News Archive

FASTRAC Satellites to Set Up and Ship Out!

March 2008

It is an exciting time for the FASTRAC satellite team and community.  The FASTRAC satellites have been officially manifested for launch and have returned to Austin for the first time since 2006! During the next few months the satellite team will be working on the satellites in the newly developed Flight Integration Lab at the Center for Space Research.  This new lab, set up in what was previously the CSR library, facilitates the FASTRAC flight build and will soon accomodate other student-built satellite projects from the University of Texas.  (The FASTRAC team would like to sincerely thank CSR for its gracious accommodations for our project.) 

During the next few months, FASTRAC will be undergoing maintenance before final delivery to the Air Force Research Laboratories in Albuquerque, New Mexico. Before April 1^st, the FASTRAC team will have completely disassembled the satellite, fixed issues including replacing fuses and installing new memory cards, and completely reassembled the satellite. In addition to working on the satellite hardware, several members of the FASTRAC team are also working on satellite software. In order to ensure a completely reliable mission, the software is being fine-tuned to make sure that all known problems are removed from the flight code. While satellite flight hardware and software is being completed, the FASTRAC ground station at the University of Texas W.R. Woolrich Laboratories is also undergoing testing and advancement. The ground station has already performed excellently in simulated satellite passes and the new, student-developed ground station software is extremely exciting.

 

On April 1^st, the satellites will be packaged in their container and shipped off to New Mexico for environmental and pre-launch testing. During this test phase, the satellites will be subjected to conditions that approximate those found during launch and in a space environment. After completing this final phase of testing, the satellites will be launched from Kodiak Island, Alaska in the fall of 2009 on a Minotaur IV rocket. After launch, the FASTRAC crew will be manning the ground station waiting to make first contact with the satellites and begin the mission!

 

 

Once again, the FASTRAC team would like to thank the many people involved in helping in this final push. Without the hard work from the team and the community as a whole, the project would never be as close to success as it is today. With a few more months of hard work, the satellites will be ready for their launch into space on the University of Texas' inaugural student-satellite spaceflight.

 

 

 

 

Absolute and Relative Estimation for Satellite Formations

December 2007

Introduction

As part of a grant from the General Dynamics corporation, I am exploring methods of implementing an estimation algorithm in the form of a SIMULINK model that will take measurement data and return an estimate of a chaser satellite relative to a host satellite. This project provides the input to a control system that can be used to generate specific relative orbits of the chaser about the host that may be necessary for a variety of missions.

Absolute Estimators

Currently there are two de-coupled estimators working to provide a position for the chaser satellite. The first estimator accepts a measurement, in the current configuration this is a range from a ground station, and uses that measurement in conjunction with an initial condition and a dynamic model to return an estimated current state. The estimator itself uses an extended form of the Kalman filter algorithm which updates state and covariance at each time step. The model was built to be flexible and can accept any rate of measurement data and an update to the dynamic model requires the editing of only a single block.

Relative Estimator

The relative estimator has nearly the same form as the absolute estimator. Instead of a ground station range, the relative estimator utilizes a range measurement between the two satellites to perform state updates. Also, since the relative estimator is based on the Clohessy-Wiltshire equations, which are linear and do not require integration of a state transition matrix, there is no integrator block in this model. The two estimations generated by the measurements can be combined to provide an absolute estimate for the chaser satellite which is required for performing course adjustments in a control algorithm. Currently only the absolute estimator includes perturbation effects, such as drag and J2, in its dynamic model. Future work will include re-deriving the CW equations with drag and J2 accounted for and merging the estimation algorithm with the control algorithm.

The full story with results and images can be found here.

 

The Impact of Multipath on BOC Modulated GNSS Signals

November 2007

by Ben Harris

 Introduction

My dissertation topic explores how M-Code can be used to perform orbit determination. As part of that investigation I've looked at how M-Code and a similar class of signals called BOC modulated codes will reject multipath. These are the results of that investigation which I completed from August to October 2007.

