65th IHC Booklet/Program (pdf - 4.9MB) - Office of the Federal ...

Surface-Reflected GPS Wind Speed Sensing Results for 2010 Atlantic Season
Michael S. Grant 1 , Stephen J. Katzberg 2 , Jason P. Dunion 3
(michael.s.grant@nasa.gov)
1 NASA/Langley Research Center, 2 NASA/Distinguished Research Associate, 3 University of
Miami/RSMAS/CIMAS-NOAA/AOML/HRD
During the 2010 Atlantic storm season, NASA-Langley GPS receiver/reflectometers deployed
on NOAA P-3 Hurricane Hunter aircraft were used to retrieve ocean surface wind speed for a
number of storms including Hurricane Earl, Tropical Storm Alex, and for a rapid intensification
flight into Hurricane Karl. The sensing technique uses surface-reflected GPS navigation signals
in a manner analogous to bistatic radar, where Global Positioning System (GPS) satellites serve
as surface illumination sources. A modified GPS receiver measures surface-reflected signal
strength at increasing path delays, relative to the geometric mirror reflection point. Surface wind
speed is inferred by monitoring ocean surface roughness (mean square slope) via the measured
scattering of the reflected signal.
Results from earlier storm seasons verified the reflected signal sensitivity over a range of wind
speeds: from nearly 0 (calm seas) to greater than 40 meters per second (m/s) [1]. Another study
reported that, for 0 – 15 m/s wind conditions and well-developed seas, GPS wind speed retrievals
were in agreement with TOPEX satellite altimeter-derived measurements to better than 1 m/s,
with a precision ~2 m/s (1-sigma) [2]. Surface wind speeds obtained using an empirical
calibration function also compared well to GPS dropsonde data, particularly for winds in the 0 –
20 m/s range [3]. In this paper, GPS reflection measurement results from the 2010 storm season
will be presented and compared to co-located measurements from the operational Stepped-
Frequency Microwave Radiometer, surface-adjusted flight level winds, and GPS dropsondes.
The GPS reflection measurements were achieved using light-weight (< 5 kg), low power,
autonomously operated GPS receivers and nadir-viewing 3.5” hemispherical antennas – a system
which requires only minimal aircraft accommodation. GPS wind speed instruments deployed on
longer-duration aircraft missions could be useful for mapping the changing surface wind field
and changes in momentum flux related to the ultimate intensity and classification of developing
storm systems.
The sensitivity of the GPS instrument to low-speed surface wind conditions (< 10 - 15 m/s),
where microwave radiometers can be less effective, implies that GPS surface-reflection
instruments can provide independent wind speed retrievals to compliment those obtained with
radiometers or other instruments currently utilized in tropical cyclone genesis and forecasting
studies. Combining wind speeds based upon GPS surface reflections (due to larger surface
slopes) with those derived from radiometric brightness temperature (involving finer surface
features) can potentially provide a more complete understanding of tropical cyclone formation,
and improve forecasts of storm intensity.
[1] Katzberg, S. J., O. Torres, and G. Ganoe (2006), ‘Calibration of reflected GPS for tropical
storm wind speed retrievals,’ Geophys. Res. Lett., 33, L18602, doi:10.1029/2006GL026825
[2] Garrison, J. L., et. al., (2002) ‘Wind speed measurements using forward scattered GPS
signals,’ IEEE Trans. Geoscience and Remote Sensing, vol. 40, no. 1.
[3] Katzberg, S. J., and J. Dunion (2009) ‘Comparison of refl. GPS wind speed retrievals with
dropsondes in tropical cyclones,’ Geophys. Res. Lett., 36, L17602, doi: 10.1029/2009GL039512.
Session 3 – Page 7