Eric C. Silverberg

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

ICESat Geolocation Validation Using Airborne Photography

NASAs ICESat satellite launched in January of 2003, carrying the Geoscience Laser Altimeter. During the initial phase of this mission, many validation procedures were implemented to verify the accuracy associated with a variety of altimetry-derived data products. Of specific interest was the need to validate the geodetic position of the ICESat footprints, which is a convolution of laser-pointing determination, satellite position, and ranging measurements. This paper describes the methodology and implementation of one effort using aerial photography to image the laser spots on the surface during a satellite overflight. The spot locations are determined based on the relative positions of accurately placed geodetic infrared-emitting markers within the overflight area and apparent in the aerial photograph. One specific overflight opportunity captured six successive ICESat footprints with the airborne camera system. The mean geolocation predictions of those spots using the ground fiducial placement in the image provide a data product validation to better than 3.1-m rms on the surface with an estimated accuracy of ±3.6 m when compared to the ICESat solution. These results are within the ICESat mission requirement of 4.5 m on the surface (1.5-arcsecond pointing knowledge) for geodetic position determination.

Relative lateration across the Los Angeles basin using a Satellite Laser Ranging System

In January of 1981 the Transportable Laser Ranging System (TLRS) developed for NASA by The University of Texas was used to conduct a four-day test of the relative lateration technique. The test evolved making repeated measurements of six lines over the Los Angeles basin varying in distance from 26 to 84 kilometers. Although the raw times-of-flight to the various targets changed typically by 5 parts in 106, their line ratios varied nearly an order of magnitude less. The test suggests that the TLRS or other pulsed laser ranging systems might be able to economically combine Lageos ranging and long baseline horizontal work to survey large areas for accumulating crustal strain.

On the Effective use of Lunar Ranging for the Determination of the Earth’s Orientation

Lunar ranging data have been routinely available since September of 1970, but many problems of a varying nature have delayed the establishment of a world-wide lunar ranging network. As a result, we must reexamine the role which this program can play in the determination of the Earth’s rotation and polar motion. Although there are many technical difficulties now inhibiting the widespread use of this technique there seems little doubt but that we can overcome these problems and achieve routine, accurate orientation determinations. The more difficult questions concern how an Earth rotation campaign should now be configured to use the equipment and resources in the best way.

Pointing angle and timing calibration/validation of the Geoscience Laser Altimeter with a ground-based detection system

The scientific goals of the laser altimeter carried on ICES at require that the laser pointing direction should be known to an accuracy of 1.5 arcseconds and the time tag of the altitude measurement should have an accuracy of 0.1 millisecond. In order to verify that the satellite altimetry instrumentation is properly characterized, the laser spot position and time tag should be independently verified. One independent approach uses a ground-based electro-optical detector array. Operation of many devices over an area will enable detection of the laser footprints when they illuminate the ground. With the coordinates of the captured spot obtained from the detector array, the laser spot position determination is compared to the position inferred from the data collected onboard ICESat. A small subset of these detectors will also be used to time-tag the footprint for further verification. These timing detectors will be hardwired to a central timing station that uses a GPS receiver to time stamp the laser pulse time of arrival. The detectors used only for positioning are designed for autonomous operation.

Low Cost Lageos Ranging System

This paper describes a Lageos laser ranging station design based on single photoelectron detection which could be used to monitor polar motion at all periods, and short-period UT1 variations with high accuracy and at moderately low cost. No comparison has been made with the costs of long baseline radio interferometry (LBI) stations, since the establishment and operation of basic international networks using both approaches seems scientifically well justified. Both LBI and lunar laser ranging results will determine intermediate and long-period UT1 variations and nutation. The LAGEOS laser ranging station design suggested here is similar to the design used in the high-mobility LAGEOS ranging station now being constructed by the University of Texas. It has been shown that 30 cm diameter transmit/receive optics with 10 arc second pointing accuracy and 50 milliwatt average laser power output are usually sufficient to give 70 signal counts in a three minute averaging time, even at 20° elevation angle. Lasers having the desired power at 530 nm wavelength with 10 pps repetition rate and 3 ns pulse length are now reported to be commercially available at moderate cost. The resulting range accuracy for normal points corresponding to the three minute averaging time is three cm, including allowances for absolute calibration of the system, atmospheric refraction correction uncertainties, and Earth tide uncertainties. Much of the pulse-timing electronics needed is commercially available, and many time and latitude stations already have good epoch determination systems. The ten arc second pointing requirement for a 30 cm diameter beam seems considerably easier to meet than the optical requirements for classical observing techniques. A simple beam director with flat mirrors appears quite feasible, with inexpensive encoders on each axis for minicomputer control if desired. A number of special features of the University of Texas high-mobility station would not be needed for a fixed station.

An estimate of the geodetic accuracy attainable with a transportable lunar laser station

A transportable lunar laser ranging station can be expected to measure the range to the lunar corner retroreflectors from a site on the Earths surface with an uncertainty of about 3 cms. The accuracy with which that station can infer its geocentric position is influenced by many factors including the uncertainties in the pole position and rotation rate of the Earth and the data loss due to weather.The results of a simplified modelling study intended to include these factors are presented in cartographic form. The modelling indicates that, assuming the successful operation of three or more widely separated fixed laser stations, the transportable station will be able to determine its relative geocentric position in all three coordinates with an accuracy comparable to the original range uncertainty.

High-Accuracy Range Measurements To The Moon

The lunar ranging station at the University of Texas McDonald Observatory has made more than 1800 range measurements to the four lunar retroreflectors during the first six years of its operation. Each range consists of a normal point constructed of from 5 to 20 single photoelectron returns. Normal point accuracies to about 4 parts in 1010 (± 10 cms) have become routine. The availability of excellent commercial timing equipment, similar to that used for nuclear time-of-flight experiments, means that the error budget for such a measurement is primarily dependent on the width of the transmitted laser pulse (currently 3 nanoseconds FWHM). Second generation systems using mode-locked, subnanosTRond lasers can probably achieve routine normal point accuracies approaching one part in 10 (± 2 cms). High speed pockel cells, capable of slicing sharp edges on relatively long laser pulses, may permit such accuracies to also be realized with conventional Q-switched lasers.