Citizen Explorer-I
New Technologies Aboard Citizen Explorer-I
Colorado Space Grant Symposium 12/18/18 / Page 1 of 8 /Scott Miller
Colorado Space Grant College
520 UCB
Boulder, CO 80309
(303) 492-5440
Katie Dunn
Colorado Space Grant College
520 UCB
Boulder, CO 80309
(303) 492-5440
Colorado Space Grant Symposium 12/18/18 / Page 1 of 8 /Abstract
Citizen Explorer-I (CX-I), designed and built by students at Colorado Space Grant College in Boulder to provide global ozone monitoring, employs a unique mission architecture and several innovative technologies during its mission. The mission architecture allows K-12 schools around the world to be involved as ground stations available to receive science data and telemetry from CX-I. Another important technology allows the spacecraft to be less reliant on ground operators. Spacecraft Command Language (SCL) allows mission designers to set constraints operations; for example, the designer may specify that the spacecraft would not be allowed to transmit to the ground if the battery power were too low. In addition to SCL, a low level of artificial intelligence will be supplied to the spacecraft through the use of the Automated Scheduling and Planning ENvironment (ASPEN). ASPEN is used to maintain a spacecraft schedule in order to achieve the objectives a mission operator would normally have to complete. Within the communications system of CX-I, Point-to-Point Protocol (PPP) is the main method for communicating with the satellite. As PPP has not been widely used in satellite communication, CX-I provides an opportunity to study the effectiveness of this Internet protocol. The Attitude Determination and Control System (ADCS) on CX-I uses a gravity gradient boom as a means of orienting the satellite’s science instruments toward nadir. The boom is constructed of Stanley tape measures, which provides several advantages. The entire 4-meter boom can be collapsed into a very small space, is lightweight, and uses its own elasticity to deploy and take its proper structural shape. The receive antenna also employs the elastic property of a tape measure to provide a simple deployment mechanism. These new technologies’ effectiveness will be tested for use on future small satellite projects within the space satellite industry.
Colorado Space Grant Symposium 12/18/18 / Page 1 of 8 /Background
The main objectives of the Citizen Explorer mission are to provide significant experimental knowledge to the science community, grant educational opportunity to K-12 schools around the world and allow undergraduate and graduate students an educational opportunity in science and engineering. CX-I has two science instruments on board, a spectrophotometer that will measure UV-B and photometer which will measure reflective light coming from the Earth. Two other instruments are needed on the ground, an aerosol detector as well as a UV meter to compare the data coming from the satellite with the conditions on the ground. Using all four of these instruments, a measurement of the ozone can be taken. Because the focus of the mission is education, an important part of the mission is its architecture. The satellite will transmit science data to schools around the world through separate school ground stations called Edu-Stations. This data can then be compared with the measurements on the ground, and sent to the mission operations center at the University of Colorado at Boulder (CU) for further analysis and compiling. CX-1 is designed for a polarsun synchronous orbit at an altitude of 705 km. It was designed for a 10:15 AM/10:15 PM equator pass.
Citizen Explorer’s Low Level Artificial Intelligence
Background
Mission operation programs and software have been utilized with other Space Grant missions in the past and in the continuing future. The 1997 launch of DATA-CHASER (Distribution and Automation Technology Advancement - Colorado Hitchhiker and Student Experiment of Solar Radiation) on STS-85 employed a program written by the artificial intelligence group at the Jet Propulsion Lab (JPL) in Pasadena, California. The DATA-CHASER Automated Planning and Scheduling System (DCAPS) was designed to exhibit the feasibility of planning and scheduling software on a satellite. This mission was successful, reducing the amount of mission operator time and increasing the amount of science data sent from the payload. Another application of low level intelligence is in Three Corner Satellite, a constellation flying combination of three satellites built by the University of Colorado at Boulder Space Grant (CSGC), Arizona State University Space Grant and New Mexico State University Space Grant. These three satellites use the Continuous Activity Scheduling Planning Execution and Replanning(CASPER) program, a real time on board planner to achieve their mission of stereoimaging using a total of twelve cameras on board. CSCG continues to investigate and explore the world of artificial intelligence in space applications through the use of the Automated Scheduling and Planning Environment (ASPEN) on CX-1.
