Minimize LARES

LARES (LAser RElativity Satellite)

Overview    Spacecraft    Launch   Mission Status    References

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), Università di Roma, and Università di Lecce. 1) 2) 3)

The scientific objective is to test a prediction following from Einstein's theory of General Relativity, in particular: 4) 5) 6) 7)

- 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.

 

Background:

Frame dragging effect: In 1918, two Austrian physicists, Josef Lense (1890‐1985) and Hans Thirring (1888‐1976), derived from Einstein's equations of GR (General Relativity) the twisting of the fabric of spacetime around a spinning object, in other words, rotating masses drag spacetime 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 LTE (Lense Thirring Effect) as shown in 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.

The orbital plane of a drag-free satellite (Figure 2, left) or of a perfect test particle (i.e. a body unaffected by non gravitational perturbations) with a motion under a central force is a kind of huge gyroscope, and thus it determines the axis of an inertial frame of reference. If the central spherically symmetric body is rotating it does not produce gravitational waves (like a charged rotating sphere does not produce electromagnetic waves), but it will produce an additional contribution to the spacetime metric and the gravitational field which, in the slow motion and weak field approximation, is formally similar to the magnetic field produced by an electric current, like the one of the charged rotating sphere. Because of this formal analogy the rotation of the Earth is said to produce a gravitomagnetic field. This field causes the rotation of the orbital plane and of the node of a satellite in the same direction of the Earth's rotation (Figure 2, right). This particular effect is called the LTE (Lense-Thirring Effect).

 

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Figure 1: Artist's view of the Lense-Thirring Effect (image credit: LARES initiative)

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Figure 2: Frame dragging (left): The rotation of the inertial reference frame is a few tens of marcsec/year in low Earth orbit. Lense-Thirring Effect (right): The shift of the node is about 0.1 arcsec/year for LARES (image credit: (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 3). 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.

Mission

Launch date

Orbit

Spacecraft mass

LAGEOS-1 (NASA)

May 4, 1976

5858 km x 5958 km
Inclination = 109.8º

406.965 kg

LAGEOS-2 (NASA, ASI)

Oct. 22, 1992

5616 km x 5950 km
Inclination = 52.6º

405.38 kg

Table 1: Some parameters of the LAGEOS missions

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Figure 3: Frame dragging on LAGEOS-1 and -2 (image credit: LARES initiative)

In 1998, the LARES experiment was proposed and selected as a phase-A study by ASI, the Italian Space Agency. 8) 9) 10)

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.

 


 

Spacecraft:

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 (Società Aerospaziale Benevento) of Benevento for the support structure, TEMIS of Milan for the telemetric system. 12) 13) 14) 15) 16)

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Figure 4: Photo of the LARES satellite (image credit: ASI)

The design of LARES minimizes the effects of non-gravitational perturbations. That means the satellite basically moves only under the effect of gravity thus allowing us to study the gravitational laws of physics as well as the terrestrial gravity field that is very important for geodesy and geodynamics. That has been achieved by designing the spherical satellite in one single piece with 92 cavities for hosting the CCRs (Cube Corner Reflectors). The small circles, visible in Figure 5 are the front faces of the CCRs.

The material chosen for the satellite was a tungsten alloy characterized by a very high density: 18,000 kg/m3. The tungsten alloy, never used for structural components of a satellite, makes LARES the densest orbiting object in the solar system but above all this characteristics makes LARES the artificial orbiting object with the smallest surface-to-mass ratio. This quantity is proportional to the acceleration produced on the satellite by the non gravitational perturbations acting on the surface of the satellite. Shortly LARES, thanks to this design choice is the best test particle ever manufactured and inserted in orbit.

The choice of using a tungsten alloy resulted in several manufacturing challenges: maintaining strict mechanical tolerances, typically used for aluminum alloy (density of 2,700 kg/m3). Manufacturing the screws of the CCR mounting system and manufacturing the hemispherical cavities to interface the separation system, to mention just a few.

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Figure 5: Illustration of the LARES satellite assembly (image credit: LARES collaboration, Ref. 34)

In Figure 5 one can see the CCR and CCR mounting system with the three tungsten alloy screws. In the bottom part, labeled with "SEP - interface hole", is one of the four hemispherical cavities mentioned above that will be engaged by the separation system brackets (Figure 6). The other four equatorial cavities are threaded holes used for handling and are labeled "EHH", these are closed with "EHH – Caps" after final assembling before the launch.

