In this report, the results of the upgrade of the German Modular Transportable Laser Ranging System MTLRS-1 are summarized. A short description of the new components and their influence on the system accuracy is given. It is shown, that the single shot accuracy of the MTLRS-1 has been improved from 5 cm to 1 cm

Amberg, L.

Beyer, L.

Blenski, G.

Etling, W.

Hessels, U.

Motz, R.

Sperber, Peter

English

NASA Technical Reports Server

N94-15610
Results of the MTLRS-1 Upgrade
P. SPERBER, L. AMBERG, L. BEYER, G. BLENSKI, W. ETLING, U. HESSELS, R. MOTZ,
INSTITUT FIJR ANGEWANDTE GEOD/_SIE
FUNDAMENTALSTATION WETTZELL
DER FORSCHUNGSGRUPPE SATELLITENGEODA$IE
D-8493 K6TZTING
Abstract. In this report the results of the upgradeof the German Modular Transportable Laser Ranging
System MTLRS_I are summarized. A short description of the new components and their influence
on the system accuracy is given. It is shown, that the single shot accuracy of the MTLRS_I has been
improved from 5 cm to 1 cm.
1. Introduction
The German Modular Transportable Laser Ranging System MTLRS#I has been operated suc-
cessfully since 1984. From 1985 to 1990 the system was employed in the Central and Eastern
Mediterranean area and in the U.S.A., contributing to the WEGENER-Medlas and the NASA
Crustal Dynamics project.
In order to improve the system reliability and the ranging accuracy from 5 cm to 1 cm a major
system upgrade was performed from December 1990 to July 1991.
This upgrade was planned and carried out in five steps:
• Exchange of the Laser
• Exchange of the Start Detector
• Exchange of the Receiving System
• "Exchange of the Single Photon Detector
• Improvements in Periphery and Software
In this paper we will summarize the technical specifications of the new systems and show first
tracking results.
2. Technical description of the upgrade
2.1.The Laser
The duration and the energy of the emitted laser pulses are critical elements in the determination
of the accuracy and the number of returns produced by a satellite laser ranging system. To improve
this two points, we exchanged the original Nd:¥AP Laser (Pulseduration: 370 ps; Pulseenergy at
536 nm: 10 mJ) for a new Nd:¥AG Laser (Pulseduration: 30 ps, Pulseenergy at 532 nm" 30 mJ).
To ensure a good reliability of the laser in field, we made use of the self filtering unstable resonator
(SFUR), described by R. Bianchi et.al, and K. Hamal et.al, which has a very simple optical layout.
This setup has three major adventages:
• insensitive to inaccurate adjustment
• homogenious spatial energy distribution
• very high pulse energy in the resonator.
Typeset by .AA_T_
12-17.
https://ntrs.nasa.gov/search.jsp?R=19940011137 2020-06-16T19:11:59+00:00Z
The realisation of the optical setup of the laser delivered by Laser Valfivre is shown in Fig. 1. To
save place, the resonator M1 - M2 is folded over the prism PR. The laser is cavity dumped through
a pockelsceil PC and a polarizer PO. An active-passive configuration (acustooptic modelocker ML
and dye cell with Kodak 9860 in 1,2-Dichlorethan) is used to generate picosecond pulses.
In the SFUR configuration, the energy of the pulses in the resonator is already reasonable high,
therefore a Single Pass Amplifier (AMP) is sufficient to generate a pulseenergy of about 100 mJ in
the infrared which gives about 30-40 mJ in the Second Harmonic.
Due to the good isolation of the laser head and the power electronic to environmental influences
and due to remote control facilities in the laser head, we are able to operate the laser over long
period without opening the shielding.
2.2. The Start Detector
To take full advantage of the short laser pulses it is neccessary to use a start diode with very
fast risetime to generate the start signal for the counter. As each component between diode and
counter gives some additional jitter we were looking for a solution with a very simple electrical
layout. Mainly we tried to find a way where it was no longer neccessary to use a constant fraction
discriminator to compensate the amplitude fluctuations of the diode signal.
Therefor we integrated an optically triggered avalanche diode (I. Prochazka et.al.) as start de-
tector. This diode generates an electrical signal, which is independent from the energy fluctuations
of the laser pulses. This signal is directly used as start signal for the counter.
The signal has a risetime of 300.400 ps (Fig. 2), the jitter between iaserpulse and electrical pulse
is less than 20 ps.
To obtain high stability and to have also the possibility to varY, the pulse energy during tracking
the new start diode is placed before the amplifier to detect the infrared signal. In the previous
design the diode detected the green light after the second harmonic generation.
2.3. Receiving Package
In contrast to other SLR System the MTLRS is using an echelle grating for the spectral filtering
of the received light. Compared to the common used interference filters this has the advantage
to be insensitive to environmental influences. Unfortunately an echelle grating generates a time
spread of a pulse due to the different light paths over the grating.
In the old grating this was compensated with an accuracy of about 1 cm by sending the diverse
parts of the beam to different mirrors. Due to this splitting of one pulse into several beams, the
outcoming beam has not the high optical quality which is necessary for focusing on the small active
area (100pro) of an avalanche diode.
In the new package a second echelle grating in opposite position is used to make full compensa-
tion. The excellent optical quality of this configuration results in a very homogeneous beam which
is necessary to generate small focal spots.
Additionally a remote controled field-of-view pinhole and a better isolation against straylight is
