frequent, exploratory communication among RTWG members is needed.



5.0 Network Issues
------------------

5.1 New stations proposed to the IGS should be described on a web
page and announced to the community, but added to the IGS network
only on the request of an AC or Coordinator.

5.2 The "Global" station designation should be discontinued.  The 99
IGb00 Reference Frame stations will be promoted on station lists and
a letter will be written to agencies operating IGb00 stations, noting
the significant effort and responsibility and requesting a reaction
to the Reference Frame station guidelines.

5.3 The analysis community should develop a plan to handle North and
East eccentricities.

5.4 The IGSMail list will be split into IGSMail (for messages such as
IGS Workshops, new IGS stations, product-related announcements, major
DC announcements, sessions at conferences, enhancements to web pages
or services, etc.) and IGSStation (for station configuration notices,
outage or repair notifications, and RINEX data replacement
notification).

5.5 Monitoring and encouraging compliance to the data recording and
transmission guidelines is encouraged.



6.0 Data Transfer and Data Centers
----------------------------------

6.1 Clarify the roles of IGS DCs with respect to real-time data and
data products

- Near term: Archive metadata and monitor quality of transmitted data streams.

- Long term: Offer end users a publicly-available mechanism for data
consumption/retrieval/subscription.

6.2 IGS operational data centers should archive raw receiver data
indefinitely and provide access to these data upon request for data
revision or scientific study on a limited basis.

6.3 Establish guidelines for GPS data file revision and define a
methodology for notification, archival, and permanent catalog of
revised data.

6.4 Work with the IGS Network Coordinator to improve timeliness of
IGS data and data products.



7.0 Integrity Monitoring of IGS Products
----------------------------------------

7.1 IGS should use its RT data streams to

- Monitor the Ultra-Rapid predicted orbits to detect and flag outlier
satellites and

- Estimate improved satellite clocks based on the RT data.

7.2 The goal is to have a near real-time (<10 minutes) product with
quality similar to the Rapid product.



8.0 GNSS Sessions
-----------------

8.1 Revise all format standards used by IGS entities (to transfer
tracking data, orbit & clock information, and derived products) to
properly exploit all opportunities offered by Next Generation
Satellite System signals.

8.2 The IGS asks for a proper calibration of GALILEO, GLONASS and
modernized GPS satellite antennas (before launch) and for providing
that data to the scientific community.

8.3 It is encouraged to put laser retro-reflectors on all GNSS satellites

8.4 In order to collocate the GALILEO Reference Frame (GRF) to ITRF
the IGS asks for a proper calibration of GRF Reference Station
antennas and for providing that data to the scientific community.

8.5 The IGS should start as soon as possible the discussion with
receiver manufacturers to explore an optimal set of signals (from
GPS, Galileo, Glonass) to be tracked by future GNSS receivers.

8.6 The IGS GNSS WG should be recognized as an interface for
information exchange and for stimulating cooperation between IGS and
entities involved in the technical set up of GALILEO, modernized GPS
and GLONASS.

- The IGS GNSS WG should develop a test plan during the next 12-18
months for collecting and evaluating test data from the first two
trial GALILEO satellites, using data from the prototype GALILEO
sensor stations that apparently will be co-located at IGS stations.

- In coordination with the Galileo Program, IGS should consider how
best to realize the Galileo terrestrial reference frame.

- The IGS GNSS WG should develop a test plan to collect and evaluate
the new GPS  civil signals as soon as possible (L2C in 2004). GPS
modernized signals should be integrated into the current IGS
processes in a continuing evaluation phase as the new constellation
of satellites is populated and capable receivers are produced and
implemented.

8.7 Access to cross-link measurements between the GNSS satellites
should be provided, this is a very important measurement type with
inestimable impact on all derived products. A white paper on this
topic will be prepared in an attempt to influence the GPS III
development.

8.8 Transition combination of GLONASS orbits to IGS AC Coordinator.



9.0 Precise Orbit Determination
-------------------------------

9.1 The position paper concludes that the processing of more GNSS
satellites (GLONASS, in the future Galileo and modernized GPS) will
probably not lead to substantial capacity problems. It is therefore
recommended that the IGS Analysis Centres consider the future
inclusion of GLONASS data processing in their POD systems, following
the example set by the CODE Analysis Centre.

9.2 The inclusion of LEO data (typically requiring high-rate data
processing) is currently posing capacity problems for most centres
that study this possibility, but these problems will be compensated
at least in the long term by increasing computer power. It is
recommended that centres that work on LEO GPS data continue their
efforts to find alternative methods of exploiting this data as long
as switching to high-rate data would remain prohibitive.

