source: EDN article
Steve Taranovich -October 20, 2015
I posed some questions about Orion’s sensors to George Schamel, Lockheed Martin Orion avionics, power & wiring system certified principal engineer and he very graciously explained them.
Absolute & Relative Navigation Sensors1
Orion’s Command Module contains a block entitled “Absolute & Relative Navigation Sensors”. This type of sensor tracks the vehicle location in a selected frame of reference by measuring and integrating vehicle linear acceleration and rotation. Relative navigation sensors track the vehicle location in relation to another object in space measuring relative distance, orientation and closing velocity.
One version of this type of sensor is a sun sensor or a star tracker. These systems successful employ celestial navigation for spacecraft and marine vehicles and are also of great interest to planetary robotists as well.
Magnetic heading detection devices are not viable in deep space because most of the planets in our solar system have negligible magnetic fields. Absolute heading detection for planetary rovers remains the most significant of the navigation parameters in terms of its influence on accumulated dead-reckoning errors. Sensors that can provide a measure of absolute heading are of extreme importance in developing long-range navigation algorithms.
Of all the celestial bodies, the sun is the most attractive for navigation. In the past twenty to thirty years, a sun sensor has been used on every satellite launched for both attitude determination and attitude control. Two key factors make the sun the most attractive celestial body for navigation. First, the sun is sufficiently bright; so it is really easy to detect without the need to discriminate among other celestial sources. Second, the sun’s angular radius is nearly orbit-independent and sufficiently small (0.267 degrees at 1 AU) that it suffices to model it as a point source1.
Another type of navigation sensor that is possible for very deep space travel is an X-ray pulsar source-based navigation and timing (XNAV)2. This approach uses observations of the X-ray emissions of highly stable, rotation powered, millisecond pulsars. These pulsars serve as a kind of natural celestial beacon. The accurate pulse time-of-arrival estimates from multiple non-coplanar sources allow simultaneous determination of both position and velocity autonomously anywhere in the solar system.
Highly stable pulsars can be viewed as nature’s celestial lighthouses2. The pulsars provide an oscillating signal that has long-term stability comparable to current state of the art atomic clocks. These signals can help resolve a spacecraft’s time and position similar to that of a GPS. Figure 1 shows a pulsar with emission jets that radiate in the high energy spectrum. These sources do have challenges however, like the fact that the signals are faint, noisy, and they are not tagged with time or Earth-centered Inertial (ECI) position of origin. These factors can be overcome by using source timing models, adequate collection time, adequate collection area, and Kalman-Filter-based signal processing. See Figure 1.
Figure 1: Pulsar Physics Yielding Regular Signals Detectable to High Precision in the X-ray Band. (Image courtesy of Reference 2)
A conceptual illustration of the XNAV measurement and solution process is provided in Figure 2.
Figure 2: The Processing Algorithms for XNAV (Image courtesy of Reference 2)
Heat shield sensors
Figure 3: Orion Heatshield instrumentation (Image courtesy of NASA)
These sensors are composed of thermocouple plugs (Aerothermal), heat shield pressure ports (Aerodynamics)/transducers, heat shield Thermal Plugs (TPS) and Radiometers.
Thermocouple plugs are placed in the heat shield to measure the aerothermal heating of the heat shield surface during reentry. See Figure 5.
Heat shield pressure ports measure the pressure distribution around the heat shield to gain an improved understanding of the aerodynamics of the vehicle during reentry. See Figure 6.
Thermal plugs (TPS = Thermal Protection System): the thermal plugs contain multiple thermocouple sensors stacked through the thickness of the heat shield to measure the heat transmission to the interior surface and then to the vehicle itself during reentry and post-landing. See Figure 4.
Figure 4: Heat shield Developmental Flight Instrumentation (DFI) sensors (Image courtesy of NASA)
Figure 5: The Thermal Plug Concept (Image courtesy of NASA)
Figure 6: Pressure Port and Transducer Concept (Image courtesy of NASA)
Radiometers measure a different type of heating phenomena that occurs due to the heat energy radiation from the aerodynamic shock wave that forms during reentry. See Figure 7.
Figure 7: Radiometer design. (Image courtesy of NASA)
Backshell DFI sensors
DFI = Development Flight Instrumentation are various types of sensors located around the vehicle to measure the vehicle environments during the mission. Data Acquisition Units (DAUs) sample and digitize the data sending it to a central data storage for post-flight analysis. See Figure 8.
Figure 8: Backshell DFI sensor locations. (Image courtesy of NASA)
Figure 9: Backshell pressure sensor locations. (Image courtesy of NASA)
Environmental control and life support system (ECLS)3
Human life in the Orion spacecraft is sustained by the Environmental Control and Life Support (ECLS) subsystem. The ECLS design team is Lockheed Martin, Hamilton Sundstrand and Paragon Space Development Corporation. See Figure 10.
Figure 10: Orion vehicle breakdown by module. (Image courtesy of NASA)
The categories of air, water, food, and thermal control are the essentials to support human life. Waste management and fire safety are needed to maintain a safe environment. Technologies also rely on vehicle and environmental interfaces to meet requirements or to provide a lower mass solution for those environments, such as the vacuum resource on International Space Station (ISS).
Table 1: Orion Technologies by ECLS Category (Image courtesy of NASA)
C3I – Standard Communications, Communication Network Router Card, Digital Video Recorder.
Communications to and from the vehicle include both deep space and ground based systems. An S-Band system is the main communications radio on the Orion on the command module through the command module/service module interface adapter. This system typically has a Phase Modulation function that communicates between the Command module and ground control. This system also has the capability to communicate between other spacecraft. See Figure 11.
The sun sensors are simple devices that provide a measure of the vehicle orientation relative to the sun so that the solar arrays and the vehicle may be oriented for both power generation and thermal management. See Figure 11 Service Module section.
Solar Array Drive Electronics (SADE)
The solar array drive electronics on the service module accept commands from the vehicle computer to re-orient the solar arrays for power generation and configuration for vehicle maneuvering. See Figure 11 on the Service Module.
Figure 11: Orion system architecture (Image courtesy of NASA)
1 Design and Analysis of a Sun Sensor for Planetary Rover Absolute Heading Detection, Ashitey Trebi-Ollennu, Terry Huntsberger, Yang Cheng, E. T. Baumgartner, Brett Kennedy, and Paul Schenker, IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 17, NO. 6, DECEMBER 2001
2 XNAV for Deep Space Navigation, P. Graven, J. Collins, S. Sheikh, J. Hanson, P. Ray, K. Wood, 31st ANNUAL AAS GUIDANCE AND CONTROL CONFERENCE, 2008
3 Project Orion, Environmental Control and Life Support System Integrated Studies James F. Russell Lockheed Martin John F. Lewis NASA, Johnson Space Center, 2008