Mark Jessop is an electronic engineering PhD student at the University of Adelaide. He is most noted for his work with the DIY space project “Tux in Space” and is currently working on TOPCAT, one of the payloads onboard the CubeSat UKube-1. Let’s take a closer look at what Jessop had to say about TOPCAT.
What is a CubeSat?
A CubeSat is a small cube shaped satellite that is based on a standard form factor of 10 cm3
or 1U. Larger versions of CubeSats are available in multiples of the standard form factor and may be referred to as 2U, 3U or larger. CubeSats have become the standard in micro-satellites because they are cheap and easy to build. For a couple hundred thousand dollars, it is now possible for anyone to build an experiment and launch it into space. The low cost is due to the ability to launch CubeSats in large batches of up to ten or twenty as secondary payloads on larger rockets. Jessop notes that another key advantage of the CubeSat is, its relatively short development cycle; in the case of TOPCAT, his team was able to bring the device from a concept to a working prototype in about two months.
UKube-1 is a 3U CubeSat funded by the UK Space Agency and was developed by Clyde Space, a division of Strathclyde University in the UK. Including TOPCAT, UKube-1 will carry four additional payloads: FUNcube, myPocketQub, Janus and CMOS Imager. FUNcube is an amateur radio repeater and back up telemetry system created by AMSAT. myPocketQub is essentially an opensource nanosatellite by UKSEDS that allows hobbyists to work on its code and contribute and participate in a space mission. Janus is an FPGA experiment by Astrium designed to demonstrate the use of space-based cosmic radiation to produce true random number generation in satellites for highly secure telecommunication applications. CMOS Imager Demonstrator will perform various imaging tasks, take images of Earth and test the effect of radiation on instruments in space.
The aim of TOPCAT is to collect information in the form of electron density measurements on the ionosphere and plasmasphere. For those unfamiliar with the ionosphere, it is a number of layers of ionized particles ranging altitudes from about 50 to 400 km above the Earth’s surface. The ionosphere affects radio waves traveling through it and is therefore responsible for the positioning errors that occur in handheld GPS devices. Understanding the ionosphere can help improve the accuracy of GPS devices. TOPCAT will quantify the state of the ionosphere by using electron density measurements and state the number of free electrons per cubic meter. This allows the team to work out various properties like refractivity, and how it affects radio waves traveling through it. Above the ionosphere is the plasmasphere which is similar but has a lesser effect on radiowaves. Radio wave delay is dependent upon two key variables: radio frequency and the electron density along the radio path. Using a transmitter that broadcasts two frequencies, TOPCAT will measure the phase difference between the two signals allowing the team to use mathematical models to figure out the electron density and create a useful map of the ionosphere.
The main core of TOPCAT is the “space rated” NovAtel Dual-Frequency GPS Receiver, which is one of the smallest dual-frequency receivers on the commercial market. Communication between the GPS and the CubeSat platform is handled using ATMega2560, selected for its large amount of RAM. A 1 megabit EEPROM is used for long term storage in place of flash because it is more robust in a high radiation environment. There are two GPS receivers involved in this project. The first is space rated and is onboard the CubeSat, the second is on the ground, and both weigh 4,000 lbs. All GPS units are required to have COCOM limitations where they will stop working at velocities greater than 500 m/second if the altitude is higher than 18km. The limitations are implemented to discourage the use of GPS in intercontinental ballistic missiles. There is one other situation where this condition is satisfied, and that is on a satellite. In order to obtain a space rated GPS that can maintain lock in Earth’s orbit, it was necessary to file a lot of paperwork. Upon receiving the GPS it was necessary to test it using CAST 1000 GPS simulator which created the fake signals necessary to confirm its ability to function at high velocities in Earth’s orbit.
Software Requirements and Implementation
The basic software requirement of TOPCAT is the ability to pass binary data from the GPS onboard UKube-1 to the receiver on the ground. The key information TOPCAT is recording is the position, time, and phase measurements amongst other relevant data. The data is packaged into 256 byte packets and stored into EEPROM for long term storage, on the timescale of a couple of hours. They want to avoid storing the data on RAM as the payload needs to be able to respond to I2C commands from the CubeSat platform within 2 milliseconds. A failsafe within the platform will shut TOPCAT down if it misses this time deadline 3 times in a role making it crucial that they meet those deadlines. The final requirement is that the software can control how many packets to send and assign certain priority levels to different types of data before sending it into the downlink buffer and eventually transmitting it to the ground. TOPCAT uses Arduino as it automates most of this process and presents an easy to use software platform for working with CubeSats. Arduino also comes with a number of benefits including hardware abstraction and digital libraries which they use to communicate with the SPIA prompt. Despite its advantages there, Jessop noted a few issues with Arduino. The GPS sends out kilobytes of data at semi random intervals and because Arduino’s Hardware Serial Library has a 128 byte long ring buffer, it is possible to lose data when talking to the platform using an I2C command. In order to remedy this, it was necessary to add a set callback function which allowed them to override Arduino’s interrupt service routine which gets called every time a new byte enters the system. Jessop wrote his own path which would strip out the data they needed and dump it into a swing buffer. The end result is having two buffers to switch between. When one fills up, the main loop begins processing that data while additional bytes are written into the second buffer.
Preparations Before Launch: Failure Modes
With all the time and money invested in this project it was necessary to be aware of the risk involved with launching a CubeSat into space. Launch failure is a primary concern, mitigated by using the Russian ICBM which has had 17 launches of which only one failed. There were ten CubeSats on that launch. The next concern was radiation, specifically the high energy particles that bombard geostationary satellites above the Earth. While these particles can prove devastating to microcontrollers and CPUs, UKube-1 orbits below the Van Allen Belt, which blocks most of these particles. The danger is of course still present, and if the flash memory or microcontroller were taken out, the mission would be a total failure. TOPCAT contains a hardware watchdog which will reset the payload in the event that the RAM is flipped or something goes wrong. As mentioned before, the CubeSat itself serves as a watchdog and will reset the payload if it fails to respond to one of the I2C commands. Jessop noted a couple interesting failure modes. In the event of total EEPROM failure, TOPCAT can store about 20 packets of data in the ATmega8 RAM and send data down from there immediately. If only a single page of EEPROM fails it is possible to isolate it and stop using it for the duration of the mission. If the GPS fails there isn’t much they can do to save the mission. The device has some aluminum shielding and the best they can do is add more aluminum to the top. The final point of vulnerability is with the uplink and downlink. There is a VHF downlink on a ham radio band 2 meters at 9600 baud that should be slow but reliable. The other downlink is a 1 megabit per second downlink on 2.4 gigahertz. The limited number of ground sites provides a small window of time to get data down each orbit and if either one fails it could prove problematic. This can be remedied by acquiring more downlink sites and possibly using the amateur radio downlink sites set up by the AMSAT FunCube project. By understanding the different failure modes associated with TOPCAT it is possible for Jessop and the team to implement countermeasures into the design and give their project the best possible chance for success. The TOPCAT team still has some time to refine the project before it takes off, UKube-1 is expected to launch with the Soyuz-2-1B/Fregat-M rocket from Baikonur in Kazakhstan on March 27, 2014.