Bundesamt für Kartographie und Geodäsie Frankfurt am Main, 2008

© ForschungsgruppeSatellitengeodäsie

TIGO's new SLR system

In April 2006 the laser system in TIGO's SLR module was profoundly modernized. The old acousto-optically mode locked oscillator and the 10-Hz flashlamp pumped amplifier system were replaced to incorporate the latest state-of-the-art technology. The generation of short pulses is now based on passive mode locking utilizing SESAM (semiconducting saturable absorbing mirrors). The new Nd:YAG pump source for the Ti:Saph amplifier system is entirely based on diode pump chambers. The combination of the new oscillator and amplifier systems, depicted in Fig. 1, yields a significantly higher pulse stability both in shape and intensity, resulting in better data quality. Also the overall operation reliability was greatly improved, which boosted the station's productivity (number of measured satellite passes). In Fig. 2a the monthly observation statistics of the TIGO-SLR system for a few months before and after the upgrade (April/May) are plotted, showing the significant increase in data yield shortly after the system upgrade. The CONL station currently ranks among the ten most productive station within the ILRS network. The simultaneous improvement of data quality can be seen in Figure 2b. The post-fit NP rms for Lageos passes measured in both the 847 and 423.5 nm channels is plotted before and after the upgrade, showing that the NP precision has improved by about a factor of 2.

SLR development at TIGOSLR system componentsApart from the routine, continuous satellite tracking, engineers of the laser group at TIGO perform development projects for the improvement of the SLR system. One of these projects is the development of a rotating shutter system for noise reduction of the SPAD detectors. The upgrade to 100-Hz operation required a new shutter system capable of simultaneously controlling phase and rotation speed of two perforated disks (one to block the transmitting oscillator pulse train, the other to block the detectors from straylight), allowing the transmitted and received laser pulses to pass, respectively. The technical solution, developed at TIGO by visiting students from Germany and further improved by TIGO engineer Alejandro Fernandez, is based on Field-Programmable-Gate-Arrays realizing high-precision delay lines programmed by a real-time software using the RTAI Linux module.

Studies of SLR signalsIn the frame of the doctoral thesis of Cesar Guatiao, the polarization of SLR signals as reflected from retrorefelctor arrays is investigated. Changes in corner-cube orientation, such as rotation and beam angle variation, produce changes in the polarization state and diffraction pattern of the returned beam. In this work these effects are experimentally verified using the special setup of TIGO's reception optics (Fig. 4a), which allows for simultaneous detection of a received beam in two independent channels. A ground-based corner cube target is illuminated under a range of incident angles. The refected photons are separated by vertical and horizontal polarization using two crossed polarizers and simultaneously counted by SPAD detectors in each channel. As a result, the polarization ratio of the received infrared beam as a function of incident angle is obtained (see Fig. 4 b and c). The results of this study are expected to verify and possibly improve models of SLR signal distortion by retrorefelctor arrays.

LIDAR experimentsIn colaboration with the University of Concepción, first experiments for construction of an atmospheric LIDAR (Light Detection and Ranging) system have been performed at TIGO. A prototype system for detection of aerosols and cloud height was set up using spare components for SLR. Laser pulses of approx. 15 ns duration at 532 nm are emitted at 45° zenith angle and the back-scattered signal detected by a small (10 cm diameter) reception telescope and a photomultiplier tube (PMT) detector. The received signal allows to determine basic cloud properties and statistical analysis of cloud cover and height above the station. An upgrade with larger reception telescope and higher pulse energies is under way and expected to enable water vapor profile retrieval using the Raman-LIDAR technique.

Fig. 5) Pictures of the first LIDAR experiments at TIGO. Right: The green (532 nm) beam emitted by a JEDI pump laser hits a thin cloud. The simple prototype system is setup on a breadboard (upper left side). The back scattered signal of this event as detected by the PMT can be seen on the oscilloscope (left below). The cloud yields a clear signature in the detected curve, showing its height and thickness.

Fig. 1: New SLR system with passive mode locking oscillator and diede pumped Nd:YAG pump laser

Fig. 2a: Number of observed satellites before and after the system upgrade

Fig. 2b: Normal point precision before and after the system upgrade

Fig. 3) Experimental setup of a rotating shutter for 100 Hz pulsed lasers. The green laser beam is transmitted through a synchronized and phase-controlled disk with 5 holes rotating at ~20 rev/sec.

Fig. 4 a) left: Basic scetch of the reception path for silmultaneous detection in orthogonal polarization directions; b) center: normalized return rates in both polarizations vs incident angle; c) right: resulting variation of polarization ratio.

1) 2) 2) 2) 2) 2)B. Sierk , C. Guaitiao , A. Fernandez , M. Avendaño , Victor Mora ,Yazmina Olmos 1) Federal Agency for Cartography and Geodesy, Frankfurt/Main, Germany,

2) Universidad de Concepción, Concepción, Chile

SLR at TIGO

FGS 2008, Bad Kötzting