LAGEOS-I (Laser Geodynamics Satellite-I) / LAGEOS-II

LAGEOS-I is a passive research satellite of NASA - the orbit of the spacecraft (and its slight perturbations) represent the geodynamic measurements. LAGEOS-I is the first NASA satellite dedicated wholly to laser ranging. LAGEOS was designed to act as a permanent reference point so that the Earth's progress could be tracked relative to the satellite (in contrast to the traditional system of tracking satellites relative to the Earth). One of LAGEOS' functions is to aid in the understanding of the Earth's crustal motions. The USGS (United States Geological Survey) as well as many institutions around the world are tracking the LAGEOS orbit to study the dynamics of the solid Earth, to analyze (deduce) continental drift (plate tectonics, crustal deformations), the Earth's gravitational field, and the "wobble" in the Earth's axis of rotation. ¹⁾

Spacecraft:

The LAGEOS spacecraft is an aluminum sphere with a brass core, built for NASA/GSFC by Bendix Aerospace Systems. The satellite has a diameter of 60 cm and a mass of 411 kg - a very massive satellite (cannonball) for its small size, with a minimal cross-section. The materials were chosen to reduce the effects of the Earth's magnetic field on the satellite's orbit. Its 426 prisms, called `corner-cube reflectors,' are imbedded in the satellites' surface. Of these, 422 are made of fused silica glass while the remaining 4 are made of germanium. As a passive satellite, there are no onboard sensors or electronics, there is no attitude control - and of course no communication with the spacecraft.

Figure 1: Illustration of the LAGEOS-1 sphere with its reflectors (image credit: NASA)

The three-dimensional prisms reflect laser beams back to the source, regardless of the angle from which they come. Pulsed laser beams transmitted from Earth ground stations are returned by the reflectors on LAGEOS; the travel times are precisely measured, permitting ground stations in different parts of the world to measure their separations (to better than 1 cm in thousands of kilometers) to determine the distance between themselves and the satellite.

Launch: The launch of LAGEOS-I took place on May 4, 1976 on a Delta-2 launch vehicle from VAFB (Vandenberg Air Force Base), CA.

Orbit: Near-circular orbit, altitude = 5858 km x 5958 km (eccentricity of 0.0045), inclination = 109.84º, period = 225 min. The orbit is of the type MEO (Mean Earth Orbit), considered to be very stable. The spherical shape (symmetry) and orbit parameters have been chosen with the purpose of minimizing the effects of the disturbing forces. The high-inclination orbit was chosen to permit tracking of many ground stations around the world. ^{2) 3)}

The first four years until 1980 were devoted to determining LAGEOS' precise orbit and to building up a global network of 14 Earth stations, i.e. SLR (Satellite Laser Ranging) stations. By accurately measuring the time for a laser pulse to travel to the satellite and return, the position of the laser system could be determined to about 10 cm. Under NASA's Crustal Dynamics Project (started in 1979), 56 investigators from12 countries were making repeated measurements between their locations and LAGEOS.

Since the LAGEOS-I orbit is known to extremely high accuracy, the location of a laser ranging station on the surface of the Earth can be determined to a precision of less than 1 cm (by measuring the time for a laser pulse to travel from the laser ranging station to the satellite and return).

Note: The technique of laser ranging to a near-Earth satellite was initiated by NASA in Oct. 1964 with the launch of the Beacon-B satellite (also referred to as Explorer-22 mission).

Applications:

In the past 30 years the Satellite Laser Ranging (SLR) technique has evolved and improved to a large extent, currently achieving a ranging precision down to a few millimeters. Moreover the growth in the size of the international network of SLR stations and the rapidly growing constellation of geodetic target satellites make the SLR a well established technique for solid Earth studies and for the related Earth subsystem sciences. The long SLR observation history has become a very important source of data for global and local changes detection and monitoring in many different fields. Following is a list of some applications: ⁴⁾

• SLR is a proven geodetic technique with significant potential for important contributions to scientific studies of the Earth/atmosphere/oceans system

• SLR is the most accurate technique currently available to determine the geocentric position of an Earth satellite, allowing for the precise calibration of radar altimeters and separation of long-term instrumentation drift from secular changes in ocean topography.