installed in the new package. The sketch of the whole receiver is seen in Fig. 3. (H. Visser)
At the input the beam coming from the telescope is directed by a prism to an optical package
in which a rotating shutter and some small prism are used for filtering and guiding of the beam.
The light has to travel through the rotating shutter and is directed to an echelle grating. After
that the pulse is spread in time and frequency. An adjustable pinhole is used for the field of view
filtering and for selection of the correct wavelength. To compensate the time spread the beam now
propagates a second time to the echelle grating before two lenses form a parallel, 8 mm diameter
output beam.
2.4. Single Photon Detector
A major improvement in the accuracy of the time interval measurement was achieved by the
exchange of the photomultiplier for a single photon avalanche diode (SPAD) (I. Prochazka et.al.,
G. Kirchner).
The signal out of the SPAD is independent of the signal strength, so the use of a constant fraction
discriminator is not necessary.
A description of the technical details is given in Sperber et.al.
3. System Accuracy
The main reason for the upgrade of the MTLRS was the demand to improve the system accuracy
from 5 cm to about 1 cm single shot r.m.s.
In Fig. 4 the developement of the accuracy during each step of the upgrade is depicted. The
main influence to the accuracy came from the exchange of the laser and the single photon detector.
The start detector has minor effect and the importance of the new receiver package can be seen in
the good optical quality which finally allows the usage of the SPAD.
The contribution of each single component of the system to the system accuracy is shown in
Fig. 5 for the old and the new configuration. The r.m.s of all components has now about the same
size. Further improvements are probably possible at the laser (10 ps) and at the counter. More
accurate start and stop detectors are not available at the moment.
The plot of a typical pass before and after the upgrade is given in Fig. 6. Here the significant
decrease of the system r.m.s is conspicious.
All this data are showing, that the single shot accuracy of the MTLRS#1 is now 1 cm or less,
if all components (mainly start diode, SPAD and laser) are optimally adjusted.
Under field conditions we demonstrated (P. Sperber, H. Hauck) a system accuracy of 1 cm - 1.5
cm and a normalpoint accuracy of a few ram.
3. Perspectives into the next years
The receiving and transmitting part of MTLRS#1 is now again state of the art. To bring the
whole system to a status comparable with modern SLR systems two additional upgrade steps are
under discussion.
• Installation of a new control electronic and software, designed for future demands to Laser
Ranging Systems. IfAG has decided to develop such a control system based on a trans-
puter network in cooperation with MTLRS#2 (OSG Kootwijk). The integration in the
MTLtLS#1/2 is scheduled for 1993 (E. Vermaat et. al.)
• The Laser and the Receiving system are now prepared to make two color ranging. Only
the telescope does not meet the specifications for two color ranging as the reflecting mirrors
are optimized only for a wavelength of 532 nm. A further upgrade for two color ranging
capability depends strongly on the demand of the international SLR community for two
color ranging data.
12-19
5. Summary
After the upgrade of MTLRS#I performed from December 1990 to Juli 1991 a system with the
following modified specifications is now available.
• 30 ps laser pulse-width with 30 mJ/pulse at 532 nm
• Optically triggered avalanche diode to start the time interval counter
• Compensated echelle grating filter
• Single Photon Avalanche Diode
• In field normal point generation
As a result of these modifications MTLRS#1 is now capable of tracking high and low satellites
day and night with a single shot r.m.s, of about 1 cm.
12-20
REFERENCES
Laser Valfivre, SpeciJicationJ "of a laser jyJtem ]or MTLRS#1 , 1989.
A. Bian_,i, A. Ferrario, P.G. Gobbi, C. Maivlcini,G.C. Reali, SPIE 701, p.132,, 1989.
K. Hamal, V. Hraskova, H. JeUnkova, CLEO'SS Proceedings p.82, WE 4, 1987.
I. Prochazka, K. Hamal,, S_bcentimeter Single Photon Rangin# All Solid St41e Detection P,_c_ege, 1990.
G. Kirc.hner, Proceedings of the seventh international Workshop on Laser Ranging Instrumentation p. 123,
1989.
P. Sperber, U. Hessels, Ft. Motz in this Proceedings, The new MTLRS#1 Receivinl Sys|em 199_.
P. Sperber, H. Hauck, in this Proceedings, Ez-perierLce and ResffillJ of the 1991 MTLRS#I USSR c=mp_@n,
199_.
E. Vermsst, J.W. Offierskl, K.H. Otten, W. Beek, C. van Es, P. Sperber, in this Proceedings, A Tr4nR_sier
B4Jed Control System for MTLRS, 199_.
H. Visser, Technisch Physische Dienste (TPD), Delft private communication, Design o/ ,_ new MTLRS#I
receiver package, 1990.
12-21
Ml - M5 Mirrors QSZ (]sc;ll_tor SS
MM Motor-MiRe s AMP AmplIFler Ap
Po Polor;zee ANL AC tlve mode-loc]_er PC
Pr Prism PNL Po s sire mode-(ocher T_¢
@ Dlognos t_c dlodes
Fig. 1 Schematic of Laser Head
SoFety shutter HVP half wove p(ate
Aperture SHG Second harmonic gen.
Pocl_els-cel( HeNe He-Ne oGgn_ent lOSer
Telescope CCD CCO torero
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Stort = 16. 75[_ ms Sto_ = 2_. 250_ ms Oelt_ T _ 3, 5_B _s
Fig. 2 - Signal of the Start Diode
12-22
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12-23
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Fig. 4: Decrease of the single shot r.m.s, during upgrade
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R,M.S, of MTLRS#I Components
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Fig. 5. Single shot r.m.s before and after upgrading
12-24
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12-25


Results of the MTLRS-1 upgrade

Results of the MTLRS-1 upgrade

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