EUROLAS WORKSHOP

*[Detecting and eliminating errors in the EUROLAS network]{.underline}*

Hosted at Herstmonceux, UK, by the Natural Environment Research Council
(NERC) Space Geodesy Facility (SGF) -- 2002 March 11-13

Fourteen representatives from 10 EUROLAS stations, one representative
from the NASA network and one from the ILRS central bureau attended the
workshop. A list of attendees is given at Appendix 2. Several people
brought Stanford timers so that direct intercomparisons could be made
between them and the Herstmonceux timers during the course of the
workshop.

The meeting was held to address issues of common interest to the EUROLAS
cluster of stations in order, for example, to exploit more fully its
unique capability for very precise, simultaneous tracking. Such
capability is seriously compromised if one or more stations generate
corrupt data from locally induced observational errors: it is
particularly destructive if these errors vary with time or the
circumstances of the pass. This capability should also be exploited in
order to improve data quality from EUROLAS, which of course will be of
benefit to the mission of the whole ILRS.

The discussion, in four half-day sessions, dealt with specific elements
of the SLR technique: Detectors; Reduction/Post Reduction errors;
Counters and Timers and Miscellaneous items. The following summary picks
out key points of the discussion and highlights recommendations. It is
hoped that some more detailed descriptions of good practice for getting
best results in specific areas can be posted on the ILRS website in due
course.

**Introduction:**

Van Husson set the scene by emphasising the keys for stations to achieve
consistent millimetre accuracy:

-   Aggressively pursue and eliminate all site biases;

-   Improve the quality of data available to analysts;

-   Comply fully with format and data centre standards.

He pointed out that one of the prime science drivers is the desire to
determine the radial component of an orbit (LAGEOS and altimetry
satellites) at the millimetre level. His definition of 1mm accuracy was
that the mean range bias for any pass should be ≤ 1mm and that the
normal point scatter should be at the same level. This is *not* the same
as saying that the single shot RMS needs to be 1mm! Thus each station's
performance goal should be 1mm accuracy and repeatability in both the
short-term (over a pass of, say, 50 minutes) and in the long-term
(pass-to-pass, day-to-day, month-to-month).

**General Principles that were accepted from the outset:**

1.  All stations should be aiming at 1 mm accuracy on a pass-by-pass
    basis;

2.  Stations should have primary responsibility for data integrity --
    the goal must be for the user community to have 100% confidence in
    the data from the stations;

3.  Stations should routinely reject suspect data instead of sending
    them to data centres for (possible) rejection by others;

4.  At the same time, stations and analysts need to continue to improve
    their monitoring and analysis techniques to enable stations to
    identify and solve subtle performance problems;

5.  Daylight ranging is essential;

6.  Measurements should be traceable through data and calibration
    records.

**[Session 1: Detectors]{.underline}**

What operational conditions, procedures and monitoring lead to the best
results?

**C-SPAD:**

The C-SPAD employs a cooled detector chip and has two output channels --
one "raw" output pulse and the other compensated for the effects of walk
with return energy. Benefits of use and features to be treated with care
were fully discussed.

-   [Multi-photon return levels]{.underline}:

> Uncompensated C-SPAD\'s outputs display time-walk at higher return
> energy levels. Even for the compensated outputs there is still a
> residual '**walk**' for high signal strength returns from satellites
> with extended arrays - including LAGEOS, ETALON, and Topex/Poseidon.
> If left uncorrected, this time walk will bias the measurement results.
> Similar results are found in PMT\'s.

