**ILRS Technical Workshop**

####### Kötzting, Germany

## October 28 - 31, 2003

![](media/image1.jpeg){width="3.779861111111111in"
height="2.9972222222222222in"}

## Agenda

**Theme:** Working toward the full potential of the SLR capability

**Areas of Attention:**

-   What are our current limitations and how do we overcome them?

-   How do we reach 1mm accuracy for each laser ranging component
    (ranging machine, refraction, spacecraft center-of-mass, data
    processing, etc)?

-   How do we monitor our improvements?

##### Tuesday, October 28 

***9:00 Opening***

**Welcome --** U. Schreiber (TUM/ILRS) (5 minutes)

D Grünreich or J. Ihde (BKG) (5 minutes)

H. Drewes (CSTG) (5 minutes)

**Introduction --** Werner Gurtner/Mike Pearlman (ILRS) (15 minutes)

Topics and goals of the workshop, schedule, etc.

**Invited --** Markus Rothacher

> The special role of SLR in view of inter-technique combinations

***10:00 -- 10:30 Break***

**Operations** (How do we make our current systems work better?)

**1. A review of station performance and data throughput (20 minutes)**

> *(CB - Mike Pearlman, Van Husson)*
>
> Is our network performance and data throughput improving? What should
> we expect? Where are the big deficiencies? Where would improvements
> have the biggest impact?

**2. Daylight ranging (1.5 hours)**

> *(NE WG - Werner Gurtner, Ulrich Schreiber)*
>
> Why is daylight ranging so important? Why is the daylight ranging so
> poor? What is the experience at the most successful stations? How do
> we improve it? What are the hardware issues? What are the software
> issues? Would better predictions help? What are the current
> limitations? Each station should come prepared to participate.

***12:30 -- 14:00 Lunch***

**3. Implementation of the new Engineering Data File (1 hour)**

> *(NE WG - Georg Kirchner, Van Husson)*
>
> What is the definition of the Engineering Data File and its
> components, desired results and expectation, status of development,
> plan for development and implementation? How do we get all of the
> information and how do we make it user friendly?

**4. Local survey monumentation and eccentricity measurement (0.5
hour)**

> *(Wolfgang Schlueter)*
>
> Summary of the IERS Meeting on the IERS Workshop on Site Collocation
>
> How do we get the long-term, stable observation monument and local
> survey control monument network for all laser ranging systems, fixed
> and transportable so we can connect our measurement over space and
> time to 1 mm? What are the recommended local survey, computation, and
> archiving methods necessary to achieve the desired accuracy (1 mm) for
> the eccentricity values. How do we design the local networks? How do
> we know that we are achieving 1 mm? Is it possible to do this using
> local permanent control methods rather than launching episodic survey
> campaigns? How long has it been since local surveys were conducted at
> stations?

***15:30 -- 16:00 Break***

**5. Improved data QC at the stations (1 hour)**

> *(NE WG - Georg Kirchner, Van Husson)*
>
> What data errors are presently seen at the Data Centers? What data
> screening do stations currently perform? What minimum standards should
> we establish to test data integrity and \"reasonableness\" of content?
> Should we have standardized routines? Should they be mandatory? Should
> we delete data that does not satisfy a minimum requirement? Each
> station should come prepared to participate.

**6. How shall we handle dynamic priorities? (1 hour)**

> *(Mike Pearlman)*
>
> How do we make localized priorities and tradeoffs? Should different
> applications be prioritized differently? How do we protect sensitive
> satellites such as ICESat and ADEOS-II? How do we address difficult
> missions like LRE? A strawman will be presented for discussion
> purposes.

***18:00 End of Session***

######### Wednesday, October 29

***9:00 Begin Session***

**7. System Calibration (2 hours)**

> *(NE WG - Ulrich Schreiber, Ivan Prochazka)*
>
> Have we made any improvements since the Florence Workshop? Are
> stations monitoring their calibrations and testing for reasonableness?
> What is our experience to date on timing unit calibration? What can we
> do to maintain the calibration consistency over periods without
> tracking? Should we be doing more frequent barometer calibrations?
> Stations should come prepared to participate.

***11:00 -- 11:30 Break***

**8. Refraction modeling (1 hour)**

> *(DFP WG - Stefan Riepl, Erricos Pavlis)*
>
> How good are our zenith delay models and our mapping functions? How
> much improvement are we seeing from newer models and how much can we
> expect? How large are the influences of horizontal refractivity
> gradients and nonlinear effects? What are we getting from the
> low-altitude tracking? Which missions need improved refraction
> corrections?

**9. Developing and implementing the new consolidated prediction format
(0.5 hour)**

**Cancelled**

***12:30 -- 14:00 Lunch***

**Modeling** (How do we make better corrections to our data?)

**10. LEO Data Submission - how fast is fast enough? (0.5 hour)**

> *(DFP WG - Graham Appleby, Rolf Koenig)*
>
> What do we really need? What kind of turnaround is reasonable?
> Practicable? How fast can all of the components of the system respond?
> What are we going to do with it? Do all LEO satellites need this?

**11. Spacecraft center-of-mass modeling (1 hour)**

> *(SP WG - Graham Appleby, Toshi Otsubo)*
>
> How does the satellite center of mass depend on station configuration
> and operating conditions? How do we develop systematic and realistic
> ways of quantifying return energy level and other pertinent
> parameters? Where do we stand in cataloguing the spacecraft
> cross-sections and center-of-mass corrections? How do we bring in
> supporting information like spacecraft attitude? How good are the
> dispersion models for dual wavelength applications? How do we make
> this user friendly for the analysts?

***15:30 -- 16:00 Break***

**12. Two-Wavelength Tracking (1.5 hour)**

> *(NE WG - John Degnan, Cinzia Luceri)*
>
> What is the experience to date with dual two-color ranging? What are
> systems getting now? What advance in refraction correction can we
> expect from the current two-wavelength systems? Emphasize the critical
> role that the current systems are playing in the development and
> evolution of the two-wavelength systems, What parameters will we need
> to reach the 1-2 mm refraction correction, etc. What satellites are
> most important for two-color work? Should we accommodate both separate
> range measurements and differential measurements? Is the calibration
> procedure well defined? What changes do we need in data flow,
> formatting, and archiving? What changes are needed in data reporting?
> What do the analysts need to use the data and make it useful? Do we
> need wavelength dependent center-of-mass corrections? Is there a
> polarization issue?

***17:30 End of Session***

***Evening***

***20:00 -- 22:00 Governing Board Meeting***

######### Thursday, October 30

***9:00 Begin Session***

**Analysis** (How do we generate the best data products?)

**13. Pilot Projects (1.5 hours)**

> *(A WG -- Graham Appleby, Mark Torrence)*
>
> Where do we stand on the Pilot Projects? How well do the different
> groups agree? How well are we doing with analytic studies of system
> bias? Are we getting consistent results from different analysis
> centers? What is the prognosis for consolidated products? What are our
> current data and modeling limitations? What are the dominant issues?

***10:30 -- 11:00 Break***

**14. New approaches (0.5 hours)**

> *(A WG -- Erricos Pavlis, Mark Torrence)*
>
> Are there new approaches for analyzing data? Multi-satellites?
> Multi-technique? How far can we reduce our observation interval
> (days?)? Why is there and offset between SLR and GPS? How is SLR used
> for altimetry? Are there better ways to use the data? What new data
> products should we provide (Geocenter time series, time varying
> gravity field, orbits, etc.)?

**15. KHz ranging and its impact (1 hour)**

> *(NE WG - Georg Kirchner, John Degnan)*
>
> Where do we stand with kHz ranging? What are the advantages and
> problems? How soon can we expect a useful flow of data? Who is
> building these systems? What is the impact on the ILRS data system?
> Can we rethink the definition of normal points?

***12:30 -- 14:00 Lunch***

**New Technologies** (Where are we going?)

**16. SLR 2000 (1 hour)**

> *(NE WG - John Degnan)*
>
> Where do we stand? What major issues still need to be resolved? When
> do we expect the prototype to be operational? What is the plan for
> replication, testing and deployment?

**17. Automation (1 hour)**

> *(NE WG - Ben Greene, Werner Gurtner)*
>
> What does automation mean? What is our experience to date? What can we
> reasonably expect? How does this interface with remote control?

***16:00 End of Session***

***20:00 - 23:00 Banquet***

# Friday, October 31 

**9:00 -- 9:20 Keynote from IVS**

*(W. Schlueter)*

**9:30 -- 12:00 Plenary Session**

> *(Mike Pearlman, Werner Gurtner)*
>
> Summary of each session by the session chairs.
>
> Evaluation of the workshop. Correct format?
>
> Discussion of Operational and Administrative Topics

***12:00 End Of Workshop***

**ILRS Workshop**

####### Kötzting, Germany

## October 28 - 31, 2003

**Workshop Summary**

Approximately every two years the ILRS holds its traditional
International Laser Ranging Workshop to examine progress in the relevant
technologies and system developments that are pacing the evolution of
SLR. This tradition has built up over the last thirty years with the
Thirteenth Workshop being held in Washington in October 2002 and the
Fourteenth planned for San Fernando, Spain in June 2004. This venue
gives the analyst, the practitioner and the station representatives the
opportunity to see what\'s new and who is doing it.

Periodically special workshops are held to focus more on seeking
solutions to engineering and operational problems or to improving
operations and performance in an organized manner across the network.
Sometimes these meetings are held in conjunction with the traditional
workshops; other times they are organized separately to concentrate our
energies on the issues at hand.

In October 1995 a meeting on New Technology was held in Wettzell. In
September 1999, a meeting on calibration issues was held in Florence. In
March 2002, a meeting on the detection and elimination of errors was
held in Herstmonceux.

The Koetzting Workshop was organized to examine how we could work more
effectively toward achieving the full potential of the SLR capability.
We wanted to ask such questions as: What are our current limitations and
how do we overcome them? How do we reach 1 mm accuracy for each laser
ranging component (ranging machine, refraction, spacecraft
center-of-mass, data processing, etc)? How do we monitor our
improvements?

We were fortunate to have Prof. Markus Rothacher as an invited speaker
from the IERS to start the meeting off with a presentation on the
special role of SLR within the context of inter-technique combinations

Topics discussed in the sessions included improved data throughput,
daylight ranging, comprehensive on-line engineering files,
inter-technique ground survey techniques, improved quality control at
both the data centers and at the stations, the implementation of dynamic
priorities, improved system calibration, better refraction models, rapid
data throughput on LEO satellites, improved spacecraft center-of-mass
corrections and documentation, two wavelength ranging, and the emerging
role of automation to SLR.