The satellite is manufactured out of a single piece of tungsten alloy. With the conventional casting technique, it is not possible to manufacture a sphere without defects. The material provider has used liquid phase sintering in which small particles of tungsten are surrounded by 5% of Ni-Cu matrix. The tungsten particles were so evenly distributed that the center of mass of the satellite was within 0.2 mm from the geometric center of the satellite. The separation system was designed specifically for LARES and special tests were devised for testing the interface and the separation. Of special relevance was the accuracy required to machine the hemispherical cavities. In fact the separation system brackets pushed on the cavity with about 27,000 N and the pressures developed at the contact area approached the admissible strength of the material. No minimum defects were acceptable and very tight tolerances were required (Ref. 34).

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Figure 6: LARES satellite on the separation system (image credit: 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. 17) 18)

- 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.

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Figure 7: 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). 19)

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. 20) 21)

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Figure 8: 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.

Secondary educational payloads of this flight were: 7 CubeSats and 1 microsatellite, ALMASat, of the University of Bologna, Italy, selected by the ESA Education Office. 22) 23) 24)

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. 26). 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.

Xatcobeo (a collaboration of the University of Vigo and INTA, Spain): a mission to demonstrate software-defined radio and solar panel deployment

Robusta (University of Montpellier 2, France): a mission to test and evaluate radiation effects (low dose rate) on bipolar transistor electronic components

e-st@r (Politecnico di Torino, Italy): demonstration of an active 3-axis Attitude Determination and Control system including an inertial measurement unit

Goliat (University of Bucharest, Romania): imaging of the Earth surface using a digital camera and in-situ measurement of radiation dose and micrometeoroid flux

PW-Sat (Warsaw University of Technology, Poland): a mission to test a deployable atmospheric drag augmentation device for de-orbiting CubeSats

MaSat-1 (Budapest University of Technology and Economics, Hungary): a mission to demonstrate various spacecraft avionics, including a power conditioning system, transceiver and on-board data handling.

UniCubeSat GG (Universitá di Roma ‘La Sapienza', Italy): the main mission payload concerns the study of the gravity gradient (GG) enhanced by the presence of a deployable boom.

Table 2: Overview of the CubeSat passenger payloads flown on the Vega-1 mission 25) 26) 27)

 

As shown in Figure 9, 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.

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Figure 9: 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. 28) 29)

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

• Sensors

• 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).

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Figure 10: 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.

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Figure 11: Illustration of the LARES separation system (image credit: LARES initiative)

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Figure 12: Photo of the LARES separation system (image credit: (image credit: LARES initiative)

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Figure 13: Photo of LARES, ALMASat-1 and the seven CubeSats before encapsulation in fairing (image credit: ESA, Ref. 27)

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.

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Figure 14: 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.

Phase of flight

Orbital maneuver

1st AVUM cutoff

The spacecraft reached the Transfer Orbit conditions as required by the optimization process

Long coasting

Barbecue mode (roll around the AVUM longitudinal axis)

2nd AVUM firing

Reached the final orbit (1450 km x 1450 km, i=69.5º)

Pointing maneuver

Satellite spin-up to 5 rpm to demonstrate the GNC capability

LARES satellite separation

LARES system separation in the opposite direction of the orbital velocity, commanded by 3 pairs of dry-loop signals sent by LV linked to the different flight events. The differential velocity between LARES and the AVUM after the separation shall be 0.75 ± 20% m/s.

Despin phase

Despinning of the AVUM stage

CCAM

Collision and Contamination Avoidance Maneuver

3rd AVUM firing

Reached the orbital conditions to meet the space debris mitigation requirements for the indirect re-entry. by decrease the perigee altitude, to reach the required disposal orbit (1450 km x 304 km, i=69.5º).

Release of microsatellites

Release of secondary payloads, driven by the LARES A&H using temporized actuation signals.

Passivation phase

Passivation in sequential order: RACS, LPS, and gas tank. The maximum designed-duration of this phase is about 700 s.

