LIST OF ABSTRACTS

Session 14/15: "Advanced Systems and Techniques: Transponders, Altimeters, and Time Transfer"

New achievements in the simulator of photon counting planetary altimeter

Josef Blazej, Ivan Prochazka
Czech Technical University in Prague
E-mail: blazej@fjfi.cvut.cz

Poster:
We are presenting new achievements in single photon counting altimeter simulator. The existing planetary altimeter simulator proposed for operational range 400 to 1400 km with one meter resolution has been extended. The new design is prepared for cooperating with map and surface relief system and results of the photon counting laser altimeter simulator are presented. The simulator is designed to be a theoretical and numerical complement for a Laser Altimeter Technology Demonstrator of the space borne laser altimeter for planetary studies built on our University.
Data describing surface height, slope, roughness, and albedo are extracted from map systems and together with calculated trajectory they are used for altimeter results simulation. The physical model of simulator is based on classical time correlated photon counting scheme. Photons from background light are received and detector registered event from its thermal noise. These presumptions are defining the static number of detected events. Due to photon counting concept the temporal distribution of events is crucial for the simulation. It can be derived from following rules: temporal position of signal from terrain is depended on range only, temporal resolution of signal is dependent on terrain slope and roughness, temporal distribution of both background and thermal noise is given by Poisson distribution, and all signals is affected by temporal resolution of detector and consequent timing electronics with Gauss distribution with presumed standard deviation.

Globally contiguous, high resolution topographic mapping of planets and moons via photon-counting

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

Planetary scientists have long expressed interest in obtaining globally contiguous, high resolution (few meter horizontal, decimeter vertical) 3D topographic maps of planets and moons. Unfortunately, achieving such a capability through a simple scaling of the laser power and/or telescope aperture from prior art NASA laser altimeters is not feasible. This is especially true of laser altimeters destined for orbit about distant planets or moons where instrument mass and prime power usage is severely constrained. Photon counting receivers permit each range measurement to be made with a single received photon, even in daylight, and the surface sampling rate of an orbiting altimeter can be increased by three to four orders of magnitude by emitting the available laser photons in a high frequency train of low energy pulses instead of the  low frequency train of high energy pulses typical of past spaceborne lidars. Targets of particular  interest to NASA are the Earth, Moon, Mars, the  Jovian moon Europa and the Saturnian moons, Titan and Enceladus.

The feasibility of the photon-counting  approach in the presence of a strong solar background was first successfully demonstrated from a high altitude aircraft under NASA's Instrument Incubator  Program in 2001. Sigma Space Corporation has subsequently developed a second generation of scanning 3D imaging and polarimetric lidars for use in small aircraft and Unmanned Aerial Vehicles (UAV's).

In late 2006,  Sigma completed a study for NASA's Jupiter Icy Moons Orbiter (JIMO) mission which concluded that the three primary Jovian moons (Ganymede, Callisto, and Europa) could be contiguously and globally mapped, at few meter horizontal resolutions,  by a photon-counting lidar in a matter of months from orbital altitudes of 100 km. Work is also underway to include a photon-counting lidar  on an Enhanced ICESat-II mission, which is presently scheduled for a 2013  launch into a 600 km orbit. The lidar would use a single low energy, high repetition rate laser (nominally 0.4 mJ @ 10 kHz = 4W at 532 nm) to measure surface topography over 16 uniformly spaced ground tracks spread over roughly 2.1 km.

Mars Laser Ranging: Science and Design

T. W. Murphy, J. Degnan, W. Farr, W. Folkner, A. Girerd, H. Hemmati, K. Nordtvedt, R. Reasenberg, S. G. Turyshev, J. G. Williams
University of California, San Diego, USA
E-mail : tmurphy@physics.ucsd.edu

A mission-concept study, primarily at the Jet Propulsion Laboratory, is investigating the science deliverables and technical practicalities of placing a laser transponder on the surface of Mars to carry out dedicated interplanetary laser ranging. A preliminary assessment of the attainable science will be presented, together with a discussion of the instrument design and link budget. The challenges of the Mars environment will also be addressed.

Analysis and prediction of altimetric sea level variations during ENSO, and normal conditions

Tomasz Niedzielski, Wieslaw Kosek
Space Research Centre, Polish Academy of Sciences, Poland
E-mail: niedzielski@cbk.waw.pl

Poster:
Recent developments in satellite altimetry are due to precise tracking and determining the satellite orbits. This is attained by such techniques as: Satellite Laser Ranging (SLR), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and Global Positioning System (GPS). These techniques were adopted by recent American-French altimetric missions TOPEX/Poseidon (T/P), and Jason-1 (J-1). Satellite altimetry provides the accurate sea surface height (SSH) measurements. The SSH estimates and the long-term mean sea level are used to compute sea level anomaly (SLA) data. The SLA time series describe the dynamic sea level change. The SLA data provide the knowledge about the large-scale ocean circulation, including El Nino/Southern Oscillation (ENSO). The altimetric data obtained from T/P and J-1 measurements are used to compute the spectra of the SLA data as well as amplitudes of the most energetic oscillations in sea level variation as a function of latitude and longitude. These spectra and amplitudes were computed by the Fourier Transform Band Pass Filter (FTBPF). The SLA time series is often used to determine the trend of sea level change. These estimates may be derived both globally and regionally (for the dissimilar ocean areas). It is also possible to predict SLA data. The SLA predictions are computed using the combination of polynomial-harmonic least-squares and autoregressive models. The SLA forecasts can be used to predict ENSO events, because El Nino and La Nina signals are very well visible in SLA time series.