We also had the opportunity to review the Analysis Working Group
progress in its Pilot Projects and their advancement toward unified ILRS
data products, new approaches in analysis techniques, and kilohertz
ranging including the SLR2000. We also had a presentation from one of
our sister services, the IVS, on its present activities and future
plans.

In this document, each of the session organizers has provided a summary
outlining the key issues and either solutions with recommendations and
actions items or a plan of how to arrive at that point.

The plans, recommendations and action items elucidated will be used by
the ILRS for guidance on how to set our priorities and how to proceed.

We wish to thank Ulrich Schreiber, Thomas Klügel, the BKG, and the
Wettzell Station for their hospitality in hosting this meeting in
Koetzting.

**The Special Role of SLR for Inter-Technique Combinations** (Markus
Rothacher)

**Introduction**

The IERS is working towards construction of a rigorous combination of
results from space observation techniques. This combination will make
use of the strengths of the individual observation techniques and profit
from co-location of instruments at both sites and satellites. The effort
is to ensure consistency between all techniques by identifying the
difference between technique-specific systematic biases and genuine
geodetic/geophysical signals. The final goal is to assure that all
common parameters of all observing techniques are rigorously combined as
consistent input to the IGGOS project (Table 1).

Table 1. Parameter Space for Combinations; "X" indicates contributes to
the parameter estimations, "(X)" indicates some contribution.

  --------------------- ---------- --------------- ----------------- --------- --------- ---------------
  **Parameter Type**    **VLBI**   **GPS/GLON.**   **DORIS/PRARE**   **SLR**   **LLR**   **Altimetry**

  Quasar Coord. (ICRF)  X                                                                

  Nutation              X          \(X\)                             \(X\)     X         

  Polar Motion          X          X               X                 X         X         

  UT1                   X                                                                

  Length of Day (LOD)              X               X                 X         X         

  Sub-Daily ERPs        X          X                                                     

  ERP Ocean Tide        X          X                                 X                   X
  Amplitudes                                                                             

  Coord.+Veloc.(ITRF)   X          X               X                 X         X         \(X\)

  Geocenter                        X               X                 X                   X

  Gravity Field                    X               X                 X         \(X\)     X

  Orbits                           X               X                 X         X         X

  LEO Orbits                       X               X                 X                   X

  Ionosphere            X          X               X                                     X

  Troposphere           X          X               X                                     X

  Time/Freq. Transfer   \(X\)      X               \(X\)                                 
  --------------------- ---------- --------------- ----------------- --------- --------- ---------------

**Station Height**

SLR is in a unique situation concerning atmospheric refraction. The
troposphere is dispersive for SLR, but not for microwaves. For SLR, the
dry delay can be modeled with pressure data at the site, and the wet
delay is quite small. All other techniques (GPS, VLBI, DORIS, altimetry,
InSAR, etc.) suffer from the same tropospheric refraction effects, and
similar ionospheric refraction effects. These other techniques have
tropospheric refraction as the or a major error source. Only SLR can
help to detect biases due to atmosphere mis-modeling common to all other
techniques. The wet delay for GPS/VLBI/DORIS can be up to 40 cm whereas
for SLR it is only up to 6 mm. The troposphere zenith delay parameters
have to be estimated for microwave techniques, but not for SLR.

SLR is the only technique where, in general, no clock corrections have
to be estimated. GPS phase analysis requires the estimation of receiver
and satellite clock parameters for every measurement epoch. VLBI
analysts typically parameterize receiver clock corrections about every
hour (between stations). DORIS analysis requires a clock bias to be
estimated. The estimation of clock parameters degrades the recovery of
station height. However the estimation of a range bias for an SLR
station corresponds to a receiver clock correction as estimated in
GPS/VLBI/DORIS analyses and "destroys" the much more favorable height
estimation of SLR.

In summary, microwave techniques have inherently less accurate height
estimation because of the extreme correlations between the estimation of
height, clock, and troposphere zenith delay. Height estimation should be
no problem for SLR; the heights should be "perfect." The SLR technique
is very important for absolute height estimation, but the estimation of
range biases should be avoided to the extent possible in SLR analysis.

**Absolute Scale**

GPS receiver and satellite antenna characteristics are generally not
accurately known, changing with elevation and azimuth. This variation
can cause GPS results to be elevation cut-off dependent. There may be
large systematic effects due to receiver and satellite antenna phase
center corrections. Additionally, any change of GPS receiver or antenna
equipment causes jumps in height estimation. GPS is too affected by
these systematic effects to allow accurate absolute height and absolute
scale determination in the global network and for ITRF. VLBI telescopes
are huge heavy structures resulting in telescope deformation. The
deformation may bias VLBI results through some elevation dependence. SLR
does not have the antenna problems of either GPS or VLBI; any
deformation should be small, and there are no phase center variations.
So SLR is well suited to define scale of ITRF, but the calibration of
the SLR system has to be very accurate and reliable.

**Geometry and Gravity**

SLR is one of the major techniques establishing a link between the three
pillars of geodesy: geometry, Earth rotation, and gravity. SLR is a
critical technology for the determination of the geocenter: the relation
between origin of ITRF and low order harmonics coefficient C~(1,0)~,
C~(1,1)~, and S~(1,1)~ of the Earth's gravity field, and the principle
axes of the Earth's inertia tensor: the relation between Earth rotation,
orientation of the gravity field (C~(2,1)~, S~(2,1)~), and ITRF
orientation. The SLR data enables the determination of gravity field
variations that are due to exactly the same Earth processes (e.g. from
geophysical fluids) as variations in Earth rotation and deformation
(geometry). SLR is a link between the three pillars of geodesy, and is
very important for the IGGOS integration concept and for IERS reference
frame definitions.

**Validation of satellite orbits**

SLR measurements are successfully used to validate GNSS (GPS, GLONASS,
in future, Galileo, etc.) orbits However, there still is a systematic
bias of about 4-5 cm between GPS orbits determined from microwave and
SLR observations; the source of this offset is still unclear. SLR may
also become important for Galileo satellite orbit validation. Much
progress has been made in precise orbit determination of LEOs thanks to
validation with SLR allowing the detection of systematic orbit errors.
SLR allows the evaluation of different estimation strategies (kinematic,
reduced-dynamic, etc.) and software packages. SLR observations are
extremely valuable for POD studies, and POD becomes ever more crucial
for new satellite missions.

**Conclusions:**

-   There are few SLR observations, but there are also few parameters to
    estimate.

-   SLR is the only optical technique (unique to assess atmospheric
    biases).

-   No troposphere estimation necessary for SLR (correlations, biases).

-   No clock correction estimation is necessary for SLR (correlations,
    biases) which allows good height estimation.

-   SLR can reliably estimate absolute scale and heights because it does
    not have theantenna problems of other techniques.

-   SLR provides an important link between gravity, Earth rotation, and
    geometry.

-   SLR is crucial for progress in POD.

SLR is a unique partner in the goal of a rigorous combination of the
space geodetic techniques (IERS and IGGOS). The individual techniques
should feel as a part of a larger whole with a common goal rather than
working in competition. Interlinked, the techniques are much stronger
and it is more difficult to isolate and cut individual techniques.

**Session 1. A review of station performance and data throughput ([Mike
Pearlman]{.underline})**

The network data yield continues to increase as more satellites are
added to the tracking roster and stations implement more automation. The
bulk of the data however are still generated by a small number of
stations. In the last quarter, five stations provided more than half the
data; sixteen stations provided 75% of the data (see below). One third
of the stations are not providing data at sufficient volume to be of
routine use to the analysts. Most of the stations are delivering their
data within 4 hours (see below). A few are in the neighborhood of 12 to
18 hours, in part due to operational constraints. Transmissions problems
also play an occasional role.

All but a few of the stations provide data of sufficient quality at the
moment, but there are still range bias issues. We must implement
engineering tests at the field stations to recognize and address
problems early and at the point of local expertise.

In response to the great diversity in data production and the relative
utility to the user, the ILRS has implemented a regime of station
qualification. As of January 1, 2004, those stations performing above
minimum qualification criteria will be categorized as Operational
Stations. Those that do not meet the criteria will be categorized as
Associate Stations. At the moment, 16 stations are performing below the
operational level. It is anticipated that the minimum qualification
criteria for Operational Stations will increase over time to keep pace
with increased demands on the network and the ILRS.

In mid-2004, the ILRS will add a requirement for a collocated IGS
receiver at each Operational SLR Station. Most of the SLR stations
presently performing at the operational level already satisfy this
requirement.

It is recognized that mobile stations may occupy sites for short periods
of time for specific applications (e.g., FTLRS in Chania for altimetry
support and TROS in Urumqi for the Chinese National Survey program).
These systems should probably be characterized some other way, since
they do not lend themselves to the long-term reference frame
requirements of SLR.

A more detailed look at the distribution of satellite support shows that
we need to do a better job tracking both the very high and the very low
satellites.

> ![](media/image2.png){width="5.997222222222222in"
> height="3.7958333333333334in"}
>
> ![](media/image3.png){width="6.0055555555555555in"
> height="3.7958333333333334in"}

**Session 2. Daylight ranging (Werner Gurtner, [Ulrich
Schreiber]{.underline})**

As one can see from the statistics below there is a considerable
imbalance between passes taken in daylight and passes taken at
nighttime. The figure below gives an overview of the network status.

![](media/image4.png){width="6.4944444444444445in" height="4.03125in"}

Turbulence as a very local condition might be contributing with as much
as 10 arcseconds of error in pointing, increasing the difficulty in
acquiring data. However this will apply only to systems with a very
collimated beam. Degradation of the telescope orientation by direct
sunlight exposure may require a mount-model based on star observations
during the day. Mount shielding helps to avoid deformation of the
telescope in the first place.

**Action Item:** The BC will work with stations that can do daylight
mount modeling such as Herstmonceux, Grasse, SLR2000 and Wettzell to
make information available about how this capability can be achieved.

Predictions are also an important issue for daylight ranging,
particularly for those stations with narrowly collimated beams. Systems
with a larger beam divergence and relatively large field of view have
less trouble ranging in daylight hours as long as the have adequate
signal to noise ratio.

