Wind Speed with Airborne GPS Bistatic Radar

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					    Measuring Ocean Wind Speed and the Ocean Surface
        Level Using Airborne GPS Bistatic Radar
                      NOAA Earth System Research Laboratory
             V. U. Zavorotny (303-497-6616;

                                   December 21, 2007

        Feasibility studies of using airborne GPS bistatic radar as a remote sensing tool
already have taken place over a decade. This novel technique, if proven, could augment
existing monostatic radar systems. The technique is based on receiving GPS signals of
opportunity onboard of an aircraft that reflect from the land or ocean surface and
measuring the correlation power as a function of the time delay between direct and
reflected signals. Multiple GPS satellites (up to 8-10) would potentially allow
simultaneous topography/altimetry, roughness, or reflectivity mapping of the area of
interest using multiple footprints. Potentially, this technique could be extended toward a
space-borne system of receivers.

        A well-developed theoretical model is available describing the characteristics of
the received GPS signals upon reflection from various types of natural covers. Many
airborne experiments using GPS reflected signals for measuring ocean wind speeds,
ocean and land topography, bare-land soil moisture, Arctic ice and snow have been
performed over the last decade. Still, there is no well-established opinion on reliability
and accuracy of this technique. Progress in this area will depend on a significant increase
in funding basic and applied research in this field, and on a better cooperation between
different institutions involved in these activities.

Background on GPS Reflection Technique

        The Global Positioning System (GPS), which was created in U.S. about 20 years
ago for global navigation, has also led to a revolution in Earth remote sensing. GPS
signals are used routinely to make measurements of Earth orientation and polar motion,
plate tectonic motion, neutral atmospheric temperature and water vapor profiles, and
ionospheric electron density profiles for global monitoring. In addition, high-accuracy
GPS navigation has enabled many other techniques through Precise Orbit Determination,
which was used in satellite radar ocean altimeters such as TOPEX/Poseidon, Jason, and

        The next 5-10 years will see another revolution in global navigation signals, and
thus in Earth remote sensing opportunities. The U.S. GPS system will be significantly
upgraded, with more and stronger signals, more transmitted frequencies, and an improved
infrastructure. In couple of years Europe will also deploy its own navigation system,
called Galileo, which will be similar in capability to the upgraded GPS system. Russia
put forward efforts to restore their GLONASS system. China and India have plans for the
next decade to deploy into orbits their own systems. The resulting Global Navigation
Satellite System (GNSS) will have from 50 to 100 transmitters, each with significant
enhancements compared to today’s GPS satellites.

        The GPS ocean altimetry concept has been proposed by Martin-Neira in 1993 [1].
Since then, a number of aircraft experiments designed to study GPS surface reflections
for purposes of ocean altimetry and scatterometry have been performed [2-101], with
published precision and accuracy results. The GPS ocean scatterometric model has also
been proposed by Zavorotny and Voronovich [11] and tested in numerous aircraft
experiments. An agreement was obtained between retrieved wind-speeds and in situ buoy
data (see, e.g., [3-5, 8, 10]). Germain et al. [10] performed the first inversion of GNSS-R
signals using the full Delay-Doppler Maps for the retrieval of the sea-surface directional
mean square slope. Soil moisture measurements using GPS signal reflections have been
performed from a stationary 300-m tall tower [12], and from aircraft [13].

       Recently, despite on weak signal levels after scattering off the ocean’s surface,
dozens of GPS reflections have been observed [14] from a UK-DMC satellite modified
with a moderate-gain down-looking antenna and a GPS receiver capable of down-linking
many seconds of open-loop data. A much improved link budget and scatterometric
measurement results are expected soon. Before UK-DMC, only a limited set of
experiments had demonstrated detection of ocean-reflected GPS from space [15-17].

         The upcoming GNSS revolution, with twice or more the number of transmitting
satellites, more frequencies, and higher transmitted power levels, will make GNSS ocean
altimetry and scatterometry a much more viable option for the ocean surface level and
ocean roughness measurements. Of course, for orbital receivers of the GPS echoes large
high-gain antennas with electronic steering of beams would be necessary. At the same
time, low-flying platforms, such as aircrafts and balloons, can use much simpler and
inexpensive low-gain antennas.

        Because the source of the GPS signal and the receiver of reflected GPS signals are
separated by a significant distance, the configuration is called bistatic radar. Measuring a
delay between the direct signal and the reflected from the ocean surface signal and
recalculating the temporal delay into the spatial intervals makes this radar an altimeter. In
order to have absolute elevation measurements, an independent accurate measurement of
the platform altitude is required and it is available from systems like POD or WAAS.
        Conventional radar altimeters are monostatic and can provide altimetric data only
along a single nadir track. Several GPS satellites (currently, from 5 to up to 10) make
possible multiple tracks separated by a distance that depends on the altitude of the
receiver of reflected signals that arrive at various angles. The higher the receiver the
wider is the swatch of surface tracks. Since GPS satellites are changing their position in
the sky slowly, the displacement of reflection spots is determined mostly by the
movement of the receiver’s platform. Because the bistatic altimetry concept is newer and
less explored, it has, until recently, significantly lagged behind other concepts in
technology development.

