LARES (LAser RElativity Satellite)
LARES is a low-cost Italian geodynamic satellite mission (managed by ASI) with a short development time that will enable achieving important scientific goals in gravitational physics, fundamental physics and Earth sciences fields. The program is a collaboration between ASI (Agenzia Spaziale Italiana), INFN (Istituto Nazionale di Fisica Nucleare), Universita di Roma, and Universita di Lecce. 1) 2) 3)
- The measurement of ‘frame-dragging or the Lense-Thirring effect’ due to the Earth's angular momentum and a high precision test of the Earth's gravitomagnetic field.
- An improved, high precision, test of the inverse square law for very weak-field gravity and test of the equivalence principle
- A measurement of the general relativistic perigee precession of LARES and a high precision measurement of the corresponding combination of the PPN (Parametrized-Post-Newtonian) parameters beta and gamma. The PPN parameters beta and gamma test Einstein's theory of gravitation versus other metric theories of gravitation.
- Measurements and improved determinations in geodesy and geodynamics.
According to agreements with ESA, the LARES satellite will be the payload of the first qualification flight of the VEGA launcher planned for early 2012.
Frame dragging effect: In 1918, two Austrian physicists, Josef Lense (1890-1985) and Hans Thirring (1888-1976), derived from Einstein's equations of General Relativity the twisting of the fabric of space-time around a spinning object, in other words, rotating masses drag space-time around themselves as they rotate. Similarly, as the Earth rotates, it pulls spacetime in its vicinity and therefore will shift the orbits of satellites near the Earth. - This effect is called the Lense Thirring (LT) effect (Figure 1), a portion of the general frame dragging phenomenon in the field of gravitomagnetism, namely a tiny nodal precession that is indeed hard to measure.
Figure 1: Artist's view of the Lense-Thirring effect (image credit: LARES initiative)
The LARES mission concept was first presented in response to an ASI (Italian Space Agency) call for ideas issued in 1997. The LARES mission concept represents an improvement of the LAGEOS-3 project proposed in 1984 by I. Ciufolini. The LAGEOS (Laser Geodynamics Satellite) series was designed to be a passive long-lived satellite with a stable, well-defined orbit. As such, it acts as a reference point in inertial space. An international ground-based network of laser ranging stations is using the orbiting LAGEOS satellites as passive reflectors to obtain ranges to the satellite by precision laser echo-bounce techniques.
Since the position of the satellites is determined by some laser ranging stations with uncertainties of < 1 cm, there was a potential for measuring the 2 m per year drift of the nodes of LAGEOS due to the LT effect (Figure 2). However, uncertainties in gravitational and non-gravitational perturbations, on one single satellite, are bigger than the LT effect. Using a combination of satellites, it is possible to reduce those uncertainties at about 10% of the LT effect. The launch of the LARES satellite can significantly reduce those uncertainties.
The LAGEOS-1 (LAser GEOdynamics Satellite-1, launch May 4, 1976) and LAGEOS-2 (launch Oct. 23, 1992) missions in MEO (Medium Earth Orbit) of NASA and ASI represent the origin of international cooperative research in geodynamics.
Table 1: Some parameters of the LAGEOS missions
Figure 2: Frame dragging on LAGEOS-1 and -2 (image credit: LARES initiative)
The mutual interest of Italian and US scientists in the field geodynamics is also continued into the LARES project. In Sept. 2003, a USNA (United States Naval Academy) low-cost project proposal within SSP (Small Satellite Program) considered the LARES satellite to be flown on their proposed mission. However, no funding could be secured for the project. 11)
In 2004, an opportunity of a free launch for LARES came about on the planned maiden flight of the new European Vega launcher, offered by ESA. However, it turned out that a much lower orbit could be achieved (~1500 km) than the originally planned polar orbit for the mission of 12270 km. After a lot of orbital analysis, it turned out that the proposed orbit on the new Vega launcher would suffice as well the mission objectives of LARES (Ref. 1).
ASI approved funding of the LARES mission in February 2008. The INFN experiment was approved in September 2007. Already in 2004, INFN started to fund R&D for LARES in view of a future construction and launch of the satellite.