9.3 Within IGS, the available information on process sizes and
processing cost is rather rudimentary, so that the Position Paper
analysis necessarily introduced certain assumption on trends and
future growth rates. It is recommended that the Analysis Centres
start collecting processing metrics in a systematic way, in
particular for the most fundamental POD process size parameters:

- Process execution times

- Memory used by each process

- Number of estimated parameters

- Number of included tracking observations

9.4 Time series of these fundamental quantities can greatly enhance
insight in the projected increase of IGS capabilities and processing
needs with time.



10.0 Antenna Effects
--------------------

10.1 Antenna/Radome Combinations:

- The use of radomes should be avoided at sites to be used for
inter-technique comparison unless needed for antenna protection.

- Only radomes that have repeatable calibrations and mountable with
reproducible physical relation to the antenna (centered position,
azimuthal orientation) should be introduced into the IGS network.

- Combinations of antennas and radomes that are already calibrated by
Geo++ and/or NGS should be introduced into igs_01.pcv (possibly at
the time of the adoption of absolute antenna phase center
corrections).

- If new radome calibrations become available, the impact on the RF
realization will have to be checked before introduction.

- If an existing non-calibrated pair is removed from a station, it
should be calibrated for any future re-analysis.

10.2 If available, physically distinct subgroups of antennas should
be introduced into the Þles rcvr_ant.tab and igs_01.pcv.

10.3 RF sites should install local antenna arrays in order to
guarantee the stability of the global terrestrial reference frame on
the (sub-)mm-level.

10.4 The ANTEX format (relative or absolute offsets and patterns)
should become the official IGS format.

10.5 Timescale for decision on absolute phase center models:

- Adopting absolute receiver and satellite antenna calibrations
should be considered according to the following plan:

   By June 2004: Reconciliation of the satellite antenna phase center
offsets and patterns and offsets between the groups generating these
results.

   Sep-Dec 2004: IGS AC submission of final products with both relative
and absolute phase center models used.

   Jan 2005: Evaluation of the effects of relative and absolute phase
center models.

   March 2005: Decision on the adoption of absolute phase center models.

- Actions:

   Values for old PRNs and GPS Block SVs (particularly Block I) are needed.

   Possible time dependence of values as fuel expended on satellites
should be explored.

   Elevation angle cut off tests with relative and absolute phase
center models (orbits free!) should be performed and evaluated.



11.0 IGS Troposphere Combination Products
-----------------------------------------

11.1 The Troposphere WG will consider alternate approaches including
the following proposal for IGS Trop product generation:

- Discontinue the current IGS Ultra Rapid trop product for lack of
use (leave operational weather forecasting applications to regional
networks)

- Replace the current IGS Final trop product with a higher quality,
higher efficiency Final product based on the IGS Combined orbit and
clock solutions

- Carry out regular comparison with AC Final trop solutions

- Carry out periodic comparison campaigns with independent techniques
(WVRs, VLBI,  radiosondes), and other GPS solutions

- Immediately reprocess 10-12 years of data from all IGS sites to
establish long-term consistent climatology


11.2 Met sensors:

- Short term: Request input from the community about interest in the
calibration problem, preference of solutions

- Long term: Work with the World Meteorological Organization on the
transfer of ownership of met packages to weather bureaus (with help
from NOAA/FSL)



12.0 IGS Ionosphere WG
----------------------

12.1 The use of the final IGS product is quite large (154,000 IONEX
files downloads in 2003, 68% from Non-IAAC users). However for the
rapid product, started in Dec 2003, very few downloads are registered
from its temporarily server at UPC. In this context to promote its
use, the next actions items have been adopted:

- To send a new e-mail to the IGS e-mail list.

- Moving the igs-iono e-mailing list to igscb.

- Moving rapid product server from UPC to CDDISA.

12.2 After receiving inputs from VLBI, Altimeter and Timing users, it
has been decided to:

- Maintain the present generation of both final and rapid IGS TEC maps.

- Include the list of GPS receivers used for timing in the list of
IAAC used stations to compute the ionospheric product, in order to
ensure IGS DCB estimations for such receivers.

12.3 There has not been consensus between the IAACs on increasing the
temporal and spatial resolution of the present ionex files, including
densification.

12.4 The Ionosphere WG should suggest a suitable method to remove or
mitigate the 2nd order ionospheric error for the ACs to apply in
their data reduction. This activity might be performed in
collaboration with the IAG ionospheric sub-commission.



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