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N94- 15 8 

SUB-CM Ranging 
and other improvements 

in Graz 


SLR Graz / Institute for Space Research 
Austrian Academy of Sciences 
Observatory Lustbiihel, Lustbiihelstr. 46 
A- 8 04 2 GRAZ, AUSTRIA 

Tel.: +43-316-472231; Fax: +43-316-462678 
E-Mail : kirchner@f lubiwOl . 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 ram RMS from ERS1 
and STARLETTE. In addition, routinely using the semi train, 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. 


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 electronics now allow us to increase this Vab to 
more than 10 V, resulting in a jitter of 5 mm RMS (fig. 1) 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 
10 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 1; 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 

10 ps 

Our standard counter 



HP 5370B 

40 ps 

12 ps 

Available for tests 


21 ps 

8 ps 

Our future counter for SLR 


20 ps 

8 ps 

Same type, available 



Table 1: 



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- 


turns on the SPAD 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 NdtYAG 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 NdrYAG 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 A0- 
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 10°, 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. 


Figure 1: Minimum Jitter from Target is about 5 

Figure 2: Noise depends on Voltage above break and temperature 





HP5370 A 

HP5370 B 

SR620 / I 

SR620 / 2 


F~~1 COARSE: >15 C ES! FINE; -27 C 


Figure 3: Calibration with different counters and alignments 


Calibration Jitter 

Calibration Jitter [mml 

AO-Modelooker Installed 

o 50 100 160 

Calibration Number 1992 

Cal Jitter oi routine passes 1/92-3/92 

Figure 4: Effect of Acousto-Optic Modelocker on Cal stability 

10.00 r LOfiflOl 1992- 2-27 START t 22 20 47-19 POINTS 72.6° ELEVATION 

**71 ; * , 

rs. ■ 

• ^ *' •. ‘‘V V \ ■ 

•••«."• ' ••; x -s.‘ ‘ — ' * 

* * Sj 

"•* • N V , . V. 

% U ’i, ^ •. 


- 10 . 00 J - 

9 5 10 15 20 25 30 35 40 45 50 55 


Figure 5: Typical LAGEOS pass, with semi train returns 



Return Increase Factor (Average Sept.91) 

ETALON 1*2 ___ 


# of Pulses per Shot used 

ETALON 1*2: 2.34 

■' LAGEOS : 1.62 
ERS-1 : 1.36 


Figure 6: Increase of return number with semitrain