LIST OF ABSTRACTS

Session 7: "High Repetition-rate Systems"

Development of Any Frequency Fire Rate SLR Control System

Cunbo FAN, Xue DONG, Xingwei HAN, You ZHAO
Changchun Observatory/NAOC, CAS, China
E-mail: fancb@cho.ac.cn

This paper presented the high repetition-rate control system which was developed by Changchun SLR group. The system can make the SLR system work at from several hertz to more than 2 kilohertz frequencies. The real-time control hardware and software under Windows XP environment are introduced in detail. The hardware control circuit includes three parts: accurate timing part, range gate control part and laser firing control part. A 2 KHz laser which borrowed from Wuhan was used to work together with the control system to test the performance. The experimental results show that the any frequency fire rate control system can work very well at or less than 2 KHz. If there is a suitable high repetition rate laser instrument (up to several KHz), Changchun SLR system can work at that freqency now.

Impact of Receiver Deadtime on Photon-Counting  SLR and Altimetry during Daylight Operations

John Degnan
Sigma Space Corporation, USA,
E-mail : John.Degnan@sigmaspace.com

Kilohertz photon-counting SLR and altimetry systems employ either Single Photon Avalanche Photodiodes (SPADs) or MicroChannel Plate PhotoMultiplier Tubes (MCP/PMTs) as detectors. Both detector types have comparable quantum efficiencies, at least at visible wavelengths. SPADs are further divided into passively or actively quenched subgroups with vastly different recovery times following a photon event. Passively-quenched SPADs have recovery times on the order of 1 to 2 microseconds which essentially makes them single-stop per pulse devices for most lidar applications. Active quenching of SPADs, on the other hand, can typically reduce the recovery time to somewhere between 50 and 100 nanoseconds. Relatively speaking, MCP/PMTs have vanishingly small recovery times, but this inherent advantage is not always maintained by the backend timing electronics. In NASA's SLR2000 (NGSLR) system, for example, the timing electronics recovery time is comparable to that of an actively-quenched SPAD receiver.
During night operations, when the noise background is dominated by detector dark counts (which vary from about 30 kHz for an MCP/PMT up to about 200 kHz for some SPADs), all of the aforementioned detector types are predicted to work quite well for single stop applications, which include SLR, transponders, or altimetry over barren terrain (i.e. devoid of vegetation). During day operations, however, the target return rate will be substantially reduced if the mean time between solar counts is comparable to the recovery time of the overall receiver. Furthermore, high solar count rates reduce MCP/PMT sensitivity (via microchannel saturation effects) as well as lifetime, which is related to the total charge transferred from the photocathode to the anode. The solar background count rate can only be managed by limiting the telescope aperture, spatial field-of-view, and/or spectral filter bandwidth. Range gating (or temporal filtering) has no impact on this effect. Sample photon-counting systems, such as SLR2000 and a proposed spaceborne altimeter, will be used to illustrate these important effects.

Transmitter Point-Ahead using Dual Risley Prisms: Theory and Experiment

John Degnan, Jan McGarry, Thomas Zagwodzki, Thomas Varghese
Sigma Space Corporation, NASA/GSFC, USA
E-mail: John.Degnan@sigmaspace.com

Conventional high energy SLR systems employ coaligned transmitters and receivers. The combination of high pulse energies, large transmitter beam divergences and even larger receiver fields-of-view ensure that a sufficient number of photons are reflected off the target and into the receiver to exceed the multiphotoelectron detection threshold. The latter is set sufficiently high to minimize false alarms under high background conditions. In contrast, eyesafe photon-counting systems operating at 532 nm in daylight must operate with over 3 orders of magnitude lower pulse energies, tight transmitter beam divergences to concentrate more of the transmitted light onto the satellite, and narrow receiver fields-of-view to reduce the solar background and improve signal contrast. Since the transmit and receive FOV's no longer overlap, NASA's Next Generation Satellite Laser Ranging System (NGSLR) is designed to point the receive telescope where the satellite was at the time the photons were reflected and independently point the transmitter ahead to where the satellite will be when the subsequent pulse arrives at the satellite. This transmitter point-ahead feature has been implemented in NGSLR via a matched dual Risley prism pair located on the optical bench in the transmitter path. The point-ahead angles, expressed in the azimuth and elevation axes of the telescope, are obtained from the orbit prediction program. Converting these az-el offsets into rotation angles for the two Risley prisms requires that we properly account for the complex and time-dependent coordinate transformations that are imposed by the various optical components and the axis rotations of the Coude mount as the transmitted pulse travels from the Risleys to the exit aperture of the telescope. In addition, one must correct for angular biases between the home positions of the servo motor drivers and the corresponding wedge orientation. Retroreflecting the outgoing transmitter pulse from a point in the common transmit/receive path on the optical bench into the focal plane of the star calibration camera provided a useful means of comparing theory and experiment prior to actual field implementation.

