We have conducted computer simulations to determine the accuracy of the geodetic parameters to be derived from spaceborne laser ranging measurements. Errors in both the distance measurement and the satellite orbital position are considered. In contrast to dynamic force modelling, we use a quasi-geometric approach in which the satellite position is given by an external measurement, such as from GPS, and tlte range measurements are used to derive the target coordinates. New features of this analysis include: (i) modeling range noise variations with the distance between the satellite and the target and the slant path through the atmosphere; (ii) analysis of the effect of errors in an externally provided satellite ephemeris; and, (iii) consideration of target field-of-view limitations. Ephemeris errors are modeled by along-track, radial, and cross-track sinusoidal orbital perturbations. Both short and long period examples are considered. The analysis is used to estimate the errors in the coordinates of retroreflector targets placed in grids with sites separated by 50 km. We find that coordinate errors due to measurement noise are a few millimeters alter several days of data collection. Horizontal and vertical errors are comparable. Errors due to range- and slant path-dependent measurement noise do not differ greatly from those due to purely random noise. A constant range bias produces a height offset in the station coordinates which may be slightly larger than the range bias itself. The target coordinate errors due to periodic orbital errors generally decrease with time as these systematic perturbations gradually take on an increasingly quasi-random character. In some cases observational geometry constrains the accuracy at 5 days but such limitations are less significant by 15 days. Eor a nearly polar orbit, along-track, short period (10 min) errors propagate primarily into latitude. Longer period (100 min) errors also propagate into latitude and, to a lesser extent, height. Radial orbit errors propagate into all three components of the target positions. Cross-track errors affect primarily the longitude. As a general rule, 10–20

The International Laser Ranging Service (ILRS) through its permanent components (Tracking Stations, Operations Centers, Data Centers, Analysis Centers, Central Bureau, and Governing Board) distributes satellite and lunar laser ranging data and derived products to support global, multidisciplinary scientific research. The ILRS Data Centers and Central Bureau serve as the primary source for information, data, and products for this global user community. The ILRS website, https://ilrs.gsfc.nasa.gov , is a key tool for communication for the service, providing background information on the ILRS, its organization and operation, and detailed descriptions of ILRS components, data, and products. Links are provided to extensive information on the supported satellite missions and ILRS network stations including performance assessments and data quality evaluations. Furthermore, the website connects users to archives of laser ranging data and derived products available through the data centers. In this paper, we discuss the development of the ILRS infrastructure, its current status, website resources, description of laser ranging data and products, and plans for future enhancements.

Quality control systems for satellite laser ranging (SLR) observations have been developed at a number of analysis institutes worldwide, using various software packages of precise orbit determination and data analysis. Satellite laser range observations, primarily from the two LAGEOS satellites but also from other satellites in low-Earth orbits and up to GNSS altitude, are being processed on a sub-daily to weekly basis. The generated quality control reports are widely used to detect various kinds of problems and quickly provide anomalous information to laser ranging stations. They have been effective in shortening the time to return to normal operations when anomalous data are detected and in quantifying the performance of laser ranging stations. Consequently, a rapid feedback loop has now been incorporated in the modern SLR operation.

The scale of the solar system is slowly changing, likely increasing as a result of solar mass loss, with additional change possible if there is a secular variation of the gravitational constant, G. The measurement of the change of scale could provide insight into the past and the future of the solar system, and in addition a better understanding of planetary motion and fundamental physics. Estimates for the expansion of the scale of the solar system are of order 1.5 cm year-1 AU-1, which over several years is an observable quantity with present-day laser ranging systems. This estimate suggests that laser measurements between planets could provide an accurate estimate of the solar system expansion rate. We examine distance measurements between three bodies in the inner solar system -- Earth's Moon, Mars and Venus -- and outline a mission concept for making the measurements. The concept involves placing spacecraft that carry laser ranging transponders in orbit around each body and measuring the distances between the three spacecraft over a period of several years. The analysis of these range measurements would allow the co-estimation of the spacecraft orbit, planetary ephemerides, other geophysical parameters related to the constitution and dynamics of the central bodies, and key geodetic parameters related to the solar system expansion, the Sun, and theoretical physics.