The PI (Principal Investigator) of the LARES mission is Ignazio Ciufolini of the University of Salento, Lecce, Italy. Various Italian and international institutions are involved in the collaborative LARES mission: INFN-LNF (National Institute of Nuclear Physics-Laboratori Nazionali di Frascati), Frascati, Italy; University of Rome Tor Vergata; CNR-IAC, Rome; University and INFN-Lecce; University of Rome, Sapienza; University of Maryland at College Park and of Baltimore County, MD, USA; NASA/GSFC, Greenbelt, MD, USA; UTA (University of Texas at Austin), TX, USA; US Naval Observatory, Washington DC, USA; ESA (European Space Agency); and the ILRS (International Laser Ranging Service) community.
In May 2009, NASA selected Erricos C. Pavlis as the US Co-PI at JCET (Joint Center for Earth Systems Technology) of the University of Maryland, Baltimore County (UMBC) for the LARES satellite mission.
The spacecraft prime contract was awarded to Carlo Gavazzi Space (CGS) with the support of numerous SMEs (Small and Medium-sized manufacturing Enterprises) located in Italy: Rheinmetall of Rome (RHI) for the separation mechanism, SAB (Societa Aerospaziale Benevento) of Benevento for the support structure, TEMIS of Milan for the telemetric system. 12) 13) 14)
Figure 3: Illustration of the LARES satellite (image credit: ASI)
The LARES System is supplied by two CGS-Saft Li-ion battery packs named ABU (Avionic and Harness Battery Unit) and SBU (Separation Battery Unit). SBU includes one module in a non-standard electrical configuration. The battery system is never recharged during flight. 15) 16)
- The ABU (called “system battery”) is composed of two battery modules and one connector support. The system battery electrical configuration is 8S2P.
- The SBU (called “separation battery”) includes one battery module and one connector support. Such a module is equipped with a non-standard PCB to realize a hot redounded 2S2P configuration.
Figure 4: Illustration of the LARES battery system (image credit: CGS)
The completely passive satellite is a dense tungsten alloy (THA-18N) sphere of 376 mm in diameter and a mass of ~ 400 kg (density of ~ 18 kg/cm3) covered with retroreflectors that allow the satellite's motion to be followed via SLR (Satellite Laser Ranging) from Earth. Once in orbit, LARES will be the known object with the highest mean density in the Solar System.
The surface of the sphere is covered by 92 CCRs (Corner Cube Reflectors) evenly distributed so that the signal strength is practically independent on satellite attitude. The LARES spacecraft, like its predecessor LAGEOS; has no protruding parts on the surface of the satellite to avoid the introduction of unknown effects on the satellite motion (due to drag). 17)
The LARES satellite is configured to conduct some of the solar energy to the dark side of the satellite. This should reduce the thermal gradients on the cubes (retroreflectors) and allow the thermal energy to be re-radiated more uniformly over the sphere, thereby reducing thermal thrusts on the spacecraft. The thermal NGPs (Non-Gravitational Perturbation) are proportional to the satellite area/mass ratio. 18) 19)
Figure 5: View of the CCR mounting scheme (image credit: INFN)
Launch: The LARES spacecraft was launched on February 13, 2012 on the maiden flight of the Vega launch vehicle of ESA (the Vega flight was designated as VV01); the launch site was Kourou in French Guiana. - The first Vega lifted off at 10:00 GMT from the new launch pad, and conducted a flawless qualification flight.
CubeSat passenger payloads: Although ESA's Education Office was providing 9 CubeSat positions on the maiden flight of Vega, only 7 CubeSats were confirmed as of December 2011 (Ref. 24). Not all universities that were were preselected for the launch opportunity in June 2008, were able to deliver their CubeSat and the requested documentation. Other CubeSat projects, like SwissCube and HiNCube, decided to be launched on commercial flights.
As shown in Figure 6, the LARES passive satellite is one of the elements of the LARES System. The LARES System also includes:
• SSEP (Separation Subsystem), which holds the LARES satellite during flight and deploys it once in orbit
• SSUP (Support Subsystem), which represents the main structure of the overall system
• A&H (Avionic and Harness Subsystem), which performs the data acquisition, telemetry, command and electrical power functions. The LARES battery packs are included in the Avionic and Harness Subsystem.
• The CubeSat dispensers containing the CubeSat microsatellites to be deployed in orbit
• ALMASat-1, an educational microsatellite of the University of Bologna to be deployed in orbit.