The HTSI predictions (which are issued daily but cover a 5 day span)
show a 10 - 15 msec timing offset at midnight UT. This is a consequence
of the IRV tuning process. HTSI has written a software repair, but it
has yet to be implemented.

**Action Item:** HTSI should implement the repair to its IRV tuning
process to correct the midnight offset problem.

An offset of up to 15 msec is observed in GFZ predictions for GRACE and
CHAMP. The AIUB server currently provides both drag functions and time
offsets for the predictions.

**Action Item:** The CB will ask GFZ if they can speed up the prediction
cycle for CHAMP and GRACE to improve data acquisition.

There is also a midnight-offset problem with the ESOC predictions on
Envisat. The problem stems from a lack of IRV tuning. NERC SGF has
alerted them about the problem.

**Action Item**: The CB will remind ESOC of the offset issue and see if
we can offer them any help.

It was also suggested that we use the CODE predictions for GLONASS and
GPS, which are generated from precise orbits based on the IGS tracking
network.

SLR systems with kHz repetition rates have demonstrated a much reduced
acquisition time.

**Recommendations:**

-   Stations: Some stations are using a narrow spectral filter and a
    frame grabber in order to adjust the pointing of the transmit
    system. Graz uses a filter of 0.3 nm around the wavelength of the
    laser, while Herstmonceux uses a 0.15 nm filter.

-   Prediction Centers: Shorten IRV prediction generation cycles for the
    difficult satellites to the shortest meaningful value!

-   Stations, Data Centers: Keep the delay of LEO data submission as
    short as possible!

-   Strict request from the Analysis group: Day and nighttime
    observations give a much better data distribution of observations
    and therefore enhance the quality of the SLR measurements.
    **Stations, please observe day and night!**

**Session 3. Implementation of the new Engineering Data File ([Georg
Kirchner]{.underline}, Kalvis Salminsh)**

The Engineering Data File (EDF) concept mainly consists of storing all
available information for each calibration record, and, in the future,
for all ranging records; the stored information should contain all the
relevant hardware descriptions, and all of the results collected during
calibration including statistical values, meteorological values, and any
additional information that is desired by the station.

There are several advantages of storing all this information:

-   A continuous, consistent technical history of the station is created
    automatically, allowing checks of continuity, jumps, drifts etc. of
    any stored parameter at the station, whether it is recorded manually
    or automatically.

-   The same files will be a valuable tool for analysts to correlate,
    results from orbital analysis with the technical history of any
    station.

-   Stations can now easily compare their specific setups and their
    respective results, allowing better insights into deficiencies,
    capabilities, possible improvements, etc.

-   All that can be done with simple software or scripts, which
    furthermore, once written and tested, can be used by other stations.

Kalvis Salminsh from Riga investigated various possibilities to
implement this EDF concept. Based on his research, he recommended that
the EDF be implemented in the Extensible Markup Language (XML) format.
XML is a standard way of describing data (i.e., independent of any
programming language or application). XML is a text-based language for
creating structured documents, and an open standard managed by the World
Wide Web Consortium (W3C). XML is also supported with a great deal of
freeware or inexpensive tools. Furthermore, new tags to define specific
data can be easily added, with full backward compatibility.

A so-called schema contains the names of the parameters which are to be
stored in the XML file; an initial version of this schema has been
developed already and will be distributed, together with installation
guide lines, examples, tools, etc., to those SLR stations which have
already agreed in Koetzting to participate in this test phase
(Herstmonceux, Concepción, Zimmerwald, Wettzell, Potsdam, and Graz)
within the next few weeks. This schema will be maintained or augmented
by Kalvis according to any requirements from the participating stations.

In a first test, using this preliminary schema, calibration data from
Herstmonceux and Graz for July 2003 were converted by simple scripts
into XML files. As a result, a simple comparison between these two data
sets showed a plot of a small, remaining calibration constant drift over
temperature, with the same scale and direction for both stations,
pointing to a similar slight temperature dependence of the SPAD power
supply used in both stations.

Such an XML based data exchange/data storage system could also be used
for other SLR issues, e.g., the ILRS Normal Point (NP) format. We should
also consider developing an XML schema for NP files, which could then be
used in parallel with the present NP format. The advantage of such an
approach is that in the XML NP file, any desired additional information
can be packed, for use by the analysts, without having to go through a
long, cumbersome, and by no means backward-compatible, change in the NP
data format. Similar advantages could be provided with XMP for a new
prediction format system, or for the site logs.

**Session 4. Local survey monumentation and eccentricity measurement
(Wolfgang Schlueter***)*

A workshop devoted to "site co-location survey objectives, methods, and
issues" was organized by the International Earth Rotation and Reference
Systems Service (IERS). The workshop took place in Matera, Italy on
October 23-24, 2003.

Presently, the quality of the station coordinates resulting from the
various space techniques is in the order of a few millimeters in the
horizontal and about 1 cm in the vertical component. Precise local ties
are the perfect link to enable a rigorously combination of all space
observing techniques by their common parameters in order

-   to study the systematics going along with each space technique and,

-   to establish a unique, high-precision terrestrial reference frame.

In previous analyses, one of the major limiting factors was the
characteristic and the availability of accurate local tie information
for all co-located sites around the globe. In 5 sessions at the
workshop, attention was given to:

-   Co-location sites and their importance for the ITRF

-   Site surveys

-   Analysis and SINEX

-   Reporting and Archiving

-   Planning for 2004

More than 30 participants from Australia, South Africa, USA, and Europe
discussed various examples, analysis methods, and survey strategies.
Finally, guidelines for co-location site surveys and report templates
were proposed. The potential availability of survey teams as well as the
planning for surveys in 2004 were investigated.

The important recommendations are the following:

-   Local ties between co-located instruments should be determined with
    an accuracy of 1 mm, with full variance/covariance information,
    available in SINEX format.

-   Local survey measurements should have the same importance as and
    should be treated like any of the space geodetic techniques. Site
    coordinates (VLBI, GPS, SLR, DORIS) should be better tied to the
    ground. The quality of local ties should be such that they can be
    assumed true for the combination.

-   All GPS sites close to other geodetic techniques should be part of
    the IGS routine processing.

-   A database will be established at IERS (Central Bureau and ITRS
    Product Centre) for all information in connection with site
    co-location (list of co-location sites, local ties in SINEX,
    co-located instruments, site maps and pictures, survey reports,
    survey status, site events and history, etc.).

To support the local tie activities an IERS Working Group on Local
Survey will be recommended for adoption by the IERS Directing Board. The
charter of the working group will be the coordination and assistance in
local tie analysis as well as SINEX file generation.

The workshop information (program, presentations, and recommendations)
is available at the IERS webpages
(http://www.iers.org/workshop_2003_matera/programme.html). The
proceedings will be published as IERS Technical Note No. 33.

**Session 5. Improved data QC at the stations ([Georg
Kirchner]{.underline}, Mike Pearlman)**

SLR has a long history of improvements in measurement capability and
ranging accuracy. It also has a long history (or "tradition") of
providing data that are sometimes biased, corrupted, or incomplete. Some
of these problems are detected at the data centers during routine checks
of incoming data, others are visible only after orbital fits, etc.

Not all of these problems can be detected by checks at the stations;
however, the most trivial errors, such as meteorological data out of
range, jumps or drifts in calibrations, noise instead of real data, etc.
can be easily seen by simple checks and by following some procedures at
the stations before sending the data. The general idea is to strive for
delivery of checked, valid data. In view of the amount of SLR data
produced today, it was also suggested that we reject any suspicious, or
obviously erroneous data to improve overall SLR data quality, and to
push SLR stations to check their data and their procedures.

A survey of stations performed some months ago showed that only a few
stations are checking normal point (NP) data before sending them to the
data centers; most stations do not apply ANY checks.

To facilitate implementation of such basic checks at all stations,
Herstmonceux agreed to upgrade its internal NP check program for easy
implementation and general use at any SLR station. A first version of
that program, including source code, implementation guide, examples for
station template, etc., will be made available shortly.

At the EUROLAS workshop in Herstmonceux in spring 2002, a recommendation
was made to require a minimum number of SLR data points per NP to
improve NP data quality by avoiding poorly defined NPs. The criteria
suggested was a minimum of six data points per NP in daylight and three
data points per NP at night. These criteria were relaxed for stations
with low data rates. The ILRS Governing Board subsequently agreed with
this policy.

However, in a lengthy discussion in Koetzting, there were some counter
arguments were made:

-   From analysts: in spite of the available amount of SLR data, they
    would like to have EVERY measured point.

-   In the case of multi-photon receivers, even single point NPs are
    well defined.

-   In the case of very small (FTLRS) or low-repetition-rate (Metsahovi)
    SLR stations, it was always agreed that this process could be
    relaxed.

-   Each NP data point includes the number of full-rate data points that
    were included in its formation, thereby providing the analysts with
    a mechanism for screening.

At the Workshop in Koetzting, we agreed that the criteria could be
relaxed and that stations could exercise their own discretion in setting
minimum criteria (even a single data point) if they felt that it
improved their data quality. Such a criteria must be documented as a
comment in their site logs.

The ILRS Governing Board has redefined the minimum data point
requirement as:

-   Stations may set a minimum data requirement too low return rates
    should select their minimum points per NP as required;

-   In general, the recommendation of 2002 (minimum points per NP: six
    for day, three for night) is still valid;

-   Stations with high return rates could select also higher criteria;
    e.g., Graz with the kHz laser system is currently using a minimum of
    100 points per NP for ALL satellites and has announced to possibly
    increase that to 600 points per NP for LEOs because of much better
    definition of the NPs in this case.

Additionally, a minimum number of NPs per pass was also discussed;
however, analysts again wanted to see all of the data especially if
single NPs could be well defined ones and data were very sparse (e.g.,
pass length limited by clouds).

**Session 6. How shall we handle dynamic priorities? ([Mike
Pearlman]{.underline})**

We have examined a number of schemes for dynamically adjusting SLR
tracking priorities to try to compensate for a dearth or an
overabundance of data from specific satellites. The basic framework was
that it must be (1) easy to understand and implement and (2)
interpretable by each station even though different stations might be
have different levels of success with different satellites.

The ILRS has traditionally prioritized satellites by orbital parameters
and application. Currently 21 satellites are given 21 different
priorities, sometimes distinguished by only a very insignificant
difference in orbital parameters.

