A lot of tests and experiments have been made in Graz during the last 2 years to increase performance and accuracy; using the SPAD from the Prag group as receiver, we have reached now about 5 mm RMS from the calibration target, and about 8 mm RMS from ERS1 and STARLETTE. In addition, by routinely using the semitrain, the number of returns has been increased significantly for most satellites

Kirchner, Georg

Koidl, Franz

English

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and
SUB--CM
other
in
N94-15603
Ranging
improvements
Graz
Georg KIRCHNER, Franz KOIDL
SLR Graz / Institute for Space Research
Austrian Academy of Sciences
Observatory Lustb6hel, Lustb_helstr. 46
A-8042 GRAZ, AUSTRIA
Tel.: +43-316-472231; Fax: +43-316-62678
E-Mail : kirchner@flubiw01, tu-graz, ac. at
1.0 Introduction
A lot of tests and experiments have been made in Graz during
the last 2 years to increase performance and accuracy; using the
SPAD from the Prag group as receiver, we have reached now about 5
mm RMS from the calibration target, and about 8 mm RMS from ERSI
and STARLETTE. In addition, routinely using the semitrain, the
number of returns has been increased significantly for most
satellites.
2.0 Experiments
In January 1991, together with the Prague group, we installed
their streak camera as a receiver in the Graz laser telescope,
and first echoes from AJISAI and STARLETTE could be recorded.
In December 1991, again together with the Prague colleagues,
2-color ranging experiments were performed, using Raman upshifted
red (683 nm) and 532 nm wavelengths; returns of both colors from
AJISAI and LAGEOS could be recorded. A detailed description of
both experiments is given elsewhere in these proceedings.
11-31
https://ntrs.nasa.gov/search.jsp?R=19940011130 2020-06-16T19:14:01+00:00Z
3.0 Accuracy Improvements
As stated already during the Matera workshop, the contribution
of the SPAD itself to the overall jitter can be decreased by
using higher voltages above break (Vab). Modifications of the
original SPAD e!ectronics now allow us to increase this Vab to
more than i0 V, resulting in a jitter of 5 mm RMS (fig. i) from
the calibration target. In this test, we used an SR620 counter,
which also contributes to the lower RMS (the start input of the
SR620 handles the output of the start Optoswitch significantly
better than our HP5370A).
The well known disadvantage of the high Vab is the increase in
noise; while a standard SPAD at 2.5 Vab at room temperature has
an acceptable noise of about 200 kHz or less, this increases at
I0 Vab to about 1 MHz (fig. 2). To reduce this, we cool the SPAD
now with Peltiers to -25°C, reaching again about 200 kHz noise.
Short before the workshop, we had 4 different counters
available for test purposes at the SLR Graz: HP5370A, HP5370B,
and 2 SR620. The RMS values of these specific instruments are
listed in table I; RMS 1 is measured with asynchronous, random
pulses, as it is the case during satellite ranging; RMS 2 is
measured with pulses synchronous to the internal frequencies, and
is listed here only for completeness.
RMS 1 RMS 2
HP 5370A 22 ps i0 ps
HP 5370B 40 ps 12 ps
SR620/I 21 ps 8 ps
SR620/2 20 ps 8 ps
Remarks
Our standard counter for SLR
Available for tests
Our future counter for SLR
Same type, available for tests
Table i: Counter Comparison
Using these counters in our station for calibration tests, we
got the results shown in fig. 3. We measured all counters with
"good alignment" (the returns focussed as good as possible on the
center of the SPAD) and "weak alignment" (focussing of the re-
11-32
turns on the S_AD far from optimum). It can be seen that good
SPAD alignment (together with optimizing all start/stop input
pulse rise times, pulse forms, trigger thresholds etc.) is of the
same importance than selecting the proper counter.
4.0 Routine use of the Semitrain
Since summer 1991, we routinely use the second half of the
pulse train (semitrain), delivered from our Nd:YAG laser, for all