• SLR's ability to measure the temporal variations in the Earth's gravity field and to monitor motion of the station network with respect to the geocenter, together with the capability to monitor vertical motion in an absolute system, makes it unique for modeling and evaluating long-term climate change by:

- Providing a reference system for post-glacial rebound, sea level and ice volume change

- Determining the temporal mass redistribution of the solid Earth, ocean, and atmosphere system

- Monitoring the response of the atmosphere to seasonal variations in solar heating.

• SLR provides a unique capability for verification of the predictions of the Theory of General Relativity. For instance: The Astro-Metrology Group at the University of Maryland at College Park has developed methods of measurement of satellite spin dynamics based on experimental measurements made by the group on the LAGEOS-I geodetic satellite using the Goddard Space Flight Center's 48" telescope. Recent interest in using these satellites to measure the Lens-Thirring effect (General Relativistic Effect) has defined the need for knowledge of the current dynamics of the satellite. ^{5) 6)}

• SLR stations form an important part of the international network of space geodetic observatories, which include VLBI, GPS, DORIS and PRARE systems

• On several critical missions, SLR has provided fail-safe redundancy when other radiometric tracking systems have failed

• The International Laser Ranging Service (ILRS) has been formed by the global SLR community to enhance geophysical and geodetic research activities (ilrs.gsfc.nasa.gov).

Operational status as of 2007 (LAGEOS-I, and -II): The passive spacecraft in MEO permit long-term tracking observations (over many decades - since the orbit of the satellite won't decay, the corner-cube reflectors may suffer a long-term decay from orbiting through the Van Allen radiation belts). The long-term availability of the passive satellites permits also the tracking with new generations of SLR tracking systems (advanced technologies) in the ground segment for more precise ranging results and consequently orbit determination (model analysis, etc.).

LAGEOS-II

LAGEOS-II is a collaborative NASA-ASI geodesy mission [Aeritalia built the LAGEOS-II for ASI (Agenzia Spaziale Italiana) based on the same design as the NASA-provided LAGEOS-I], a follow-up of LAGEOS-I. LAGEOS-II has a mass of 405 kg, a diameter of 60 cm, and a total of 426 laser reflectors. LAGEOS-II is an identical S/C to LAGEOS-I.

Launch of LAGEOS-II: The satellite was deployed from the Space Shuttle (Columbia, STS-52) launch from Cape Canaveral: Oct. 22, 1992 (NASA). Italy developed and provided IRIS (Italian Research Interim Stage), a solid-fueled booster, which carried the satellite from the Shuttle's parking orbit into the required LAGEOS II orbit. ^{7) 8) 9)}

Orbit: LAGEOS-II and LAGEOS-I are deployed in prograde (LAGEOS-II: 52.64º inclination) and retrograde (LAGEOS-I: 110º inclination) orbital planes, respectively. LAGEOS-II has a near-circular orbit with a perigee of 5616 km and an apogee of 5950 km, period of 223 minutes.

The orbit of LAGEOS-II was selected to provide more coverage of seismically active areas, such as the Mediterranean Basin and California; it may help scientists understand irregularities noted in the motion of LAGEOS 1-I. The design of the 52.6º inclination orbit provides gravity and tidal sensitivity to improve on the advances from LAGEOS-I in a shorter time. The nodal precession period of LAGEOS-II is about one half that of LAGEOS-I's three year period. The related effect on the Earth's shadowing of the satellite gives rise to a signal in LAGEOS-II's acceleration pattern with a shorter period than LAGEOS-I. ¹⁰⁾

Figure 2: Illustration of the LAGEOS-II satellite

Objectives:LAGEOS-II is an integral part of the Crustal Dynamics Project (CDP). Study of the Earth's crust in the Mediterranean region. Research in solid Earth geophysics [study of global and local tectonic processes, polar motion and Earth rotation, determination of Universal Time (UT1) and its variations, the recovery of Earth and ocean tidal parameters, and geopotential modelling].

