Stephen T. Lowe

2‐cm GPS altimetry over Crater Lake

Dierences in electromagnetic path delay, be- tween direct Global Positioning System (GPS) signals and those reflected from the surface of Crater Lake, have led to lake surface height estimates with 2-cm precision in 1 sec- ond. This is the rst high-precision altimetric demonstra- tion with GPS from sucient altitude (480 m) to probe fundamental experimental errors, which bear on future air- and spaceborne passive GPS altimetry. It also serves as the rst demonstration of a new approach to altimetric remote sensing in the coastal region, an area that is poorly mea- sured by conventional radar altimetry. Time-series analy- sessuggest thattroposphericandthermalnoisefluctuations dominate the altimetric error in this experiment. Estimat- ing the dierential delay from several simultaneously visi- ble satellites may enable tropospheric error estimation and correction. Thermal noise on the reflected signal will be reduced with fully polarimetric observations and larger an- tenna apertures.

A delay/Doppler-mapping receiver system for GPS-reflection remote sensing

A delay/Doppler-mapping receiver system, developed specifically for global positioning system (GPS)-reflection remote sensing, is described, and example delay/Doppler waveforms are presented. The high-quality data obtained with this system provide a more accurate and detailed examination of ground-based and aircraft GPS-reflection phenomenology than has been available to date. As an example, systematic effects in the reflected signal delay waveform, due to nonideal behavior of the C/A-code auto-correlation function, are presented for the first time. Both a single-channel open-loop recording system and a recently developed 16-channel recorder are presented. The open-loop data from either recorder are postprocessed with a software GPS receiver that performs the following functions: signal detection; phase and delay tracking; delay, Doppler, and delay/Doppler waveform mapping; dual-frequency (L1 and L2) processing; C/A-code and Y-code waveform extraction; coherent integrations as short as 125 /spl mu/s; navigation message decoding; and precise observable time tagging. The software can perform these functions on all detectable satellite signals without dead time, and custom signal-processing features can easily be included into the system.

Sensitivity analysis of wind vector measurements from ocean reflected GPS signals

The power received by a GPS receiver located on an airplane flying over the ocean and collecting the GPS signal reflected off the surface is estimated for varying wind speed and wind direction, platform altitude, satellite elevation angle, and receiver integration time. By accounting for the wind-induced anisotropy on the surface, it is shown that the trailing edges of computed waveforms change with both wind speed and direction. When the receiver integration time is properly chosen, substantial differences in the waveforms at different wind directions are exhibited even at the beginning of the trailing edge, particularly for reflections from high-elevation satellites. These differences are associated with the extent and directional properties of the scattered fields as a function of reflection angle. An analysis of the interdependence of the observational (geometric) parameters, the coherence time, and the receiver integration time is presented to suggest a suitable arrangement for enhanced measurement sensitivity to the wind direction.

Wetland monitoring with Global Navigation Satellite System reflectometry

Abstract Information about wetland dynamics remains a major missing gap in characterizing, understanding, and projecting changes in atmospheric methane and terrestrial water storage. A review of current satellite methods to delineate and monitor wetland change shows some recent advances, but much improved sensing technologies are still needed for wetland mapping, not only to provide more accurate global inventories but also to examine changes spanning multiple decades. Global Navigation Satellite Systems Reflectometry (GNSS‐R) signatures from aircraft over the Ebro River Delta in Spain and satellite measurements over the Mississippi River and adjacent watersheds demonstrate that inundated wetlands can be identified under different vegetation conditions including a dense rice canopy and a thick forest with tall trees, where optical sensors and monostatic radars provide limited capabilities. Advantages as well as constraints of GNSS‐R are presented, and the synergy with various satellite observations are considered to achieve a breakthrough capability for multidecadal wetland dynamics monitoring with frequent global coverage at multiple spatial and temporal scales.

Direct Signal Enhanced Semicodeless Processing of GNSS Surface-Reflected Signals

This paper presents an aircraft demonstration of direct-signal enhanced semicodeless processing of global navigation satellite systems (GNSS) signals reflected from the Earths surface. Comparisons are made between this new method and an interferometric approach to GNSS reflectometry. Results show that this technique produces waveforms with greater signal-to-noise compared with the interferometric approach for all GNSS signals currently in use or planned for the near future. Alternatively, the semicodeless technique can have similar performance with smaller antennas for lower hardware costs. The semicodeless approach also has the advantage that different signals along with their different surface spatial resolutions are processed separately, each signals coherent integration time can be optimized, and ground/aircraft experiments and tests are free of spurious signals. The signal processing demands of the semicodeless approach are shown to be proportional to the number of signal components processed when integrated with a GNSS precise orbit determination (POD) receiver.

Statistical Studies of Giant Pulse Emission from the Crab Pulsar

We have observed the Crab pulsar with the Deep Space Network Goldstone 70 m antenna at 1664 MHz during three observing epochs for a total of 4 hr. Our data analysis has detected more than 2500 giant pulses, with flux densities ranging from 0.1 kJy to 150 kJy and pulse widths from 125 ns (limited by our bandwidth) to as long as 100 μs, with median power amplitudes and widths of 1 kJy and 2 μs, respectively. The most energetic pulses in our sample have energy fluxes of approximately 100 kJy μs. We have used this large sample to investigate a number of giant pulse emission properties in the Crab pulsar, including correlations among pulse flux density, width, energy flux, phase, and time of arrival. We present a consistent accounting of the probability distributions and threshold cuts in order to reduce pulse-width biases. The excellent sensitivity obtained has allowed us to probe further into the population of giant pulses. We find that a significant portion, no less than 50%, of the overall pulsed energy flux at our observing frequency is emitted in the form of giant pulses.