Citizen Explorer Artificial Intelligence
Citizen Explorer uses a combination of the Spacecraft Command Language(SCL), the Automated Scheduling and Planning Environment (ASPEN) and to generate low level autonomy for the spacecraft. ASPEN is a planning and scheduling tool created by the artificial intelligence group at JPL. The program allows for mission operators to use lower level commands to achieve high level science requirements. The seven keys components that make up the ASPEN structure are activities, state variables, parameters, parameter dependencies, temporal constraints, reservations, and resources. Activities are considered tasks that occur over an interval of time and that effect the spacecraft in some way. Several activitiesprogrammed on CX-I’s schedule are transmitting to both the ground station at CU Boulder and the schools Edu-station’s around the world, changing the satellite’s power source from batteries to solar panels depending on the point in the orbit and turning on and off different components of the satellite depending on the resources available and the needs of the spacecraft. State variables, much like the name, declare a state of a component on the spacecraft. These state variables can be intricately woven together to form a web of constraints for the spacecraft. A parameter is a variable with a restricted domain.Parameter dependencies occur when there is a association or dependency between two parameters. Temporal constraints are the dependencies of one activity on another. For example, Citizen Explorer must have sufficient power before transmitting to the ground. The temporal constraint would first be checked to be sure there was adequate power for a transmission and then proceed to transmit if power wasavailable. Reservations are requirements that an activity may have on any function of the spacecraft. When CX-I takes a science scan, it requires a certain amount of voltage; this would be a reservation the activity requires to achieve its goal. Lastly, resources represent physical values on the spacecraft. The upper and lower bounds of that resource are given as well and ASPEN can coordinate which resources are used in different places on the spacecraft. Two types of resources exist;depletableand non-depletable resources. The depletable resources represent physical resources such as power and memory, both of which can be exhausted over time. Non-depletable resources are used in a particular activity as a reservation. Using the structure stated above, a schedule is fabricated for a typical day in the life or week in the life type scenario for the spacecraft.
Once the schedule or model is created, the mission operators can then observe the different behaviors of the spacecraft using the ASPEN model as well as different mission operations scenarios that they might devise. The ASPEN model is formulated on the ground, but once a suitable schedule has been created, it is loaded into the software of the satellite’s flight computer. At this point many of the other mission operation’s tools help to create a working schedule on the flight computer. The schedule is transferred into SCL in order to download onto the satellite’s computer. This is the language the satellite uses on its computer, so the ASPEN model must also “speak” this language. SCL is also useful for long term tracking of the satellite’s operation, finding the glitches that can not be seen in the ASPEN model.
ASPEN is a distinctivesoftware program in the mission operations field of satellite command. ASPEN has flown successfully on the Earth Orbiter-1 satellite and is continually being researched and developed atJPL. The field of artificial intelligence of spacecraft has far reaching implications for the future of space flight. Artificial intelligence plays a key role in deep space exploration. The further away from the Earth humans decide to explore, the more complicated it becomes to communicate with satellites, rovers and landers. Increased artificial intelligence of these systems can reduce the communication needed with the mission operators on the Earth. Another aspect of this intelligence is to devise decision making abilities on these spacecraft. This will allow for the spacecraft to decide on the data that is sent back to the scientists on Earth, allowing for only the interesting scientific data to be transmitted. A new level of scientific knowledge can be achieved with artificial intelligence on spacecraft. The use of a small, low level beginnings to this artificial intelligence, such as ASPEN on CX-I, is a stepping stone for additional research and development in the field of artificial intelligence.
Mission Architecture
CX-I has a unique mission architecture. The use of ASPEN in the mission operations pertaining to this architecture is actually quite helpful considering how it is structured. The concept involves having many ground stations throughout the world that will collect data from the orbiting satellite. As mentioned above, CX-I will downlink its data to any available ground station it passes over. The different schools working on the project will know when the passing of the satellite is to occur and can watch as the data comes streaming down from the satellite as it passes overhead.
The data taken from these ground stations will be analyzed initiallyby the software given to the schools through the Edu-station package. The data will be used for educational purposes in each classroom then transferred via the Internet to the main mission operations center at CU. Each educational facility will have a log in and password to their own page on the Space Grant website in which they can transfer the data they collected, both from the satellite and from their ground instruments. The data will be compiled into a database at CU and available for the scientific community toexperience.
This way of collecting data with multiple ground stations is not used in the satellite community. The multiple ground station approach has many advantages. The amount of data that is received will be immense. With data being taken as the satellite passes over a location everyday at the same time, the data should be able to be compared throughout the days, months and even years to gain an understanding of the ozone’s behavior. The educational value of watching and teaching students about the data that is streaming down into their ground stations ispriceless. Students will be able to observe a task they might only dream about or see on television right in their own school setting.
