Robert O. Nellums

Utilization of flash ladar for cooperative and uncooperative rendezvous and capture

An ideal Rendezvous and Capture (R&C) sensor on a seeker Space Vehicle (SV) would provide accurate relative 6 degree of freedom data for the Guidance Navigation and Control System (GNCS) from far and near, operate autonomously, and provide multifunctional capability. Flash LADAR has the potential to fulfill these requirements. Sandia has developed Scannerless Range Imaging (SRI) LADAR sensors for a multitude of applications. One of the sensors, LDRI, flew onboard the STS97 mission to install the P6 truss and solar panels on the International Space Station. When compared to scanning LADAR, Scannerless LADAR is smaller, lighter, not mechanically complex, and has a much faster image acquisition time. Recently Sandia has demonstrated Flash Scannerless Range Imaging. Flash LADAR enables the capture of a full scene 3-D range image in one acquisition, thus, enabling freeze motion. The technology’s proven ability to accurately image an object as well as capture the image on the move has the potential to provide very accurate static and dynamic position data for the target vehicle relative to the seeker SV. Since no specific requirements are imposed on the target vehicle, the sensor will work equally well on cooperative and uncooperative target vehicles. This sensor technology can also provide docking feature inspection data and perform a detailed inspection of the target vehicle. This paper will describe the applicability of a Flash LADAR sensor for on-orbit cooperative and uncooperative rendezvous and capture.

A Miniature, High-Resolution Laser Radar Operating at Video Rates

We are developing a laser radar to meet the needs of NASA for a 5-lb, 150 in3 image sensor with a pixel range accuracy of 0.1-inch. NASA applications include structural dynamics measurements, navigation guidance in rendezvous and proximity operations, and space vehicle inspection. The sensor is based on the scannerless range imager architecture developed at Sandia. This architecture modulates laser floodlight illumination and a focal plane receiver to phase encode the laser time of flight (TOF) for each pixel. We believe this approach has significant advantages over architectures directly measuring TOF including high data rate, reduced detector bandwidth, and conventional focal plane array (FPA) detection. A limitation of the phase detection technique is its periodic nature, which provides relative range information over a finite ambiguity interval. To extend the operating interval while maintaining a given range resolution, a LADAR sensor using dual modulation frequencies has been developed. The modulation frequency values can be scaled to meet the resolution and range interval requirements of different applications. Results from the miniature NASA sensor illustrate the advantages of the dual-frequency operation and the ability to provide the range images of 640 by 480 pixels at 30 frames per second.

3D scannerless LADAR for Orbiter inspection

The Space Shuttle Program requires on-orbit inspection of the thermal protection system which covers the Orbiter spacecraft, including the critical leading-edge surfaces. A scannerless ladar system mounted on a 50-foot boom extension of the robotic arm provides this capability. This paper describes the sensor and ground processing system, which were developed by Sandia National Laboratories to meet the requirements of the Return to Flight mission in July of 2005. Mission operations for this sensor system are also reviewed.

Characterization of scannerless ladar

Scannerless laser radar (LADAR) is the next revolutionary step in laser radar technology. It has the potential to dramatically increase the image frame rate over raster-scanned systems while eliminating mechanical moving parts. The system presented here uses a negative lens to diverge the light from a pulsed laser to floodlight illuminate a target. Return light is collected by a commercial camera lens, an image intensifier tube applies a modulated gain, and a relay lens focuses the resulting image onto a commercial CCD camera. To produce range data, a minimum of three snapshots is required while modulating the gain of the image intensifier tubes microchannel plate (MCP) at a MHz rate. Since November 1997 the scannerless LADAR designed by Sandia National Laboratories has undergone extensive testing. It has been taken on numerous field tests and has imaged calibrated panels up to a distance of 1 km on an outdoor range. Images have been taken at ranges over a kilometer and can be taken at much longer ranges with modified range gate settings. Sample imagery and potential applications are presented here. The accuracy of range imagery produced by this scannerless LADAR has been evaluated and the range resolution was found to be approximately 15 cm. Its sensitivity was also quantified and found to be many factors better than raster- scanned direct detection LADAR systems. Additionally, the effect of the number of snapshots and the phase spacing between them on the quality of the range data has been evaluated. Overall, the impressive results produced by scannerless LADAR are ideal for autonomous munitions guidance and various other applications.

