Attitude Maneuvers with the CubeSat Control System

The CubeSat control system is designed to work with either thrusters or reaction wheels. It has a number of handy built in maneuver modes such as pointing at the sun, nadir pointing or pointing at a specific latitude and longitude on the ground. Here is the spacecraft shown in the VisualCommander interface.

CSCS

The movie in the link below shows attitude maneuvers in the VisualCommander interface. The interface has pages for the various subsystems and attitude control system functions. We start by seeing the spacecraft in a polar orbit on the Summary page. The solar arrays are reorienting themselves so that their cell faces are pointed at the sun. We switch the 3D display to look along the boresight of the telescope. We then go to the ACS page and select a sun pointing maneuver. We go back to the Summary page and see that the sun appears in the display. We then return to the ACS page and command nadir pointing. The remainder of the movie shows the reorientation maneuver to nadir pointing.

CSCS Reorientation Movie

For more information on our simulation frameworks including our real-time control system framework, ControlDeck, go to Simulation Framework page.

For more information on VisualCommander go to VisualCommander page.

You can also send us an email to find out more about our CubeSatControl System. All of these products are available now.

Spacecraft CAD Design in the Spacecraft Control Toolbox

AutoDesk Inventor and SolidWorks are powerful software packages for the computer-aided design of spacecraft. Ultimately you need to use one of those packages for the mechanical design of your satellite, but what about the preliminary design phase when you are still determining what components you even need? The CAD software in the Spacecraft Control Toolbox can provide you with a valuable tool to do your conceptual layouts and early trade studies, and the same model can be used as the basis for disturbance analysis in later design phases.

A CAD model in SCT is built in a script which allows you to build your models algorithmically. You can call design functions, use for loops and revision-control your source code. For example, within the script you can do an eclipse analysis and compute the battery capacity. This number can generate the volume of your batteries which you can then use to size your spacecraft.

The function BuildCADModel provides the model-building interface. The CreateComponent function is used to generate the individual components using parameter pairs as arguments. Components are grouped into bodies to allow for rotation and articulation. A GUI displays your finished model and allows you to visualize it in 3D. You then store your finished models as mat-files. Our disturbance model uses every triangle in your model for disturbance analysis.

The example figure shows a solar sail design, with the spacecraft bus in the middle. BuildCADModel allows you to group components into subsystems as on the left-hand side, which can then be highlighted using transparency.

Subsystem

The figure below shows the BuildCADModel GUI which allows you to verify the body and component properties.BuildCADModel-Vehicle

There are many examples of spacecraft models in the SCT to help you get started, and a lengthy chapter in the User’s Guide discussing the finer points of component location, orientation, and physical properties such as drag and optical coefficients. Your CAD model essentially functions as a database for your entire spacecraft model!

Why Use Princeton Satellite Systems’ MATLAB Toolboxes?

Almost all aerospace organizations have extensive libraries of software for simulation, design and analysis. Why then should they use our MATLAB toolboxes?

I’ve been working in the aerospace business since 1979. My experience includes:

  1. The Space Shuttle Orbiter Dynamics Analysis
  2. The GPS IIR control system design
  3. The Inmarsat 3 control system design
  4. The GGS Polar Platform control system design
  5. The Mars Observer delta-V control system
  6. The Indostar-1 control system
  7. The ATDRS momentum management system
  8. The PRISMA formation flying safe mode guidance

Continue reading

Optical Navigation for Geosynchronous Transfer Orbits

Optical navigation, using the Earth’s chord width and angles between nadir and stars, is an alternative to GPS based navigation for autonomous spacecraft. The Optical Navigation System, developed under a NASA contract, is well suited for this application.

In the Spacecraft Control Toolbox, we provide an easy-to-use demo script in the 2014.1 release that shows you how to implement optical navigation. The system uses Unscented Kalman Filters (also known as sigma point filters) with non-linear dynamics and measurement models.

This demo uses our new UKF functions shown below:

ukf.t = t;
ukf = UKFPredict( ukf );
ukf = UKFUpdate( ukf );

ukf is a data structure that includes all filter information. Measurements are passed as data structures to UKFUpdate which have pointers to the measurement functions. In this way, any type of measurement can be used in the filter and introduced at any time.

The following plots show some results from the script. The first shows the orbit and the estimated orbit which are essentially the same.

