LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN






Author’s Instructions for the 2009 IEEE Aerospace Conference

LMRST-Sat: A Small, High Value-to-Cost Mission


Courtney B. Duncan, Matthew S. Dennis

Jet Propulsion Laboratory,

California Institute of Technology

4800 Oak Grove

Pasadena, California 91109 USA

818-354-8336

[email protected]


Andrew E. Kalman, Kevin Anand Stein,

Yonas Tesfaye, Bryan I-Ming Lin,

Eddie Truong-Cao, Cyrus Foster

Space and Systems Development Laboratory

Department of Aeronautics and Astronautics

Durand 377

Stanford University

Stanford, California 94306 USA


Abstract—The Communications, Tracking, and Radar Division at NASA's Jet Propulsion Laboratory (JPL) and the Space and Systems Development Lab (SSDL) at Stanford University are collaborating to fly a nanosat-class mission for costs usually associated with small technology development tasks, a few $100K. The mission hosts a JPL-developed Low Mass Radio Science Transponder (LMRST) on a university-class CubeSat bus as a satellite that occupies a total volume of two liters plus deployable antennas. In low earth orbit, the LMRST payload will provide a far-field source for calibration of 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. 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.

This paper describes the work completed thus far, "Phase One": development of the LMRST, satellite bus, and integrated testing; and outlines the work planned for "Phase Two": acceptance testing, launch, and operations. 1 2















Table of Contents



1. Introduction

Most spacecraft weigh hundreds of kilograms and consist of a variety of complex subsystems. In addition to several science instruments, sensors, or other payload equipment they will usually also have a propulsion system and fuel, a precision stabilization system, actuated appendages, power, communications, computers(s) and a spaceframe. Their size and complexity, their relative uniqueness, the costs and risks of launch, and the fact that a spacecraft must work for a long time but may never be repaired or maintained after launch, guarantees a steep price tag usually in the tens to hundreds of millions of dollars. While this expense is necessary and apparently unavoidable for the class of missions that characterize NASA’s Jet Propulsion Laboratory, there is an emerging class of space technologies for which the approach and mindset are so different and the costs so comparatively miniscule that they are overlooked or misunderstood by the current generation of spacecraft builders. “Microsats” or “Nanosats” offer the space exploration community previously unthinkable capabilities far out of scale with their relatively small cost. Lower cost enables smaller communities, such as university departments, to be players in the space exploration and utilization community. The innovations already emerging within this broader base are intriguing. It is expected that a revolution in tiny, versatile spacecraft will have some parallels with the electronics and digital revolutions of recent decades.

This paper describes the first of two phases of a nanosat demonstration project undertaken by JPL in partnership with the Space and Systems Development Laboratory at Stanford University under modest technology development funding. Costs for Phase One have been $168K. Phase Two and launch costs are expected to each cost a similar amount making the cost of the entire mission approximately $500K.

2. Single Instrument

The Instrument

LMRST-Sat consists of a single scientific instrument supported by a spacecraft bus, both in the CubeSat standard form factor. [1] [2]

An X-Band version of the JPL-developed Radio Science Transponder Instrument (RSTI) has been renamed to Low Mass Radio Science Transponder (LMRST) with this packaging. With either name, 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.

The Customer

X-Band measurements performed at the Deep Space Network (DSN) 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. 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 instrumental offsets obtained will be more precise than those available from presently available calibration techniques. This is the first purpose of LMRST-Sat

By restricting the mission to a single purpose development, costs are minimized and the project is tractable for a handful of workers. In this case, it is also useful as a training exercise for aerospace graduate students.

3. Small Spacecraft Challenges

Minimums

Even with a mission of limited scope, a self-contained spacecraft still has a certain set of minimum needs. The payload must be powered. Its orientation, if not controlled, must at least be estimated to some degree for operational purposes. A method of commanding and monitoring the instrument is required as is a means of limiting the operational and non-operational temperature of components.

On LMRST-Sat, these goals are accomplished by joining a 1U LMRST payload with a 1U CubeSat bus at a simple interface. The bus provides command, telemetry, and power.

The configuration has a volume of about two liters and a total surface area of about 1000 square centimeters. 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 payload.





LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Figure 1. Exploded View of LMRST-Sat

Limited Resources

In a low earth polar orbit with typical eclipsing of 35%, this configuration is capable of collecting and storing about 2.4 watts power, orbit-average. In a terminator orbit illuminated near full time, it will collect 3.7 to 5.0 watts, orbit average, depending on the time of year.

LMRST consumes 8-9 watts 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 LMRST-Sat 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.

Omnis

LMRST-Sat is not actively stabilized and by design does not have pointing requirements. CubeSats have been actively pointed, but the added equipment and complexity is not needed for this mission. The satellite 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.

LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Figure 2. Orientation of LMRST-Sat for Several Geographic Latitudes

The command and telemetry system accommodates this variable orientation through the use of omni-directional antennas. 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.

4. Low Mass Radio Science Transponder

JPL developed the RSTI to support resource-limited applications such as determination of the gravity field of Europa from a small platform in order to better analyze the structure of a possible sub-surface ocean, or improvement of the ephemeris and internal structure knowledge of a planet on a landed mission. RSTI has a mass of about one kilogram and consumes up to fifteen watts depending on configuration. It has been developed in both X-Band and Ka-Band versions.

While the trend in transponder development has been towards more and more capable, digital and software defined units, these are too expensive in terms of mass and power for such missions.

LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN Figure 3. LMRST Block Diagram


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. Transponder bandwidth is sufficient to handle low earth orbit to ground Doppler shifts of +/- 200 kHz.

The next step in the engineering development of the RSTI is a demonstration flight in space. This is the second purpose for LMRST-Sat. The X-Band RSTI was chosen for this demonstration since from earth orbit it can meet a real need of the X-Band portion of the DSN.

5. CubeSat

LMRST is hosted on LMRST-Sat by a 1U CubeSat Kit™ bus. The CubeSat concept and the CubeSat Kit family of products are extensively documented at the CubeSat and CubeSat Kit websites. [1] [2]

CubeSat Kits require customization for specific missions. For LMRST-Sat, a payload to CubeSat Kit mechanical and electrical interface was specified by SSDL.

Mechnical Interface

In keeping with CubeSat standards, SSDL specified dimensions and external properties of LMRST and a mechanical attachment interface.

LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Figure 4. CubeSat to LMRST Mechanical Interface

Electrical Interface

The 20 watt-hour CubeSat battery nominally provides 8.3 volts DC. LMRST agreed to add a power distribution board in order to operate from this voltage. SSDL provided a redundant switch capable of handling the expected 1.0 amp current drain of the payload. 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 volts DC to the CubeSat flight computer’s analog to digital converter (ADC) interface.

The telemetry values are:

- 8.3 volt DC raw battery voltage supplied,

- current supplied,

- temperature of X-Band final amplifier,

- temperature of crystal oscillator,

- AGC (proportional to uplink signal amplitude), - static phase error voltage (“lock detect”), and

- temperature of baseplate.


Computing Requirements

The flight computer in the CubeSat utilizes a Texas Instruments 16-bit MSP430 ultra-low power microcontroller. The computer manages all aspects of satellite operation during ground testing and flight including:

The satellite will not necessarily be in view of a ground station when it is time to activate the LMRST for a DSN pass or deactivate it after the pass. The flight computer is responsible for maintaining an on-board real-time clock and operating both transponder and command passes according to a preloaded schedule.

During the mission, command and telemetry are handled via a web 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.

LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Figure 5. Ground Software Block Diagram.



LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Figure 6. Vizon™ Telemetry Browser Interface



Antenna Deployment

After launch, CubeSats in 1U, 2U or 3U configurations are deployed from a container called a P‑POD (Poly Picosat Orbital Deployer [3]). Proper operation of the P-POD mechanism drives the requirements on dimensions and external properties of all CubeSats. VHF and UHF antennas are too large to fit into the P-POD, so they are wrapped around LMRST-Sat and deployed after the satellite is released into space. The flight computer’s first job is to deploy the antennas and begin transmitting “aliveness” telemetry while listening for commands from the ground.

Pre-deployment, these antennas are held in place by fishing line which is in contact with a nichrome wire. When the CubeSat is ready to deploy the antennas, it uses a switch similar to the payload activation switch to pass sufficient current through the nichrome wire to heat it to a temperature that will melt the fishing line. When the lines melt the antennas spring into place under their own straightening force.

6. Integrated Phase One Results

On May 27, 2009, the teams from SSDL and JPL met to integrate LMRST with the CubeSat and perform integrated testing. Up to that time, SSDL had been working with an empty LMRST chassis provided by JPL for mechanical fit checking, simulated transponder electrical loading, and assembly practice.



LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Figure 7. CubeSat and LMRST During Integration



A successful VHF / UHF antenna deployment demonstration was performed followed by de-integration with the fit check chassis and integration with the actual LMRST. [5]

Using an SSDL-provided mobile ground station, LMRST-Sat was commanded, the payload was activated and deactivated, and telemetry was observed. While active, LMRST was tested against various transponder performance requirements.





LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Figure 8. LMRST-Sat Before VHF / UHF Antenna Deployment.



LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Figure 9. LMRST-Sat



In Phase One, X-Band patch antennas for LMRST and solar cells were not installed. The side panels pictured in Figure 9 are fitted with accurately sized placeholders for these components. LMRST testing was conducted with coaxial cables, not in a radiated environment but the CubeSat command and telemetry system was tested both over the air and over the USB interface.

