The GPS Laser Retroreflector Array Project

Systematic co-location in space through the precision orbit determination of GPS satellites via satellite laser ranging will contribute significantly towards improving the accuracy and stability of the international terrestrial reference frame. NASA recently formed the GPS Laser Retroreflector Array Project to develop and deliver retroreflectors for integration on the next generation of GPS satellites. These retroreflectors will be an important contributor to achieving a global accuracy of 1.0 mm and 0.1 mm/year stability in the international terrestrial reference frame. We report here the current status of the GPS Laser Retroreflector Array Project

The NASA Space Geodesy Network

NASA successfully demonstrated next generation SLR, VLBI, and GNSS using the GGAO prototypes.Tier 1 deployment of next generation stations is underway: KPGO (Kokee Park, Hawaii); VGOS station operational; VGOS station at McDonald Observatory, Texas nearly operational; build of long-lead items for first 3 SLR stations underway. Planning and discussions with international partners for Tier 2 deployment underway. Plan for Decadal Survey responsive deployment acceleration under review by NASA

Tests of gravity Using Lunar Laser Ranging

Lunar laser ranging (LLR) has been a workhorse for testing general relativity over the pa~t four decades. The three retrorefiector arrays put on the Moon by the Apollo astronauts and the French built array on the second Soviet Lunokhod rover continue to be useful targets, and have provided the most stringent tests of the Strong Equivalence Principle and the time variation of Newton's gravitational constant. The relatively new ranging system at the Apache Point :3.5 meter telescope now routinely makes millimeter level range measurements. Incredibly. it has taken 40 years for ground station technology to advance to the point where characteristics of the lunar retrorefiectors are limiting the precision of the range measurements. In this article. we review the gravitational science and technology of lunar laser ranging and discuss prospects for the future

Hollow Retroreflectors for Lunar Laser Ranging at Goddard Space Flight Center

Laser ranging to the retroreflector arrays placed on the lunar surface by the Apollo astronauts and the Soviet Luna missions have dramatically increased our understanding of gravitational physics along with Earth and Moon geophysics, geodesy, and dynamics. Although the precision of the range measurements has historically been limited by the ground station capabilities, advances in the APOLLO instrument at the Apache Point facility in New Mexico is beginning to be limited by errors associated with the lunar arrays. At Goddard Space Flight Center, we have developed a facility where we can design, build, and test next-generation hollow retroreflectors for Lunar Laser Ranging. Here we will describe this facility as well as report on the bonding techniques used to assemble the retroreflectors. Results from investigations into different high reflectivity mirror coatings, as well as dust mitigation coatings will also be presented

Truncated Icosahedral Gravitational Wave Antenna.

A spherical gravitational wave detector can be equally sensitive to a wave from any direction, and also able to measure its direction and polarization. We derive a set of equations to describe the mechanics of a spherical antenna coupled to an arbitrary number of attached mechanical resonators. A special arrangement of 6 resonators is proposed, which we term a Truncated Icosahedral Gravitational Wave Antenna, or TIGA. An analytic solution to the equations of motion is found for this case. We find that direct deconvolution of the gravitational tensor components can be accomplished with a specified set of linear combinations of the resonator outputs, which we call the mode channels. We develop one simple noise model for this system and calculate the resulting strain noise spectrum. We conclude that the angle-averaged energy sensitivity will be 56 times better than for the typical equivalent bar-type antenna with the same noise temperature. We have constructed a prototype TIGA. This shape was machined from an Al 6063 cylindrical bar, is 84 cm in diameter, has its first quadrupole resonances near 3200 Hz, and is suspended from its center of mass. The frequencies of the lowest seven multiplets were found to closely match those calculated for a sphere. We observed the motion of the prototype\u27s surface using 6 accelerometers attached to its surface in the symmetric truncated icosahedral arrangement. We have tested a first order direction finding algorithm, which uses fixed linear combinations of six accelerometer responses to first infer the relative amplitudes of the quadrupole modes and from these the location of the impulse. The six accelerometers were then replaced by six mechanical resonators. A strain gauge was used to monitor the radial motion of each resonator. The frequency response of the of coupled system was measured and compared to the eigenvalue solutions of the equations of motion. It was concluded that deviations from perfect symmetry have a second order effect on our ability to observe the prototype\u27s quadrupole modes and thus determine the location and direction of the initial excitation

A Comparison of Fabrication Techniques for Hollow Retroreflectors

Despite the wide usage of hollow retroreflectors, there is limited literature involving their fabrication techniques and only two documented construction methods could be found. One consists of an adjustable fixture that allows for the independent alignment of each mirror, while the other consists of a modified solid retroreflector that is used as a mandrel. Although both methods were shown to produce hollow retroreflectors with arcsecond dihedral angle errors, a comparison and analysis of each method could not be found which makes it difficult to ascertain which method would be better suited to use for precision-aligned retroreflectors. Although epoxy bonding is generally the preferred method to adhere the three mirrors, a relatively new method known as hydroxide-catalysis bonding (HCB) presents several potential advantages over epoxy bonding. HCB has been used to bond several optical components for space-based missions, but has never been applied for construction of hollow retroreflectors. In this paper we examine the benefits and limitations of each bonding fixture as well as present results and analysis of hollow retroreflectors made using both epoxy and HCB techniques

Errors on the inverse problem solution for a noisy spherical gravitational wave antenna

A single spherical antenna is capable of measuring the direction and polarization of a gravitational wave. It is possible to solve the inverse problem using only linear algebra even in the presence of noise. The simplicity of this solution enables one to explore the error on the solution using standard techniques. In this paper we derive the error on the direction and polarization measurements of a gravitational wave. We show that the solid angle error and the uncertainty on the wave amplitude are direction independent. We also discuss the possibility of determining the polarization amplitudes with isotropic sensitivity for any given gravitational wave source.Comment: 13 pages, 4 figures, LaTeX2e, IOP style, submitted to CQ

The TIGA technique for detecting gravitational waves with a spherical antenna

We report the results of a theoretical and experimental study of a spherical gravitational wave antenna. We show that it is possible to understand the data from a spherical antenna with 6 radial resonant transducers attached to the surface in the truncated icosahedral arrangement. We find that the errors associated with small deviations from the ideal case are small compared to other sources of error, such as a finite signal-to-noise ratio. An in situ measurement technique is developed along with a general algorithm that describes a procedure for determining the direction of an external force acting on the antenna, including the force from a gravitational wave, using a combination of the transducer responses. The practicality of these techniques was verified on a room-temperature prototype antenna.Comment: 15 pages, 14 figures, submitted to Physical Review