Basics of BOC Modulation

M-Code belongs to a new class of GNSS signals that are based on Binary Offset Carrier or BOC modulation. BOC modulation is conceptually similar to the common amplitude modulation associated with consumer radio, and with the Binary Phase Shift Key or BPSK modulation used to encode the standard GPS signals, however there is a twist. The BOC modulated signal is modulated with an additional square wave that overlays the pseudorandom code. BOC signals are denoted BOC(α,β), where α is the period of the square wave in multiple of the chipping rate of C/A, and β is the chipping rate of the pseudorandom part of the code. The following image depicts standard BOC and BPSK modulations

Note that M-Code will be modulated using BOC(10,5).

Signal Multipath

Because GNSS receivers are omnidirectional (generally speaking), each GNSS signal is subject to interference from its own reflections. The following image depicts this phenomenon which is called multipath.

Two modes of multipath are shown: specular and diffuse. Diffuse multipath spreads the reflected energy in many directions, creating essentially noise. Specular multipath redirects the energy in a uniform direction to create a consistent signal similar to the LOS signal. This is also known as coherent reflection. Though the reflections are always delayed with respect to the line of sight or LOS signal, the induced error in tracking can be positive or negative. It is possible to model the value of that error, when the reflector is coherent and if the additional distance traveled by the multipath signal is known.

At the point of reflection two affects occur. First the multipath signal is shifted in phase relative to the LOS signal. Second the amplitude is reduced. The distance added to the signal due to the reflection also provide an opportunity for the signal to continue changing in phase relative to the LOS signal.

Additional affects change the multipath signal compared to the LOS. The multipath signal pierces the antenna from a different direction, and is therefore received with a different gain. This is because GNSS antennas do not uniformly pickup signals from all directions. There are other affects that cause the signal to distort in phase as it is received at the antenna.

In the end, the error introduced by the reflected signal can be productively modeled as a function of three variables:

1. the delay caused by the reflection,
   
2. relative phase, and
   
3. relative amplitude.
   

Multipath Error Envelope for BOC Modulated Codes

The impact of a single, specular reflector on any BOC modulated code can be solved. The solution requires tracing the influence of the reflected signal through the chain of processes that occur inside a GPS receiver. These processes include correlation and discrimination. Often the error is presented in the form of an error envelope. In the envelope, the amplitude of the multipath reflection is fixed, and the error is plotted as a function of distance for two different phases, 0° and 180°. Here is the multipath error associated with BOC(10,5) signal, with an amplitude of 0.5.

The red lines correspond to multipath error associated with a relative phase of 180°. Blue is for 0° relative phase. These values were generated from a numerical simulation written for my dissertation. The green values are generated by the analytical model I have solved. More detail about the model will of course be presented in my dissertation. Also we have submitted an abstract to the IEEE/PLANS 2008 conference on this topic.

 

Research Begins on Cislunar Navigation

October 2007

In the Fall 2007 Semester, the Lightsey Research Group began work on cislunar navigation for the Orion/CEV Program. As NASA prepares to send humans to the Moon, the need arises for precise orbit determination – this is especially true during the return trip from the Moon. Due to entry, descent, and landing (EDL) constraints, precise knowledge (and control) of the spacecraft’s position and attitude is required at entry interface.

During operations in Low Earth Orbit (LEO), the data required for navigation may be obtained through an onboard Global Positioning System (GPS) receiver or through other traditional navigation methods used for crewed spacecraft. As the distance between the spacecraft and Earth increases, these methods become problematic due to design (e.g. GPS signals are designed to transmit towards the Earth) and/or poor geometry. To address this difficulty, past spacecraft operating in cislunar space have employed a combination of inertial measurements (e.g. data from accelerometers and gyros) and inertial state updates from ground tracking. The current lunar architecture and concept of operations, however, makes it desirable for the Orion vehicle to be capable of on-board inertial navigation updates.

The Lightsey Research Group is currently investigating a number of potential solutions that would allow for on-board inertial navigation updates. At the present time, research is focused on solutions that rely primarily on optical measurements. In such a scenario, the Orion vehicle would detect satellites for which precise position data is available (e.g. GPS satellites, many geostationary satellites, etc). Then, by coupling accurate angular measurements with precise satellite position information, the resulting data may be used to produce an estimate of the Orion’s inertial position.  Concerns regarding satellite geometry and satellite visibility (i.e. how easy is it to detect the satellite with the camera and at what distance can this reliably be done) are also being addressed.

