B. E. Schutz

ICESat's laser measurements of polar ice, atmosphere, ocean, and land

The Ice, Cloud and Land Elevation Satellite (ICESat) mission will measure changes in elevation of the Greenland and Antarctic ice sheets as part of NASA’s Earth Observing System (EOS) of satellites. Timeseries of elevation changes will enable determination of the present-day mass balance of the ice sheets, study of associations between observed ice changes and polar climate, and estimation of the present and future contributions of the ice sheets to global sea level rise. Other scientific objectives of ICESat include: global measurements of cloud heights and the vertical structure of clouds and aerosols; precise measurements of land topography and vegetation canopy heights; and measurements of sea ice roughness, sea ice thickness, ocean surface elevations, and surface reflectivity. The Geoscience Laser Altimeter System (GLAS) on ICESat has a 1064 nm laser channel for surface altimetry and dense cloud heights and a 532 nm lidar channel for the vertical distribution of clouds and aerosols. The predicted accuracy for the surfaceelevation measurements is 15 cm, averaged over 60 m diameter laser footprints spaced at 172 m alongtrack. The orbital altitude will be around 600 km at an inclination of 94 � with a 183-day repeat pattern. The on-board GPS receiver will enable radial orbit determinations to better than 5 cm, and star-trackers will enable footprints to be located to 6 m horizontally. The spacecraft attitude will be controlled to point

The Joint Gravity Model 3

An improved Earth geopotential model, complete to spherical harmonic degree and order 70, has been determined by combining the Joint Gravity Model 1 (JGM 1) geopotential coefficients, and their associated error covariance, with new information from SLR, DORIS, and GPS tracking of TOPEX/Poseidon, laser tracking of LAGEOS 1, LAGEOS 2, and Stella, and additional DORIS tracking of SPOT 2. The resulting field, JGM 3, which has been adopted for the TOPEX/Poseidon altimeter data rerelease, yields improved orbit accuracies as demonstrated by better fits to withheld tracking data and substantially reduced geographically correlated orbit error. Methods for analyzing the performance of the gravity field using high-precision tracking station positioning were applied. Geodetic results, including station coordinates and Earth orientation parameters, are significantly improved with the JGM 3 model. Sea surface topography solutions from TOPEX/Poseidon altimetry indicate that the ocean geoid has been improved. Subset solutions performed by withholding either the GPS data or the SLR/DORIS data were computed to demonstrate the effect of these particular data sets on the gravity model used for TOPEX/Poseidon orbit determination.

Precision orbit determination for TOPEX/POSEIDON

The TOPEX/POSEIDON mission objective requires that the radial position of the spacecraft be determined with an accuracy better than 13 cm RMS (root mean square). This stringent requirement is an order of magnitude below the accuracy achieved for any altimeter mission prior to the definition of the TOPEX/POSEIDON mission. To satisfy this objective, the TOPEX Precision Orbit Determination (POD) Team was established as a joint effort between the NASA Goddard Space Flight Center and the University of Texas at Austin, with collaboration from the University of Colorado and the Jet Propulsion Laboratory. During the prelaunch development and the postlaunch verification phases, the POD team improved, calibrated, and validated the precision orbit determination computer software systems. The accomplishments include (1) increased accuracy of the gravity and surface force models and (2) improved performance of both the laser ranging and Doppler tracking systems. The result of these efforts led to orbit accuracies for TOPEX/POSEIDON which are significantly better than the original mission requirement. Tests based on data fits, covariance analysis, and orbit comparisons indicate that the radial component of the TOPEX/POSEIDON spacecraft is determined, relative to the Earths mass center, with an RMS error in the range of 3 to 4 cm RMS. This orbit accuracy, together with the near continuous dual-frequency altimetry from this mission, provides the means to determine the oceans dynamic topography with an unprecedented accuracy.

The ICESat-2 Laser Altimetry Mission

Satellite and aircraft observations have revealed that remarkable changes in the Earths polar ice cover have occurred in the last decade. The impacts of these changes, which include dramatic ice loss from ice sheets and rapid declines in Arctic sea ice, could be quite large in terms of sea level rise and global climate. NASAs Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), currently planned for launch in 2015, is specifically intended to quantify the amount of change in ice sheets and sea ice and provide key insights into their behavior. It will achieve these objectives through the use of precise laser measurements of surface elevation, building on the groundbreaking capabilities of its predecessor, the Ice Cloud and Land Elevation Satellite (ICESat). In particular, ICESat-2 will measure the temporal and spatial character of ice sheet elevation change to enable assessment of ice sheet mass balance and examination of the underlying mechanisms that control it. The precision of ICESat-2s elevation measurement will also allow for accurate measurements of sea ice freeboard height, from which sea ice thickness and its temporal changes can be estimated. ICESat-2 will provide important information on other components of the Earth System as well, most notably large-scale vegetation biomass estimates through the measurement of vegetation canopy height. When combined with the original ICESat observations, ICESat-2 will provide ice change measurements across more than a 15-year time span. Its significantly improved laser system will also provide observations with much greater spatial resolution, temporal resolution, and accuracy than has ever been possible before.