PPP
Using PPP will result in lower cost for the flight software, and a practical communications design, hence the satellite is simply a node on a wireless Internet connection. Using a TCP/IP network along with the PPP will allow the Citizen Explorer Mission Operations Center to have easy access to the CX-Ithrough remotely located ground stations located in Colorado, New Mexico, and Alaska. These remote ground stations will act as gateways between the Mission Operations Center and the satellite. Human intervention at the remote ground stations will not be required during operation. CX-Iwill offer an excellent operational analysis of protocols used for data communications.
A group of layered protocols are used for computer-to-computer communication. These various protocol layers are classified by their particular purpose. PPP will be used as the data link layer for ground-satellite communications.
The network and transport protocol layers are identical for ground-ground and extraterrestrial communications. These protocols create a TCP/IP based network interconnecting Citizen Explorer Mission Operations to the satellite through various ground routes. These routes are dependent on the location of the satellite and which ground station is in the satellite's footprint. This network relies on the integrity of the data link protocol, which must endure bit errors produced by the RF link.
The lowest level protocol is the physical layer. Citizen Explorer's network will use Ethernet between terrestrial computers. Riding on the physical layer is the data link layer. The data link layer must endure data flaws introduced by the physical layer such as packet corruption or network congestion. Ethernet, commonly used for terrestrial communications, is a proven data link protocol and not of concern to the mission. However, many factors of satellite communication may affect the performance of PPP as well as the higher level protocols. Some of these factors include:
Noise - A low signal-to-noise ratio may result from a weak RF signal. The result is a high bit error rate (BER) in the data stream. The number of correctly received packets reduces as the BER increases. The packets must be re-transmitted slowing the effective data rate of the link.
Variable feedback loop - The propagation delay of the satellite channel varies from approximately 2.4 - 10.4 milliseconds over a single pass.
Intermittent connection - Packet loss may result as the satellite connection is transferred from one ground station to another.
Delay-bandwidth product (DBP) - The amount of data a protocol should have transmitted but has not yet been acknowledged is critical to maximize the channel capacity and will vary over the course of a pass.
The effect of the satellite link's BER on PPP will result in corrupt packets. In order to have an effective communications system, a reliable stream of non-corrupt packets must be received. Corrupt packets are thrown away by PPP and a packet re-transmission is requested. The effective data rate decreases as the number of corrupt packets increase. If the BER is high enough, essentially all packets will contain corrupted data and communication with the satellite will not be possible. Current testing allows simultaneous bi-directional data flow, user-selectable error rates, and unbalanced forward and return transmission rates. Further goals of the project include providing time-variable error rates over several minutes and adding a .25 second channel delay. The result of the BER test is promising for PPP as a space protocol. Using a data rate of 9600bps and the TCP/IP based file transfer protocol (ftp), files containing random text of size 1KB, 10KB, 100KB and 1000KB were transferred between computers with user defined BERs of 0, 10-6, 10-5, and 10-4. Each case was run ten times and the average run time of each transmission was recorded.
In general it was not possible to transfer files using a BER of 10-4 consequently the file transfer times are not displayed on the graph. However, a BER of 10-6 produced results nearly as good as the file transfer run with a BER of 0. As the BER was increased to 10-5 the transmission times also increased.
Eric Darnel, of the University of Alaska, has completed a link budget analysis for the Citizen Explorer's uplink and downlink. The link budget, a theoretical analysis of the satellite links, includes BERs expected for the Citizen Explorer communication link. The current link budget implies that a BER of 10-6 is highly realistic.
Costs reduction and easy operation of satellites over the Internet can all be accomplished by using the common terrestrial protocol PPP for space applications. Using virtual instrument techniques it has been shown PPP maintains its integrity for BER rates above those generally seen in satellite communications. Citizen Explorer should provide important lessons learned about using PPP in extraterrestrial applications.
Gravity Gradient Boom
Background and Requirements
CX-I is required to point toward nadir to conduct science experiments and communicate with ground stations. To make CX-I a nadir-pointing satellite, the satellite deploys a gravity gradient boom to align itself toward nadir. A satellite tends to align itself along the axis of the minimum moment of inertia, which becomes the axis of the boom once it is deployed.
The satellite requires a boom moment of inertia of at least 40kg*m2 and the best option would be 100kg*m2. With a tip mass of 2.5kg, the boom length should be 4 meters at minimum. The boom also has to take up a volume only 6”x5”x4” when stowed.