Status of the Zimmerwald SLR Station

M. Ploner, P. Lauber, P. Ruzek, M. Prohaska, P. Schlatter, J. Utzinger, T. Schildknecht, A. Jäggi

Astronomical Institute, University of Bern

16 cm

Photo sensor 1+2for prelim. speedand position check

2 holes

Photo sensor 3 „OPEN“

ROTATING SHUTTERThe rotating shutter opens the receiving pathfor the time of reception only. The shutter isimplemented as a thin aluminum disk withtwo diametrically placed holes of 6 mmdiameter each at a radius of 70 mm. It isdriven by a DC servo motor with integratedcontroller operated in stepper mode at aspeed of about 3000 rpm (‐>100 Hz opticalchopper freq.) and is precisely synchronizedwith the actual firing rate of the laser. Theshutter is open during about 300 μs with anuncertainty w.r.t. the expected time ofreception of +/‐50 μs. Frequency and phaseare controlled by a PC via the FPGA card.

GNSS ConstellationSince summer 2010 the complete Glonassconstellation (currently 24 satellites) isranged from Zimmerwald. The number ofobservations per normal point (#OBS/NP)is almost a factor of two higher forGlonass satellites equipped with uncoatedreflectors than for those with coatedreflectors. The difference in #OBS/NP is aresult of the higher return rate forsatellites with uncoated reflectors. Thelower RMS of the normal points foruncoated reflectors can be explained bythe higher #OBS/NP. No significantdifferences can be seen between differenttypes of Glonass satellites (Glonass,Glonass‐M and Glonass‐K).

DETECTORThe field of view (FOV) can be limited by variouspinholes (diameters from 8 to 40 arcsec) inorder to reduce the noise, especially duringdaytime operation. A beamsplitter separatesthe infrared and green light. A variable neutraldensity filter can be used to decrease thenumber of photons passing through a Fabry‐Perot filter (FWHM of 0.15 nm) and a bandpassfilter (531 nm – 533 nm) to the CompensatedSingle Photon Avalanche Diode (CSPAD)detector.

21 mJOscillator(100 MHz)

Regenerative Amplifier

DoublepassAmplifier

Attenuator

SHG (KDP)

Beam Splitter0-100 %

2–98 % transmission

9 mJ max.

19 mJ max.

Year Observation minutes

2009 (from 20th July)  41

2010 164

2011 756

2012 399

2013 (until 25th October) 799

Bistatic Experiment Graz‐"CZ‐2C"‐Zimmerwald

CZ‐2C 18 June 2013, 21:17 ‐ 21:25 UT: Returns measured at Zimmerwald

Bistatic Experiment Graz‐ENVISAT‐Zimmerwald

Envisat 28 March 2012, 20:40 – 20:50 UT: Returns measured at Zimmerwald

TRANSMIT/RECEIVE SWITCHA specially coated mirror serves as switchbetween the transmit and the receive paths.This mirror has an anti‐reflective coating onthe back side (towards the laser). On thefront face there is a reflective coating exceptat the place where the transmit beam passesthe glass plate. This portion of the mirror hasan anti‐reflective coating and is also used toreflect a small percentage of the receivedcalibration pulse into the receiving path.

For further information visit our website:

http://www.aiub.unibe.ch

Poster compiled by P. Ruzek, November 2013Astronomical Institute, University of Bern, Bernpeter.ruzek@aiub.unibe.ch

13‐Po43

In case of range observations to the LunarReconnaissance Orbiter (LRO) the lasersystem has to operate in a special mode.At the spacecraft the observation windowhas a width of 8 ms and a frequency of 28Hz. The pump diodes of the amplifiers canbe triggered with a variable rate betweenapproximately 9 and 11 ms in steps of 10microseconds. By means of the pockelscell selecting the pulses to be amplified,the actual firing rate can be reduced byan additional integer factor. For the LROone‐way ranging experiment the laserfires at approximately 14 Hz, i.e. with abasic pump interval of 10.200 ms

(corresponds to a frequency of 98 Hz) anda reduction factor of 7. The firing timecan be empirically corrected in steps of 1ms in case of unsuccessful observations.Especially during daytime operation theactual pointing offset of the telescopedue to thermal effects is empiricallyestimated using GNSS satellites passingnearby. On 20th July 2009 the SLR stationZimmerwald was the first European SLRstation that successfully carried outobservations to LRO. Since this successmore than 2300 observations minuteswere collected at Zimmerwald.

