INSTRUMENT_HOST_DESC |
Instrument Host Overview
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For typical Radio Science experiments on other missions, the link
from the spacecraft to the DSN stations contains the primary science
content of the experiment, which makes the DSN a part of the
extended Radio Science instrument. For GRAIL this is partly true.
There are two data types, the primary is from the links between the
spacecraft, and those data become available via telemetry. The
second type is a one-way X-band Doppler link from each spacecraft to
the DSN stations. This science link supplements the LGRS on-board
observations and, again, make the DSN a part of the GRAIL science
instrumentation. The following sections provide an overview of the
spacecraft and their science instruments followed by the DSN ground
system.
Instrument Host Overview - Spacecraft
=====================================
Gravity Recovery and Interior Laboratory Spacecraft
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The GRAIL spacecraft design was based on the Lockheed Martin (LM)
Experimental Small Satellite-11 technology demonstration mission
for the United States Air Force, and the avionics were derived
from NASA's Mars Reconnaissance Orbiter. A single-string
architecture met this short mission's reliability requirements.
The resulting design met all GRAIL mission and science
requirements with ample technical margins, providing flexibility
to solve problems that might arise during development and which
met or exceeded design principles established by the Jet
Propulsion Laboratory.
Each of the two GRAIL spacecraft, GRAIL-A and GRAIL-B, was about
the size of a washing machine and had about 200 kg of mass. They
were nearly identical; but the need to point antennas at one
another required differences in the MoonKAM mounting and in the
angles of the star trackers used for attitude control and the
antennas through which the orbiters measured the changing
distance between them. These factors also required that GRAIL-B
precede GRAIL-A in lunar orbit.
Each spacecraft bus was a rectangular composite structure. The
science payload ranging antennas were in thermal enclosures and
were mounted so that they were nominally on the line between the
centers of mass of the two spacecraft. The other components of the
payload instrument were on a single interior bus panel for easy
integration and testing.
Two non-articulated solar arrays of XSS-11 heritage were deployed
just after separation from the launch vehicle. Warm gas systems,
identical to those on XSS-11, provided delta-V for maneuvers and
unloading of the 3 reaction wheels. Additional attitude sensing
components included an inertial measurement unit (IMU), a sun
sensor and a star tracker.
Command and Data Handling (C&DH), power management, and the
lithium ion battery also had XSS-11 heritage. The S-band
telecommunications sub-system for communication with the DSN used
components with heritage from Themis and Genesis.
The spacecraft was built and the science payload was integrated
and tested at Lockheed Martin's Denver facility. LM used two
system-level spacecraft test labs (STL) and one software simulator
(SoftSim) testbed with unlimited copies that enabled integration
and verification of all hardware and software throughout the
Assembly Test and Launch Operations (ATLO) cycle.
See [HOFFMAN2009] for more information about the spacecraft.
Spacecraft Configurations
-------------------------
During its mission, GRAIL needed to operate in four distinct
mission configurations.
Launch: During launch, the two spacecraft had to be fitted together
within the nose cone, or payload fairing, of the launch vehicle.
Large parts, such as the solar arrays, were designed to be folded
up.
Cruise: As soon as the two spacecraft were clear of the launch
vehicle, the body-fixed solar arrays were deployed to begin
producing power. The high-gain antenna also became operational.
Lunar Orbit Insertion: The lunar orbit insertion required numerous
maneuvers to circularize the two orbits. No science observations
took place, as the spacecraft were not in formation flying yet.
Science Operations: During the Science Phase, the two spacecraft
were placed in a precision formation flying configuration in order
to point to each other and exchange two radio links at Ka- and
S-bands.
Coordinate Systems
------------------
Two coordinate systems are used to reference the various GRAIL
instruments. The definitions are summarized below.
1) Mechanical Frame (MF): This is defined by the spacecraft
manufacturer. It is the reference frame for such things as KBR
horn location, center of mass, and thruster locations.
+X = Parallel to, and in opposite direction from, the solar array
normal vector
+Z = Normal to star tracker bus plate
+Y = +Z ? +X
An onboard attitude control sub-system approximately orients the
mechanical frame with -Z along the line of flight and -/+ Y
pointed towards the moon.
2) Science Reference Frame (SRF): This is the Mechanical Frame as
realized by the Star Tracker. If the Star Tracker were perfectly
aligned, MF would equal SRF. SRF is the reference frame for GRAIL
science measurements.
Major Spacecraft Components
---------------------------
Science Payload Instruments:
There are two payload elements on each GRAIL orbiter: the Lunar
Gravity Ranging System (LGRS) which is the science instrument, and
the MoonKAM lunar-imager which is used for Education and Public
Outreach. The LGRS is based on the instrument used for the Gravity
Recovery and Climate Experiment (GRACE) mission, which has been
mapping Earth's gravity since 2002. The LGRS is responsible for
sending and receiving the signals needed to accurately and
precisely measure the changes in range between the two orbiters.
The LGRS consists of an Ultra-Stable Oscillator (USO), Microwave
Assembly (MWA), a Time-Transfer Assembly (TTA), and the Gravity
Recovery Processor Assembly (GPA).
The USO provides a steady reference signal that is used by all of
the instrument subsystems. Within the LGRS, the USO provides the
reference frequency for the MWA and the TTA. The MWA converts the
USO reference signal to the Ka-band frequency, which is
transmitted to the other orbiter.
