CU-E3 (University of Colorado Boulder - Earth Escape Explorer)
The University of Colorado Boulder Earth Escape Explorer (CU-E3) is a nanosatellite of the 6U CubeSat format that will demonstrate long-distance communications while in heliocentric orbit. The mission is focused upon advancing deep space CubeSat communication techniques using an innovative reflect array antenna, an X-band transmitter for downlink and a C-band antenna for uplink. 1) 2)
As a competitor in the NASA CubeQuest Challenge Deep Space Derby, the CU-E3 team's goal is to demonstrate it is possible to build a deep space communications system that is small, powerful, and (relatively) low cost. This means not just the hardware on the satellite but also the ground station. On the satellite side, University of Colorado Boulder students have developed custom hardware to interface with an AstroDev Li-2 radio for C-band uplink. For downlink, CU-E3 will be using an X-band radio developed for low earth applications at the University of Colorado Boulder under the NASA Small Satellite Technology Development program. For ground station services, the CU-E3 team will be partnering with a commercial provider, ATLAS. 3)
Student staffing for CU-E3 is provided primarily by students enrolled in a two-semester graduate projects course sponsored by the University of Colorado Boulder Aerospace Engineering Sciences program. After completing graduate projects, some students continue to work on the project through an independent study course, while others continue as volunteers. Most students are completing master's degrees in either aerospace or electrical engineering. There are also a few upper level undergraduate students from both aerospace and electrical engineering participating.
CU-E3 Overview: CU-E3 is a 6U CubeSat consisting of transmit and receive communications chains, a set of batteries and power routing electronics, and a Blue Canyon Technologies XB1. BCT (Blue Canyon Technologies) is a rapidly growing small satellite company founded by CU-Boulder alumni that is partnering with the CU-E3 team to provide an XB1, which is a combined command and data handling (CDH) and attitude determination and control (ADCS) unit. The XB1 will point the satellite accurately, interpret commands and manage power for the CU-E3 payload. CU-E3 has no propulsion system. Reaction wheels provide attitude and pointing control. Deployables include three solar arrays and a primary feedhorn antenna paired with a reflect array for signal amplification. The solar panels provide power to the spacecraft and enable the cubesat to utilize solar radiation pressure to desaturate the reaction wheels. CU-E3 also contains a secondary feedhorn antenna for communication during the initial portion of the mission.
Figure 1: Internal Model of the CU-E3 CubeSat (image credit: CU)
Figure 2: Model of CU-E3 CubeSat with primary feedhorn/reflect array and solar cells deployed (image credit: CU)
CU-E3 won a ride to space on the Artemis-1 mission (formerly SLS-EM1) by finishing in the top three in ground tournament four of the NASA CubeQuest Challenge. Artemis-1 will have five "bus stops" where CubeSats can be deployed. CU-E3 will be getting off at the last one, bus stop 5, and coasting for the one year mission duration. Once the CubeSat reaches a distance of four million kilometers from Earth (as determined by the Deep Space Network), the Deep Space Derby competition of the CubeQuest Challenge will begin. The XB1 will generate random data according to NASA's specification and transmit it to the ATLAS ground station. From there, the data will be transferred to NASA, where a committee will review the transmitted data for errors and determine if it meets the threshold for any of the prize categories, which include largest volume of data transmitted in a 30 minute window, largest volume of data transmitted in a 28 day window, and longest transmission distance.
Challenges of Deep Space Communications
The farther away you are from Earth, the weaker your transmitted signal becomes. To overcome this, deep space missions have traditionally used large antennas mounted to large spacecraft. Then Jet Propulsion Laboratory's MarCO (Mars Cube One) 6U CubeSat demonstrated it was possible to downsize the spacecraft and still close the deep space communications link.
On the ground side, NASA-funded deep space missions have relied on the DSN (Deep Space Network). DSN has high gain antennas at three points on the globe with approximately 120 degrees of longitude separation from each other. This means deep space missions using DSN are in view of at least one antenna at all times.
However, there are two problems with DSN for non-governmental missions. The first is cost. Compatibility testing alone runs in the range of $40,000 to $50,000. Then there is an additional cost for aperture time once the mission is underway.
The second problem is demand. The increasing sophistication of scientific instruments on deep space missions has led to an increase in demand for DSN's services. According to JPL's Office of the Chief Scientist and Chief Technologist, "NASA estimates that the deep space communications capability will need to grow by nearly a factor of 10 each of the next three decades." 4)
CU-E3 is addressing the spacecraft side of the problem by using a high power amplifier in conjunction with a high gain, low volume reflect array to amplify the transmitted signal.
For the ground station side of the problem, CU-E3 is partnering with a commercial provider, ATLAS Space Operations Inc. of Traverse City, MI, USA. While ATLAS does not have the same level of coverage as the Deep Space Network, it does have access to a worldwide network of antennas with the capability to transmit to and receive signals from deep space. CU-E3's communications window will be limited to the time when the ATLAS ground station is in view. For our current launch date and ground station antenna in Brewster, Washington, we estimate the communications window to be approximately six hours at the beginning of the mission. That time will change over the course of the mission, ranging from 5 to 9 hours across the length of the mission. Additionally, if the launch date changes, the Artemis-1 SLS and CU-E3 trajectory will change, affecting the length of the communications window. Balancing all these factors to produce a communications plan for the mission will be a critical part of the mission going forward.