Table 3: AVUM qualification flight phases (Ref. 14)

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Figure 15: View of the AVUM module and LPS (image credit: ESA)

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Figure 16: 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:

• September 2016: After four years in orbit, analysis of the SLR (Satellite Laser Ranging) data to LARES in combination with the SLR data from the LAGEOS 1 and 2 missions show an improvement in the measurement of Earth's frame-dragging. The frame-dragging phenomenon, predicted by General Relativity and induced by Earth's rotation, twists the space-time fabric and drags inertial frames with it. The LARES mission experiment aims at improving the previous measurement obtained with the two LAGEOS satellites by almost one order of magnitude. The SLR data are collected by the ILRS (International Laser Ranging Service) network. 30)

- The main objective of the LARES mission is to improve the accuracy in the measurement of the LT (Lense-Thirring) effect, a particular manifestation of frame-dragging, predicted by the theory of General Relativity. Currents of mass-energy produce a space-time deformation. In the case of the Earth, its rotation affects the orbital motion of the satellite that can be detected by looking at the motion of the node of the satellite orbit. This effect vanishes with the third power of distance so that it is higher for LARES, which orbits at 1450 km, than for the two LAGEOS satellites which orbit at about 6000 km altitude (Figure 17).

- The effect produces a displacement of the node of the order of few meters/year. We then expect to reduce the uncertainty on the measurement of the LT effect to a few cm/year. The measurement technique used, satellite laser ranging, is able to reach that accuracy, but the main problem are the non-gravitational perturbations and the deviation of the gravitational field from a perfect spherical field. Both need to be modelled with high accuracy. Using a combination of orbital parameters from LAGEOS 1, LAGEOS 2 and LARES, along with the accurate determination of the Earth gravitational field based mainly on the GRACE mission results, it is possible to eliminate the effect on the node motion of J2 and J4 (the first two even zonal harmonics). Consequently, there remain only the effects on the node of the higher order even zonal harmonics whose contribution to the total measurement uncertainty is estimated to be at few percent level. The nongravitational perturbations are dealt with by an optimal design of the satellite and careful modelling. Other objectives of the LARES mission are in the field of geodesy, geodynamics and environmental monitoring.

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Figure 17: Node shift due to Earth frame-dragging: 118.4 milliarcsec/y on LARES (green, longer arrow) and 30.7 milliarcsec/y on LAGEOS satellites (red, shorter arrow), image credit: LARES collaboration

Results: The combination of the two LAGEOS and LARES satellite data allows the project team to eliminate the static effects due to errors in J2 and J4, and the J2 and J4 tides. By further filtering of the effects of first six most important tides on the three satellite nodes, one obtains the secular trend due to the LT effect. 31)

µ = (0.994 ± 0.002) ± 0.05

where µ is the frame-dragging parameter: a value of 1 corresponds to the General Relativity prediction. The results show an improved agreement with respect to what was obtained with the use of the two LAGEOS satellites only. The addition of LARES data has an impressive impact on the reduction of the formal 1σ error that stands now at only 0.2%. The analysis of the estimate of the systematic errors, amounting to 5%, can be found in (Ref. 31); we consider this estimate to be conservative. The overall error on the LT effect measurement is reduced to about 5%, a factor-of-two improvement over previous analyses using LAGEOS and LAGEOS 2, without a contribution from LARES, in spite of the fact that we have only about four years of LARES data. Figure 18 illustrates the signal of interest, which is the node displacement due to General Relativity as a function of time. It is easy to see that the node drift increases linearly with time.

The next few years will allow a further reduction of the error because of the availability of further improved gravitational field models, and because the longer time series will help to better separate the linear signal from the various periodic effects such as tides or other time variable gravity variations. The new analysis will also benefit from the fact that the tracking station positions and velocities will come from an improved realization of the International Terrestrial Reference Frame, ITRF2014

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Figure 18: Lense-Thirring effect, experimental results (image credit: LARES collaboration) 32)

In summary, the overall error achieved, including systematic errors, is about 5%, thus we obtained a factor two improvement over the previous tests using the LAGEOS satellites only. A few more years of data analysis are required to eliminate the effects of the uncertainties of some periodic perturbations of the node of LARES and LAGEOS satellites. This, together with the future improved measurements of the Earth gravitational field, will allow further improvements of the test of frame-dragging (Ref. 30).

