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
courtney.duncan@jpl.nasa.gov
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 JPLdeveloped 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.
TABLE OF CONTENTS
1. INTRODUCTION ................................................................ 1
2. SINGLE INSTRUMENT....................................................... 2
3. SMALL SPACECRAFT CHALLENGES ............................... 2
4. LOW MASS RADIO SCIENCE TRANSPONDER .................. 4
5. CUBESAT .......................................................................... 4
6. INTEGRATED PHASE ONE RESULTS ................................ 5
7. THE FUTURE .................................................................... 6
8. POSSIBILITIES FOR THIS CLASS OF SPACECRAFT........... 7
REFERENCES........................................................................ 7
ACKNOWLEDGMENTS.......................................................... 7
BIOGRAPHY ......................................................................... 8
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. INTRODUCTION
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
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
1978-1-4244-3888-4/10/$25.00
2
©2010 IEEE.
IEEEAC paper #1228, Version 3, Updated October 31, 2009
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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.
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.
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.
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.
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
2
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.
Figure 1. Exploded View of LMRST-Sat
Figure 2. Orientation of LMRST-Sat for Several
Geographic Latitudes
Limited Resources
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.
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
3
CubeSat Kits require customization for specific missions.
For LMRST-Sat, a payload to CubeSat Kit mechanical and
electrical interface was specified by SSDL.
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.
Mechnical Interface
In keeping with CubeSat standards, SSDL specified
dimensions and external properties of LMRST and a
mechanical attachment interface.
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.
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.
Figure 4. CubeSat to LMRST Mechanical Interface
Electrical Interface
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.
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.
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]
The telemetry values are:
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- 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:
-
Controlled release of the antennas from their
stowed positions after ejection from the P-POD.
-
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 Card; and relay
to the ground via the VHF telemetry transmitter or
USB interface.
-
Power and battery management.
-
Other housekeeping as required.
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.
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.
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.
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.
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.
Figure 5. Ground Software Block Diagram.
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Figure 7. CubeSat and LMRST During Integration
Figure 9. LMRST-Sat
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]
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.
Using an SSDL-provided mobile ground station, LMRSTSat was commanded, the payload was activated and
deactivated, and telemetry was observed. While active,
LMRST was tested against various transponder performance
requirements.
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.
Figure 8. LMRST-Sat Before VHF / UHF Antenna
Deployment.
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8. POSSIBILITIES FOR THIS CLASS OF
SPACECRAFT
REFERENCES
[1] http://CubeSatkit.com/
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.
[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
Applications that have been flown on CubeSats or seriously
studied include:
-
Communications transponders and beacons from
0.007 to 24 GHz,
-
Cameras, pointed or “calculated opportunity,”
-
Reaction wheels for precise pointing and other
attitude control and determination systems,
-
GPS receivers for time and position,
-
Tethers,
-
Space qualification of new technologies, and
-
Biology experiments.
[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.
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.
SSDL
Dhackson Muthulingam, Joseph Johnson, Brian Thompson,
Dawn Wheeler, Nicolas Lee, Randy Lum, Steven Pifko, and
Mark Vallee.
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.
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BIOGRAPHY
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.
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