LMRSat (Low Mass Radio Science Transponder Satellite)
The Communications, Tracking, and Radar Division at NASA/JPL (Jet Propulsion Laboratory) and the SSDL (Space and Systems Development Laboratory) at Stanford University are collaborating to fly a nanosat-class mission LMRSat. The LEO (Low Earth Orbit) mission hosts the JPL-developed LMRST payload on a 2U CubeSat bus. Note: The mission is also referred to as LMRST-Sat.
The objective of the LMRST payload is to provide a far-field source for calibration of the DSN (Deep Space Network) X-band equipment in the form of an integer turnaround X-band transponder with support for ranging modulation. The CubeSat bus, provided by SSDL, supplies power, structural support, and command and telemetry while on orbit. The CubeSat development and operations are conducted as a student project.
In addition to the payload functions, mission goals include space qualification of the LMRST, demonstration of nanosat capabilities and costs within NASA, and expansion of student-class projects toward eventual deep space missions. 1) 2) 3)
Figure 1: Illustration of the 2U nanosatellite with antennas deployed (image credit: SSDL, JPL)
LMRSat consists of a single scientific instrument supported by a spacecraft bus Each component - i.e. instrument (payload) and bus - is packaged into a 1U CubeSat form factor which are joined by a simple interface. Hence, the LMRSat configuration has a volume of about two liters and a total surface area of about 1000 cm2. Solar cells are mounted on the outside except where surface space is needed for antennas. The CubeSat bus contains a battery and charge control regulator that powers the spacecraft and the payload.
The LMRST payload is hosted on LMRSat by a 1U CubeSat Kit™ of Pumpkin Inc., San Francisco, CA. CubeSat Kits require customization for specific missions. For LMRSat, a payload to CubeSat Kit mechanical and electrical interface was specified by SSDL. In keeping with CubeSat standards, SSDL specified dimensions and external properties of LMRST and a mechanical attachment interface.
Figure 2: Exploded view of the LMRSat (image credit: SSDL, JPL)
Passive stabilization of the spacecraft: LMRSat is not actively stabilized and by design does not have pointing requirements. The nanosatellite is constructed with permanent magnets in the long axis and hysteresis material that, in orbit, synchronizes its tumble with Earth’s magnetic field and results in twice per orbit revolutions. This allows all faces to receive some solar exposure and some exposure to deep space for radiative cooling.
Figure 3: Orientation of LMRSat for several geographic latitudes (image credit: SSDL, JPL)
EPS (Electrical Power Subsystem): In a typical low Earth polar orbit, the spacecraft is capable of collecting and storing about 2.4 W of power, orbit-average. In a terminator orbit illuminated near full time, it will collect 3.7 to 5.0 W, orbit average, depending on the time of year.
The LMRST payload consumes 8-9 W when active, so it cannot be active continuously. The spacecraft bus computer, command receiver, and telemetry transmitter also have power needs which must be balanced against available resources. In the LMRSat mission design, the transponder payload is only activated a few minutes before a DSN site pass. Since this is a turn-around transponder without its own precision frequency reference, a long warm up or stabilizing time is not needed. In addition, the telemetry transmitter cannot be used at the same time as the payload since the two together draw more peak power than is available. The command receiver and spacecraft computer are always active, though the latter consumes very little power while in sleep mode, which is most of the time.
EPS features a 20 Whr CubeSat battery which provides nominally 8.3 VDC. The electrical interface also includes seven telemetry points to monitor LMRST performance. All telemetry sensors are implemented by JPL inside the LMRST. These read-only values are provided as voltages in the range 0.0 – 2.5 V DC to the CubeSat flight computer’s ADC (Analog to Digital Converter) interface.
OBC (On-Board Computer): A Texas Instruments 16-bit MSP430 ultra-low power microcontroller is utilized. The computer manages all aspects of satellite operation during ground testing and flight including:
- Controlled release of the antennas from their towed positions after ejection from the P-POD (Poly Picosat Orbital Deployer) system
- Reception of commands via UHF command receiver or USB interface
- Activation and de-activation of the payload
- Collection of telemetry from the payload once per second when active; storage to SD (Secure Digital) memory card; and relay to the ground via the VHF telemetry transmitter or USB (Universal Serial Bus) interface
- Power and battery management
- Other housekeeping as required.
During the mission, command and telemetry are handled via an Internet interface to SSDL’s VHF / UHF Earth station at Stanford University. With proper permissions, these activities can be conducted or monitored from anywhere on the World Wide Web.
RF communications: For both the UHF uplink and the VHF downlink, two orthogonal antenna elements are fed in quadrature giving a satisfactorily omni-directional radiation pattern. The LMRST transponder employs two patch antennas on opposite faces of the spacecraft that are also fed in quadrature. Some directions are not well covered by this arrangement, but much of the sphere is illuminated. The ranging accuracy requirement for the transponder is one meter, much larger than any dimension of the satellite or antenna system.
Figure 4: System diagram of the LMRSat (image credit: SSDL, NASA/JPL)
Figure 5: Photo of LMRSat with the antennas wrapped around the nanosatellite (image credit: SSDL, JPL)
Launch: LMRSat is a secondary payload on the CRS-3 (Cargo Resupply Services-3) flight provided by the SpaceX CRS-3 spacecraft scheduled for March 2014. It will be the fifth flight for SpaceX's uncrewed Dragon cargo spacecraft and the third SpaceX operational mission contracted to NASA under a Commercial Resupply Services contract. The launch vehicle is the Falcon-9v.1.1 of SpaceX and the site is Cape Canaveral, FL. 4) 5)
The primary payloads on this flight are OPALS (Optical PAyload for Lasercomm Science) and HDEV (High Definition Earth Viewing).