• October 2015: The surface of the satellite is covered with 92 Cube Corner Reflectors (CCRs) that allow its precise positioning through the measurements of the ILRS (International Laser Ranging Service). By measuring the time of flight of the laser pulses sent towards the satellite it is possible to reach ranging accuracies of few millimeters from the best stations. LARES is passive and as such it does not have thermal control. Thermal deformations of the CCRs can be calculated if power input, boundary conditions and thermal heat transfer parameters are known. The reflecting performances of CCRs are typically evaluated through the analysis of the FFDP (Far Field Diffraction Pattern) which provides information on the energy distribution, of the reflected laser pulse, on the ground. The CCR deformations can change the FFDP thus reducing the probability to have good laser returns to the station. Due to its particular CCR mounting system, that minimizes contact with the CCR, heat transfer of the CCR is mainly governed by radiation. It is therefore important to evaluate experimentally the solar absorptivity αs and the infrared emissivity ε. 33)

• March 2015: The LARES mission objective is to measure the LTE (Lense-Thirring Effect) with an accuracy of about 1% which is an improvement of one order of magnitude better than the previous measurements. It is worth noting that the objectives are not limited to testing general relativity, because LARES will give significant contributions to geodesy and geodynamics as well. 34) 35) 36)

- LARES is being tracked by the ILRS (International Laser Ranging Service). Operations associated with each mission are the responsibility of the satellite owner or scientific mission project office. The main operations performed for the LARES mission are the orbital predictions and the ranging measurements. The orbital predictions are performed by accredited centers that are in charge of providing the ILRS with the position of the satellite as a function of time. In the case of the LARES satellite, ISTARC (International Space Time Analysis Research Center), located in Sapienza University of Rome (1) generates tracking predictions that are delivered to the ILRS Operations Center and (2) performs a precise data analysis for orbital determination of LARES using the data collected from the entire network. This second point is not strictly related to satellite operations, but is more relevant to the scientific exploitation. 37)

- ISTARC is providing at an almost daily basis the orbital prediction for LARES to ILRS. Those predictions are prepared in semi-automatic procedures and are basically obtained by propagating the orbit of LARES starting from initial conditions determined using the actual laser ranging data. The high rate at which those predictions are provided is due to the high pointing accuracy required by the laser stations: the predictions are affected by small modeling errors that introduce small unmodeled accelerations that, after a few days, bring the satellite off the predicted track by a few meters (Figure 19). As can be seen, the deviations for LARES are very small; however, the actual deviations are bigger than that reported because, in Figure 19, all the modeled effects due to non-gravitational perturbations have been removed. This, together with other preliminary results, show the very good behavior of LARES as a test particle thus proving the good design of the satellite.

- Fifty-six operating stations, organized under the ILRS, provide accurate ranging data as normal points that are readily available to the scientists. In particular, the LAGEOS and LARES data are being processed at the ISTARC in Rome to improve the accuracy of the Lense-Thirring effect predicted by general relativity down to the level of about 1%.

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Figure 19: Estimated deviation of satellite orbit from the ideal behavior of a perfect test particle (vertical axis). The values in the horizontal axis are the along-track deviation from a theoretical geodesic of space-time (image credit: LARES collaboration)

- SLR (Satellite Laser Ranging): Extremely short (~10-300 ps) laser pulses are transmitted at various rates (10 Hz to 10 kHz) through a telescope toward the targets. An extremely accurate counter or an event timer allows to measure the round-trip time of flight with extreme accuracy thus providing single-shot distance measurements with an accuracy of a few millimeters. Laser ranging data acquired at each station are initially preprocessed locally at the stations, forming the so-called NPs (Normal Points), by averaging the data over pre-specified time intervals determined on the basis of the altitude of the targets, to suppress random noise while preserving any dynamic signals. The process reduces by several orders of magnitude the number of data to be analyzed later on, and improves the precision of the NPs to about 1 mm or less. The SLR data collected by the ILRS network are subsequently used by analysis groups for POD (Precision Orbit Determination) analysis, geophysical parameter estimation, experimental tests, the study of satellite dynamics, etc. The data and products of these analyses are distributed to ILRS researchers through the two data centers: 38)

• CDDIS (Crustal Dynamics Data Information Center) at NASA/GSFC (http://cddis.nasa.gov/)

• EDC (Eurolas Data Center) at DGFI in Munich (http://edc.dgfi.badw.de/en/)

The main products are full rate data, i.e. the single laser pulse time of flight, and the NPs. The NP data are in fact the most commonly used by scientists in their studies. The orbital predictions are produced starting from the past satellite orbits based on the NP rather than the full-rate data. An example of the quality of the predictions is demonstrated in Figure 20 where it is shown the maximum error being less than a meter after seven days.

