PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2012-08-31, D. Kahan, initial; 2012-09-05, J. Ward, minor edits; 2012-09-19, R. Simpson, edits; 2012-11-29, D. Kahan, peer review revisions; 2012-11-30, J. Ward, formatting; 2013-08-22, D. Kahan, RDR peer review revisions." RECORD_TYPE = STREAM OBJECT = INSTRUMENT_HOST INSTRUMENT_HOST_ID = "GRAIL-A" OBJECT = INSTRUMENT_HOST_INFORMATION INSTRUMENT_HOST_NAME = "GRAVITY RECOVERY AND INTERIOR LABORATORY A" INSTRUMENT_HOST_TYPE = "SPACECRAFT" INSTRUMENT_HOST_DESC = " Instrument Host Overview ========================= 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 --------------------------------------------------- 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. " END_OBJECT = INSTRUMENT_HOST_INFORMATION OBJECT = INSTRUMENT_HOST_REFERENCE_INFO REFERENCE_KEY_ID = "HOFFMAN2009" END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO OBJECT = INSTRUMENT_HOST_REFERENCE_INFO REFERENCE_KEY_ID = "KLIPSTEINETAL2009" END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO END_OBJECT = INSTRUMENT_HOST END