Laser Altimetry and Transponder Ground Based Simulation Experiment

U. Schreiber, M. Hiener, B. Holzapfel, A. Neidhardt, P. Lauber, K.H. Haufe, N. Brandl, J. Oberst, H. Michaelis
Technische Universitaet Muenchen, Germany
E-mail: schreiber@fs.wettzell.de

We have designed and built a compact demonstrator unit for the investigation of altimetry and transponder applications. A small light-weight breadboard carries a compact frequency doubled Nd:YAG laser, afocal beam expansion optics, a small receiver telescope with spectral and spatial filter arrangement and a photosensitive detection device (SPCM, PMT or SPAD). The output laser energy can be as high as 45 mJ with a pulse-width of 5 ns and the receive telescope aperture is 12 cm. Simulations suggest that the link margin for LEO satellites is comfortable and that it may be possible to obtain echoes from a dual-station experiment in several different configurations. This paper outlines details of the experimental setup and presents the obtained results from a number of targets including LAGEOS.

BepiColombo Laser Altimeter Simulator

M. Hiener, U. Schreiber, U. Hugentobler
Technische Universitaet Muenchen, Germany
E-mail: schreiber@fs.wettzell.de

The BepiColombo Laser Altimeter (BELA) is currently under development as part of ESA's space exploration program. Apart from the hardware design, also a number of control functions have to be investigated and implemented. Our BELA simulator allows the development and testing of suitable software modules for echo identification and range-gate setting. Parameters like orbit height, orbit uncertainty, variable unknown terrain profiles and the altimeter hardware specifications can be investigated in independent simulation runs. A shot by shot error vector visualizes the effectiveness of the software strategy under test. This talk gives an introduction to the simulator concept and shows some results.

One Way System Calibration Techniques

Toby Shoobridge, David Benham
NERC Space Geodesy Facility, UK
E-mail: tooo@nerc.ac.uk

It has always been necessary to make a two-way calibration to account for the delays in an SLR system. This is done by comparing the ranged measurement to a ground target of a known distance. With the current work in T2L2 and LRO it is now in addition necessary to provide as accurate an epoch as possible for the pulse as it passes the System Reference Point. Determining a one-way calibration for the transmit side delay proves difficult as the current two-way method cannot simply be divided so a new method is required.
Our ongoing work to determine the transmit-side delay is presented. This includes measurements of the laser path from the start diode to the system reference point and the electronic delays in cables and equipment from the start diode to the epoch timer. The distance taken by the laser pulse path through the coudé path was measured using three techniques. The first used a laser distance measurer, more commonly used in building surveying. The second method was to make time-of-flight measurements of timed Hz laser pulses passing through the coudé and the third method used a combination of the telescope drawings and physical measurement. The laser distance measurer initially seems to have given the most repeatable and accurate result given that the manufacturers specify an accuracy of at worst 1.5mm over a 30m distance. The other independent methods will provide a good comparative check on this. The largest error contribution to the final result comes from the physical measurements made in the laser bed.

Preliminary Results of the Laser Time Transfer (LTT) Experiment

Yang Fumin¹, Huang Peicheng¹, Zhang Zhongping¹, Chen Wanzhen¹, Zhang Haifeng¹, Wang Yuanming¹, Meng Wendong¹,
Wang Jie², Zou Guangnan², Liao Ying², Wang Luyuan², Ivan Prochazka³, Zhao You⁴, Fan Cunbo⁴, Han Xingwei⁴
1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, China
2 China Academy of Space Technology, Beijing, China
3 Czech Technical University in Prague, Czech Republic
4 National Astronomical Observatories / Changchun Observatory, CAS, China
E-mail: yangfm@shao.ac.cn

The LTT payload onboard the Chinese experimental navigation satellite COMPASS-M1 with an orbital altitude of 21500 km launched on April 13, 2007 included dual-SPAD-detector, dual-timer based on TDC device , DSP, power supply and a LRA with the total mass of 7.05 kg (including LRA's 2.45 kg) and the power consumption of 18 W. The time transfer experiment at Changchun SLR station has started since August 2007. The experiment has shown that the time and relative frequency differences between the ground hydrogen maser and the China-made space rubidium clocks have been obtained with the time precision of about 300ps for single measurement and the uncertainty of frequency difference of about 3×10^-14 in 2000 seconds. After 17 months orbital flight, the LTT payload has kept its good performance.

Preliminary Results of Laser Ranging to Un-cooperative Targets at Shanghai SLR

Yang Fumin¹, Zhang Zhongping¹, Chen Juping¹, Chen Wanzhen¹, Wu Zhibo¹, Zhang Haifeng¹, Ivan Prochazka²
1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, China
2 Czech Technical University in Prague
Email: yangfm@shao.ac.cn

The laser returns from the un-cooperative targets have been obtained at the Shanghai SLR station in July 2008. These targets are the discard Soviet and US rockets with the ID 1987-38B and 2007-006G respectively. The return signals from the targets with the range of 900 km were quite strong. The performance of the laser used for the experiment was introduced on the Canberra Workshop in 2006.

One-way Ranging to the Planets

Maria Zuber (MIT), Dave Smith (GSFC)
E-mail: zuber@mit.edu

Often the increase in mission complexity of flying an active laser system to the planets limits the opportunities for attempting laser tracking of planetary spacecraft. The best and most accurate method is generally considered to be the transponder approach that involves active laser systems at both ends of the link.  But because of the increased complexity, risk and cost of a two-way system we have been forced to consider the value of a one-way measurement in which most of the complexity and costs are at the Earth terminal, and therefore more palatable and 'fixable' should issues arise. This was the choice for LRO and hence the development of the LR system which was minimal in cost and required almost no additional spacecraft resources. The advantage of 'one-way' is clear for distances of several AU if the issues of precision versus accuracy can be resolved and the opportunities for flight are greater.