The new scheme proposed and approved by the Governing Board place the
satellites in four categories, with the satellites in each category
having equal priority. In each category, stations should try to equalize
the number of passes on each satellite over the course of a few days. If
the station has had many passes on one satellite on Category 1 and few
passes on another, a little greater stress should be place on the
satellite with weaker tracking to try to increase its data yield.

  ----------------- ----------------- ----------------- -----------------
  **Category 1**    **Category 2**    **Category 3**    **Category 4**

                                                        

  GRACE-A           GFO-1             Starlette         Etalon-1

  GRACE-B           Envisat           Stella            Etalon-2

  CHAMP             ERS-2             Beacon-C          GLONASS-89

                    Jason             Ajisai            GLONASS-87

                    TOPEX/Poseidon    LAGEOS-1          GLONASS-84

                    Meteor--3M        LAGEOS-2          GPS-35

                                                        GPS-36
  ----------------- ----------------- ----------------- -----------------

The Central Bureau will provide a daily index (+1/-1) to raise/lower the
priority within each category of satellites that need more/less
attention. The indices will be based on a minimum number of passes that
we anticipate every 2 -3 days.

**Action Item:** CB will post the priority table daily with the updated
indices on the AIUB server, so it will be available when stations access
their prediction updates.

**Session 7. System calibration ([Ulrich Schreiber]{.underline}, Ivan
Prochazka)**

1.  Gross errors must be detected and diagnosed at the stations:

-   Meteorological data

-   Calibration values

> Unless something has been adjusted, no change in the calibration value
> should occur between passes within a few hours. The stations should
> use the history of past measurements to crosscheck for data integrity.

2.  Quite a few blunders have appeared over time in the submitted data.
    This raises the question: "If Van can find them, then why not the
    stations?"

**Action Item:** All stations should generate a procedure to monitor
consistency of their (calibration) measurements. Van Husson will provide
a set of tests to help identify problems and possibly standardize
procedures but monitoring the time history of key parameters can be a
very useful diagnostic tool.

3.  Target structure and geometrical telescope properties are not a big
    problem for stable calibration. If they were you would not receive a
    signal through the telescope to the detector.

> **Action Item:** Verify the survey of your system and target at least
> every two years.

4.  Electronics introduce time dependent errors. Those enter the
    measurements as range biases. The smaller the time dependence, the
    higher is the probability of unbiased data.

5.  Asymmetry between calibrations and ranging conditions (e.g., signal
    level differences) must be understood and remedied.

6.  Signal return properties such as mean, peak-mean, skewness and
    kurtosis are key indicators of performance and may serve in the
    process of checking consistency.

# Action Item: Use this extra information

7.  Redundancy: A powerful way of checking system performances is system
    redundancy. There are many ways of creating redundant systems and
    use them in comparison.

-   Co-location (costly)

-   Cluster operation

-   Portable Calibration Standard (flexible, cheap)

-   Inter-System redundancy (Engineering Data File)

The potential of the Portable Calibration Standard so far has not been
fully utilized.

8.  Inter-system redundancy is based on comparing the engineering data
    files exchanged with the other stations (future music).

> Action Item: Definition of essential parameters and setting up a team
> to explore these capabilities

General Calibration Recommendations from the workshops in Florence and
Toulouse:

-   Make the calibration procedure friendly (no manual system changes or
    re-adjustments).

-   Calibrate the SLR system at frequent intervals. Each satellite pass
    should have the corresponding pre and post calibration with maximum
    time span of one hour.

-   Keep a calibration history file of all relevant parameters for
    consistency verification.

-   Collect a sufficient number of valid echoes.

-   Plot the calibration histogram and compare with a Gaussian fit.
    Check for anomalies in the distribution.

-   Use optically correct calibration target(s).

-   Use efficient spatial filtering.

-   Use multiple targets at different azimuths and ranges to check the
    calibration setup and survey.

-   Ensure a perfect alignment of the SPAD optics.

-   Apply the gate for the detector early enough (50 ns and 100 ns for
    APD's.

-   Keep an appropriate echo data rate \< 15% for APD's \< 80% for
    C-SPAD.

-   Interpret properly the echo data rate.

**Session 8. Refraction modeling ([Stefan Riepl]{.underline}, Erricos
Pavlis)**

The session on Refraction Modeling featured the following contributions:

1.  Introduction Stefan Riepl

2.  Using ECMWF-Data for Refraction Modeling Ruediger Haas/Stefan Riepl

3.  Ciddor/Mendes Zenith Delay and Analysis of Low Elevation

> Tracking Data Erricos Pavlis

4.  Absorption Lines and Nonlinear Refractivity Effects Yuri Galkin

5.  Refraction Modeling Fritz Brunner

6.  Using GPS Determined Water Vapor to Estimate Tropospheric

Wet Path Delay Attie Combrink

According to presentations by Erricos Pavlis and Cinzia Luceri the
recently derived and tested refraction models show improvements in
comparison to the traditional Marini and Murray model. According to
Virgilio Mendes, a new formulation of the SAASTOMOINEN zenith delay
formula leads to a minimization of systematic errors on a global scale.
Attie Combrink presented a technique for local estimation of the wet
path delay using GPS tomography.

Yuri Galkin outlined the inclusion of nonlinear absorption line effects
in the determination of signal velocity. Fritz Brunner focused on a
rigorous derivation of a refraction model for inter-technique comparison
and two-color laser ranging. Such a rigorous derivation may also serve
as a base to formulate a horizontal refraction gradient model.

As a result of the nonlinear absorption line effects and the stringent
accuracy requirements for two color laser ranging, the Governing Board
was requested to initiate an investigation program focusing on the
feasibility of modeling the laser pulse propagation velocity in the open
atmosphere with a relative accuracy of 1 part in 10E9.

**Recommendations**: The Ciddor-Mendes refraction model should be
approved as the standard refraction model for SLR. Low elevation
tracking should be enhanced to improve refraction model testing.

**Session 9. Developing and implementing the new consolidated prediction
format (Cancelled)**

**Session 10. LEO data submission - how fast is fast enough? (Graham
Appleby, [Rolf Koenig]{.underline})**

The theme of this session was to explore the practicalities and
scientific need for rapid turnaround of laser tracking data from the
network into the data Centers. If there are scientific or operational
requirements for very fast availability of data which are perhaps not
being accomplished, then it is important to understand where the
hold-ups might be and what steps can be taken to improve the situation.
If, on the other hand, there are no compelling reasons for improving
upon the current cycle speed, then that situation should also be
recognized. It is likely, however, that some improvements for some
satellite missions will be identified.

# Requirements

Koenig presented a science case that would benefit from the rapid
availability of CHAMP laser tracking data. A GFZ operational program
takes observations from CHAMP of GPS satellites undergoing occultation
by the Earth's atmosphere, from which can be derived information on the
state of the atmosphere that is of value in numerical weather
prediction. The technique is shown schematically in the GFZ figure
below.

The essence of the technique is a fast turnaround of the products of the
analysis. Part of this process is precise orbit determination, which is
accomplished by a combination of GPS and SLR tracking data. However, the
situation at present is that only a few percent of the actual global
ILRS laser tracking data reaches GFZ in time to be included in this
particular, time-critical analysis. To improve upon the situation, SLR
data will have to be uploaded to and available at the data centers
within an hour of the observations being made.

The GRACE mission also includes an occultation experiment, which will be
turned on in the upcoming months. Koenig also introduced the TerraSAR-X
mission to start in 2006, where fast data availability will also be
needed for near-real time processing of orbits and derivation of
resultant SAR products of value in Earth surface monitoring as, for
example, rapid deformation measurements after earthquakes.

Gibbs (NSGF) presented some results comparing the quality of predictions
for LEO satellites as a function of availability of tracking data. The
rapid availability of SLR data is very important in feeding the time
bias solutions that are used in the Herstmonceux-Berne Time Bias Server.
Even for the now-standard daily IRV predictions, regular update of the
along-track error in the predictions (time bias) has a significant
impact on their quality. It was made clear from comments during the
session that, particularly for the low-signal systems that have to work
with narrow range gates, the quality of the predictions can make the
difference between ranging success and failure. This particularly of
course applies to the very low, high-drag satellites CHAMP and GRACE,
but to a large degree also to all the low-altitude altimeter satellites
GFO-1, ERS-2 and Envisat.

It was also noted (Appleby) that discussions within the Analysis Working
Group on providing a weekly coordinate and Earth orientation product may
generate a need for fairly rapidly available data from the LAGEOS and
Etalon geodetic satellites in order that the best products may be
computed.

![](media/image5.png){width="3.847916666666667in"
height="3.740972222222222in"}

> Overview of the operational infrastructure for measurements, data
> reception, transfer, analysis and distribution of CHAMP\'s radio
> occultation experiment (from Wickert, J., Beyerle, G., Schmidt, T.,
> Marquardt, C., Koenig, R., Grunwaldt, L., Reigber, C.: \"GPS radio
> occultation with CHAMP,\" in Reigber, C., Luehr, H., Schwintzer, P.
> (Eds.), First CHAMP Mission Results for Gravity, Magnetic and
> Atmospheric Studies, Springer, Berlin, Heidelberg, pp. 371-383, 2003)

**Practicalities**

Comments from those station personnel present at the session implied
that a range of issues determines the rates at which the stations
transmit the observations. For some groups it is normal to send the data
to the operations centers immediately after each pass; others have pre-
and post-pass calibration runs scheduled at specific times and as a
result have variable transmission delays of up to a few hours. For at
least one station the data processing is carried out during the daytime
following each night's observing session, implying a delay of several
hours. Neither ILRS data center (CDDIS, EDC) has any problem in
principle making the data available on a rapid turn-around, with EDC
planning to implement a 15-minute turn around cycle.

# Conclusions

There are scientific as well as ILRS-operational reasons for improving
the speed of availability of normal point laser data. Specific cases
involving CHAMP, the high geodetic satellites and the altimetry
satellites were cited and discussed.

**Recommendations:**

-   All ILRS elements should work towards making the data available to
    customers through the data centers as rapidly as possible, ideally
    within an hour of the observations being obtained.

-   The CB should put out a request to the stations to give the highest
    priority to processing observations of CHAMP, GRACE, GFO-1, ERS-2,
    and Envisat.

