C. E. Webb

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.

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

Radiation Pressure Modeling for ICESat Precision Orbit Determination

A macro-model was developed for modeling radiation forces in ICESat POD. It consists of a six-sided box and two flat plates, representing the body and the solar panels. The optical properties assigned to each of these surfaces should yield radiation-induced forces that match those experienced in orbit. Prior to the launch, these forces were simulated using a micro-model developed by Ball Aerospace. After generating force histories with the micromodel over the full range of orbit and satellite orientations, a least-squares fit was performed to determine the macro-model optical properties. In this study, the performance of the macro-model was evaluated.

GLAS PAD Calibration Using Laser Reference Sensor Data

Launched in January 2003, the Ice, Cloud and land Elevation Satellite (ICESat) has conducted several periods of science operations with the Geoscience Laser Altimeter System (GLAS). The second extended for 55 days, between September and November 2003. During this period, data from the GLAS Laser Reference Sensor (LRS) was used to correct for apparent, possibly thermally induced, motion of the Instrument Star Tracker (IST). The use of this calibrated IST data in precision attitude determination (PAD) yields significant improvement in estimates of the laser-pointing direction.

Effect of GPS Orbit Errors on ICESat Precision Orbit Determination

To achieve the orbit determination requirement of the EOS ICESat, which is 5 cm and 20 cm in radial and horizontal components, respectively, using the GPS tracking system, it is required to use the precise GPS ephemeris. Three different levels of GPS orbit errors were generated through a simulation, and the effect of these GPS orbit errors on ICESat POD was investigated when the GPS ephemeris is fixed in the process of ICESat POD. Two different simulation cases were investigated. First, GPS orbit errors only case was studied to isolate the GPS orbit error effect from the effect of other errors on ICESat POD. It was found that as the GPS orbit accuracy improved, the resulting ICESat orbit accuracy improved quadratically. The effect of radial, transverse, and normal components of the GPS orbit errors on the ICESat orbit was considered individually. It was found that the transverse GPS orbit errors contributed the most to the ICESat orbit errors. Second, combined errors case was investigated to assess the effect of the GPS orbit errors in the presence other errors. The effect of the GPS orbit errors on the gravity tuning was evaluated. Significant improvement in the radial and the horizontal orbit accuracies were demonstrated from the tuned gravity field using betterdetermined GPS ephemeris.