 

Supported GPS Receiver Launched into Space

June 2007

 

Consortium for Autonomous Space Systems

December 2006

 

 

This fall semester The University of Texas at Austin and Texas A & M University have started a very interesting collaboration sponsored by the Air Force Research Laboratories in the form of the new Consortium for Autonomous Space Systems. CASS will be focused in five major areas which are:

1. 1. Novel Spacecraft Designs
2. 2. Smart Sensors
3. 3. Autonomous Spacecraft Control Systems
4. 4. Cooperative Control of Satellite Formations
5. 5. Enhanced Education, University, Industry and Government Teaming, and Technology Transfer

Each of the universities has been chosen to explore key technology in these five areas which will be very helpful for the Air Force in future DoD missions. UT will be involved in the following major tasks,

1. 1. Orbit Determination for Microsatellite Clusters
2. 2. Small Satellite Propulsion System
3. 3. Autonomous Rendezvous Maneuvers
4. 4. New Methods in Satellite Design
• Autonomous Formation Conops Design
• New Testing Methods for Rapid Response Satellite Design
5. 5. Satellite Proximity Sensors

The research for CASS will be conducted by professors, staff, and students at each university. It has to be noted that apart from CASS both universities are also involved in other programs for satellite developement such as the Nanosat program.

 

 

Application of conceptual docking technology to university satellite programs

November 2006

How can two spacecraft dock and separate in orbit? Conventional methods incorporate a mechanical linkage system, but the PARADIGM (Platform for Autonomous Rendezvous and Docking with Innovative GN&C Maneuvers) satellite design team at UT-Austin is investigating a different type of separation mechanism: electromagnetic docking.

An electromagnetic dock research investigation was originally proposed and explored by the EGADS (Electromagnetically Guided Autonomous Docking and Separation in Zero-Gravity) Microgravity University team and flown onboard the C-9 experimental aircraft in March 2006. The concept proved relevant to PARADIGM, a pico-scale satellite program, in cooperation with Texas A&M University and sponsored by NASA JSC, that aims to separate and dock two 5-inch cube satellites. An electromagnetic dock would allow the two picosatellites to perform both maneuvers without complicated mechanical equipment or precise alignment requirements.

The EGADS experiment hopes to fly onboard the C-9 again in 2007 to test additional docking and separation capabilities that could potentially apply to PARADIGM. Students interested in participating in either project can contact Jessica Williams (jessica-williams@mail.utexas.edu) for additional information.

 

Fall Presentation Schedule Posted

September 2006

The Research Group is meeting in WRW 410 every other Wednesday from 4-5 pm. The format of the meeting is usually brief announcements followed by a presentation from a student or staff member. The schedule for Fall 2006 is posted here for future reference.

9/6/2006 Mariela Gunn, Web Site Intro
9/20/2006 Benjamin Harris, GPS Research
10/4/2006 Andreas Mogensen, Mars Approach Navigation
10/18/2006, Dr. Key-Rok Choi, topic TBD
11/1/2006 Tena Wang, Sensor Fusion
11/15/2006 Thomas Campbell, Beyond FASTRAC: Communications
11/29/2006 Jamin Greenbaum, FASTRAC@AFRL & Conops

Additionally the following members will contribute news articles to the web site in the following months:

September, Daero Lee
October, Jessica Williams
November, Sebastian Munoz
November, Jack Goetz
December, Eric Rogstad
January, Cinnamon Wright

 

GPS Sensor Featured in Ready To Commercialize Workshop

September 2006

A single antenna GPS attitude sensor developed by the Lightsey Research
Group is being featured in a University of Texas Ready to Commercialize
Workshop on October 12. The workshop brings investors to The University
of Texas to learn about products that could be licensed and
commercialized. The Single Antenna GPS Attitude (SAGA) sensor was
developed for the Formation Autonomy Satellite with Thrust Relnav
Attitude and Crosslink (FASTRAC) spacecraft, which is expected to launch
in 2007. Although the device was originally designed for operation on a
satellite, it can work with other kinds of vehicles and platforms as
well. SAGA's potential for miniaturization makes it very promising for
applications where the sensor size must be very small.

For more information about the UT Ready to Commercialize Workshop, see:
http://www.otc.utexas.edu/Events/Oct2006/index.jsp

For more information about the FASTRAC mission, see:
http://fastrac.ae.utexas.edu

 

Greg Holt finishes Ph.D.