In addition to the limitations on transmit time imposed by the communications window, there are two other technical factors limiting the transmit time:
1) heat dissipation from the transmitter and high-power amplifier. Results from current testing and analysis allow for the maximum transmission time to be estimated as approximately four hours.
2) desaturation of the reaction wheels. As CU-E3 lacks a propulsion system, the reaction wheels cannot be quickly desaturated. Therefore, throughout the mission the satellite will have to rotate into a desaturation attitude that uses solar radiation pressure to slowly decrease the stored reaction wheel momentum. The reaction wheel desaturation maneuver will require the satellite to be oriented in a way that directs the antenna away from the earth. The goal is to perform reaction wheel desaturation during the hours when the ATLAS ground station is not available.
Since these limitations mean CU-E3 would not be able to communicate with Earth 24 hours a day, the reduced ground station coverage is an acceptable trade-off for the significantly reduced cost of aperture time.
Launch: CU-E3 is a selected NASA nanosatellite mission to demonstrate long-distance communications while in heliocentric orbit. It will fly as a secondary payload mission on the first flight of the SLS (Space Launch System), namely the Artemis-1 mission [former EM-1 (Exploration Mission-1)], expected to launch in the second half of 2021. 5)
CO-E3 Communication System
To minimize costs and to provide learning opportunities for students on the project, the CU-E3 transmit and receive chains are a combination of student-developed hardware and commercial off the shelf (COTS) parts. The uplink is C-band (5182 MHz), while the downlink will be in X-band (8447.6 MHz).
Due to the distances at which CU-E3 will be operating, initially we had planned to use BPSK for both uplink and downlink to maximize the accuracy of data transfer. However, due to difficulty getting our Lithium 2 to demodulate a BPSK signal, we are now looking at using GMSK for uplink, while still using BPSK for the downlink.
Receive chain: The CU-E3 receive chain consists of four pieces: a C-band patch array antenna, low noise amplifier, a downmixer board to reduce the uplink frequency to UHF, an Astrodev Lithium 2 radio, and the XB1 central avionics unit (CAU). Power connections to some components in the chain are routed through an expansion board.
The patch array antenna is student-designed. It receives C band signals at a frequency of 5182 MHz and provides amplification of approximately 9.8 dB. The low noise amplifier (LNA) is a Pasternack PE15A1010 which amplifies the incoming signal by up to 39 dB with a noise figure of 0.9 dB. The receive downmixer board is another student-designed and built part. It downconverts the received signal to an intermediate frequency of 420 MHz that is within the acceptable range for the Lithium 2 radio. The Lithium 2 radio converts the analog signal to digital form and demodulates it, then sends a bitstream to a Blue Canyon Technologies XB1 CAU for further processing.
Power to the low noise amplifier and Lithium are provided via an expansion board. The phase lock loop voltage controlled oscillator on the receive downmixer board is programmed by the XB1.
Figure 3: CU-E3 communications system block diagram (image credit: CU)
Transmit chain: Like the receive chain, the CU-E3 transmit chain is a combination of student-designed and COTS parts. The transmit chain consists of the XB1, an Xtx transmitter, a high power amplifier, a microwave switch, two hybrid couplers, a primary feedhorn antenna/reflect array, and a secondary feedhorn antenna.
The XB1 generates a random bitstream according to the competition specifications provided to the project by NASA. Additionally, the RF switch allows the transmitting antenna to be switched between the high gain reflectarray and low gain secondary feedhorn antenna. This switch is accomplished through an uplink command to the XB1.
The Xtx transmitter is an X-band radio developed for low earth applications at the University of Colorado Boulder under the NASA Small Satellite Technology Development program. It will transmit a signal at 8447.6 MHz with an accuracy of plus or minus 1 MHz. Recent testing suggests even greater accuracy, with frequency drift of less than 10 kHz over its operating temperature range of -20ºC to 50ºC.
The high power amplifier was developed and built by students. It amplifies the signal by approximately 30 dB, up to a signal power level of approximately +35 dB (3 W).
The DowKey 401T-420832A-ROHS microwave switch directs the signal either to the primary feedhorn and reflect array or to the secondary feedhorn.
The Anaren 1E0018-3 hybrid couplers create a circularly polarized signal which is then sent to an antenna for transmission.
The secondary feedhorn has a measured gain of 12.8 dB. It will be used for initial commissioning of the satellite, pre-competition, at distances less than four million kilometers. The primary feedhorn with reflect array has a measured gain of 22.3 dB. It will be set up prior to reaching the competition distance of 4 million kilometers, and then used for the competition transmissions.
The expansion board provides data and power connections for the Xtx. It also provides power connections and thermistor output for temperature monitoring of the high power amplifier, and power and control switching for the microwave switch.