Figure 6: Accommodation of the various payloads on the LARES mission of Vega (image credit: CGS)
A special separation subsystem (SSEP) is being used for the transport and deployment of the LARES spacecraft in the launch process. Due to its large mass and compact size, the satellite cannot be positioned right at the launcher interface but must be located about one meter above it so that the position of its center of mass can be representative of that one of a more typical satellite. But the main problem is represented by the high level of acceleration during launch combined with the scientific requirements that allow only minimal impact on the satellite surface. 26) 27)
The LARES A&H has been conceived as a new generation avionic system for space transportation, self standing and highly independent module to be embarked on launch vehicle with negligible impacts on external interfaces. It is composed of:
• Acquisition & Processing Equipment, A&P/EQ
• Distribution & Separation Equipment, D&S/EQ
• Battery Pack Equipment, BAP/EQ
• Telemetry Equipment, LM/EQ
• Internal Camera for payload release monitoring
• External Camera for launch vehicle stages separations view.
The core part of the Avionics is the Acquisition & Processing Equipment. In order to properly support the launcher qualification, the basic functions of the A&H subsystem for the separation of the satellite and of the payload passengers have been enhanced to acquire the environmental data inside the fairing by the additional telemetry equipments. In particular, the A&P/EQ includes:
• PCSU (Processing Control and Storage Unit)
• VAU (Video Acquisition Unit)
• CAU (Conditioning and Acquisition Unit)
• FAU (Fast Acquisition Unit)
• DSU (Data Storage Unit).
Figure 7: Architecture of the LARES A&H subsystem (image credit: ASI, ESA)
The PCSU encapsulates telemetry streams from other boars on A&H/SS into a 1 Mbit/s CCSDS compliant PCM-NRZ-L format telemetry output to feed the RF transmitter. VAU acts as an independent acquisition system which communicates with PCSU for configuration parameters. It controls the video digitalizing processor and the hardware JPEG2000 codec. The video resolution and frame rate can be reduced so as to fulfil the remnant telemetry bandwidth after the sensor transmission in order to meet the requirement of a data rate transmission of 1 Mbit/s. The CAU/FAU boards acquire various types of sensor/transducers such as acoustic and aerodynamic pressures, heat flux density, shock acceleration (low and high frequency levels) and temperature.
Figure 8: Illustration of the LARES separation system (image credit: LARES initiative)
Figure 9: Photo of the LARES separation system (image credit: (image credit: LARES initiative)
Figure 10: Photo of LARES, ALMASat-1 and the seven CubeSats before encapsulation in fairing (image credit: ESA, Ref. 25)
Orbit of primary payload: Circular orbit, altitude of 1450 km x 1450 km, inclination = 69.5º, period = 114.7 minutes.
Note: Orbital analyshas shown that any orbit higher than 1300 km can be used for the LARES satellite. However the optimal one would be a supplementary orbit with respect to LAGEOS-1 (i.e. 6000 km altitude and 70º inclination). The Vega launcher capability on the first flight permits only an apogee of 1450 km.
Orbit of secondary payloads: Elliptical orbit, altitude of 354 km x 1450 km, inclination = 69.5º, orbital period = 103 minutes (14 revolutions/day), eccentricity = 0.075. About 75% of the orbit is in sunlight.
Figure 11: Photo of the payload (LARES, AlmaSat, CubeSats and P-POD) integration for the 1st Vega launch (image credit: ESA,Arianespace)
AVUM (Attitude and Vernier Upper Module):
The Vega launcher is a single body vehicle composed of three SRM (Solid Rocket Motor) stages, a liquid propulsion upper module, referred to as AVUM (Attitude and Vernier Upper Module), and a fairing. The three SRM stages perform the main scent phase while the fourth stage, the AVUM, compensates the solid propulsion performance scattering, circularizes the orbit and executes the final deorbiting maneuvers of the stage. AVUM is itself composed of a Propulsion Module and an Avionics Module which contains three subsystems: GNC (Guidance Navigation and Control), SAS (Electric Safeguard Subsystem), and TMS (Telemetry Subsystem)). The AVUM provides attitude control and axial thrust during the final phases of Vega’s flight to allow the correct orientation and orbit injection of multiple payloads.
The accurate insertion of the payload into the target orbit is accomplished by the Vega AVUM by means of RACS (Roll and Attitude Control System) and the LPS (Liquid Propulsion System). AVUM is capable to perform a complex orbital sequence, such as payload pointing, barbeque mode etc., in order to test the GNC algorithms design in terms of functionalities and performances.
The AVUM qualification flight (Table 3) is based on three boosts with the LARES satellite release between the second and third AVUM boost. After the third AVUM cut-off, the launcher reaches assigned orbit for the secondary payload release. After the de-orbiting phase, the AVUM system starts the passivation phase, characterized by the unused propellant depletion relevant to RACS, LPS, and inert gas tank.