August 2006

Dr. Greg Holt successfully defended his Ph.D. thesis on August 8, titled "Generalized Approach to Navigation of Spacecraft Formations Using Multiple Sensors." Greg has successfully completed all of his Ph.D. degree requirements and graduated in Summer 2006. Congratulations, Greg!

During his time as a student at The University of Texas at Austin, Greg was part of the pulse of the Aerospace Department. A partial list of his accomplishments includes: leading the Formation Autonomy Satellite with Thrust Relnav Attitude and Crosslink (FASTRAC) as the first student manager (he was also the GPS lead and delivered the software for the Orion GPS receivers that are on the satellite), and teaching his fellow students officially as an Assistant Instructor for ASE 167M Flight Dynamics Lab and unofficially as a volunteer Indiana Jones on many GPS Scavenger Hunts for K-12 students. Greg also received a National Science Foundation scholarship during the years from 2001-2004.

Greg has accepted a job at NASA Johnson Space Center and has moved to the Houston area with his wife Amanda. We will miss Greg, but we are very proud of him wish him all the best in his new job! Hook'em!

 

GPS Flight Software Delivered to TerraSAR-X Satellite

July 2006

Jacob Williams and Key-Rok Choi recently delivered their build of GPS receiver flight software to a German radar satellite mission known as TerraSAR-X in July 2006.

The mission, which is scheduled to be launched in late 2006, will make radar maps of the Earth surface from space. The commercial data product is made possible with highly accurate knowledge of the satellite's position from space using a dual frequency BlackJack receiver manufactured by Broad Reach Engineering. UT-Austin was contracted to make software modifications to the receiver to perform precise orbit determination and occultation measurements of the Earth's ionosphere. For more information, see information about our TerraSAR-X project.

 

Williams Wins NASA Fellowship

March 2006

Jacob Williams has been awarded a NASA Earth System Science (ESS) Fellowship award for his Ph.D. proposal "Extending the Capabilities of the BlackJack GPS Receiver for Earth Science Applications." The fellowship is one out of 55 selected from more than 200 proposals. Good job, Jacob!

More information on Jacob Williams

 

Balloon Payload Sets SDL Altitude Record

March 2006

Greg Holt, Shaun Stewart, Tom Campbell, and Michael Linford designed and flew a camera payload (pictured left) to more than 100,000 feet on a high altitude balloon in Summer 2003. The camera took the image shown above right. The workshop, held in Boulder Colorado, was part of the University Nanosatellite program. The team was noted for their enthusiasm about the great state of Texas!

More information on the Satellite Design Lab

More information on our Nanosatellite Project, FASTRAC

 

Madsen, Monda Graduate In Spring 2003

February 2006

Congratulations to Jared Madsen (pictured) and Eric Monda for receiving their graduate degrees in 2003!

Jared Madsen successfully defended his Ph.D. dissertation, "Robust Spacecraft Attitude Determination Using Global Positioning System Receivers." Dr. Madsen has accepted a position with Sandia Laboratories and moved to Albuquerque, NM. More information on Dr. Jared Madsen

Eric Monda completed his Masters thesis, "Investigation of Real-Time GPS Pseudolite Relative Navigation." Eric is continuing on at UT in pursuit of a Ph.D.

 

Navigating with Satellites at Explore UT

February 2006

Students from the research group recently participated in the University-wide open house, ExploreUT. As part of the College of Engineering section, the group gave a series of talks and demonstrations on the subject "Navigating With Satellites". Participants were treated to a hands-on demonstration of Global Positioning System technology and research. The talk was geared toward high school students interested in pursuing engineering fields in college.

Go to UT College of Engineering Page
Go to ExploreUT page

 

Demo Introduces High School Students to GPS

January 2006

Group members recently put on a hands-on demonstration of the Global Positioning System for visiting high school students. As representatives of the MITE (Minorities Introduction To Engineering) and NexTech (Young Technology Leaders) programs, these students were given the opportunity to hear an explanation of GPS from grad students Tifanie Smart and Greg Holt. Then, they were taken on a GPS archaeological hunt led by Greg as Indiana Jones!



Hands-on experience using the GPS receivers during the archaeological hunt

A brief talk before heading out


The 1-minute history of navigation...

Live GPS data right in the conference room!

You're almost there, keep looking!

The intrepid team poses for a shot