Both LAGEOS satellites are tracked by a global network of fixed and transportable lasers from some 65 sites. The ILRS (International Laser Ranging Service) provides global satellite and lunar laser ranging data and their related products to support geodetic and geophysical research activities as well as IERS (International Earth Rotation Service) products important to the maintenance of an accurate ITRF (International Terrestrial Reference Frame). For the IERS activities, the ILRS focuses on the tracking collected on Lageos-I, Lageos-II and the moon.

The data available to the investigators consist of both preprocessed and analyzed data (i.e. station positions, baselines, and Earth rotation parameters as a function of time). The data are being archived in the Crustal Dynamics Data Information System (CDDIS) at NASA/GSFC.

Frame dragging: An international team of NASA and university researchers has dramatically improved the accuracy of the first direct evidence that the Earth drags space and time around itself as it rotates. The measurements used the latest gravity models obtained from the GRACE mission.

The research, reported in the journal Nature, is the most accurate direct measurement to date of the Lense-Thirring Effect - an effect of general relativity, which predicts a rotating mass will drag space around it. The Lense-Thirring Effect is also known as frame dragging (first predicted by two Austrian physicists Josef Lense (1890-1985) and Hans Thirring (1888-1976) in 1918), a consequence of Einstein's Theory of General Relativity, published in 1916.

Ignazio Ciufolini (University of Lecce, Italy) and Erricos C. Pavlis (Joint Center for Earth Systems Technology, Maryland) report to have made the first reasonably accurate measurement of frame dragging. They tracked the orbits of the LAGEOS-I and LAGEOS-II satellites for 11 years using the SLR technique. They state that Earth's rotation twists the fabric of space enough to displace the satellites by 1.9 m per year from where they would otherwise be, matching the amount predicted by general relativity with a measurement precision of about 10%. ¹¹⁾

Note: A first direct measurement of the frame dragging and geodetic effects and their magnitudes will be provided by GP-B (Gravity Probe-B), a NASA mission with a launch Apr. 20, 2004. GP-B carries precision gyroscopes to measure the frame-dragging effect on its one-year mission.

2) G. B. Afonso, F. Barlier, C. Berger, F. Mignard, J. J. Walch, "Reassessment of the charge and neutral drag of LAGEOS and its geophysical implications," Journal of Geophysical Research, Vol. 90, 1985, pp. 9381-9398.

3) G. B. Afonso, F. Barlier, M. Carpino, P. Farinella, F. Mignard, A. Milani, A. M. Nobili, "Orbital effects of LAGEOS seasons and eclipses," Annalae Geophysicae, Vol. 7, 1989, pp. 501-514

4) "International Laser Ranging Service," http://ilrs.gsfc.nasa.gov/docs/slrover.pdf

5) D. G. Currie, K. E. Kissell, P. V. Avizonis, D. D. Wellnitz, "Measurement of the LAGEOS-I Spin Axis," URL: http://www.physics.umd.edu/rgroups/am/lageos.html

6) R. Wood, T. Otsubo, R. Sherwood, "Lageos 2 spin rate and orientation," http://cddis.nasa.gov/lw13/docs/papers/target_wood_1m.pdf

7) http://www.earth.nasa.gov/history/lageos/lageos2.html

8) "Columbia Successfully Lofts Italian LAGEOS Satellite," Space News, Oct. 26-Nov. 1, 1992, p. 13

9) NASA/ASI LAGEOS II brochure

10) P. Dunn, M. Torrence, R. Kolenkewicz, D. Smith, "Observations of Surface Forces on the LAGEOS Satellites," Geophysical Research Abstracts, Vol. 5, 07162, 2003

11) I. Ciufolini, E. C. Pavlis, "A confirmation of the general relativistic prediction of the Lense-Thirring effect," Nature, Vol. 431., Oct. 21, 2004, pp. 958-960, doi:10.1038/nature03007