OpNavSim

The second shows the position errors. Of course, actual errors would depend on the accuracy of the sensors, particularly the Earth sensor. Great care needs to be taken when setting up the UKF parameters. As you can see, the largest errors are at perigee and if the UKF parameters are not set properly, the filter might think a hyperbolic orbit was a valid solution!

OpNavErrors

Check out our Spacecraft Control Toolbox page for more information on the 2014.1 release! More information about optical navigation can be found on our Deep Space Navigation page.

SunStation in Operation!

SunStation is in operation! The system produces a peak of 7.8 kW solar power. It has 14 kWh of lithium batteries. The SunStation electronics are shown below. The inverter is on the left. The batteries are in the cabinet on the right.

SunStation

The SunStation has a web interface. You can see that when this screen shot was made the SunStation was selling 5.1 kW back to the power company! The batteries are fully charged. Usage was very small. The house has a whole house ventilator that is drawing most of the power. The homeowners also own a Nissan Leaf and a Toyota Prius Plugin. The solar array is enough to fully recharge those cars and run the house electrical devices when the air conditioning is not on.

SunStationWebInterface

Unlike gasoline, diesel or natural gas systems SunStation provides power year round! There is no noise and no toxic emissions. SunStation has no moving parts and is zero maintenance. Solar power systems are eligible for Solar Renewable Energy Credits which are cash payments for having an operating solar power system. It is estimated that this system will bring in $3700/year revenue between selling power and SRECs.

SunStation Installed!

The first SunStation installation is done! This system includes 14 kWh of Valence lithium batteries and an Outback inverter. Unlike most other solar power systems, the solar panels will deliver power to the house with or without the grid active.

SunStationInstalled

An existing 3.8 kW array was augmented with the new Panasonic solar panels on the right. They are about the same size as the older Sharp panels but much more efficient. In the bottom picture you can see the Outback inverter. The cabinet on the lower right houses the Valence batteries. The system will power the entire house in an outage with the exception of the central air conditioning system.

Unlike gasoline, diesel or natural gas systems this system provides power year round! There is no noise and no toxic emissions. SunStation has no moving parts and is zero maintenance. Solar power systems are eligible for Solar Renewable Energy Credits too which are cash payments for having an operating solar power system so you save money two ways.

Check out our SunStation page for more information!

DFD for Europa Exploration

Europa-luna

The Galileo moons of the Jovian system are of great interest for future space exploration due to the belief that three of the four of the largest moons (Europa, Ganymede, and Callisto) contain water (in liquid and/or ice form). So far the eight spacecraft that have visited the vicinity of Jupiter are Pioneer 10 and 11, Voyager 1 and 2, Ulysses, Galileo, Cassini, and most recently New Horizons. NASA has ambitions to send another probe to further study Europa.

At Princeton Satellite Systems, in collaboration with Dr. Samuel Cohen at the Princeton Plasma Physics Laboratory, we’ve been working on the Direct Fusion Drive (DFD) engine, an advanced technology for space propulsion and power generation. Using the DFD, we have simulated two potential missions to Europa, an orbiter mission and a lander mission. The simulations were completed in MATLAB using functions contained within our Spacecraft Control Toolbox.

Continue reading

Adaptive Cruise Control

The automotive industry continues to incorporate advanced technology and control systems design into new vehicles. Features such as adaptive cruise control, lane keep assist, autonomous park assist, and adaptive lights are becoming more common in the automotive market. These exciting technologies greatly increase vehicle safety!

Adaptive cruise controls measure the distance and speed of nearby vehicles and adjust the speed of the vehicle with the cruise control to maintain safe following distances. Typically a system will use a radar that measures range, range rate and azimuth to vehicles in its field of view.

A typical situation is shown below. The car with adaptive cruise control is traveling near three additional vehicles. Two cars have been tracked for awhile but a third is passing and plans to insert itself into the space between the tracking car and one of the tracked cars. How does the cruise control keep the three cars straight?

MHTAuto

Every measurement has uncertainty. The following drawing shows the uncertainty ellipsoids for the three vehicles. As you can see they overlap so a measurement could be associated with more than one car.

