Richard T. Howard

Orbital Express Advanced Video Guidance Sensor

In May 2007 the first US-sponsored fully autonomous rendezvous and capture was successfully performed by DARPAs Orbital Express (OE) mission. For the following three months, the Boeing ASTRO spacecraft and the Ball Aerospace NEXTSat performed multiple rendezvous and docking maneuvers to demonstrate some of the technologies needed for satellite servicing. MSFCs advanced video guidance sensor (AVGS) was a near-field proximity operations sensor integrated into ASTROs Autonomous Rendezvous and Capture Sensor System (ARCSS), which provided relative state knowledge to the ASTRO GN&C system. AVGS was one of the primary docking sensors included in ARCSS. This paper provides an overview of the AVGS sensor that flew on orbital express, a summary of the AVGS ground testing, and a discussion of AVGS performance on-orbit for OE. The AVGS is a laser-based system that is capable of providing bearing at midrange distances and full six degree- of-freedom (6-DOF) knowledge at near ranges. The sensor fires lasers of two different wavelengths to illuminate retro- reflectors on the long range target (LRT) and the Short Range Target (SRT) mounted on NEXTSat. The retro- reflector filters allow one laser wavelength to pass through and be reflected, while blocking the other wavelength. Subtraction of one return image from the other image removes extraneous light sources and reflections from anything other than the corner cubes on the LRT and SRT. The very bright spots that remain in the subtracted image are processed to provide bearing or 6-DOF relative state information. AVGS was operational during the Orbital Express unmated scenarios and the sensor checkout operations. The OE unmated scenarios ranged from 10 meters to 7 kilometers ending in either a docking or a free-flyer capture. When the target was pointed toward the AVGS and in the AVGS operating range and field-of-view (i.e. along the approach corridor of the NEXTSat), the AVGS provided full 6-DOF measurements. The AVGS performed very well during the sensor check-out operations, effectively tracking beyond its 10-degree Pitch and Yaw limit-specifications. AVGS also provided excellent performance during the unmated operations, effectively tracking its targets, and showing good agreement between the SRT and LRT data. The AVGS consistently exceeded the tracking range expectations for both the SRT and LRT. During the approach to re-mate in scenario 3-1 recovery the AVGS began tracking the LRT at 150 m, well beyond the OE specified operational range of 120 meters, and functioned as the primary sensor for the autonomous rendezvous and docking. For all scenarios, the AVGS was used while ASTRO was in the approach corridor to NEXTSat, and during close proximity operations and docking.

Automated Rendezvous and Capture System Development and Simulation for NASA

The United States does not have an Automated Rendezvous and Capture/Docking (AR&C) capability and is reliant on manned control for rendezvous and docking of orbiting spacecraft. This reliance on the labor intensive manned interface for control of rendezvous and docking vehicles has a significant impact on the cost of the operation of the International Space Station (ISS) and precludes the use of any U.S. expendable launch capabilities for Space Station resupply. The Marshall Space Flight Center (MSFC) has conducted pioneering research in the development of an automated rendezvous and capture (or docking) (AR&C) system for U.S. space vehicles. This AR&C system was tested extensively using hardware-in-the-loop simulations in the Flight Robotics Laboratory, and a rendezvous sensor, the Video Guidance Sensor was developed and successfully flown on the Space Shuttle on flights STS-87 and STS-95, proving the concept of a video- based sensor. Further developments in sensor technology and vehicle and target configuration have lead to continued improvements and changes in AR&C system development and simulation. A new Advanced Video Guidance Sensor (AVGS) with target will be utilized as the primary navigation sensor on the Demonstration of Autonomous Rendezvous Technologies (DART) flight experiment in 2004. Realtime closed-loop simulations will be performed to validate the improved AR&C systems prior to flight.