**Session 11. Spacecraft center-of-mass modeling ([Graham
Appleby]{.underline}, [Toshi Otsubo]{.underline})**

**Spherical Satellites**

The main issue addressed by this session was how to pull together work
that continues to be done on the dependence of satellite center of mass
(CoM) corrections on station configurations and operating procedures.
The spherical geodetic satellites are of most concern since they are
both physically large and of fundamental importance in exploitation of
the full potential of the SLR technique in defining the scale and origin
of the terrestrial reference frame.

The CoM issue is shown schematically in the drawing above. A Gaussian
pulse is transmitted from a ground station and reflected by all those
cube corner reflectors that 'see' the pulse. Because of the curvature of
the satellite, the reflected pulses will be both shifted in time
relative to one another and vary in amplitude according to the apparent
cross section and reflectance of the cubes as seen from the station. The
result, if one considers the effect averaged over many such
measurements, is a return pulse that is distorted and stretched with
respect to the transmitted Gaussian pulse.

![](media/image6.pct){width="2.9944444444444445in"
height="1.913888888888889in"}

Theoretical work was presented (Otsubo and Appleby) to compute the
return distributions from LAGEOS, Etalon, and Ajisai and to derive CoM
corrections that depend upon tracking system parameters such as laser
pulse length, detector (APD, MCP) and return signal strength (number of
photons). It was re-iterated that the most accurate CoM value could be
computed only for those systems working in the single photon regime. For
multi-photon systems, not only are the numbers of returning photons most
likely varying during passes, but also the effects on CoM corrections of
internal delays within discriminating electronics are very difficult to
ascertain. For the MCP systems it is likely that signal-strength
variations result in less systematic range errors than for the CSPAD
systems, but the CoM correction cannot be determined unambiguously.

Through orbital analyses for the Etalon satellites (Pavlis), it was
demonstrated that CoM corrections are indeed system dependent, and an
earlier CoM correction of 610 mm assumed appropriate for the MCP systems
was found to be too large by as much as 20 mm for most of them. An
investigation using full-rate data to estimate average return rates for
the major systems for LAGEOS and Etalon (Wilkinson) suggests that there
are significant differences among systems that may give some insight
into the causes of mm-level bias determined for each system. The effect
was graphically demonstrated in an analysis of LAGEOS-2 full-rate range
residuals from the MLRO system at Matera (Bianco, Arnold, *et al*). The
high return rate and very high precision of the data shows that the
range measurements are modulated at the 5 mm level by satellite-attitude
dependent variations in the mean reflection surface. The modulations
were used to infer the spin rate of the satellite.

**It was recommended** that all station managers should attempt to
quantify the effect on their range data of variable signal strength.
Test ranging should be carried out to LAGEOS, Ajisai and (say) Envisat
during which return levels are switched in a controlled way between
minimum and maximum return levels within the systems' dynamic ranges:

Results could be presented and discussed at the Laser Ranging Workshop
in June 2004 in San Fernando, Spain.

It was concluded that for all systems apart from those working strictly
in the single-photon regime, which will include the upcoming SLR2000
systems and for which accurate CoM values can be computed, analysts
should continue to estimate a system-dependent CoM offset, constrained
at say the 10 mm level. It is emphasized that this effect is not an
instrumental range error or 'bias', but merely an uncertainty at the mm
level in correcting very precise observations to the centers of mass of
the satellites. More details of this research are given on the ILRS
website at

> <http://ilrs.gsfc.nasa.gov/working_groups/signal_processing_wg/spwg_activities/>

**GNSS arrays**

Orbital analyses imply that the smaller arrays on the newer GLONASS
satellites cause less systematic elevation-dependent bias, at the
expense of some reduction in link budget (Otsubo). In a related study
using laser data and IGS-determined orbits it was concluded (Appleby and
Otsubo) that an offset persists between the IGS-determined and
SLR-measured heights of the two laser-tracked GPS satellites. Range data
taken when the satellites are close to the local zenith give an accurate
assessment of the radial offsets in the IGS orbits, which appear to be
different for the two satellites; from a two-year analysis, for GPS-35
the offset is -6.2 ±0.7cm and for GPS-36 the value is -2.1 ±0.4cm.

**Reflectivity Cross Sections**

In a signal-level related study, a theoretical analysis has been carried
out to compute cross-section values for the arrays of all existing and
past ILRS-tracked missions (Arnold). The origins and quality of
previously published values are unclear and this work places all cross
section values on the same rigorous theoretical footing. The table of
results will be placed on the ILRS website.

**Session 12. Multi-wavelength tracking ([John Degnan]{.underline},
[Cinzia Luceri]{.underline})**

**Hardware (John Degnan)**

The need and accuracy requirements for multi-wavelength ranging are
driven by the quality of the atmospheric models used to correct for the
atmospheric delay in single wavelength systems. The ultimate performance
of [any]{.underline} model depends on inputs to the model, e.g., the
accuracy of the in situ measurements of pressure, temperature, and
relative humidity. For example, temperature and relative humidity
measurement accuracies produce sub-mm range errors in the Marini-Murray
(M-M) model but a 0.1 mbar pressure error (best case) can produce about
0.8 mm of range error at zenith and 2.4 mm of range error at 20^o^
elevation. However, even if the model inputs had zero error, one must
ask the question: How well does the model represent the physical
atmosphere?

The effect of deviations of the vertical structure from hydrostatic
equilibrium on M-M have been estimated to be less than 1 cm at
elevations above 20^o^ elevation \[Hauser, 1989\] as are the effects of
horizontal gradients \[Abshire and Gardner, 1985\]. The M-M model is
static (time-independent) whereas random fluctuations in delay due to
turbulence effects are typically a few mm but can be a few cm at 10^o^
elevation under conditions of strong turbulence (Abshire & Gardner,
1985). Although some competing atmospheric models are believed to be
better than M-M at wavelengths below 500 nm due to better physics and at
low elevation angles due to better mathematical approximations, they are
all unable to take into account the aforementioned effects. With
potential un-modeled errors at the cm level, the only way to
unequivocally achieve mm-accuracy range measurements is through direct
measurement of the atmospheric delay via multi-wavelength ranging.

Theoretical analysis has suggested that the optimum wavelength pairs for
two-color ranging are the 2^nd^ and 3^rd^ harmonics of Nd:YAG at 532 and
355 nm respectively or the 1^st^ and 2^nd^ harmonics of the tunable
Ti:Sapphire laser, nominally at 800 and 400 nm. This "Figure of Merit"
analysis \[Degnan, 1993\] takes into account the wavelength dependence
of the dispersion and transmission of the atmosphere, detector quantum
efficiencies, nonlinear conversion efficiencies, and the beam divergence
from fixed transmit and reflector apertures. In order to achieve 1 mm
absolute accuracy in the atmospheric correction or even to validate
existing models at the mm level, the Differential Time-Of-Flight (DTOF)
between pulses at 532 and 355 nm must be measured with an absolute
accuracy of less than 0.6 psec. Although alternative wavelength pairs
that do not take advantage of the high atmospheric dispersion in the UV
have even more stringent differential timing requirements (\<0.4 psec at
846 and 423 nm), they often have other offsetting advantages such as
better transmission through the atmosphere.

Various groups have performed multi-wavelength ranging experiments to
satellites via the following approaches:

-   Low repetition rate, high SNR systems using Photo-Multiplier Tubes
    (PMT's) or Single Photoelectron Avalanche Detection (SPAD's): GSFC,
    Wettzell/TIGO, Graz, Zimmerwald, EOS Australia, Grasse

-   Low repetition rate, high SNR systems augmented by streak cameras:
    GSFC, Wettzell/TIGO, Matera

-   Low repetition rate, waveform averaging of streak camera profiles:
    Wettzell

-   Three or more wavelengths: Graz in collaboration with Czech
    Technical University, CNES, etc

Unfortunately, none of these experiments has yet resulted in DTOF
measurements of sufficient accuracy to support mm satellite ranging. In
fact, DTOF measurements based on single pulse pairs are rendered useless
for most of the existing satellites, regardless of the type or temporal
resolution of the range receiver (PMT, SPAD, Streak Camera), by phase
and polarization mixing of the multi-cube returns. This mixing results
in large shot-to-shot variations in the return waveforms at a single
wavelength, and, as demonstrated by a series of two color streak camera
waveforms taken at GSFC for various satellites from 1996 to 1997, the
waveforms at different wavelengths are highly uncorrelated (e.g.,
different numbers of peaks on a single shot) thereby negating possible
pulse pair convolution approaches to determining the DTOF.

An exception was the short-lived ADEOS/RIS experiment, which consisted
of a single large retro; although the latter had a delta function
impulse response, it also had a limited cross-section, range, and field
of view. The decommissioned WESTPAC satellite produced single cube
returns, but the design also resulted in large modulations of the target
cross-section with orientation, causing the satellite to "wink out"
periodically. New large radius satellites with recessed cubes (e.g., a
Super-LAGEOS) would approximate a flat panel array at normal incidence
(very narrow impulse response) at all satellite orientations while
providing a constant high optical cross-section for long-distance
ranging to low-drag geodetic altitudes \[Degnan, 1993\].

It appears that producing single wavelength range residuals and
averaging over a single normal point interval or pass before computing
the DTOF seems to be the most promising route in the short term. By
averaging over a sufficient number of returns, the temporal phase and
polarization modulations produced by the multi-cube response of the
target would be expected to average to zero and produce an impulse
response based on an incoherent sum of the individual cube responses
averaged over an allowed range of satellite orientations. In addition,
there are a few LEO satellites with quasi-single cube returns that
should prove useful in near term two-color experiments designed to
either test multi-wavelength hardware configurations, evaluate competing
atmospheric models, or compute key unknown parameters within a
particular model. An example of the latter was provided by Stefan Riepl,
who computed a differential zenith delay by fitting the two color
residuals from an entire satellite pass to an atmospheric model. Here
the model is used as a "crutch" to estimate a single defining parameter,
the differential zenith delay, while the dependence on satellite
elevation is provided by the model. The parameter estimation benefited
statistically from the use of all the data within a pass whereas the low
laser fire rate precluded meaningful estimates of the DTOF within a
single normal point interval.

Of course, the ultimate goal of multi-wavelength ranging is a
"model-free" measurement of the atmospheric delay within each normal
point. Such a capability frees us from assumptions regarding the
instantaneous vertical and transverse structure of the atmosphere.
Multicolor versions of kHz photon-counting systems currently under
development at GSFC and Graz offer the potential of high statistical
return rates (tens of thousands of range measurements per LAGEOS normal
point at each color) and freedom from amplitude bias. As a result, while
we await the future launch of satellites truly designed for mm-accuracy
ranging, two-color kHz systems may offer the greatest near-term hope of
achieving few mm absolute accuracies without requiring high repetition
rate, high pulse energy lasers to feed streak cameras and their
cumbersome attendant technologies (variable optical delay lines, etc.).