SLR measurements. The software has been modified to allow for au-
tomatic "folding" of the returns. Fig. 5 shows an example pass of
LAGEOS, with totally 7 tracks (or pulses from the semitrain)
identified later by the software.
By using all returns from the semitrain, the number of returns
for most satellites could be increased significantly (fig. 6);
for LAGEOS the increase in returns is more than 50%; for the
ETALON's, it is more than doubled.
5.0 Acousto-Optic Modelocker
In February 1992, we installed an Acousto-Optic Modelocker in
our old Nd:YAG laser oscillator, inceasing reliability and shot-
to-shot reproducibility of the laser pulses. Besides increasing
the number of valid shots from our previous average of about 70%
(it was an old, purely passiv modelocked oscillator!) to more
than 99%, the much better stability of the pulses had a notice-
able effect on the jitter (Fig. 4); the routine cal. values
showed better stability and even slightly lower RMS (the AO-
Modelocker was installed after calibration number 98, in fig. 4).
The AO-Modelocker requires temperature stabilization; this is
done by slighly heating it to 27° , using the hot side of Peltier
elements; the cold side of these elements is used at the same
time to cool the Dye Cell to about i0 ° , which should result in a
longer lifetime of the Dye. Results are promising (we used the
same dye from February to May), but still have to be verified
during the next years.
11-33
I00 _ SPAD #5 (-25°C)/ SR623
JITTER + MEAN CAL vs. VAB
?
_
5
JITTER [mini MEAN' CAL VALUE [pl] * 0-740.5 WJl
I | I I I I i I |
2 3 4 5 6 7 8 9 10 11
SPAD' VOLTAGE ABOVE BREAK IV]
12
300
250
200
150
100
50
0
-- CAL JITTER [ram] -'_ MEAN CAL VALUE [ps]
1992-05-15
Figure i: Minimum Jitter from Target is about 5 mm
I000
800
600
406
200
1992-05-15
-10 i.I SPAD #5
_oise Temperature Dependance
Noise [kHz]
0
-30
SPAD #5 / SMD #1 10 V
10 Hz Gating
...................._'Lov ....................
......._-,......; ...i .........'i.. ._'_:
-25 -20 -16 -10 -5 0 5 10 15
SPAD TEMPERATURE [¢C]
,KG/KF,
Figure 2: Noise depends on Voltage above
11-34
break and temperature
C DUNTER COMPARISON
EXT, CALIBRATION JITTER
EXTERNAL CALIBRATION JITTER [mm]
10 ............................................... 9.00- ...........................................................................................................
HP5370 A HP5370 E SR620 / 1 SR620 / 2
$FAD ALIGNMENT iCOARSE; 4-15 C _'_ FINE; -27 C
1992-05-13
Figure 3: Calibration with different counters and alignments
LASER STATION GRAZ
Calibrationdi_ter
10
9
8
7
6
Calibration Jitter [mini
AO-Modeloolmr Installed
° °_[] .............................
........ 0 .................................................... 1131............. _:._...........................
o o o rmsO 0 0 _ 0
.r.rm,Ol_..O ................ u ................ _ "u ....... _ ...................................... I_" --
o
.........................................................................................................................................................................
......................................................'oo.,,,,,..I°oIiiiii);;iiiiii/
......................................................... I
I
1 , I i
0 50 100 160 200 250
Calibration Number 1992
Cal Jitler ol routine passes 1/92-3/92
Figure 4: Effect of Acousto-Optic Modelocker on Cal stability
11-35
'_ to. oa
in
I1!
5.00
-S. O0
LOSROI Igg2- 2-._7 START I 22 20 474g POIIITS 72.6 ° ELEVATIO:!
-10.00
0 5 I0 15 20 25 30 35 40
.]-IIt_TES _FTER START
45 50 55
<KG>
Figure 5: Typical LAGEOS pass, with semitrain returns
SLR GRAZ: SEMI-TRAIN
0HCF_EA$I_OF _ETL_N NILIII_IBIEF_
_eturn Increase Factor (Average Sept.91)
2.5
2 ETALON 1-2
05 _ _ l _ ; T _ .
1 2 3 4 5 6 7 8
# of Pulses per Shot used
ETALON 1-2:2.34
STARLETTE : 1.28
lggl-og-30
Figure 6: Increase of return number with semitrain
11-36


Sub-cm ranging and other improvements in Graz

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