Challenges and Lessons Learned
Staffing and turnover represent a significant challenge for the project. Because students are often on the project for only one or two semesters, and are not around to train in their successors, maintaining continuity between semesters is difficult. The project has attempted to address this by requiring students to write documentation as part of their coursework. However, documentation quality varies, and even when documentation is well-written, understanding it often requires incoming students to have a level of knowledge they do not yet possess. The effects of student turnover are limited by retaining upper level graduate students in project management rolls after their completion of the two-semester course, who can then serve as sources of reference on previously completed work.
Additionally, finding students to work on the communications subsystem of CU-E3 poses a particular challenge. While CU Boulder does have a strong program in radio frequency (RF) engineering, most of the students in that program are PhD students who are committed to projects in the electrical engineering department. Hence, they are not available to work on CU-E3.
A related problem is making a match between what the project needs to have done (for example, antenna design and digital communications) and skills that students available to work on the project have. Because CU Boulder's electrical engineering program does not specialize in digital communications, students must learn "on the job" while working on CU-E3's communications system. A high level of selfmotivation and willingness to seek out resources is required.
Some technical challenges have been created or exacerbated by budget and/or staffing problems. For example, the team looked at purchasing a C-band patch array antenna. Such components typically cost several thousand dollars, and the project did not have the budget for that purchase. Consequently, that part became part of the student-designed hardware. After eighteen months of development, the team found that the students' design was too thick to be manufactured. This necessitated a redesign after the original students involved in the antenna project had left. This caused a significant delay in the production and testing of that part, and represents the difficulty.
Challenges that are more purely technical include adapting hardware originally designed for low Earth or near Earth orbit to function in deep space. An example of this is the Lithium 2 radio that is part of CU-E3's receive chain. The Lithium had been used on other CU Boulder CubeSat missions, but with a GMSK modulation scheme suitable for low Earth to near Earth orbit. With a firmware update, the Lithium theoretically supports BPSK. However, at the distances we are operating in, switching from BPSK to GMSK may significantly increase the bit error rates, posing a risk to successfully commanding the satellite.
CU-E3 Ground Systems Support Research Process
Many, perhaps most, satellite missions have a single ground system which provides telemetry, command, and ranging services. CU-E3 is different in that we will be using two separate ground systems. The NASA competition we are participating in requires that we use the DSN (Deep Space Network) for ranging. For budget reasons, CU-E3 needed to find another provider for telemetry and command services.
The team conducted research on potential ground station partners at the 2017 SmallSat conference. After team member John Sobtzak presented a poster on the CU-E3 communications system at the conference, the team was approached by the University of Alaska Fairbanks about working with them for ground station support. CU-E3 also talked with ATLAS Space Operations at the SmallSat conference.
ATLAS buys time on antennas around the world, and serves as an aperture time broker service. Antennas in the ATLAS network have transmit and receive capabilities at various frequencies, including the C-band uplink and X-band downlink capability that CU-E3 needs for its mission. Users access the antenna network using ATLAS's FreedomTM platform.
ATLAS is also developing a second service, the LINKSTM electronically steered array system. This system achieves the power provided by a 70 m parabolic dish antenna. The LINKSTM system is the basis of the Interplanetary Satellite Communications Network (ISCN), which can track and communicate with satellites operating up to 18 million kilometers from Earth using VHF, UHF, S-band, and X-band frequencies. 6)
Ultimately, CU-E3 selected ATLAS for telemetry and commanding. ATLAS has the necessary technical capabilities and was willing to donate aperture time in return for the possibility of receiving a portion of any prize winnings.
1) "Cube Quest Challenge Team Spotlight: CU-E3," NASA, 1 June 2017, URL: https://www.nasa.gov/directorates/spacetech/centennial_challenges/cubequest/CU-E3
2) "CU-Boulder Students Win $30,000, Advance In CubeSat Competition," Colorado Space News, 21 March 2016, URL: https://www.coloradospacenews.com
3) Sarah Withee, Gabriel Altman, Wesley Caruso, Charles Gillard, John Sobtzak, Brodie Wallace, Kyle Wislinsky, "Closing the Deep Space Communications Link with Commercial Assets," Proceedings of the 33rd Annual AIAA/USU Conference on Small Satellites, August 3-8, 2019, Logan, UT, USA, paper: SSC19-WKVIII-02, URL: https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=4428&context=smallsat
4) "Deep Space Communications," NASA/JPL, URL: https://scienceandtechnology.jpl.nasa.gov/research/research-topics-list
5) "The First Artemis Launch has Been Delayed Until Mid-to-Late 2021," Universe Today, 5 March 2020, URL: https://www.universetoday.com/145243
6) Scott Schaire, Serhat Altunc, Yen Wong, Obadiah Kegege, Marta Shelton, George Bussey, Marcus Murbach, Howard Garon, Yudhajeet Dasgupta, Steve Gaines, Edward McCarty, Sean McDaniel, Wesley Faler, Peter Celeste, Trish Perrotto, Matt Batchelor,"Investigation into New Ground Based Communications Service Offerings in Response to SmallSat Trends,"Proceedings of the 32nd Annual AIAA/USU Conference on Small Satellites, Logan UT, USA, Aug. 4-9, 2018, paper: SSC18-SI -0, URL: https://digitalcommons.usu.edu
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).