RAX-2 stopped transmitting beacons over the weekend and is not responsive to normal commands. The cease in normal operations happened mid-day on Saturday, April 20, and the cause of the issue is currently under investigation. Over the next few days, we will be recording the spectrum during passes and sending up some custom commands in an attempt to determine the cause.
RAX-2 has been operating on-orbit for 540 days, has completed its scientific goals, and has surpassed it’s planned one-year scientific mission. We’ll be posting updates on the status as we investigate over the next few weeks.
Since our last update about radar experiments (January), RAX-2 has completed 6 experiments with the incoherent scatter radars in Poker Flat, Alaska, and Resolute, Canada. Although space weather conditions were active during some of the experiments, no FAI backscatter has been detected. The RTI plots from the two most recent experiments are shown below (expt 1031 completed March 16 and expt 1032 completed April 14). Since the space weather was active, we’ve been downloading raw samples from expt 1031 for analysis of the scintillation of the direct radar pulses.
Results from experiment 1032, completed with PFISR on April 14.
Results from experiment 1031, completed with PFISR on March 16.
If you’re an amateur radio operator tracking RAX-2, you may have noticed that the GS client stopped working after a recent Java update. We have fixed the bug, and the updated version of the GS client works with Java 1.6 and 1.7. You can download the updated ground station client from the Beacon Decoding Software page.
A paper on the design and implementation of the GPS subsystem on the RAX satellites is now available in the journal Acta Astronautica. We hope that this paper is helpful for other small satellite developers. The publication details and abstract are given below, and a complete list of papers on MXL research and spacecraft is available on the MXL website.
S.C. Spangelo, M.W. Bennett, D.C. Meinzer, A.T. Klesh, J.A. Arlas, J.W. Cutler, Design and Implementation of the GPS Subsystem for the Radio Aurora Explorer, Acta Astronautica, Volume 87, June-July 2013, Pages 127-138, ISSN 0094-5765, 10.1016/j.actaastro.2012.12.009
Abstract: This paper presents the design and implementation of the Global Positioning System (GPS) subsystem for the Radio Aurora eXplorer (RAX) CubeSat. The GPS subsystem provides accurate temporal and spatial information necessary to satisfy the science objectives of the RAX mission. There are many challenges in the successful design and implementation of a GPS subsystem for a CubeSat-based mission, including power, size, mass, and financial constraints. This paper presents an approach for selecting and testing the individual and integrated GPS subsystem components, including the receiver, antenna, low noise amplifier, and supporting circuitry. The procedures to numerically evaluate the GPS link budget and test the subsystem components at various stages of system integration are described. Performance results for simulated tests in the terrestrial and orbital environments are provided, including start-up times, carrier-to-noise ratios, and orbital position accuracy. Preliminary on-orbit GPS results from the RAX-1 and RAX-2 spacecraft are presented to validate the design process and pre-flight simulations. Overall, this paper provides a systematic approach to aid future satellite designers in implementing and verifying GPS subsystems for resource-constrained small satellites.
We’ve been quiet on the blog so far this month, but RAX-2 science operations have continued as usual in 2013. Earlier this month, we finished downloading all the science data of interest that has been stored onboard since previous science experiments. We have had a backlog of data to download since the first FAI detection in March 2012 and detection again in subsequent experiments, and we’ve now completed all the data downloads! In the meantime, we are watching the space weather forecasts and running experiments when conditions of interest arise. We ran an experiment with PFISR on February 4, but no FAI were detected. Two experiments are planned this week with RISR. If space weather conditions remain calm, we’ll use the time in between experiments to collect and download some extended bus telemetry for analysis.
RAX-2 carried out a radar experiment with the Poker Flat Incoherent Scatter Radar, PFISR, in the early hours of January 18 UT, but no echos from FAI were detected. This was the first experiment using GPS time synchronization following GPS testing in December. The RTI from the experiment is shown below.
We’ve now received over 250,000 beacons from RAX-2! This has been accomplished with the support of amateur radio enthusiasts around the world. Locations of ground stations that have received RAX-2 data are shown below. Any amateur radio operator can join the RAX ground station network by using our beacon decoding software.
Locations of ground stations that have received RAX-2 data.
252,426 beacons have been received, with 131,106 received by our primary ground stations here at the University and at SRI, and 121,320 were received by HAMs in the network. Each beacon contains satellite health data and telemetry. RAX-2 transmits a beacon every 10 seconds, reduced from the initial period of 20 seconds in March 2012. The cumulative number of beacons received over time is shown below. During the first few months of the mission, we scheduled downloads of the onboard beacon archives over various ground stations. These downloads stopped in January 2012 due to the SD card anomaly. Beacons since then have all been received real-time.
Cumulative number of RAX-2 beacons received over time
The periodic beacons are independent of data file downloads from the spacecraft, which composed primarily of science data but also include higher fidelity satellite bus data. The cumulative size of these file downloads is shown below. 162 MB have been received using the satellite’s 9600 bps radio, with 105 MB received by MXL/SRI and 57 MB received by the global HAM community. As you can see, the contribution from the amateur radio community has been tremendous. To all the ground station operators in the network, thanks for your continued support!
Cumulative sum of data downloads (independent of beacons).
In other news, daily satellite operations through the semester break and into the new year have consisted of continued downloads of science data to the primary ground stations at MXL and SRI as well as the global HAM network. The next radar experiment is expected to take place in late January.
Over the past week we tested the RAX-2 GPS subsystem, and it acquired lock with the GPS constellation and performed as expected. This was the first GPS test competed without use of RAX’s primary SD card. The data was logged to the PIM using a custom program uploaded to the spacecraft. Position accuracies of well under 5 meters were demonstrated!
Details of the design and implementation of RAX’s GPS subsystem will soon be available in the journal Acta Astronautica. The paper has been accepted and the proofs are being developed for publication.
Accuracy of satellite position estimates (meters) over approximately 5.5 orbits.
Satellite position estimates in an Earth-centered Earth-fixed Cartesian coordinate system.
RAX-2 was included in the recent article Smallsats on the Rise as an example of a small satellite that has made unprecedented scientific measurements. From the article:
Another example is the University of Michigan’s Radio Aurora Explorer 2 (RAX-2) satellite, a 3U CubeSat launched in October 2011 to study plasma irregularities in the ionosphere. ”It has made fundamentally new measurements. No one has ever made these measurements ever,”said Cutler, who led development of RAX. A National Science Foundation program specifically designed to support development of CubeSats for space weather and related research funded the satellite.