Figure 12: View of the AVUM module and LPS (image credit: ESA)
Figure 13: Vega maiden flight trajectory (image credit: ASI, ESA)
After LARES separation, the AVUM and the remaining part of the LARES system will be injected into a disposal orbit, in compliance with space debris mitigation guidelines requiring a lifetime after the satellite operational phase, less than 25 years. The disposal orbit parameters were optimized in order to avoid collision between AVUM, the LARES satellite, and the secondary payloads. For the LARES satellite, whose orbital life in the so-called space debris protected region, is very much longer than 25 years, a request for waiver was issued by ASI and accepted by ESA.
Status of LARES mission:
• In the fall of 2012, almost all the stations are acquiring LARES and the first analysis are currently in progress. Due to the periodicity of some classical perturbations (few months periods) it is required to analyze the data for a longer period of time, in order to perform an accurate measurement of the Lense-Thirring effect. From the analysis of the first six months of data, it can be stated that the satellite is performing perfectly both in terms of signal returns and orbital stability, thanks to the good optical and structural design of the satellite. Furthermore from the ranging data of some stations it can be retrieved, with good accuracy, the spin rate of the satellite. 28)
• In March 2012, LARES has started its operational phase. First measurements have been made by the ASI Center in Matera, Italy. 29)
- As soon as a reliable set of LARES orbital elements was published on the ILRS site and several global network stations linked up with the satellite, the Moblas 5 site at Yarragadee in Australia was the first site to link up with the satellite. This was followed by others including the ASI MLRO (Matera Laser Ranging Observatory), which has observed over 20 passages with a ‘single shot’ rms precision of about 3 mm. LARES has a very efficient set of reflectors, this makes it an easy target for all global network stations.
However, long-term tracking data sets (at least several years) of observations are needed for analysis to accomplish the objectives of measuring the Lense-Thirring effect (also referred to as the frame-dragging effect) in Earth orbit, with the desired precision of 1%.
• The entire VV01 flight took place entirely in nominal conditions; in particular, the AVUM stage was correctly reignited three times: the first firing allowed the ballistic trajectory towards the LARES target orbit, the second firing allowed the circularization of the LARES orbit and the last one, located AVUM on an elliptical orbit with a perigee at 340 km. Just after reaching the perigee of the elliptical orbit, the secondary payloads, Almasat-1 and the seven Cubesats, were released in sequence with a time separation of ten seconds one from the other (Ref. 28).
The successful orbit insertion of LARES satellite was an exceptional positive reward for the risk of launching a real satellite with a high science potential return on a maiden flight.
As LARES orbits the Earth, laser beams are emitted from a number of ground stations around the Earth, the International Laser Ranging Service (ILRS), and reflected by the CCRs on LARES to the ground stations. The time delay between emission and arrival of the laser beam provides a measure of the round-trip distance to LARES, allowing a highly accurate orbit determination. Correcting for a number of effects, most importantly the deviation of the Earth gravitational field from an ideal sphere, yields the frame-dragging effect.
The fundamental idea of this experiment is based on two considerations:
• Position measurements of laser–ranged satellites, of the LAGEOS type, that are accurate enough to detect the very tiny effect due to the gravitomagnetic field: the Lense–Thirring precession
• To “cancel out” the enormous perturbations due to the non-sphericity of the Earth’s gravity field, a new satellite (LARES) is needed with an inclination supplementary to that of LAGEOS, and with the other orbital parameters, α and ε, nearly equal to those of LAGEOS..
Figure 14: Schematic view of LAGEOS-1, LAGEOS-2 and LARES in orbit for the measurement of frame dragging (image credit: INFN)
Figure 15: Illustration of the ILRS network (image credit: ILRS, Ref. 28)
The data analysis of the LARES mission will be conducted by the following institutions:
• University of Rome, La Sapienza, Rome, Italy
• INFN (National Institute of Nuclear Physics), of the University of Salento, Lecce, Italy
• JCET (Joint Center for Earth Systems Technology) at UMBC (University of Maryland, Baltimore County), MD, USA
• NASA/GSFC, Greenbelt, MD
• UTA (University of Texas, Austin)
• Helmholtz-Zentrum Potsdam, GFZ (GeoForschungsZentrum), Potsdam, Germany.
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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.