In June 2009 the Lunar Reconnaissance Orbiter (LRO) spacecraft was launched to the Moon. The payload consists of 7 science instruments selected to characterize sites for future robotic and human missions. Among them, the Lunar Orbiter Laser Altimeter (LOLA) was designed to obtain altimetry, surface roughness, and reflectance measurements. The primary phase of lunar exploration lasted one year, following a 3-month commissioning phase. On completion of its exploration objectives, the LRO mission transitioned to a science mission. After 7 years in lunar orbit, the LOLA instrument continues to map the lunar surface. The LOLA dataset is one of the foundational datasets acquired by the various LRO instruments. LOLA provided a high-accuracy global geodetic reference frame to which past, present and future lunar observations can be referenced. It also obtained high-resolution and accurate global topography that were used to determine regions in permanent shadow at the lunar poles. LOLA further contributed to the study of polar volatiles through its unique measurement of surface brightness at zero phase, which revealed anomalies in several polar craters that may indicate the presence of water ice. In this paper, we describe the many LOLA accomplishments to date and its contribution to lunar and planetary science. (C) 2016 Elsevier Inc. All rights reserved.

We describe the results of the Laser Ranging (LR) experiment carried out from June 2009 to September 2014 in order to make one-way time-of-flight measurements of laser pulses between Earth-based laser ranging stations and the Lunar Reconnaissance Orbiter (LRO) orbiting the Moon. Over 4,000 h of successful LR data are obtained from 10 international ground stations. The 20-30 cm precision of the full-rate LR data is further improved to 5-10 cm after conversion into normal points. The main purpose of LR is to utilize the high accuracy normal point data to improve the quality of the LRO orbits, which are nominally determined by the radiometric S-band tracking data. When independently used in the LRO precision orbit determination process with the high-resolution GRAIL gravity model, LR data provide good orbit solutions, with an average difference of similar to 50 m in total position, and similar to 20 cm in radial direction, compared to the definitive LRO trajectory. When used in combination with the S-band tracking data, LR data help to improve the orbit accuracy in the radial direction to similar to 15 cm. In order to obtain highly accurate LR range measurements for precise orbit determination results, it is critical to closely model the behavior of the clocks both at the ground stations and on the spacecraft. LR provides a unique data set to calibrate the spacecraft clock. The LRO spacecraft clock is characterized by the LR data to a timing knowledge of 0.015 ms over the entire 5 years of LR operation. We here present both the engineering setup of the LR experiments and the detailed analysis results of the LR data. (C) 2016 Elsevier Inc. All rights reserved.

Network stations provided system configuration documentation upon joining the ILRS. This information, found in the various site and system log files available on the ILRS website, is essential to the ILRS analysis centers, combination centers, and general user community. Therefore, it is imperative that the station personnel inform the ILRS community in a timely fashion when changes to the system occur. This poster provides some information about the various documentation that must be maintained. The ILRS network consists of over fifty global sites actively ranging to over sixty satellites as well as five lunar reflectors. Information about these stations are available on the ILRS website (http://ilrs.gsfc.nasa.gov/network/stations/index.html). The ILRS Analysis Centers must have current information about the stations and their system configuration in order to use their data in generation of derived products. However, not all information available on the ILRS website is as up-to-date as necessary for correct analysis of their data.

One-way laser ranging is being performed routinely from International Laser Ranging Service ground stations to the Lunar Orbiter Laser Altimeter, an instrument onboard NASA’s Lunar Reconnaissance Orbiter. We developed software to process this novel type of tracking data and gathered information e.g. on characteristics and distribution in a preliminary analysis. By incorporating the high accuracy spacecraft range measurements into orbit determination, one expects the positioning and thereby the accuracy of further derived data products to improve. We used the one-way laser ranging measurements within an estimation software based on the Tudat library for carrying out an orbit determination for the Lunar Reconnaissance Orbiter. Thereby the results from the preliminary analysis on tracking data coverage, quality and quantity were used for inputs into the estimation and for evaluation of the results.