Geodetic measurements of convergence at the New Hebrides island arc indicate arc fragmentation caused by an impinging aseismic ridge

Global positioning system (GPS) measurements in 1990 and 1992 from two sites on the southern New Hebrides island arc give convergence rates with the Australian plate of 103 ± 5 mm/yr and 118 ± 10 mm/yr. In contrast, GPS measurements in the central New Hebrides indicate anomalously low convergence rates of ≈42 mm/yr. On geologic time scales, the mean central New Hebrides convergence rate has been 85–132 mm/yr. Elastic fault models with a locked interplate thrust zone indicate that maximum possible rates of horizontal elastic strain are insufficient to account for the anomalously slow convergence. Therefore, we propose that the central New Hebrides segment is moving eastward relative to adjacent arc segments at a rate of ≈36–83 mm/yr. This displacement is accommodated by crustal shortening at the eastern margin of the arc and strike-slip faults crosscutting the arc. Resistance to subduction of the aseismic D9Entrecasteaux Ridge system is the likely cause for horizontal forces sufficient to shove a large segment eastward and fragment the arc. This process demonstrates that subducting bathymetric features can impose fundamental structural modifications on an arc that may represent the initial stages of arc polarity reversal.

Dynamic orbit determination using GPS measurements from TOPEX/POSEIDON

The GPS data acquired by the TOPEX/POSEIDON (T/P) Demonstration Receiver (DR) have been used in a dynamic orbit determination, which was based on the description of the gravitational and nongravitational forces in the equations of motion. The GPS carrier phase data were processed in a double difference mode to remove clock errors, including the effects of Selective Availability. Simultaneous estimation of the T/P orbit and GPS orbits was performed using five 10-day cycles in the interval between December (1992) and April (1993). The resulting T/P orbits have been compared with the orbits determined from Satellite Laser Ranging, the French one-way Doppler tracking system, DORIS, and with the JPL reduced dynamic orbits obtained from the GPS/DR data. Using similar dynamic orbit determination strategies and force models with the GPS/DR to those used with SLR/DORIS, the radial component of the T/P orbit (based on JGM-2) was found to agree to better than 30 mm (rms) and 35 mm with the JPL reduced dynamic orbit. An experimental gravity tuning was accomplished using four cycles of GPS/DR data. The resulting GPS/DR-orbits, determined by the dynamic technique with the experimental gravity field, are in better agreement with the JPL reduced dynamic orbits in both the radial component (21–25 mm) and altimeter crossover residuals than the JGM-2 orbits.

Comparison of VLBI and SLR geocentric site coordinates

The geocentric coordinates for 18 pairs of SLR and VLBI sites are compared. After a seven-parameter frame adjustment, the two coordinate sets have weighted rms differences of 15, 22, and 22 mm for X, Y, and Z, respectively, consistent with the formal errors being too small by a factor of about two.

ICESat Altimetry Data Product Verification at White Sands Space Harbor

Three unique techniques have been developed to validate the Ice, Cloud, and Land Elevation Satellite (ICESat) mission altimetry data product and implemented at White Sands Space Harbor (WSSH) in New Mexico. One specific technique at WSSH utilizes zenith-pointed sensors to detect the laser on the surface and enable geolocation determination of the altimeter footprint that is independent of the data product generation. The system of detectors also registers the laser light time of arrival, which is related to the data product time tag. Several overflights of the WSSH have validated these time tags to less than 3plusmn1 mus. The ground-based detector system also verified the laser illuminated spot geolocation to 10.6 m (3.5 arcsec) plusmn4.5 m on one occasion, which is consistent with the requirement of 3.5 m (1sigma). A third technique using corner cube retroreflector signatures in the altimeter echo waveforms was also shown to provide an assessment of the laser spot geolocation. Although the accuracy of this technique is not equal to the other methodologies, it does offer position determination for comparison to the spacecraft altimetry data product. In addition, elevation verifications were made using the comparison of the ICESat elevation products at WSSH to those acquired with an airborne light detection and ranging. The elevation comparisons show an agreement to within plusmn34 cm (plusmn6.7 cm under best conditions) which indicate no significant errors associated with the pointing knowledge of the altimeter

Autonomous Navigation of Global Positioning System Satellites Using Cross-Link Measurements

The Global Positioning System (GPS) Block IIR satellites, which will replace the current Block II/IIA satellites, will have satellite-to-satellite communication capabilities that will allow intersatellite ranging between the Block IIR satellites. The cross-link pseudorange measurements will be used by onboard computers to update the stored navigation messages, which are based on trajectories predicted over an extended period of time, by ground-based processing of tracking data. During normal operations, the cross-link pseudorange measurements will provideimproved satellite states, which then can be broadcast to theusers. One oftheerrorsourcesin the updated navigation messages, which cannot be corrected by cross-link measurements, is found in the Earth orientation parameter (EOP) errors. As a consequence, as the age of the Earth rotation parameter increases, the performance of the autonomous navigation system will degrade. The effect of this error source on the Block II GPS autonomous navigation accuracy is described. This work is based on simulated cross-link pseudorange measurements. Realistic force, measurement, and reference frame models are used in the analysis to account for additional major error sources. Cross-link measurementsfora period of oneday, generated at the end ofeach of three different prediction intervals, are used to update the predicted trajectory. The estimated solutions then are compared to true solutions to evaluate the effect of prediction errors. With the current EOP prediction errors, the user range errors (URE), computed from improved trajectories and clock differences for a 90-day prediction, exceed 9 m, and for a 180-day prediction, they exceed 17 m. Finally, results of processing measurements from ground stations, instead of cross links, are discussed wherein the URE for the 180-day prediction case are shown to be about 3.1 m.

The geoscience laser altimeter system (GLAS)

GLAS is a space-based lidar designed for NASA’s Mission to Planet Earth (MTPE) Laser Altimeter Mission (LAM). The GLAS instrument will precisely measure the heights of the polar ice sheets, to profile areas of the Earth’s land topography, and to profile the structure of clouds and aerosols on a global scale. The LAM mission utilizes a small dedicated spacecraft in a polar orbit at 598 km altitude with an inclination of 94 degrees. GLAS is being developed to launch in 2001 and to operate continuously for a minimum of 3 years with a goal of 5 years.