The New 100-Hz Laser System in Zimmerwald: Concept, installation and first experiences

W. Gurtner, E. Pop, J. Utzinger
Astronomical Institute, University of Bern, Switzerland
E-mail: gurtner@aiub.unibe.ch

In spring 2008 we replaced the Titanium-Sapphire laser by a new 100 Hz Nd:YAG system. One of the requirements for the new laser was a high flexibility in the actual firing rate and epochs to allow for synchronous operation in one-way laser ranging experiments. Firing order and range gate generation are controlled by a Graz-provided PC card with field-programmable gate array. The protection of the receiver from backscatter through our monostatic telescope was realized with a synchronized rotating shutter. First experiences show a very stable operation of the laser without any re-adjustment needs for many months, a much better efficiency regarding return rates to high satellites, and a single shot rms to well-defined targets of about 5 mm in single-photon mode.

Medium Resolution Event Timer and Range Gate Generator in Graz FPGA Card

Farhat Iqbal, Franz Koidl, Georg Kirchner
Austrian Academy of Sciences
E-mail: farhat_ieee@yahoo.com

Poster:
The Graz kHz Control System is running completely asynchronously; that means the software is waiting for a start event time, then reads this Event Time, calculates the Range Gate, and loads the Range Gate Generator on the FPGA board.
Because the Graz E.T. needs about 400 μs to fix the event time (2.5 kHz max repetition rate), we implemented now a 250-ps-resolution FAST event timer within the Altera FPGA Apex 20K chip on the Graz FPGA card; with this circuitry, any event time can be determined now within 20 ns, and with a resolution of 250 ps - more than adequate for Range Gate setting.
In addition, we implemented also a completely digital Range Gate Generator with 500 ps resolution on the same Altera FPGA chip of the Graz FPGA card; this replaces the presently used Programmable Delay chip, improving linearity and stability.

Graz kHz SLR LIDAR: First Results

Georg Kirchner, Franz Koidl, Daniel Kucharski
Austrian Academy of Sciences, Institute for Space Research, Austria
E-mail: Georg.Kirchner@oeaw.ac.at

Following an idea presented by NERC in Canberra 2006, we developed a kHz SLR LIDAR for the Graz station: Photons of each transmitted laser pulse are backscattered from clouds, atmospheric layers, aircraft vapour trails etc. A Single-Photon Counting Module, installed in the main receiver telescope, detects these photons. Using our FPGA card, these detection times are stored with a 100 ns resolution (15 m slots in distance). Event times of any number of laser shots can be accumulated in up to 4096 counters (according to > 60 km distance).
These LIDAR distances are stored together with epoch time and telescope pointing information; any reflection point is therefore determined with 3D coordinates, with 15 m resolution in distance, and with the angular precision of the laser telescope pointing.
First test results to clouds in full daylight conditions - accumulating up to some 100 laser shots per measurement - yielded high LIDAR data rates (20 points per second) and excellent detection of clouds (up to 10 km distance at the moment). Our ultimate goal is to operate the LIDAR automatically and in parallel with the standard SLR measurements, during day and night.