**Analysis** (Vincenza Luceri)

Two-color data are available from a few stations of the network:
Zimmerwald (423 and 846 nm), TIGO at Concepción (423.5 and 847 nm), and
MLRO (532 and 355 nm). The data are supplied as normal points and
full-rate data in two separate data sets; no differential delay is
actually submitted by the stations. Most of the analyses are, of course,
performed using the two wavelengths separately. Differential delay
analysis was presented by V. Luceri as first results coming from the
two-color data collected by MLRO. Comparisons between the differential
delays data and differential delays computed by the available
tropospheric correction models showed an improvement, with respect to
the Marini-Murray, in the "Mendes+Ciddor" model (available at the
beginning of this year) for the UV wavelength but a bias is still
present, above all at higher elevation. A larger improvement is expected
from a new model that is being tested and will be soon available.

E. Pavlis presented the analysis of wavelength data from Zimmerwald and
Concepción (primarily Zimmerwald), taken from 2001 to March 2003. The
data were analyzed as independent ranges and corrected for atmospheric
refraction using both the Marini-Murray and the Mendes-Ciddor model that
were described during the 13^th^ ILRS Workshop (i.e., NOT the latest one
presented in Kötzting). The residual differences were examined,
primarily for the case of the blue color (423 nm), since the data in the
infrared (846 nm) were very sparse. Of the total \~1800 NPs, \~1500 were
at 423 nm and \~300 at 846 nm. Across wavelengths, we did not see
different overall scatter behavior. The global rms of either group was
at 12 mm.

The talk given by R. Koenig showed the results of the analysis of 378
NPs in blue and 411 NPs in infrared from Zimmerwald to CHAMP in terms of
residuals in POD. The construction of NPs does not lead to common time
tags for the NPs of the two colors, so a direct difference of the
residuals is not readily available. By removing the orbit error by
proper parameterization, the systematics between blue and infrared are
preserved and differences can be built between pairs spaced up to 3 s.
However no systematic effects between the two colors are detectable for
the Marini-Murray refraction model. A slight degradation of the quality
of the observations can be seen for low elevations.

A range bias comparison was performed by V. Husson. Using the August
2003 Zimmerwald 2-color data and the Marini-Murray refraction model,
there are pass-by-pass, mm level systematic differences that are
satellite dependent. The differences could either be caused by the
Marini-Murray model, un-modeled satellite center of mass variations,
and/or problems related to system calibration. More data over a longer
period of time are required to help determine the root cause of these
differences.

**Open Issues:**

-   Low elevation data are strongly necessary to improve models.

-   Different CoM corrections for different wavelengths; the difference
    in the range correction for different wavelengths can be one
    millimeter.

-   Following the standard procedure to construct the normal points with
    two-wavelength data can introduce a misalignment of the
    corresponding NP epochs in the two data sets.

-   The quarterly report card gives a report of the passes,
    distinguishing the two wavelengths; a report of the two-color passes
    (two-color simultaneous ranging) would be helpful.

-   The differential delay is immediately available at the stations and
    should be delivered to the data centers.

**Session 13. Pilot projects ([Graham Appleby]{.underline}, Mark
Torrence)**

**Pilot project** **(POS+EOP)**

This activity is the principal project being run by the ILRS Analysis
Working Group (AWG) under the chairmanship of Ron Noomen. Invitations to
submit solutions have been distributed to ILRS Analysis Centers and
Associate ACs, with approximately nine groups being involved.

Current products are **28-day solutions** using LAGEOS and Etalon to
derive station coordinates and daily EOPs (x,y~pole~, LoD). Since August
2003, up to 3 or 4 independent solutions continue to be routinely
available at the CDDIS/EDC by Tuesday each week.

In addition, two combination centers (ASI, DGFI) are routinely providing
**combined products** (coordinates, EOP, summary files); a third center
(NCL) is ready to proceed. The combination products from these two
groups are available at the data centers by Wednesday each week.

It is clear that, while good progress is being made, there is still a
need for more contributed solutions in order to improve the quality of
the final product. Both analysis groups and combination groups are
encouraged to get involved.

**Analysis Software Benchmarking**

Prior to being accepted as an analysis group, analysts must submit their
software to a set of benchmarking tests. There has been good progress
with this exercise, but some issues are still to be resolved. A further
test orbit (GM and gravity field to J~4~ only) had been agreed upon
during the two-day AWG prior to the ILRS Workshop.

It was recommended that by the end of November 2003 **all analysis
groups** must have submitted the required benchmark solutions. The
Benchmark 'judges' (Pavlis, *et al*) were tasked to report the results
by the end of 2003.

**QC Harmonization Pilot Project**

Very good progress has been made (Husson, Torrence) in producing an
unambiguous product to act as an aid to system diagnosis. Most of the
contributing agencies, as a result of this PP, are now working with the
same standards. For example, ITRF2000 is being used by most groups to
define the reference frame. A combination product is under development.

**Proposed next steps**

Evidence presented during the AWG meeting and summarized during the
session (Pavlis, Altimimi, Luceri) strongly suggests that for scale and
origin resolution, a more useful product will be provided by using a
seven-day data arc for the analyses. An example of a series of results
from such an analysis, computed in this case by a single analysis centre
(ASI, Luceri *et al*), is shown in the Figure. Given are weekly
solutions for Helmert translations and scale differences from ITRF2000.

![](media/image7.png){width="2.877083333333333in"
height="1.9986111111111111in"}

The epoch of the proposed weekly solutions was also considered. The
current Pilot Project solutions use 28-day arcs beginning on Tuesday
each week, which then provides enough time for the analysis and
combinations groups to make available an ILRS EOP product for the IERS.
However, it was noted that the ILRS combination product could be more
readily combined with those of the other services if it is time-aligned
to the GPS week; i.e., the seven-day data-span should begin at 0hrs
Sunday and end at 24hrs on Saturday.

**Recommendations**

-   The session heard that the AWG recommends that the analysis
    coordinators implement these two changes (7-day data arc, epoch
    aligned to GPS week) to the POS+EOP Pilot Project and proposes that
    the changes to the CfP take effect from mid-November 2003, via
    emails to the contributing groups.

-   **Still to be decided, by consultation between the analysis
    coordinators and IERS, is how best to provide a weekly, timely ILRS
    EOP product for the IERS rapid service.**

**Session 14. New approaches ([Erricos Pavlis]{.underline}, Mark
Torrence)**

The chair reviewed first the state of the analysis working group\'s
current and future plans and procedures. At the moment the Pilot
Projects (PP) for the analysis of LAGEOS-1 and -2 and ETALON-1 and -2
SLR data in 28-day batches is completed, although the results for the
ETALON case have not been fully validated and evaluated by a substantial
number of ACs. The new approach adopted during the Koetzting workshop
was also described: Complete the benchmark PP by the end of the year,
and start the support of the EOP+STA PP with weekly arcs coincident with
the definition of a GPS-week (i.e., Sunday 0h UTC to Saturday 24h UTC).
Continued support of the traditional SLR customers (e.g., NEOS and JPL)
was also adopted, with the use of a fixed datum (ITRF2000).

The chair presented a short example where LAGEOS data were used at JCET
to estimate the location of the FTLRS at Chania, from a sparse SLR data
set and independently, at CSR/UTEX, John Ries used a much stronger set
of JASON SLR data to estimate the same location. The two positions were
compared and found to agree within 2 cm or so. Suggestions from the
floor hinted that we may be ripe enough to start including more targets
in the process that produces the weekly products. The GFZ experience is
that a simultaneous analysis of SLR and GPS data from CHAMP produces a
far more robust and accurate set of EOP products. Werner Gurtner pointed
out the on-going IGS LEO PP. The chair suggested that it may be
worthwhile to pursue a joint PP in the near future. This will be a
discussion point for the next AWG meeting.

The chair pointed out that there are several changes in the modeling,
conventions, and standards areas where we need to focus our attention in
order to meet the schedule for required alignment of our modus operandi
with the newly adopted models, etc. It was also pointed out that part of
the success of our work, especially if we adopt new approaches and
include new targets in the near future, hinges on the timely and as
accurate as possible adoption of target signature models for the SLR
satellites (commonly known as Center of Mass (CoM) offsets). In the area
of improved refraction models, the chair accepted an action item to make
publicly available a routine that implements a new version for the
computation of the Zenith Delay and the Mendes mapping function,
applicable to all SLR-used wavelengths (i.e., from 355 nm to 1064 nm),
with millimeter accuracy. In connection with this issue, the chair
presented a new approach for the near real-time computation of
refraction corrections on the basis of global geophysical fields derived
from missions such as the AIRS instrument on the AQUA EOS platform. The
automatic benefit of such an approach is that it will naturally account
for any horizontal gradients in the atmosphere surrounding each tracking
site. Additional action items were the clarification of adopted models
for the tidally coherent signals in EOP and the geocenter. It was
suggested that none of the ACs starts producing results with new models,
prior to their official adoption by the AWG (to avoid confusion).

Finally, it was pointed out that under the newly adopted procedures, ACs
should consult the ILRS web site, and in particular the QC reports, for
up-to-date information on station yield/quality, and adjust accordingly
their weighting scheme, with a minimum number of 10 NPs (L1 + L2) from
each site over the 7 days as a ticket for accepting that site in the
weekly solution.

**Action Items:**

-   The AWG will discuss at its next meeting the possibility for
    additional PP to investigate the data-level combination of
    techniques.

-   The new zenith delay model (Ciddor-Mendes) to be released as soon as
    the associated paper is submitted for publication, possibly by the
    end of 2003 (Mendes and Pavlis).

-   The currently used by the ILRS community \"background\" models
    (according to IERS conventions) for tidally coherent signals in the
    geocenter and EOP to be reviewed and reported back to the AWG and
    IERS Conventions group (Pavlis).