Advanced Video Guidance Sensor (AVGS) Development Testing

NASAs Marshall Space Flight Center was the driving force behind the development of the Advanced Video Guidance Sensor, an active sensor system that provides near-range sensor data as part of an automatic rendezvous and docking system. The sensor determines the relative positions and attitudes between the active sensor and the passive target at ranges up to 300 meters. The AVGS uses laser diodes to illuminate retro-reflectors in the target, a solid-state camera to detect the return from the target, and image capture electronics and a digital signal processor to convert the video information into the relative positions and attitudes. The AVGS will fly as part of the Demonstration of Autonomous Rendezvous Technologies (DART) in October, 2004. This development effort has required a great deal of testing of various sorts at every phase of development. Some of the test efforts included optical characterization of performance with the intended target, thermal vacuum testing, performance tests in long range vacuum facilities, EMI/EMC tests, and performance testing in dynamic situations. The sensor has been shown to track a target at ranges of up to 300 meters, both in vacuum and ambient conditions, to survive and operate during the thermal vacuum cycling specific to the DART mission, to handle EMI well, and to perform well in dynamic situations.

Orbital Express Advanced Video Guidance Sensor: Ground Testing, Flight Results and Comparisons

Orbital Express (OE) was a successful mission demonstrating automated rendezvous and docking. The 2007 mission consisted of two spacecraft, the Autonomous Space Transport Robotic Operations (ASTRO) and the Next Generation Serviceable Satellite (NEXTSat) that were designed to work together and test a variety of service operations in orbit. The Advanced Video Guidance Sensor, AVGS, was included as one of the primary proximity navigation sensors on board the ASTRO. The AVGS was one of four sensors that provided relative position and attitude between the two vehicles. Marshall Space Flight Center was responsible for the AVGS software and testing (especially the extensive ground testing), flight operations support, and analyzing the flight data. This paper briefly describes the historical mission, the data taken on-orbit, the ground testing that occurred, and finally comparisons between flight data and ground test data for two different flight regimes.

Active sensor system for automatic rendezvous and docking

NASAs Marshall Space Flight Center has developed an active sensor system, the ideo guidance sensor (VGS), to provide near-range relative position and attitude data. The VGS will be part of an automatic rendezvous and docking system. The VGS determines the relative positions and attitudes between the active sensor and the passive target. It works by using laser diodes to illuminate the retro-reflectors in the target, a solid-state camera to detect the return from the target retro-reflectors, and a frame grabber and digital signal processor to convert the video information into relative positions and attitudes. The current sensor design is the result of several years of development and testing, and it is being built to fly as an experiment payload on the space shuttle. The VGS system is designed to operate with the target completely visible within a relative azimuth of +/- 10.5 degrees and a relative elevation of +/- 8 degrees. The system will acquire and track and target within that field-of-view anywhere from 1.0 meters to 110 meters range at any relative roll angle and relative pitch and yaw attitudes of up to +/- 10 degrees. The data is output from the sensor at 5 Hz, and the target and sensor software have been designed to permit two independent sensors to operate simultaneously for redundancy.

DART AVGS flight results

The Advanced Video Guidance Sensor (AVGS) was designed to be the proximity operations sensor for the Demonstration of Autonomous Rendezvous Technologies (DART). The DART mission flew in April of 2005 and was a partial success. The AVGS did not get the opportunity to operate in every mode in orbit, but those modes in which it did operate were completely successful. This paper will detail the development, testing, and on-orbit performance of the AVGS.

Automated Rendezvous and Docking Sensor Testing at the Flight Robotics Laboratory

The Exploration Systems Architecture defines missions that require rendezvous, proximity operations, and docking (RPOD) of two spacecraft both in Low Earth Orbit (LEO) and in Low Lunar Orbit (LLO). Uncrewed spacecraft must perform automated and/or autonomous rendezvous, proximity operations and docking operations (commonly known as Automated Rendezvous and Docking, AR&D). The crewed versions may also perform AR&D, possibly with a different level of automation and/or autonomy, and must also provide the crew with relative navigation information for manual piloting. The capabilities of the RPOD sensors are critical to the success of the Exploration Program. NASA has the responsibility to determine whether the Crew Exploration Vehicle (CEV) contractor-proposed relative navigation sensor suite will meet the CEV requirements. The relatively low technology readiness of relative navigation sensors for AR&D has been carried as one of the CEV Projects top risks. The AR&D Sensor Technology Project seeks to reduce this risk by increasing technology maturation of selected relative navigation sensor technologies through testing and simulation, and to allow the CEV Project to assess the relative navigation sensors.