-   C-SPAD should be **tuned** for zero-walk with respect to return
    signal strength for calibration ranging - tuning is effective only
    for a single pulse length (and the pulse length must be one
    available to the tuner -- Graz);

-   Stations should confirm the tuning of their C-SPADs by conducting
    calibration ranging experiments at many different return energies;
    (see, for example, Appleby and Gibbs, *Proc 9^th^ Int. Workshop on
    Laser Ranging Instrumentation,* *Canberra*, pp 92-102)

-   If signal strength can be measured, shot-by-shot correction for this
    walk is theoretically possible;

-   If signal strength cannot be measured directly, return rate, if
    correctly computed (see Appendix 1), can be used as proxy of signal
    strength, either to apply corrections or to maintain a particular
    regime of return energy;

-   C-SPAD stations should routinely compute return rates, at the very
    least in order to monitor its variability and to give an indication
    of potential range bias;

-   C-SPAD stations should perform high-low signal strength experiments
    during a test calibration and on a LAGEOS and ERS-2/Stella/Starlette
    pass (again, see *Proc 9^th^ Int. Workshop on Laser Ranging
    Instrumentation,* *Canberra*, pp 92-102); NERC SGF to process the
    data from these experiments - contact <Graham.Appleby@nerc.ac.uk> to
    discuss details.

```{=html}
<!-- -->
```
-   [All C-SPAD users]{.underline}

```{=html}
<!-- -->
```
-   **Pre-arming** -- arm the C-SPAD well before the satellite 'track'
    to avoid variable walk (\>50ns for the *uncompensated* channel;
    \>100ns for the *compensated* channel):

    -   Stations should conduct C-SPAD arming tests to verify local
        practice;

-   Optimising **alignment** of the detector minimises range bias
    effects and jitter;

    -   Stations should conduct alignment tests to quantify the effects;

-   **Temperature** - C-SPAD electronics are partially compensated for
    temperature change, but there is a residual walk with temperature
    change.

    -   Stations should calibrate frequently when the ambient
        temperature is changing rapidly.

# PMT:

-   For photo multiplier/constant fraction discriminator systems
    stations should:

```{=html}
<!-- -->
```
-   keep returns well after (\>20ns) the CFD gate;

-   control return rate to minimise \'**walk**\' effects (\<30%) and/or
    measure return pulse energy to correct times of flight with
    previously determined time walk model.

## **Session 2: Reduction/Post Reduction Errors**

What can be done at stations **after** the pass to maximise data
quality?

**⊗ *= Send as a recommendation to an appropriate ILRS Working Group.***

Stations should:

-   Compute useful parameters for each satellite pass in order to assess
    data quality. For example, NERC SGF compute (using DISTRIB
    subroutine, Sinclair 1995, available through EDC at
    ftp://ftp.dgfi.badw-muenchen.de/pub/laser/software/DISTRIB) and
    archive useful parameters (*e.g.* RMS, skew, kurtosis, LEHM-peak,
    LEHM = Leading Edge Half Maximum) from each reduction and flag
    values that fall outside the normal range for each satellite;

-   Be very careful about accepting pass segments widely spaced in time,
    especially segments with few returns, since fitting programs are
    flexible enough to accommodate noise;

-   Limits normal points to a minimum number of returns:

```{=html}
<!-- -->
```
-   in daytime 6;

-   at night 3,

> unless, for purely practical reasons, these are too high (e.g.
> low-repetition rate systems; some LEO satellites with very compact
> arrays); **⊗**

-   Compute (and control for better consistency) the return rates
    (keeping to the same regime) for both calibration and satellite
    ranging, and again at reduction time (to reject data outside return
    rate/energy limits);

-   Use a formalised algorithm for return rate calculation (outlined
    above and to be distributed as subroutine) **⊗**

```{=html}
<!-- -->
```
-   Long and Short-arc quality control is carried out **daily** by NERC
    SGF and shows up problems at levels of **20mm** for some stations
    and for some passes.

    -   For feed back on recent passes, stations should regularly
        consult <http://nercslr.nmt.ac.uk/slrweb/auto_analysis.html>

Data Centres should:

-   Reintroduce full rate data archiving in MERIT II format; **⊗**

-   Formalise a procedure for withdrawal (*i.e.* complete removal from
    data centre) of bad-pass data: (stations and analysts to agree!).
    **⊗**

**[Session 3: Counters and Timers.]{.underline}**

How can stations get the best out of their timers? Most of the
discussion centred on the Stanford SR620 time interval counters.