**Session 15. KHz ranging and its impact (Georg Kirchner, [John
Degnan]{.underline})**

John Degnan began the session by presenting an overview of kHz laser
ranging including its advantages, special requirements, normal point
computation, and performance expectations for the NASA SLR2000 system.
At present, only two groups are working with kHz systems: GSFC (SLR2000)
and Graz. KHz laser ranging systems take advantage of recently developed
microchip and SESAM lasers, which are simpler, ultra-compact, and less
expensive than mode-locked lasers for picosecond pulse generation and
naturally operate at kHz frequencies. Since the oscillator output
energies are orders of magnitude higher than that of mode-locked lasers,
they require fewer amplifier stages, and can be designed without the
need for high speed, high voltage electro-optic switches. Since the
number of range returns per normal point can increase by about two
orders of magnitude relative to conventional 5 to 10 Hz systems, the
normal point precision can improve by over an order of magnitude and
[may]{.underline} allow meaningful two color measurements of
differential atmospheric delay using high speed PMT's or SPAD's. Sample
calculations for the photon-counting SLR2000 system ranging to LAGEOS in
a Standard Clear atmosphere (23 km visibility) suggest about 60,000
successful ranges per two-minute normal point near zenith (25% return
rate) and about 5000 per normal point at 20^o^ elevation (2% return
rate). Since the RMS of the LAGEOS impulse response is about 15 mm, the
RMS precision of the centroid (normal point) would have a lower limit of
0.2 mm during acquisition and 0.05 mm near zenith. This "best case"
calculation ignores any statistical broadening due to the system impulse
response or the minimum timing resolution of the receiver. Differencing
the centroids of the orbital residuals at two wavelengths with these
kinds of statistical precisions [may]{.underline} support mm accuracy
two color measurements of the differential time of flight - at least at
the higher elevation angles (but just barely).

Clearly, the higher repetition rate compensates for single pulse photon
deficit and allows single pulse output flux to be reduced to eyesafe
levels thereby eliminating the need for safety observers or aircraft
radars. Furthermore, single photon range measurements are free of signal
amplitude biases, and the orbit range residuals within a single normal
point period are expected to reproduce the impulse response of the
overall system (laser, satellite, and detector).

Special requirements of kHz systems include: kHz gating circuits and
range gate generators; the use of event timers rather than time interval
units due to multiple pulses in flight; more sophisticated ranging
software required to extract the signal from the background noise and to
link the proper return signal to each start pulse in computing the pulse
time of flight; faster interfaces between the ranging hardware and
system CPU; and more aggressive background noise reduction and fast
receiver recovery times in the case of photon-counting systems.

Georg Kirchner then presented some exciting results obtained at Graz
with their new 2 kHz laser (400 microjoules per pulse at 532 nm). After
roughly two weeks of effort, Graz obtained more than 200,000 range
measurements in a single LAGEOS pass (\>45,000 per normal point) in
general agreement with the theoretical expectations previously reported
by John Degnan. More than 1 million measurements in a single Ajisai pass
were also reported. Because of the very short pulsewidth of 10 psec,
there is little electromagnetic interference between the various
retroreflectors so that returns from individual cubes could be resolved
on a number of targets. Georg also reported that satellite acquisition
was easier at the high repetition rate and the system tracked down to
10^o^ elevation, day or night.

Ivan Prochaska then reported on additional experiments to a ground
target at 6 km distance which used the 2 kHz Graz system to measure the
range jitter induced by atmospheric turbulence (0.4 mm) as well as
atmospheric seeing (\~2 arcseconds). He also reported on ESA-sponsored
activities on a spaceborne altimeter for mapping extraterrestrial bodies
(e.g., Mercury) from planetary orbit. (see also recent articles by John
Degnan in J. Geodynamics, November 2002 and J. e&I Electrotechnik und
Informationstechnik, April 2002, which consider a photon-counting global
mapping mission at Mars). Finally, Karel Hamal reported on a new 2 kHz
version of their Portable Picosecond Event Timer (P-PET).

**Session 16. SLR 20000 ([John Degnan]{.underline})**

SLR2000 is an autonomous and eyesafe photon-counting satellite laser
ranging station with an expected absolute range accuracy better than one
cm and a normal point (time-averaged) range precision better than 1 mm.
The system will provide continuous 24 hour tracking coverage to an
existing constellation of approximately two dozen artificial satellites
equipped with passive retroreflector arrays. Replication costs are
expected to be on the order of \$1.5M per system, and the system will be
about 75% less expensive to operate and maintain than current manned
systems. Computer simulations have predicted a daylight tracking
capability to retro-reflector equipped GPS (altitude 19,000 km) and
lower satellites. Computer and hardware simulations have demonstrated
the ability of our current correlation range receiver and auto-tracking
algorithms to extract mean signals as small as 0.0001 photoelectrons per
pulse from solar background noise during daylight tracking.

The SLR2000 system concept was first proposed in 1994\], but significant
funding for the SLR2000 project was not provided by NASA until August
1997. The first detailed overview of the SLR2000 system concept and
proposed technical approach was presented at the 1997 Fall Europto
Meeting in London, UK and has evolved with time. These periodic system
level reviews as well as more detailed papers on individual subsystems,
algorithms, and software packages can be found in the proceedings of the
last three biannual International Workshops on Laser Ranging, which were
held in Deggendorf, Germany in 1998; Matera, Italy in 2000; and
Washington DC in 2002. The reader is referred to these earlier system
overviews and to the workshop articles referenced herein for additional
background and technical detail. The SLR2000 project also maintains a
web site at:

> <http://cddisa.gsfc.nasa.gov/920_3/slr2000/slr2000.html>

where additional SLR2000-related hardware and software publications and
presentations are available online along with photos and test results
for the various subsystems.

During the first two years of NASA funding, prototypes of several
\"enabling\" components, without which the system is not feasible, were
successfully developed. These include: (1) a sensitive, high speed,
quadrant microchannel plate photomultiplier for simultaneous ranging and
pointing correction; (2) oscillator-only and oscillator/amplifier
versions of a microchip-laser based transmitter; (3) a \"smart\"
meteorological station which includes an upgraded all-sky cloud sensor;
(4) a multi-kHz rate range gate generator; and (5) a multi- kHz rate, 1
mm resolution event timer. Once the key specifications on these advanced
components were largely met and system feasibility had therefore been
established, attention then turned to the detailed engineering design
and procurement of more conventional elements of the system such as the
shelter and protective dome, arcsecond precision tracking mount,
telescope, and optical transceiver. The principal challenge during this
phase was to choose reliable but low cost approaches to meeting our
technical requirements and goals. As of this writing, prototypes of the
essential SLR2000 components and subsystems have been developed,
successfully tested at the subsystem level, and integrated into the
prototype system.

Field alignment and testing of the prototype has been underway at the
Goddard Geophysical and Astronomical Observatory (GGAO) since shortly
after the Washington workshop. Star calibrations had consistently been
below 3.5 arcseconds RMS using an 18 element mount model and 50 stars,
but very recent improvements have reduced the overall RMS error to 2.2
arcseconds. Factory and field tests of the tracking mount demonstrated a
tracking precision (actual vs. command) of better than 1 arcsec for all
but the highest angular velocity tracks. Within the limitations of the
low-resolution (4.2 arc minute) star camera, the system appeared to
track sunlit satellites in the field. Recent ground ranging tests to the
calibration target are being evaluated, and the system is being prepared
for actual satellite ranging tests during this calendar year. A Request
for Proposals (RFP) for system replication is currently projected for
late summer 2004. However, the funding, management, and manpower
uncertainties currently being endured by the NASA SLR network put these
schedule projections at some risk.

Beyond the development of the basic SLR2000, we have been investigating
potential future upgrades and applications of the system. For example,
one may be able to take advantage of bias-free photon-counting range
measurements and kilohertz laser fire rates and difference the normal
points (time-averaged data) at two wavelengths (nominally 532 and 355
nm) to compute the atmospheric refraction delay. This upgrade would
require the addition of a UV wavelength channel to the receiver.

The feasibility of utilizing SLR2000 as an Earth station in a two-way
laser transponder link for precise interplanetary ranging and time
transfer has also been examined. Detailed mathematical models and
operational scenarios have been developed and applied to a two-way
Earth-Mars link.

NASA headquarters has also directed GSFC to work with the Jet Propulsion
Laboratory (JPL) in investigating the feasibility of using SLR2000 as
the ground terminal in a space-to-ground laser communications link while
simultaneously ranging to retroreflectors onboard the spacecraft. The
motivation for initiating this activity comes from an emerging suite of
new Earth sensors which require substantial information bandwidths for
high speed data transfer to the ground. These include hyperspectral
imagers, Interferometric Synthetic Aperture Radars (In-SAR), and 3D
imaging lidars based on our photon-counting techniques. SLR2000 is
already designed to perform the majority of field operations needed for
an automated laser communications station and has a sufficiently large
telescope to accommodate high bandwidths transmitted from Earth orbit.
For example, it automatically assesses local weather and cloud cover
conditions and performs necessary system and personnel health and safety
functions. In addition, SLR tracking provides a highly accurate orbit
for rapid spacecraft acquisition, independent confirmation of target
satellite acquisition via the retroreflector returns, and a visible
beacon for the spaceborne communications terminal to lock onto. A simple
wavelength splitter within the SLR2000 optical transceiver would divert
the incoming communications photons to a separate lasercom receiver.
Recent analyses conducted in support of some proposed NASA missions has
indicated that SLR2000 can support several Gbps lasercom links from
geostationary satellites and several tens of Mbps over lunar distances
with about 2 watts of transmitter power at the remote terminal. It is
hoped that such a diversification of SLR2000 functions will attract new
users and additional funding for a global multifunction SLR2000C (for
Communications) network.

**Session 17. Automation (Ben Greene, [Werner Gurtner]{.underline})**

**Topics for Automation**

The following list contains topics that can be subject for automation at
a tracking station:

-   Mount model generation (star calibration)

    -   generation of a list of well-distributed catalogued stars

    -   coarse telescope pointing

    -   CCD imaging and subsequent image processing or fine pointing
        using some star sensor

    -   computation of the mount model

-   Prediction handling

    -   extraction of respective e-mail messages or ftp downloads from
        data centers

    -   data base maintenance)

-   Satellite passes

    -   generation of pass lists for a certain time period (e.g., one
        week)

    -   ephemerides for passes to be tracked

    -   observation and calibration schedule (proposal for observers or
        schedule for automated operation)

    -   a priori sun interference handling

    -   horizon masks

-   General power-up of station (e.g., laser)

-   Start of tracking

    -   Telescope

    -   laser

    -   dome

    -   electronics

    -   optical components

    -   pass and calibration schedule

-   Satellite acquisition

    -   search algorithms (along-track, spirals)

    -   signal detection (signal/noise separation)

    -   time bias computation

    -   range gate/window settings

-   Track keeping: Keep laser beam on target during the tracking

    -   time bias adjustment

    -   fine tuning of pointing

    -   signal level control

-   Pass interleaving

    -   using a priori schedule

    -   realtime adjustment of schedule according to current conditions
        (weather conditions such as cloud coverage, used acquisition
        times, prediction quality,...)