On-orbit testing of the video guidance sensor

The Video Guidance Sensor (VGS), part of NASAs Automated Rendezvous and Capture program, was flown on Shuttle mission STS-95 in October of 1998 to test on-orbit the functional characteristics of the VGS. This was the second flight of the VGS (the first flight was in 1997 on STS-87), and this time long-range tracking data was gathered during the experiment. The flight experiment sensor was designed to operate from 1.5 meter range out to 110 meter range, with a field-of-view of 16 by 21 degrees. The VGS tracked its target at a 5 Hz rate and returned 6-degree-of-freedom information on the targets position and attitude relative to the sensor. The VGS was mounted in the Shuttle cargo bay, and its target was mounted on the Spartan spacecraft being carried on this mission. The orbital testing of the VGS included operations with the target on the Shuttles Remote Manipulator System (RMS) at the start of the 10-day mission, long-range data during the Shuttle rendezvous with the Spartan two days later, and some more RMS operations later in the mission. The data returned from the orbital testing included VGS diagnostics, acquisition, and tracking data, RMS positions, hand-held laser range data, tapes of the data from the VGS video camera, and orbital positioning data from the Spartan and the Shuttle to allow correlation of the VGS data with orbital best- estimate-of-truth data. The Video Guidance Sensor performed well in all phases of the testing, and the VGS is being incorporated into the ground testing of a complete automated rendezvous and docking system. Work on the development of the next generation VGS is continuing.

Orbital Express AVGS Validation and Calibration for Automated Rendezvous

From March to July of 2007, the DARPA Orbital Express mission achieved a number of firsts in autonomous spacecraft operations. The NASA Advanced Video Guidance Sensor (AVGS) was the primary docking sensor during the first two dockings and was used in a blended mode three other automated captures. The AVGS performance exceeded its specification by approximately an order of magnitude. One reason that the AVGS functioned so well during the mission was that the validation and calibration of the sensor prior to the mission advanced the state-of-the-art for proximity sensors. Some factors in this success were improvements in ground test equipment and truth data, the capability for ILOAD corrections for optical and other effects, and the development of a bias correction procedure. Several valuable lessons learned have applications to future proximity sensors.

Video guidance sensor flight experiment results

An active video sensor system for determining target range and attitude was flown on STS-87. The Video Guidance Sensor (VGS), developed at NASAs Marshall Space Flight Center, demonstrated its capability in space and collected performance data. The VGS was designed to provide near-range sensor data as part of an automatic rendezvous and docking system. The sensor determines the relative positions and attitudes between the active sensor and the passive target. The VGS uses laser diodes to illuminate retro-reflectors in the target, a solid-state camera to detect the return from the target, and a frame grabber and digital signal processor to convert the video information into the relative positions and attitudes. The system is designed to operate with the target within a relative azimuth of +/- 9.5 degrees and a relative elevation of +/- 7.5 degrees. The system will acquire and track the target within the defined field-of- view between 1.5 meters and 110 meters range, and the VGS is designed to acquire at relative attitudes of +/- 10 degrees in pitch and yaw and at any roll angle. The sensor outputs the data at 5 Hz, and the target and sensor software and hardware have been designed to permit two independent sensors to operate simultaneously. This allows for redundant sensors. The data from the flight experiment includes raw video data from the VGS camera, relative position and attitude measurements from the VGS, solar angle data, and Remote Manipulator System position data to correlate with the VGS data. The experiment was quite successful and returned significant verification of the sensors capabilities. The experience gained from the design and flight of this experiment will lead to improved video sensors in the future.