7. The Future

Phase One Summary

In Phase One, the work reported here, an RSTI was repackaged into a CubeSat 1U form-factor with mechanical and electrical interfaces appropriate to mating with a 1U CubeSat bus. The two halves were integrated into a functional satellite, save the solar panels and X-Band patch antennas. The integrated satellite was successfully tested demonstrating basic functionality.

Phase Two Plans

Funding for Phase Two is still pending. During Phase Two, the side panels will be upgraded with real solar panels and X-Band patches. A non-flight test battery will be replaced with a flight battery and the re-integrated stack will be environmentally tested (vibration, thermal vacuum) to CubeSat standards for spaceflight.

Phase Two also contains the post launch operational demonstration where JPL, coordinating with the DSN, and SSDL, will collaborate to activate the LMRST over DSN sites, providing the desired X-Band calibration signal relay. At a minimum, several calibration demonstrations will be performed over a few months.

8. Possibilities for this Class of Spacecraft

Space applications that can be contained in one to three liters and can operate for a few watts of continuous power (or more non-continuously as in the case of LMRST-Sat) can fly in space in a CubeSat stack such as the one described here. There are also other competing small satellite standards emerging in industry that may prove to be similarly enabling.

Applications that have been flown on CubeSats or seriously studied include:

Navigation transponders such as LMRST or beacons utilizing ultra stable oscillators (USO) could be part of a system to perform atmospheric or gravitational science at solar system bodies other than earth and can also provide the data used to determine ephemerides and geological structure of such bodies.

Launchers from converted ICBMs to standard expendable launch vehicles have been fitted with P-PODS for the purpose of carrying CubeSats into space and deploying them. Space-X’s Dragonlab [4] has ample capacity for numerous P-PODs.

Moving CubeSats into Deep Space

It is possible that this or some future CubeSat form-factor mission will fly beyond earth orbit. In order to do this, certain enhancements or hardenings of the CubeSat design may be required, such as radiation hardening. For truly deep space applications, command and telemetry systems will need to be improved. Modest ground stations will not be adequate at current data rates, although techniques for increasing the range of a modest station by decreasing the data rates can be employed. Also, some groups [6] have employed government surplus ground station equipment to extend their command and telemetry capabilities beyond earth orbit.

References

[1] http://CubeSatkit.com/

[2] http://www.CubeSat.org/

[3] http://CubeSat.atl.calpoly.edu/media/Documents/Launch%20Providers/ppod_mk1_icd.pdf

[4] http://www.spacex.com/dragon.php

[5] http://www.youtube.com/watch?v=2T-OvYl9_Cs

[6] http://www.veron.nl/actueel/downloads/VenusBounce.pdf

Acknowledgments

This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

The authors wish to acknowledge the following students and co-workers who made significant contributions to the LMRST-Sat work in Phase One.

JPL

Bill Folkner, Narayan Mysoor, Fernando Aquirre, Amy Boas, Mike Settember, and Armond Matevosian.

SSDL

Dhackson Muthulingam, Joseph Johnson, Brian Thompson, Dawn Wheeler, Nicolas Lee, Randy Lum, Steven Pifko, and Mark Vallee.























Biography

LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN LMRSTSAT A SMALL HIGH VALUETOCOST MISSION COURTNEY B DUNCAN

Courtney Duncan has been involved in small satellites and ground operations for over thirty years. As AMSAT-North

America Vice President for Operations 1988-1991, he instituted and managed a worldwide volunteer command station

organization for four amateur radio Microsats. These Microsats, produced for about $300,000 each with significant donated engineering labor, are direct ancestors of today’s university CubeSats. He contributed real-time, embedded software to determine orbit, time, and coarse attitude, and to autonomously schedule the GPS atmospheric sounding science on GPS-MET, an instrument on the Orbital Sciences MicroLab-1. He was Instrument Manager for the Black Jack GPS Receivers flown on the Shuttle Radar Topography Mission (SRTM, STS-99), overseeing development, qualification, and delivery of flight hardware and software. Currently he works as a system engineer on the GRAIL lunar gravity mission.


Andrew Kalman is Pumpkin's Chief Technical Officer and the Director of Stanford University’s SSDL. He entered Silicon Valley in the mid-1980's and ended up co-founding a successful pro audio company. In 1995 he founded Pumpkin with an emphasis on software quality, performance and applicability to a wide range of microcontroller-based applications. In 2000 he created the CubeSat Kit in response to Professor Bob Twiggs' desire to provide a rapid design & development platform for CubeSat missions. Andrew’s areas of expertise include embedded hardware/software co-design, design for manufacturability, multiprocessor systems design and real-time operating systems. His current interests include the application of commodity components to the design of low-cost, rapidly deployable small satellites.



1978-1-4244-3888-4/10/$25.00 ©2010 IEEE.

2 IEEEAC paper #1228, Version 3, Updated October 31, 2009

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