The function of the TTA is to provide a two-way time-transfer link
between the spacecraft to both synchronize and measure the clock
offset between the two LGRS clocks. The TTA generates an S-band
signal from the USO reference frequency and sends a GPS-like
ranging code to the other spacecraft. The GPA combines all the
inputs received from the MWA and TTA to produce the radiometric
data that are downlinked to the ground. In addition to acquiring
the inter-spacecraft measurements, the LGRS also provides a
one-way signal to the ground based on the USO, which is
transmitted via the X-band Radio Science Beacon (RSB). The
steady-state drift of the USO is measured via the one-way Doppler
data provided by the RSB.
The LGRS instrument is summarized below.
2 X-band beacon antennas for Doppler ranging measurements
when the spacecraft was visible from Earth. The X-band
signal was carrier-only and not
1 S-band time-transfer system antenna, which sent a
time-synchronization code back and forth between the
spacecraft
1 Ka-band ranging antenna for precision distance measurement
between the spacecraft
For more information on the GRAIL radio systems, see
[KLIPSTEINETAL2009].
Structures:
The solar panels and antennas were body fixed, so there were no
moving parts in the spacecraft structure that would affect science
observations.
Telecommunications Sub-System:
The telecom sub-system included the following on each spacecraft:
2 S-band transponder antennas to communicate with Earth
Each of the pairs of S-band transponder antennas had one antenna
mounted on the sunny side of the spacecraft and one mounted on the
dark side. The sunny-side antennas pointed to Earth during the
full moon and the dark-side antennas pointed to Earth during new
moon. This design obviated the need to mechanically rotate the
antennas during the mission, which would have moved the
spacecraft's center of mass with respect to the Ka-Band ranging
and X-Band beacon paths, disturbing the science measurements.
Propulsion Sub-System:
The propulsion sub-system on each spacecraft included:
A propellant tank, which could hold up to 103.5 kilograms of the
monopropellant hydrazine.
A hydrazine catalytic thruster for lunar-orbit insertion and
trajectory changes, and a warm-gas system with 8 thruster valves
for attitude control and other small maneuvers.
Command and Data-Handling Sub-System:
The C&DH sub-system controlled all spacecraft functions. This
system:
* managed all forms of data on the spacecraft;
* executed commands (including maneuver commands) sent from
Earth;
* prepared data for transmission to Earth;
* managed collection of solar power and charging of the
batteries;
* collected and processed information about all sub-systems
and payloads;
* kept and distributed the spacecraft time;
* calculated spacecraft position in orbit around the Moon;
* autonomously monitored and responded to any onboard
problems that occurred.
The key parts of this system were:
Space Flight Computer
Flight Software
Solid State Recorder
Attitude Control Sub-system:
The Attiude Control Sub-system (ACS) controlled the orientation of
the orbiter as it traveled through space and maintained knowledge
of where celestial bodies were located -- for example, Earth and
the Sun. This knowledge was critical for the spacecraft to perform
the correct maneuvers to get to the Moon, to keep its solar arrays
pointed toward the Sun for battery charging, and to keep its
S-Band antenna pointed toward the Earth in order to maintain
communications.
Once in orbit around the Moon, the ACS also maintained constant
knowledge of where the spacecraft was in its orbit.
The Attitude Control sub-system provided three-axis stabilized
control and consisted of a sun sensor, a star tracker, reaction
wheels, and an inertial measurement unit.
Electrical Power:
The electrical power sub-system was responsible for generating,
storing, and distributing power to the orbiter systems. The
electrical power system included two solar arrays and a lithium
ion battery. Each solar array was capable of producing 700 watts
at the end of the mission. The arrays were deployed shortly after
separation from the launch vehicle and remained fixed throughout
the mission. Each battery had a capacity of 30 amp-hours and was
used to provide power when the spacecraft was in the Moon's
shadow.
Solar panels: The only source of replenishable power is sunlight.
Solar panels are mounted one side of each orbiter and capable in a
body-fixed position.
Thermal Sub-Systems:
The thermal sub-system maintained the right temperatures in all
parts of the spacecraft. It employed several conduction- and
radiation-based techniques for thermal control. Its components
included:
Radiators
Surface coatings
Thermal blankets
Heaters
Instrument Host Overview - DSN
==============================
Radio Science investigations utilized instrumentation with elements
both on the spacecraft and at the NASA Deep Space Network (DSN).
Much of this was shared equipment, being used for routine
telecommunications as well as for Radio Science.
The Deep Space Network was a telecommunications facility managed by
the Jet Propulsion Laboratory of the California Institute of
Technology for the U.S. National Aeronautics and Space
Administration.
The primary function of the DSN was to provide two-way
communications between the Earth and spacecraft exploring the solar
system. To carry out this function the DSN was equipped with
high-power transmitters, low-noise amplifiers and receivers, and
appropriate monitoring and control systems.
The DSN consisted of three complexes situated at approximately
equally spaced longitudinal intervals around the globe at Goldstone
(near Barstow, California), Robledo (near Madrid, Spain), and
Tidbinbilla (near Canberra, Australia). Two of the complexes were
located in the northern hemisphere while the third was in the
southern hemisphere.
The network comprised four subnets, each of which included one
antenna at each complex. The four subnets were defined according to
the properties of their respective antennas: 70-m diameter, standard
34-m diameter, high-efficiency 34-m diameter, and 26-m diameter.
These DSN complexes, in conjunction with telecommunications
subsystems onboard planetary spacecraft, constituted the major
elements of instrumentation for radio science investigations.
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