-   Ideally timers should be located in a temperature stabilised
    environment and should never be switched off;

-   If they are only switched on for operational use, **recommend** that
    they be given a warm-up period of at least one hour before being
    used for data taking;

-   Input threshold settings on counters should be set by **software**
    and **not** via the front panel controls;

```{=html}
<!-- -->
```
-   The outline procedure and results of counter tests carried out
    during the workshop are given at
    <http://nercslr.nmt.ac.uk/current_rnd/workshop_sr.html>

-   Stations should apply the results from their SR620 counter tests to
    their routine data as soon as possible;

-   Graz/Herstmonceux **agreed,** as a matter of routine, to archive and
    analyse counter differences on many real passes so as to monitor
    long-term stability and range dependencies;

-   Matera **agreed** to make direct shot-by-shot comparisons between
    their event timer and their SR620 to characterise the range
    dependency against a linear device;

-   **Agreed** that, wherever possible, the \'Herstmonceux\' counter
    tests should be repeated at other sites;

-   Short-arc quality control could be used to analyse "raw" and
    "counter-corrected" LAGEOS passes from all stations on a test basis
    -- **recommend** that stations send relevant pass data to NERC SGF
    for analysis.

### **Proposal from Ivan Prochazka**

The Technical University of Prague has developed and demonstrated the
portable standard concept through its P-PET system.

EUROLAS Stations agree to:

-   collaborate on a proposal to the EU to procure a P-PET European
    portable standard; and

-   support P-PET comparisons at as many stations as possible once a
    European standard is available.

#### 

#### Session 4: Miscellaneous

Other issues relating to establishment of Europe-wide good practice were
discussed.

**General:**

-   Encourage station personnel to visit each other for the purposes of
    information exchange, collaboration, specialised advice and
    assistance and the spread of \'good practice\' in all matters
    relating to data acquisition and processing.

**Meteorological Measurements:**

Blunders in meteorological measurements are not uncommon and in
particular, errors in atmospheric pressure can have a very significant
impact on data accuracy.

-   Stations should conduct regular comparisons of local measurements
    with duplicate sensors or nearby meteorological stations;

```{=html}
<!-- -->
```
-   NERC SGF will prepare its second Herstmonceux Druck digital pressure
    sensor for on-site comparisons at as many of the EUROLAS sites as
    possible, designing the experiments and posting the device to the
    nearest station for a round Europe trip.

**Epoch Measurements:**

Most stations use a local source of epoch driven either by a 10MHz
output from GPS (synchronised by 1pps) or by a local time service that
contributes to and is monitored against UTC. There appear to be no
problems at any station in achieving the requisite sub-microsecond level
of accuracy for epoch. However since timing errors of this magnitude
would be difficult to detect, stations should:

-   Check their clocks routinely against an independent source of UT.

**Laser Stability:**

NERC SGF has carried out extensive tests on the effects on system
precision and accuracy of laser temperature, dye-strength and repetition
rate and found that keeping each within strict limits improves
consistency - outside these limits, the observer is prevented from
operating the system. Building from this and similar experiences in the
network, stations are urged to:

-   Carefully monitor system parameters as a health check on their
    operation.

**Calibration techniques/station practices:**

Changes in calibration value indicate that something has happened at the
station that may bias the range data. Stations should:

-   Investigate (and understand) any significant changes in calibration
    value (there must be a reason for it);

-   Calibrate at least once per hour (unless they are doing real-time
    calibration), even if this means interrupting (long) satellite
    passes;

-   Perform calibration ranging in the same signal strength regime as
    used for satellite ranging;

-   Perform calibration and satellite ranging through the same optical
    and electronic paths.

**EUROLAS near-real-time display:**

AIUB has established a near real-time display of station tracking status
to: expedite recovery of manoeuvred satellites, check on \'best\' IRV
set and time-bias to use, dynamically prioritise satellite passes
(adjust to the activities of other stations in the cluster, and exchange
messages. For information on this utility, see SLRMail 372,
\"Near-real-time status exchange\", by W. Gurtner, 13^th^ July 1999.