LRO

In the so called ‘bistatic’ experiment the SLRstation Graz is firing laser pulses to spacedebris objects using a powerful laser (200mJ @ 532 nm, 3 ns pulse length, 80 Hz)provided by DLR (Deutsches Zentrum fürLuft‐ und Raumfahrt) Stuttgart. On 28thMarch 2012 the diffusely reflected photonson the satellite ENVISAT were successfullydetected and time‐tagged at theZimmerwald SLR station for the very firsttime. About one year later (18th June 2013)photons reflected on a space debris object(upper stage CZ‐2C) with a considerablysmaller radar cross section than ENVISATcould be detected. For bistatic rangemeasurements the receiving componentshave to be synchronized to the Graz firingtimes within a few microseconds. Knowingthe exact firing times of the Graz laser in

advance, the expected arrival times of theGraz photons in Zimmerwald are calculatedand the detector is activated accordingly.The collected data can be used to calculateimproved orbits of the tracked debrisobjects. But this would ask for measuringthe time stamps of the stop pulses with anabsolute accuracy below 1 nanosecond. Thisis not possible so far due to the followingunsolved issues. On the one hand, the timesynchronization of the event timer with UTCis done by a GPS receiver with an accuracyof 100 ns. On the other hand, the delaybetween the reference point of thetelescope (intersection point of thetelescope axis) and the event timer is onlyestimated with an accuracy of a fewnanoseconds.

BISTATIC EXPERIMENT

The laser system is a diode pumpedsolid state Nd:YAG lasermanufactured by Thales Laser,France, with a primary wavelengthof 1064 nm and a pulse ratebetween 90 – 110 Hz. The pulseswith an energy of about 21mJ(before doubling) and a pulse widthof 58 ps are generated by a 100MHz oscillator, a regenerativeamplifier and a double‐passamplifier. A KDP (PotassiumDihydrogen Phosphate) crystal inthe second harmonic generator(SHG) produces a 9 mJ pulse at 532nm. Theoretically the infrared

beam after the SHG could be usedfor two‐color ranging, but thequality of the infrared beam isinsufficient. In case of two‐color orinfrared‐only operation, the SHGcan be bypassed with a selectablefraction of the infrared energy.Laser trigger signals for pumpdiodes and enabling/disablingsignals for the pockels cell, as wellas the signals for range gategeneration and rotating shuttercontrol, are generated by the FieldProgrammable Gate Array (FPGA)card prepared and programmed bythe Technical University of Graz.

LASER

The 1‐meter Zimmerwald Laser and Astro‐nometry Telescope (ZIMLAT) was installed in1997. It allows for state‐of‐the‐art satellite laserranging (SLR) and also serves as astronomicaltelescope for the optical observation ofastrometric positions and magnitudes ofnear‐Earth objects, such as space debris,using Charge Coupled Device (CCD) or Com‐plementary Metal Oxide Semiconductor(CMOS) cameras. The telescope is monostaticw.r.t. SLR (transmit and receive paths areidentical between the primary mirror and thetransmit/receive mirror located at the lowerend of the Coudé path). The detectors areprotected from the backscatter of the transmitbeam by a rotating shutter.

ZIMLATThe dichroic mirror located in thefork of the mount allows for the useof tracking cameras simultaneouslywith SLR observations. The formerlyused CCD Camera with an interlineCCD was replaced in 2013 by aso‐called scientific CMOS Cameramanufactured by Andor. Themaximum frame rate of 100 fpsallows the exposure of the sensorbetween the transmitted laserpulses. The FPGA card, responsiblefor the timing of the laser, transmitsa pre‐pulse with variable delay andpulse width. This pulse can be usedfor triggering the exposure of thecamera and for controlling theexposure time. It is not possible tomake use of the full time span ofabout 10 ms for chip exposure due

to fluorescence effects in thedichroic mirror after transmitting alaser pulse. The camera is wellsuited for bright objects like LowEarth Orbiters. For fainter objects,the exposure time must beincreased by co‐adding severalshort exposures. There is oneremarkable disadvantage comparedto the formerly used interline CCDcamera manufactured by PCOwhere the photons of all sub‐exposures were accumulated on‐chip and read out only once. This isnot possible with the new CMOScamera. The sub‐exposures must beread out individually and co‐addedby software which degrades thesignal‐to‐noise ratio.

TRACKINGCAMERA

Beam

Spl

itter

RotatingShutter

PinholeFOV

Fabry-Perot0.15nm

VNDFilter

CSPAD

Bandpass1063-1065nm

VNDFilter

Fabry-Perot0.15nm

?

Bandpass531-533nm