High-resolution global gravity field models of the Moon were obtained from precise Ka-band range-rate measurements between the twin Gravity Recovery And Interior Laboratory (GRAIL) spacecraft. We assess the geodetic improvements in tracking data fit and orbit reconstruction quality for independent lunar orbiters, with the latest degree and order 660 spherical harmonics gravity field model developed at NASA GSFC from the primary mission data. We focus on the Lunar Reconnaissance Orbiter (LRO), but also discuss results for Lunar Prospector and the Japanese SELENE mission. In the case of LRO, in addition to radiometric tracking, we use altimetric data from the Lunar Orbiter Laser Altimeter (LOLA) to provide independent estimates of the position reconstruction accuracy.

Observations from the Lunar Orbiter Laser Altimeter reveal the Moon’s Shackleton crater to be an ancient, unusually well-preserved simple crater whose interior walls are younger than its floor and rim; the relative brightness of the floor at 1,064 nanometres is most readily explained by minimal volatile accumulation since crater formation and decreased space weathering due to permanent shadow. The Shackleton crater, which is situated close to the Moon's south pole in almost permanent shadow, has been the target of numerous Earth- and spacecraft-based analyses because of the possibility that it contains water ice. So far, however, results have been inconclusive. Here, Maria Zuber and co-authors present detailed maps of the Shackleton crater based on observations from the Lunar Orbiter Laser Altimeter. The data confirm the crater as one of ancient origin, an unusually well preserved, simple crater with interior walls fresher than the floor or rim. Deposits on the crater floor are nearly the same age as the rim, suggesting little floor deposition since the crater's formation more than 3 billion years ago. The authors conclude that their observations are best explained by downslope movement of regolith on the walls exposing fresher underlying material, rather than by significant amounts of water ice within the crater. The relatively bright crater floor may be a product of decreased space weathering because to shadowing — although a one-micrometre-thick layer containing about 20% surficial ice remains a possibility. Shackleton crater is nearly coincident with the Moon’s south pole. Its interior receives almost no direct sunlight and is a perennial cold trap1,2, making Shackleton a promising candidate location in which to seek sequestered volatiles3. However, previous orbital and Earth-based radar mapping4,5,6,7,8 and orbital optical imaging9 have yielded conflicting interpretations about the existence of volatiles. Here we present observations from the Lunar Orbiter Laser Altimeter on board the Lunar Reconnaissance Orbiter, revealing Shackleton to be an ancient, unusually well-preserved simple crater whose interior walls are fresher than its floor and rim. Shackleton floor deposits are nearly the same age as the rim, suggesting that little floor deposition has occurred since the crater formed more than three billion years ago. At a wavelength of 1,064 nanometres, the floor of Shackleton is brighter than the surrounding terrain and the interiors of nearby craters, but not as bright as the interior walls. The combined observations are explicable primarily by downslope movement of regolith on the walls exposing fresher underlying material. The relatively brighter crater floor is most simply explained by decreased space weathering due to shadowing, but a one-micrometre-thick layer containing about 20 per cent surficial ice is an alternative possibility.

Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury's topography occurred after the earliest phases of the planet's geological history.

Radio tracking of the MESSENGER spacecraft has provided a model of Mercury's gravity field. In the northern hemisphere, several large gravity anomalies, including candidate mass concentrations (mascons), exceed 100 milli-Galileos (mgal). Mercury's northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins. The low-degree gravity field, combined with planetary spin parameters, yields the moment of inertia C/MR(2) = 0.353 ± 0.017, where M and R are Mercury's mass and radius, and a ratio of the moment of inertia of Mercury's solid outer shell to that of the planet of C(m)/C = 0.452 ± 0.035. A model for Mercury's radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron-sulfide layer and an iron-rich liquid outer core and perhaps a solid inner core.

The International Laser Ranging Service (ILRS) website, http://ilrs.gsfc.nasa.gov, is the central source of information for all aspects of the service. The website provides information on the organization and operation of ILRS and descriptions of ILRS components, data, and products. Furthermore, the website and provides an entry point to the archive of these data and products available through the data centers. Links are provided to extensive information on the ILRS network stations including performance assessments and data quality evaluations. Descriptions of supported satellite missions (current, future, and past) are provided to aid in station acquisition and data analysis. The current format for the ILRS website has been in use since the early years of the service. Starting in 2010, the ILRS Central Bureau began efforts to redesign the look and feel for the website. The update will allow for a review of the contents, ensuring information is current and useful. This poster will detail the proposed design including specific examples of key sections and webpages.