EUROLAS recommends that:

-   Other stations/networks join; and

-   The current refresh rate of display be changed from 30 **seconds**
    to 15 seconds to aid capture of fast LEO satellites;

**Quality of predictions:**

It was agreed for reasons of flexibility, that when available, several
options for prediction sets should be accessible by the stations. For
information, NERC SGF uses:

-   GFZ IRVs (for ERS-2); IRVs + drag functions (for CHAMP, GRACE);

-   CODE/NERC IRVs (for GPS, GLONASS);

-   MCC IRVs (for REFLECTOR);

-   HTSI daily IRVs (for all other satellites);

-   (NERC SGF -- daily/weekly - for backup).

### **Time bias corrections:**

A proposal by Werner Gurtner to set up a server to compute instantaneous
time bias values was warmly welcomed:

-   Time biases will be evaluated in near real-time using the time-bias
    functions routinely computed and frequently updated by NERC SGF;

-   Values will be computed for all satellites for all available
    prediction sets, subdaily, daily, weekly and monthly; and will use
    GFZ drag functions as appropriate;

-   Service will be run from AIUB (in a similar form to the EUROLAS
    near-real-time display) with NERC time bias functions updated every
    15 minutes.

NB -- **already implemented!** -- See SLRMAIL 921 for details.

**Collocation results:**

An analysis by Van Husson of the ITRF2000 coordinates of the reference
points for the various geodetic instruments located at SLR stations
showed that nearly every site has problems (at the few mm level) in at
least one coordinate of the local tie vectors that need to be resolved.
In the laborious task of resolving these issues:

-   Every significant ILRS sites should be collocated with a GPS
    receiver contributing to the IGS global network

###### Appendices

1.  [**Computation of return rates**:]{.underline}

Count the number of laser shots, ***s***, in a given time interval - say
15 seconds;

-   For each of these shots, check whether a noise event has been
    detected \'before\' the true return (*i.e.* count noise below the
    'track') - each of these reduces by one the effective number of
    laser shots - get ***s~corrected~***;

-   Count the number of \'true\' returns, ***r***, during the same time
    interval;

-   Then the 'true' return rate = (***r***/ ***s~corrected)\ ~***× 100
    (%).

2.  **[Participants -- Herstmonceux -- 2002 March 11-13]{.underline}**

**Name Station email**

Graham Appleby Herstmonceux Graham.Appleby@nerc.ac.uk

Aldo Banni Cagliari Banni@ca.astro.it

Carmelo Belza San Fernando CBelza@roa.es

David Benham Herstmonceux David.J.Benham@nerc.ac.uk

Giuseppe Bianco Matera Giuseppe.Bianco@asi.it

Jorge Garate San Fernando JGarate@roa.es

Philip Gibbs Herstmonceux Philip.Gibbs@nerc.ac.uk

Ludwig Grunwaldt Potsdam Grun@gfz-potsdam.de

Werner Gurtner Zimmerwald Werner.Gurtner@aiub.unibe.ch

Karl-Heinz Haufe Wettzell Haufe@wettzell.ifag.de

Van Husson Honeywell Van.Husson@Honeywell-TSI.com

Georg Kirchner Graz Kirchner@flubpc04.TU-Graz.ac.at

Franz Koidl Graz Koidl@flubpc04.TU-Graz.ac.at

Cinzia Luceri Matera Luceri@hp835.mt.asi.it

Matti Paunonen Metsahovi geodeet@csc.fi

Michael Pearlman Harvard/ILRS MPearlman@cfa.harvard.edu

Christopher Potter Herstmonceux Christopher.Potter@nerc.ac.uk

Ivan Prochazka Prague TU Prochazk@troja.fjfi.cvut.cz

Stanislaw Schillak Borowiec Schillak@cbk.poznan.pl

Robert Sherwood Herstmonceux Robert.Sherwood@nerc.ac.uk

Johannes Utzinger Zimmerwald Johannes.Utzinger@aiub.unibe.ch

David Walters Herstmonceux David.Walters@nerc.ac.uk

Roger Wood Herstmonceux Roger.Wood@nerc.ac.uk