Millimeter Accuracy from Centimeter Targets

Georg Kirchner, Daniel Kucharski, Franz Koidl
Austrian Academy of Sciences, Institute for Space Research, Austria
E-mail: Georg.Kirchner@oeaw.ac.at

Spherical satellites like LAGEOS-1, LAGEOS-2 or AJISAI introduce a significant satellite signature (> 300 mm for AJISAI, > 80 mm for LAGEOS) into SLR data, if the return signal cannot be kept at a constant level (which is difficult to achieve due to fast pointing fluctuations etc.). These variations cause a significant scatter of the NPs around any assumed mean reflection point.
kHz SLR however allows - due to its inherent data density - an easy and accurate determination of the leading edge (LE) of returns (those returns coming from the nearest retros). Using this LE as a reference line, and accepting only returns between LE and LE + 20 mm, the scatter of NPs is reduced from several cm to less than 1 mm.

16 years of LAGEOS-2 Spin Data: From launch to present

Daniel Kucharski, Georg Kirchner, Franz Koidl
Austrian Academy of Sciences, Institute for Space Research, Austria
daniel.kucharski@oeaw.ac.at

Using full rate data of all SLR stations, as well as 2 kHz data from Graz, we extracted spin period data of LAGEOS-2 from launch in 1992 up to 2008. About 6600 spin data points were calculated, allowing detailed analysis of Spin Rate Slow Down during these 16 years.
In addition, we processed LAGEOS-1 data as well, deriving a set of spin period values from 1986 to 1993 (600 points). The results of this work confirm that SLR technology provides much more scientific information than usual range data, especially when high repetition rate SLR results are available.

NGSLR: Sharing Eye-safe Kilohertz SLR with Transponder Ranging

Jan McGarry¹, Thomas Zagwodzki¹, Tom Varghese², John Degnan³, Donald Patterson⁴, John Cheek⁵, Christopher Clarke⁴, Anthony Mann⁴, Peter Dunn⁵, Randall Ricklefs⁶, Anthony Mallama⁵
1 NASA Goddard Space Flight Center
2 Cybioms
3 Sigma Space
4 Honeywell Technology Solutions Incorporated
5 Raytheon Information Systems
6 University of Texas at Austin
E-mail: Jan.McGarry@nasa.gov

NASA's Next Generation Satellite Laser Ranging System (NGSLR) is expected to be operational in early 2009 with collocation testing occurring later this year. The system is currently ranging with the eyesafe laser (120 microJoules at 2 khz) from Low Earth Orbiting (LEO) satellites to LAGEOS, and has successfully ranged to GLONASS-95. Satellite passes are normally tracked using a tight laser divergence (~ 4 arcsec) with hands-off operation, and requiring no biases. Many LEOs have been tracked down below 15 degrees elevation. The telescope is pointed behind, in the direction of the returning light from the satellite, and the Risley Prism wedges are independently controlled to point the laser ahead to where the satellite will be when the laser pulse arrives.
With a simple drop-in mirror and toggle switch, the system can switch from eyesafe laser ranging to ranging with the 50 milliJoule, 28 hz, 6 nanosecond pulsewidth laser. This higher power laser was added to the system to perform uplink-only ranging to the Lunar Reconnaissance Orbiter (LRO-LR). For LRO the laser fire is controlled to ensure that the pulses arrive at the spacecraft when the Range Window is open and can accept events. Ground laser fire times are recorded and transmitted to a central facility where they are matched with the spacecraft events to form ranges.
Laser ranging to earth-orbiting satellites with the higher power laser is performed as a calibration for LRO ranging. With the LRO laser the system has successfully ranged to ETALON-1, GLONASS-95, LAGEOS 1 and 2, and many LEOs. In this configuration the telescope is pointed ahead and the Risleys are not used. The receive system is common to both laser configurations. Ranging with the LRO laser requires the use of a similar aircraft avoidance radar to the ones used by NASA's MOBLAS and TLRS systems.
During the LRO mission, when the moon is above 20 degrees, and the spacecraft is on the near side of the moon, NGSLR will point at LRO and fire the 28 hz laser. LRO will be on the near side of the moon one hour out of every two. During the off-hours the system will switch back to the eyesafe laser and perform two-way ranging to earth-orbiting satellites.

kHz Single-Photon Ranging: A Precise Tool to Retrieve Optical Response of Satellites

Toshimichi Otsubo (Hitotsubashi University), Philip Gibbs and Graham M Appleby (NERC)
E-mail: t.otsubo@srv.cc.hit-u.ac.jp

Single-photon laser ranging has been proven to be useful for retrieval of optical pulse shapes of satellite returns (Otsubo and Appleby, 2003). We had to accumulate a huge number of passes of observation data to create a usable residual profile, with the previous-generation Herstmonceux laser system of 10 or 20 Hz repetition and 100-ps pulse width. kHz single-photon ranging with a 10-ps-pulse laser currently realised at Herstmonceux is much more powerful to precisely retrieve a optical pulse shape, with just one or few passes. A quick look of actual data profile will be presented for various kind of satellites.