Ladar measurements of the International Space Station

The International Space Station (ISS) is an extremely large and flexible structure that requires validated structural models for control and operation. We have developed a 5-lb, 150 in3 laser radar to remotely measure vibration of the ISS structure and determine the structural mode frequencies and amplitudes. The Laser Dynamic Range Imager (LDRI) specifications include a 40-degree field of view, range resolution of 0.1 inches, images of 640 by 480 pixels, and a 7.5 Hz update rate. The sensor flew on the Space Shuttle in December of 2000 and provided range video of the newly installed P6 truss and solar array panels during thruster firing. Post flight analysis constructed motion time histories from selected structures. The measured vibration spectra captured the desired mode frequencies and amplitudes with a resolution of 0.02 to 0.1 inches. Additional measurements of curvature in the solar array panels demonstrated the potential for on-orbit characterization or inspection of structures.

Performance of a gated scannerless optical range imager against volume and bottom targets in a controlled underwater environment

An SRI breadboard designed specifically for use in the underwater environment was assembled, and a second series of underwater imaging experiments were performed in the swimmer delivery vehicle test tank at Coastal Systems Station in November-December 1999. The objective of these experiments was to collect information for a critical assessment of the effect of selected system parameters on image quality, to help quantify the effective underwater performance of the SRI under controlled test conditions, and to obtain data in support of an active modeling effort. The data from these experiments are currently being analyzed. A brief description of these experiments is presented along with a discussion of selected results.

A low-cost, high-resolution, video-rate imaging optical radar

Sandia National Laboratories has developed a unique type of portable low-cost range imaging optical radar (laser radar or LADAR). This innovative sensor is comprised of an active floodlight scene illuminator and an image intensified CCD camera receiver. It is a solid-state device (no moving parts) that offers significant size, performance, reliability, and simplicity advantages over other types of 3D imaging sensors. This unique flash LADAR is based on low- cost, commercially available hardware, and is well suited for many government and commercial uses. This paper presents an update of Sandias development of the Scannerless Range Imager technology and applications, and discusses the progress that has been made in evolving the sensor into a compact, low cost, high-resolution, video rate Laser Dynamic Range Imager.

Building accurate geometric models from abundant range imaging information

We define two simple metrics for accuracy of models built from range imaging information. We apply the metric to a model built from a recent range image taken at the laser radar Development and Evaluation Facility, Eglin AFB, using a scannerless range imager (SRI) from Sandia National Laboratories. We also present graphical displays of the residual information produced as a byproduct of this measurement, and discuss mechanisms that these data suggest for further improvement in the performance of this already impressive SRI.

The Potential for Imaging In-Situ Damage in Inflatable Space Structures

NASA is investigating the use of inflatable habitat structures for orbital transfer and planetary applications. Since space structures are vulnerable to damage from micrometeoroid and orbital debris, it is important to investigate means of detecting such damage. This study is an investigation into methods for performing non‐destructive evaluation (NDE) on inflatable habitat modules. Results of this work showed that various electromagnetic imaging modalities from microwaves to terahertz imaging have the greatest potential for a viable, portable, NDE tool which could possibly be deployed aboard an inflatable habitat module.

Image-processing method of modeling underwater imagery from gated and gain-modulated scannerless range imagers

An Underwater Scannerless Range Imager illuminates a wide field-of-view with a broadbeam laser pulse and captures the entire scene with an image intensified solid state camera. By imaging the received light onto a microchannel plate (MCP) receiver whose gain is modulated, and focusing the CCD camera on the phosphor screen that fluoresces upon excitation by the MCP output pulse, a sequence of images differing in the phase of the modulation waveform can be formed, and high precision target ranges can be inferred for each pixel of the viewed scene. When the medium between transmitter and target is obscured, as by turbid water, the return signal is temporally extended so that the inferred range picks up a bias owing to backscatter. The intimate relationship between the spatial and temporal behavior of the signals (near targets produce different temporal profiles than distant ones) adds complexity that cannot be handled by point spread functions, as is common for CW illumination and range-gated systems with constant gain. The method described here breaks the propagation problem into four channels depending on whether the light is scattered by the medium on the way to or from the target (or both, or neither), and calculates arrays to represent mean pathlengths and their variances. A fairly rigorous sensor model based on the various layers in a particular implementation (photocathode, MCP, phosphor, CCD array, A/D converter) and on receiver modulation transfer characteristics completes the prescription for generating realistic synthetic USRI images in moderate turbidity.