        The main problem for researchers who want to use GPS/GNSS bistatic radars for
their projects is that reliable GNSS bistatic radar (including an antenna system) is rather
expensive. To design their own instrument is a challenging cross-disciplinary task that
requires in-depth knowledge of the GPS system as well as a digital receiver design.

Ocean Wind Speed Measurements                                                                   Formatted: Font: 14 pt

        As it was mentioned above, GPS bistatic radar technique includes a possibility of
measurements of surface roughness characteristics from which the rms of wave slopes,
wind speed, and direction could be determined. There were performed several airborne
experiments [3-5, 10] to measure ocean surface roughness using reflected GPS signals,
and to retrieve from them the wind speed (an absolute value of the wind vector) along the
aircraft flight track. In the paper [8], reflected GPS measurements that were collected
using aircraft with a Delay-Mapping GPS receiver were used to explore the possibility of
determining ocean surface wind vector during flights to Hurricanes Michael and Keith in
October 2000. To interpret the GPS data, a theoretical model from [11] was used to
describe the correlation power of the reflected GPS signals for different time delays as a
function of geometrical and sea-roughness parameters. The model employs a simple
relationship between surface-slope statistics and both a wind vector and wave age or
        Therefore, for situations when this relationship holds there is a possibility of
indirectly measuring the wind speed and the wind direction using reflected GPS signals
obtained simultaneously from several GPS transmitters. Wind direction estimates were
based on a multiple-satellite nonlinear least squares solution. The estimated wind speed
using surface-reflected GPS data collected at wind speeds between 5 and 10 m s21 shows
an overall agreement of better than 2 m s21 with data obtained from nearby buoy data
and independent wind speed measurements derived from TOPEX/Poseidon, European
Remote Sensing (ERS), and QuikSCAT observations. GPS wind retrievals for strong
winds in the close vicinity to and inside the hurricane are significantly less accurate.
Wind direction agreement with QuikSCAT measurements appears to be at the 30° level
when the airplane has both a stable flight level and a stable flight direction. Discrepancies
between GPS retrieved wind speeds/directions and those obtained by other means were
discussed and possible explanations were proposed.

NOAA/FIRST RF altimetric experiment

        GPS-reflected signals can be used both for bistatic ocean altimetry and sea
roughness/wind scatterometry. GPS bistatic altimetry can be based either on phase delay
[6, 17] or on waveform delay measurements [16, 28-22]. The former approach, though
highly accurate, requires a strong coherent component in the reflected signal that can be
achieved for either rather calm seas or low grazing angles. The latter approach is less
accurate but it can be used for incoherently scattered GPS signals. Several aggravating
factors limit the accuracy of GPS altimetry based on waveform delay measurements.
Among them are long wavelength and wide waveform compared to traditional radar
altimeters, the inevitable speckle noise, and bias due to surface roughness.
         On 28 September 2006, FIRST RF Corporation operated a bistatic GPS radar-
altimeter system they had developed and installed on one of the NOAA Aircraft
Operations Center WP-3D research aircraft (N42RF, “Kermit”) during a flight over the
Gulf of Mexico. The goal of the flight was to test the FIRST RF system and to obtain the
first altimetric data with this equipment above the sea surface to study the accuracy of
such measurements using the P(Y)-encrypted GPS signal. The smallest possible
waveform width of 30 m provided by the P(Y)-encrypted signal and the relatively
quiescent sea-state conditions of the present study allowed focusing on the effect of
speckle noise, and, therefore, on the accuracy of GPS altimetry based on waveform delay
measurements [22].
         Data obtained in that experiment over a three-hour period provided direct and
reflected waveforms, and, correspondingly, time delays between direct and reflected GPS
signals. Due to low sea-surface roughness, the fluctuations of the waveform power did
not reach a complete regime of Rayleigh statistics, and a coherent component of the
signal was observed. Smoothing of the raw waveforms was used to better fit the leading
edge and the maximum of the reflected signal. The altitudes calculated from the time
delays were plotted against the smoothed altitudes obtained by the aircraft X-band radar
altimeter. Comparison of these two data sets showed that altitude derived from the GPS
reflections followed most of the aircraft’s altitude variations. The rms of the difference
in height between the smoothed aircraft altimeter data and the GPS height determination
based on a 0.4-s – averaged sea-surface – reflected waveform was on the order of 1 m.
An additional averaging over 1 min reduces the rms of the altimetric measurement using
a P(Y)-encrypted signal to about 20 cm.

References                                                                                    Formatted: Font: 14 pt

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