-   Stop tracking

    -   put telescope to park state

    -   close dome

    -   switch off laser (or certain components only)

    -   switch off certain electronic components

-   Post-processing

    -   apply calibration values

    -   data cleaning (elimination of noise returns, outliers)

    -   normal point generation

    -   reformating (quick look exchange format

    -   data submission to data center

    -   reporting and protocols

-   General power-down of station

**Status of Automation in the Network**

Many stations have realized at least some degree of automation. An
example (**Matera MLRS**) was explained by G. Bianco.

However, there are currently two systems only with the capability of
**around-the-clock automation**, i.e. completely unattended operation
for days or weeks in a row:

-   **Mount Stromlo**: Realized for the station destroyed in the bush
    fires early 2003, again planned for the replacement station
    currently under construction. The observation scenario would be
    prepared for a week ahead of time. As the system operates in a
    sealed dome there are no provision taken to account for bad weather:
    The system just follows the preprogrammed schedule.

-   **SLR-2000**: The system is still under development. John Degnan
    explained the current status of various subsystems needed for the
    realization of a fully automated system. The system will include a
    cloud monitor and a rain sensor, the former allowing for the
    optimization of the tracking schedule in real time by taking into
    account the actual cloud distribution.

**Zimmerwald** station is run in a **fully automated** mode at about 20
percent of the time, mainly to extend the shifts or to bridge gaps
between two shifts. Thus a 24-hour coverage can usually be realized with
two shifts per day. The data yield during the automated operation is
comparable to the interactive mode.

**ILRS Workshop**

####### Kötzting, Germany

## October 28 - 31, 2003

**List of Participants**

+---------------+---------------+--------------------------------------+
| **Last Name** | **First       | **E-Mail**                           |
|               | Name**        |                                      |
+---------------+---------------+--------------------------------------+
| > Altamimi    | > Zuheir      | > altamimi@ensg.ign.fr               |
+---------------+---------------+--------------------------------------+
| > Appleby     | > Graham      | > graham.appleby@nerc.ac.uk          |
+---------------+---------------+--------------------------------------+
| > Biancale    | > Richard     | > biancale@calc-gen3-ci.cst.cnes.fr  |
|               |               | > \*                                 |
+---------------+---------------+--------------------------------------+
| > Bianco      | > Giuseppe    | > giuseppe.bianco@asi.it             |
+---------------+---------------+--------------------------------------+
| > Brandl      | > Nikolaus    | > nikolaus.brandl@bkg.bund.de        |
+---------------+---------------+--------------------------------------+
| > Brunner     | > Fritz       | > sandra.schmuck@tugraz.at           |
+---------------+---------------+--------------------------------------+
| > Carman      | > Randall     | > moblas@midwest.com.au              |
+---------------+---------------+--------------------------------------+
| > Carter      | > David       | > David.L.Carter@nasa.gov            |
+---------------+---------------+--------------------------------------+
| > Combrinck   | > Ludwig      | > ludwig@hartrao.ac.za               |
+---------------+---------------+--------------------------------------+
| > Combrink    | > Attie       | > attie@hartrao.ac.za                |
+---------------+---------------+--------------------------------------+
| > Davila      | > Jose Martin | > mdavila@roa.es                     |
+---------------+---------------+--------------------------------------+
| > Dawson      | > John        | > john.dawson@ga.gov.au              |
+---------------+---------------+--------------------------------------+
| > Degnan      | > John        | > john.degnan@sigmaspace.com         |
+---------------+---------------+--------------------------------------+
| > Fumin       | > Yang        | > yangfm@center.shao.ac.cn           |
+---------------+---------------+--------------------------------------+
| > Galkin      | > Yury        | > galkin@mgul.ac.ru                  |
+---------------+---------------+--------------------------------------+
| > Garate      | > Jorge       | > jgarate@roa.es                     |
+---------------+---------------+--------------------------------------+
| > Gibbs       | > Philip      | > pgib@nerc.ac.uk                    |
+---------------+---------------+--------------------------------------+
| > Govind      | > Ramesh      | > Ramesh.Govind@ga.gov.au            |
+---------------+---------------+--------------------------------------+
| > Greene      | > Ben         | > bengreene@compuserve.com           |
+---------------+---------------+--------------------------------------+
| > Grunwaldt   | > Ludwig      | > grun@gfz-potsdam.de                |
+---------------+---------------+--------------------------------------+
| > Guaitiao    | > Cesar       | > stefan.riepl@bkg.bund.de           |
+---------------+---------------+--------------------------------------+
| > Gurtner     | > Werner      | > gurtner@aiub.unibe.ch              |
+---------------+---------------+--------------------------------------+
| > Hamal       | > Karel       | > prochazk@mbox.cesnet.cz            |
+---------------+---------------+--------------------------------------+
| > Haufe       | > Karl-Heinz  | > karl-heinz.haufe@bkg.bund.de       |
+---------------+---------------+--------------------------------------+
| > Herbetz     | > Stefan      | > stefan.herbertz@bkg.bund.de        |
+---------------+---------------+--------------------------------------+
| > Herold      | > Guenther    | > guenther.herold@bkg.bund.de        |
+---------------+---------------+--------------------------------------+
| > Husson      | > Van         | > van.husson@honeywell-tsi.com       |
+---------------+---------------+--------------------------------------+
| > Ihde        | > Johannes    | > johannes.ihde@bkg.bund.de          |
+---------------+---------------+--------------------------------------+
| > Johannsen   | > Ingo        | > ingojohannsen@highqlaser.at        |
+---------------+---------------+--------------------------------------+
| > Kirchner    | > Georg       | > kirchner@flubpc04.tu-graz.ac.at    |
+---------------+---------------+--------------------------------------+
| > Koenig      | > Rolf        | > koenigr@gfz-potsdam.de             |
+---------------+---------------+--------------------------------------+
| > Koidl       | > Franz       | > koidl@flubpc04.tu-graz.ac.at       |
+---------------+---------------+--------------------------------------+
| > Kunimori    | > Hiroo       | > kuni@crl.go.jp                     |
+---------------+---------------+--------------------------------------+
| > Lauber      | > Pierre      | > lauber@wettzell.ifag.de            |
+---------------+---------------+--------------------------------------+
| > Luceri      | > Vincenza    | > cinzia.luceri@asi.it               |
+---------------+---------------+--------------------------------------+
| > Maeda       | > Maki        | > maeda.maki@jaxa.jp                 |
+---------------+---------------+--------------------------------------+
| > Mareyen     | > Maria       | > mamy@bkg.bund.de \*                |
+---------------+---------------+--------------------------------------+
| > Motz        | > Rudolf      | > rudolf.motz@bkg.bund.de            |
+---------------+---------------+--------------------------------------+
| > Müller      | > Horst       | > mueller@dgfi.badw.de               |
+---------------+---------------+--------------------------------------+
| > Nurutdinov  | > Kostyantyn  | > nurut@mail.ru \*                   |
+---------------+---------------+--------------------------------------+
| > Otsubo      | > Toshimichi  | > otsubo@crl.go.jp                   |
+---------------+---------------+--------------------------------------+
| > Pavlis      | > Erricos     | > epavlis@JCET.umbc.edu              |
+---------------+---------------+--------------------------------------+
| > Pearlman    | > Mike        | > mpearlman@cfa.harvard.edu          |
+---------------+---------------+--------------------------------------+

**ILRS Workshop**

####### Kötzting, Germany

## October 28 - 31, 2003

**List of Participants (Continued)**

+---------------+---------------+--------------------------------------+
| **Last Name** | **First       | **E-Mail**                           |
|               | Name**        |                                      |
+---------------+---------------+--------------------------------------+
| > Pierron     | > Francis     | > Francis.pierron@obs-azur.fr        |
+---------------+---------------+--------------------------------------+
| > Prochazka   | > Ivan        | > prochazk@mbox.cesnet.cz            |
+---------------+---------------+--------------------------------------+
| > Ramirez     | > David       | > stefan.riepl@bkg.bund.de           |
+---------------+---------------+--------------------------------------+
| > Richter     | > Bernd       | > richter@iers.org                   |
+---------------+---------------+--------------------------------------+
| > Riepl       | > Stefan      | > stefan.riepl@bkg.bund.de           |
+---------------+---------------+--------------------------------------+
| > Rothacher   | > Markus      | > rothacher@bv.tum.de                |
+---------------+---------------+--------------------------------------+
| > Salminsh    | > Kalvis      | > kalvis@lanet.lv                    |
+---------------+---------------+--------------------------------------+
| > Schillak    | > Stanislaw   | > sch@cbk.poznan.pl                  |
+---------------+---------------+--------------------------------------+
| > Schlicht    | > Anja        | > anja.schlicht@bkg.bund.de          |
+---------------+---------------+--------------------------------------+
| > Schlüter    | > Wolfgang    | > wolfgang.schlueter@bkg.bund.de     |
+---------------+---------------+--------------------------------------+
| > Schreiber   | > Ulrich      | > schreiber@wettzell.ifag.de         |
+---------------+---------------+--------------------------------------+
| > Seemueller  | > Wolfgang    | > seemueller@dgfi.badw.de            |
+---------------+---------------+--------------------------------------+
| > Tatevyan    | > Ruben       | > gleb@geod.ru                       |
+---------------+---------------+--------------------------------------+
| > Torrence    | > Mark        | > mtorrenc@magus.stx.com             |
+---------------+---------------+--------------------------------------+
| > Utzinger    | > Johannes    | > johannes.utzinger@aiub.unibe.ch    |
+---------------+---------------+--------------------------------------+
| > Wilkinson   | > Matthew     | > Matwi@nerc.ac.uk                   |
+---------------+---------------+--------------------------------------+
| > You         | > Zhao        | > youzhao@public.cc.jl.cn            |
+---------------+---------------+--------------------------------------+

> \* Attended AWG only