UncertaintyEllipsoids

The Princeton Satellite Systems Target Tracking Module for MATLAB implements track oriented Multiple Hypothesis Testing (MHT). MHT is a Bayesian method for reliably associating measurements with tracks. The system is shown below:

MHTSystemSimplified

The system includes a powerful track pruning algorithm that eliminates the need for ad-hoc track pruning. Without track pruning the number of tracks maintained would grow exponentially. The system generates hypotheses that are collections of tracks that are consistent, that is the tracks do not share any measurements. Measurements are incorporated into tracks and tracks are propagated using Kalman Filters. The MHT system also can handle multiple sensors for automobiles with cameras and radar.

Check out what all our MATLAB toolboxes have to offer!
Core Control Toolbox
Aircraft Control Toolbox
CubeSat Toolbox
Spacecraft Control Toolbox

6U CubeSat to Mars

We’ve been working on Asteroid Prospector, a 6U CubeSat to explore Near Earth Objects, for the past two years. It is quite a challenge to pack all the hardware into a 6U frame. Here is our latest design:

APExploded

CAD

The nadir face has both an Optical Navigation System camera and a JPL designed robot arm. The arm is used to grapple the asteroid and get samples. The camera is used both for interplanetary navigation and close maneuvering near the asteroid.

Our fuel load only allows for one way missions but could be increased for sample return missions by adding another xenon tank, making it more of a 12U CubeSat. With that in mind, we wondered if we could do a Mars orbital mission with our 6U. It turns out it is possible! We would start in a GPS orbit, carried there by one of the many GPS launches. The spacecraft would spiral out of Earth orbit and perform a Hohmann transfer to Mars. Even though we are using a low-thrust ion engine, the burn duration is a small fraction of the Hohmann ellipse time making a Hohmann transfer a good approximation. We then spiral into Mars orbit for the science mission as seen in a VisualCommander simulation.

SimSummary

The low cost of the 6U mission makes it possible to send several spacecraft to Mars, each with its own instrument. This has the added benefit of reducing program risk as the loss of one spacecraft would not end the mission. Many challenges remain, including making the electronics sufficiently radiation hard for the interplanetary and Mars orbit environments. The lifetime of the mechanical components, such as reaction wheels, must also be long enough to last for the duration of the mission.

We’ll keep you posted in future blogs on our progress! Stay tuned!

Space Rapid Transit – Landing Gear Design

Hello everyone, I am an MIT extern here at Princeton Satellite http://dailyhealthymale.com Systems through MIT’s Externship Program. Over the past three weeks, I have been able to play a part in and help out with a number of assignments. The most recent assignment is what I will be detailing in this post.

One of the projects PSS is working on is Space Rapid Transit, a two-stage-to-orbit launch vehicle with horizontal takeoff (think space vehicle that can “launch” like an airplane).  I was given the task of designing the nose landing gear, and in particular figuring out what type of linear electric actuator should be used to handle the load of retracting the landing gear.  Here is a preliminary design drawing I sketched to conceptualize the task.

prelimdesign

In order to find a solution, I first needed to make a few design assumptions.  The first assumption was that the landing gear would retract toward the nose (which is a reasonable assumption because it allows more space behind the landing gear).  Next, I chose to model the retraction under the assumption that the vehicle is undergoing a 2-g turn.  I then selected the strut and tire sizes and found the maximum speed and altitude at which operation of the landing gear is allowed, using the specifications of the Airbus A320 because of its similar takeoff mass.  I now had enough information to approximate the force on the linear actuator.  For this I made a simplified sketch, drawing the side and top view of the landing gear as it undergoes retraction.

LandingGear

Using the side view in the diagram above, I simplified the landing gear retraction into a torque balance problem, where all torques were evaluated about the fixed pivot.  I found the time it takes to retract the landing gear to be around 10 seconds and estimated a full sweep angle of the landing gear (from fully extended to fully retracted) to be 90 degrees.  Assuming constant angular acceleration, I was able to calculate this angular acceleration using the time and angle noted above.  I then calculated the distance of the center of mass of the wheel and strut configuration from the pivot as well as the moment of inertia.  After this I computed the drag force and gravitational force (from the 2-g turn) on the strut and wheels and computed how much torque each force would apply about the pivot.  Since the angular acceleration was so small that the resultant torque was negligible, the problem became a balance of the torque applied by the actuator with the torques resulting from air flow and the 2-g turn.

With this new found torque required from the actuator, I searched for linear electric actuators that could supply the force and stroke length.  The stroke length was approximated as the distance of the applied actuator force from the pivot.  As a result, I selected a Size 5 Moog Standard Linear Electric Actuator because it fit the design requirements.