Orbit: Near-circular orbit, altitude of ~400 km to ISS, inclination =51.6°.
In addition to the primary payload, a Dragon cargo capsule resupply space transport mission to the ISS, the CRS-3 Falcon 9 mission will carry the following secondary payloads:
• All-Star/THEIA, a 3U CubeSat of COSGC (Colorado Space Grant Consortium), 2401.700 MHz
• Hermes-2, a 1U CubeSat of COSGC, 437.425 MHz
• KickSat, a femtosatellite of Cornell University, Ithaca, N.Y.
• LMRSat (Low Mass Radio Science Transponder Satellite), a 2U CubeSat of JPL (Jet Propulsion Laboratory)
• TechCube-1 (Technology Demonstration CubeSat-1), a 3U CubeSat of NASA/GSFC, Greenbelt, MD.
Sensor complement: (LMRST)
LMRST (Low Mass Radio Science Transponder):
X-band measurements performed at the DSN (Deep Space Network) suffer from lack of a far-field calibration source that is near enough to Earth to avoid most interplanetary plasmas. Low, medium, or high earth orbit (LEO, MEO, or HEO) are suitable locations for such a calibration source. 6) 7)
System L1 requirements:
• LMRST-Sat shall consist of a LMRST suitable for operation in LEO (Low Earth Orbit) hosted by a CubeSat bus.
• LMRST shall provide an X-band uplink, X-band downlink Doppler and ranging capability with 20 cm turn-around ranging precision.
• LMRST-Sat shall be positively controlled via the CubeSat bus via UHF TT&C.
In this mission, the orbiting LMRST locks to an uplink signal from a DSN X-band station and coherently retransmits it at another X-band frequency. The DSN receives this retransmission and by accurately measuring the Doppler and range may calculate transponder ephemerides and DSN instrumental offsets. The DSN receives this retransmission and by accurately measuring the Doppler and range may calculate transponder ephemerides and DSN instrumental offsets. The instrumental offsets obtained will be more precise than those available from presently available calibration techniques. This is the first purpose of LMRSat.
LMRST instrument: With this packaging into a 1U CubeSat form factor, an X-band version of the JPL-developed RSTI (Radio Science Transponder Instrument) has been renamed to LMRST. The instrument is an M/N turnaround transponder that receives on frequency “R” and retransmits coherently on frequency “R * M/N” where M and N are integers. The transponder also has the capability of locking to ranging tones present on the received carrier and coherently re-modulating them onto the transmitted carrier.
LMRST receives an uplink carrier in the vicinity of 7.2 GHz and can lock onto signals with a level of -110 dBm. The carrier is coherently regenerated with a frequency ratio of 880 / 749 and is retransmitted in the vicinity of 8.45 GHz at a power level of 20 dBm. The transponder bandwidth is sufficient to handle low earth orbit to ground Doppler shifts of ± 200 kHz.
Figure 6: Block diagram of the LMRST payload (image credit: NASA/JPL)
Figure 7: LMRST X/X-band module (image credit: NASA/JPL)
Figure 8: Photo of the X-band patch antenna (image credit: NASA/JPL)
Figure 9: LMRSat operations concept (image credit: NASA/JPL)
1) Courtney B. Duncan, Matthew S. Dennis, Andrew E. Kalman, Kevin Anand Stein, Yonas Tesfaye, Bryan I-Ming Lin, Eddie Truong-Cao, Cyrus Foster, “LMRSat: A Small, High Value-to-Cost Mission,” Proceedings of the 2010 IEEE Aerospace Conference, Big Sky, MT, USA, March 6-13, 2010
2) Courtney Duncan, “Low Mass Radio Science Transponder — Navigation Anywhere,” 1st Interplanetary CubeSat Workshop, Cambridge, MA, USA, May 30, 2012, URL: http://icubesat.files.wordpress.com/2012/06/icubesat-org-2012-c-3-2-_presentation_duncan_201206060133.pdf
3) C. B. Duncan, M. S. Dennis, A. E. Kalman, “Cubesat Mission: Low Mass Radio Science Transponder,” Poster, 2009, URL: http://www.astro.caltech.edu/~srk/MiniSat/KISS/09Poster_151_LMRST-Sat_approved.pdf
5) Patrick Blau, “Dragon SpX-3 Cargo Overview,” Spaceflight 101, URL: http://www.spaceflight101.com/dragon-spx-3-cargo-overview.html
6) Courtney Duncan, “Low Mass Radio Science Transponder – Navigation Anywhere,” 3rd Interplanetary CubeSat Workshop, Pasadena, CA, USA, May 27-28, 2012, URL of presentation: http://icubesat.files.wordpress.com/2012/06/icubesat-org-2012-c-3-2-_presentation_duncan_201206060133.pdf
7) Courtney Duncan, Fernando Aguirre, Eric Archer, Maxime Bize, “Low Mass Radio Science Transponder – Satellite TRL Raising Mission for LMRST, Solar System Navigation and Radio Science,” JPL Summer Intern CubeSat Symposium, Aug. 9, 2012, URL: http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/43018/1/12-3689_A1b.pdf
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.