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Figure 20: LARES orbital predictions accuracy (image credit: LARES collaboration, Ref. 37)

- Environmental monitoring with geodetic satellites: The NPs are used to measure the kinematics and long-term dynamics of the solid Earth, oceans and atmosphere through the estimation of EOP (Earth Orientation Parameters), gravity model parameters, station positions and velocities, etc. Figure 21 illustrates the daily differences of the polar motion and LOD (Length Of Day) series obtained using the data from the LAGEOS satellites with respect to the official IERS (International Earth Rotation Service) series derived primarily from GPS data (Ref. 38).

- In Figure 22 and Figure 23 the graphs show the weekly variations of the instantaneous Earth center of mass with respect to the conventional origin of the tracking network and the weekly variation in scale, all being the result of analyses of the LAGEOS satellites SLR data.

- The time series of the Earth center of mass (geocenter) are dominated by a strong annual signal that is climate driven, the change of seasons between the two hemispheres. There is also a much smaller semiannual and seasonal (quadrennial) signal. The annual amplitude for the X and Y components is about 3 mm, while the much more strongly affected Z component is closer to 5 mm. In addition to these periodic signals, transients at various times are the effect of other environmental phenomena such El Nino/La Nina events, excessive melting of icecaps over the Arctic Ocean, Antarctica and Greenland, etc. The same phenomena are also responsible for apparent scale variations as noted in Figure 23.

LARES_Auto4

Figure 21: Daily EOP differences of the SLR-derived series from the official IERS series over the past two decades (image credit: LARES collaboration)

LARES_Auto3

Figure 22: Weekly SLR-derived series of the coordinates of Earths center of mass over the past two decades (image credit: LARES collaboration)

LARES_Auto2

Figure 23: Weekly SLR-derived series of the network scale over the past two decades (image credit: LARES collaboration, Ref. 38)

• October 2014: The preliminary data analysis of LARES laser ranging has shown that LARES, once its non-gravitational perturbations are accounted for, is the best test particle available in the solar system. That is due to its particular design that provides the satellite with no protruding parts and with a surface-to-mass ratio smaller than that of any other artificial orbiting object. The orbital determination obtained with the laser ranging data has shown that LARES is subject to the smallest residual accelerations of any orbiting body in the solar system. This characteristic along with the good quality of the reflected laser returns proves the goodness of the design and the value of the complex thermovacuum tests performed on the CCRs (Corner Cube Reflectors). 39)

- This experimental determination and simulations performed in the recent past, strongly support the possibility of reaching the accuracy of about 1% in the measurement of the Lense-Thirring effect predicted by General Relativity. This achievement will be possible with both the combined use of LARES and the two LAGEOS satellites and the improved accuracy of the Earth gravitational field with the GRACE mission.

Satellite

Perigee/apogee

Inclination

Eccentricity

Residual acceleration

LAGEOS-1

5858/5970 km

109.8º

0.0045

1-2 pm/s2

LAGEOS-2

5616/5950 km

52.6º

0.0135

1-2 pm/s2

LARES

1437/1451 km

69.49º

0.0009

0.4 pm/s2

STARLETTE

812/1114 km

49.83º

0.0206

40 pm/s2

Table 4: LARES LAGEOS and STARLETTE orbital parameters and residual accelerations

• 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. 40)

• In March 2012, LARES has started its operational phase. First measurements have been made by the ASI Center in Matera, Italy. 41)

- 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. 40).

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.

 


 

Measurement concept:

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.

LARES_Auto1

Figure 24: Schematic view of LAGEOS-1, LAGEOS-2 and LARES in orbit for the measurement of frame dragging (image credit: INFN)

LARES_Auto0

Figure 25: Illustration of the ILRS network (image credit: ILRS, Ref. 40)

 

Data analysis:

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 (herb.kramer@gmx.net).

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