Vertical Scales of Turbulence at the Mount Wilson Observatory

The vertical scales of turbulence at the Mount Wilson Observatory are inferred from data from the University of California at Berkeley Infrared Spatial Interferometer (ISI), by modeling path length fluctuations observed in the interferometric paths to celestial objects and those in instrumental ground-based paths. The correlations between the stellar and ground-based path length fluctuations and the temporal statistics of those fluctuations are modeled on various timescales to constrain the vertical scales. A Kolmogorov-Taylor turbulence model with a finite outer scale was used to simulate ISI data. The simulation also included the white instrumental noise of the interferometer, aperture-filtering effects, and the data analysis algorithms. The simulations suggest that the path delay fluctuations observed in the 1992-1993 ISI data are largely consistent with being generated by refractivity fluctuations at two characteristic vertical scales: one extending to a height of 45 m above the ground, with a wind speed of about 1 m/ s, and another at a much higher altitude, with a wind speed of about 10 m/ s. The height of the lower layer is of the order of the dimensions of trees and other structures near the interferometer, which suggests that these objects, including elements of the interferometer, may play a role in generating the lower layer of turbulence. The modeling indicates that the high- attitude component contributes primarily to short-period (less than 10 s) fluctuations, while the lower component dominates the long-period (up to a few minutes) fluctuations. The lower component turbulent height, along with outer scales of the order of 10 m, suggest that the baseline dependence of long-term interferometric, atmospheric fluctuations should weaken for baselines greater than a few tens of meters. Simulations further show that there is the potential for improving the seeing or astrometric accuracy by about 30%-50% on average, if the path length fluctuations in the lower component are directly calibrated. Statistical and systematic effects induce an error of about 15 m in the estimate of the lower component turbulent altitude.

Analysis of GNSS-R Altimetry for Mapping Ocean Mesoscale Sea Surface Heights Using High-Resolution Model Simulations

The capability of global navigation satellite system reflectometry (GNSS-R) altimetry to map mesoscale sea surface height (SSH) fields is analyzed using synthetic measurements derived from a high-resolution (1/10°) numerical model of the North Pacific. As an example, we consider the GPS and GLONASS constellation transmitters and assume six reflection-capable receivers onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites in high-inclination (720 km, 72°) orbits. An individual GNSS-R measurement has a ~10-km footprint. The SSH measurement error is simulated as a function of the incidence angle and the error in the delay measurement between the transmitter and receiver. The delay measurement error is assumed to have Gaussian white noise distribution with a root-mean-square error (RMSE) of 1.0 or 2.0 m. Two days of synthetic measurements are used to reconstruct SSH fields using a two-dimensional variational algorithm. For the 1.0-m delay error, the basin-wide RMSE of the mapped field is 2.3 cm and the spatial correlation between the mapped and true mesoscale fields is larger than 0.9. For the 2.0-m delay error, the basin-wide RMSE is 3.8 cm and the spatial correlation is larger than 0.8. Spectral and synoptic analyses suggest that two days of measurements can reproduce mesoscale features down to 100 km. The result demonstrates the ability of GNSS-R altimetry to suppress large measurement errors due to the high density of measurements and the potential to constrain mesoscale features down to scales beyond what the constellation of existing nadir-altimeters allows.

Altimetry with reflected GPS signals: results from a lakeside experiment

In the fall of 1999 an experiment was performed at Crater Lake, Oregon, to demonstrate the feasibility of surface altimetry with GPS. A GPS antenna was directed at the lake its axis pointing slightly downward-from a rocky precipice. This arrangement allowed collection of both the direct GPS signal as well as the signal reflected off the lake surface. The relative delay and carrier phase rates between direct and reflected signals are used to infer the height of the lake surface. The site was chosen for its elevation, resulting in clear separation of direct from reflected signal waveforms much of the time. The paper discusses the experimental setup, the data processing steps and the findings of the investigation to determine feasibility and accuracy of this new type of altimetric measurement. Thermal-noise error contributions of 1 cm in 40 seconds can be inferred based on the analysis of carrier phase signals. Delay measurements using the Coarse Acquisition (CA) signals give 1-cm thermal-noise error in about 13 hours. These measurements uncover the systematic instrumentation, processing, and modeling errors germane to future airborne and spaceborne measurements over the ocean.

Spaceborne GNSS-R from the SMAP Mission: First Assessment of Polarimetric Scatterometry over Land and Cryosphere

This work describes the first global scale assessment of a Global Navigation Satellite Systems Reflectometry (GNSS-R) experiment performed on-board the Soil Moisture Active Passive (SMAP) mission for soil moisture and biomass determination. Scattered GPS L2 signals (1227.6 MHz) were collected by the SMAP’s dual-polarization (Horizontal H and Vertical V) radar receiver and then processed on-ground using a known replica of the GPS L2C code. The scattering properties over land are evaluated using the Signal-to-Noise Ratio (SNR), the Polarimetric Ratio (PR), and the width of the waveforms’ trailing and leading edges. These parameters show sensitivity to the effects of the Earth’s topography and Above Ground Biomass (ABG) even over Amazonian and Boreal forests. These effects are shown to be an important factor in precise soil moisture and biomass determination. Additionally, it is found that PR shows sensitivity to soil moisture content over different land cover types. In particular, the following values of the PR are found over: (a) tropical forests ~−1.2 dB; (b) boreal forests ~0.8 dB; (c) Greenland ~2.8 dB; and (d) the Sahara Desert ~3.2 dB.