Aerospace Control Systems

Spacecraft GNC Systems Engineering


Capabilities in Spacecraft ACS/GNC Systems Engineering

ACS/GNC experience and capabilities include systems engineering related to GNC, precision attitude determination and sensor calibration, attitude control system design and analysis, operations planning and support, and proposal support. Contact ACS if these capabilities match your engineering needs.

ACS/GNC Systems Engineering

A systems engineering approach to attitude determination and control system (ADCS) design is essential to achieve performance objectives, minimize risk, and to meet program schedule. Systems engineering is essential because ADCS design is strongly coupled with most other spacecraft subsystems as well as mission requirements and constraints. For example, environmental factors such as thermal control affect attitude sensor performance, structural modes and propellant slosh can cause jitter and interfere with control system stability, and calibration maneuvers can be severely limited by pointing and mission constraints.

Systems engineering services include

  • Requirements analysis and definition
  • Pointing control and knowledge error budgeting
  • Trade studies, feasibility studies
  • Technology and risk assessment
  • Design Review and IV & V
  • ICD development and analysis
  • Concept of Operations (ConOps) development and evaluation
  • System architecture development and evaluation
  • GNC subsystem design and development

Precision Attitude Determination and Sensor Calibration

Aerospace Control Systems is a leader in research, development, and practical application of Kalman filtering for precision attitude determination and attitude sensor and gyro calibration. We also have an intimate understanding of principles of operation and performance characteristics of gyros, star trackers, sun sensors, horizon sensors, and magnetometers. This knowledge proved invaluable on several spacecraft projects where timing was an issue. In one case, ACS unraveled a time tag problem that resulted from a variable frame rate in a star tracker. In other cases ACS identified a time synchronization problem that threatened the missions because it required a hardware fix. ACS also recognizes the importance of environmental factors and mission constraints on attitude determination performance and on calibration accuracy.

ACS has significant experience with or knowledge of a variety of attitude hardware, including gyros, star trackers, sun sensors, and horizon sensors. Gyros include the Northrop-Grumman Space Inertial Reference Unit (SIRU, SSIRU),  Litton LN200S and LN200, Honeywell GG1320 and YG9666 Miniature Inertial Measurement Unit (MIMU), and the Honeywell DigFOG and HP-FOG. Star trackers that ACS has experience with include the Ball HAST, CT-602, and CT-631; Goodrich GR-1004, HD1003, and HD1005; the Sodern SED-16, and the Galileo Avionica Autonomous Star Tracker (ASTR).

The RADICAL Redundant IMU (RIMU) Attitude Determination and Calibration filter has been used to calibrate attitude sensors and gyros from telemetry, to trend calibration parameters, it has been used as a diagnostic tool and as a benchmark to evaluate the performance of other attitude determination systems.The RADICAL filter is designed for real-time operation, automated and semi-automated ground-based processing, and desktop design and analysis. It employs the most advanced algorithms available to fully calibrate IMUs with three or more sense axes, and it estimates three-axis relative alignments between two or more star trackers or other attitude sensors.

ACS can provide attitude determination filters that accept any of a variety of sensors such as star trackers, horizon sensors, sun sensors, magnetometers, GPS-based attitude, and any type of gyro. ACS can deliver enhanced attitude determination performance for star trackers that output star centroids instead of the usual quaternion measurement.

Attitude Control System Design and Analysis

ACS has participated in the design and analysis of pointing control systems for several Earth imaging and interplanetary spacecraft. Experience includes

  • Sensor and actuator sizing, selection, modeling, and simulation
  • Controller design, analysis, and simulation
  • Attitude determination and calibration system design, analysis, and simulation
  • Gyro-less control system design
  • Dynamics modeling and simulation
  • Pointing command and control algorithms for scanning, targeting, tracking, and maneuvering
  • Slew maneuver optimization
  • Attitude control design with Control Moment Gyros (CMG)
  • Pointing error, displacement, stability, and jitter analysis
  • Structural model reduction
  • Control-Structure Interaction (CSI) analysis
  • GPS-based precision orbit determination
  • Algorithm and Software development and implementation
  • Integration and Test
  • Polarity/Phasing Test and Verification

ACS developed software tools to support control-structure interaction (CSI) analysis and structural model order reduction. CSI analysis and design has been applied to several spacecraft with no evidence of modal instability or ringing in orbit, and was used to resove a CSI problem on the TIMED spacecraft. We can also design and analyze controllers for optical instruments. ACS has also developed pointing error algorithms and software to evaluate pointing error metrics (pointing accuracy, displacement, jitter, and stability).

ACS utilizes a variety of control system methodologies from classical, modern, and robust control theory, and from nonlinear and digital control theory, to design control systems. Matlab and several Matlab toolboxes are used extensively for analysis and simulation, along with specialized software written in Matlab and C.

Control system design experience includes a variety of reaction wheels, torque rods, and thrusters, in addition to the attitude sensors listed above.

Operations Planning and Support

ACS has demonstrated experience in

  • In-flight operation and contingency planning
  • In-flight performance assessment and diagnosis
  • Anomaly analysis and resolution
  • Ground-based telemetry processing for attitude determination, attitude sensor alignment, and gyro calibration

Proposal Support

ACS can also support proposal efforts, including working sample problems in accordance with requirements of the RFP, performing preliminary analysis to support a proposed concept, and task planning and costing. ACS has the technical writing skills needed to clearly and concisely convey your proposal.