Minimization of systematic errors in satellite laser ranging with high pulse repetition rate

Sadovnikov M.A.
IPIE, Russia
E-mail: natalia.n@g23.relcom.ru

Poster:
An analytical model of single-photon satellite laser ranging (SLR) is presented, allowing to estimate the systematic error of ranging caused by fluctuations of photon numbers in the return pulse. It is demonstrated that with a sufficiently small number of photoelectrons in the in the pulse the return pulse intensity fluctuations practically do not affect the ranging systematic error value. It is also demonstrated that this error value may be reduced to a given level by reduction of the number of photoelectrons in the pulse and increase of the repetition rate. An estimate of the minimum required pulse repetition rate is presented.

High speed Pockels cell shutter and the Herstmonceux MCP-PMT detector

Matthew Wilkinson
NERC Space Geodesy Facility, UK
E-mail: matwi@nerc.ac.uk

Daytime kHz satellite laser ranging at the NERC Space Geodesy Facility in Herstmonceux, UK experiences a large amount of daylight noise. The C-SPAD detector in operation needs a minimum delay of 50ns after gating to avoid any range measurement error and only one detection is made for each laser fire. Therefore a noise point detected in this post-gating time period is a lost opportunity for a satellite laser measurement. An estimate of the loss for Lageos and HEO satellites due to daytime noise varies from 20% to as high as 50% of shots fired.
Introducing a high speed Pockels cell shutter before the C-SPAD allows the detector to be armed in darkness. The shutter then opens ~10ns before the satellite track, resulting in a reduced amount of daytime noise and an increased number of successful return signal detections. The polarisers in the Pockels cell however reduce the return signal intensity by a factor of more than 50%.
Advantages and practical considerations for this application are discussed and compared with an alternative fast gating MCP-PMT detector.

Development of the Electronic Circuit in High Frequency SLR Based on FPGA

Cunbo FAN, Zhenwei LI, You ZHAO
Changchun Observatory/NAOC, CAS, China
chenchong105@mails.gucas.ac.cn

Poster:
Increase of the laser firing frequency will significant improve the performance of Satellite Laser Ranging (SLR) system. To meet the requirement of high frequency SLR, an implementation of control circuit in single FPGA chip was designed and developed in this paper. SOPC (System On Programmable Chip) system was proposed to solve these problems. To realize the system, a control circuit custom component was designed and simulated. Then, the component was integrated into a SOPC system. Cooperated with software, the circuit has the ability to control the SLR system running at high frequency. Finally, the system was simulated in the Quartus software and NIOS IDE provided by Altera and implemented in an Altera EP1S10 development kit.

The Experiment of kHz Laser Ranging with Nanosecond Pulses at Shanghai SLR

Zhang Zhongping, Yang Fumin, Chen Juping, Zhang Haifeng, Wu Zhibo, Qin Si, Li Pu
Shanghai Astronomical Observatory, Chinese Academy of Sciences, China
E-mail: zzp@shao.ac.cn

The paper presents some progress on kHz ranging at Shanghai SLR station: design and build a range gate generator with 5ns resolution with a FPGA chip for kHz ranging; measure time of flight with a Riga A032-Event Timer; use two computers: one is for registering start and stop events and match them, display, identify returns, store data; another is for tracking and controlling, range gating; establish pre-processing software for handling kHz data on Windows Operating System.
In April 2008, we borrowed a diode-pumping Q-switched Nd:YAG laser with 50ns pulse width, 2 mJ in 532nm and 1kHz repetition from the NCRIEO in China. This paper gives the preliminary results of satellite ranging with the ns-pulses laser.