IEEE Geoscience and Remote Sensing Magazine - December 2014 - 56

receiver (DMR) has been flown on "hurricane hunter" aircraft operated by the National Oceanic and Atmospheric
Administration (NOAA) for over 10 years. Empirical models
have been derived from these campaigns, through comparing the surface roughness retrievals to dropsonde measurements [8]. These airborne experiments, and the resulting
empirical models, formed the foundation for the CYGNSS
mission, described in the next section.
Soil moisture has also been measured from aircraft
and fixed towers using GNSS-R, with a technical maturity
approaching that of altimetry and ocean winds [9]. The
principle behind soil moisture measurements is the reflectivity of water is much higher than that of dry soil, so that
the strength of the reflected GNSS-R signal would increase
with the moisture content of the soil. This measurement
is complementary to passive microwave radiometry, since
the reflectivity, C and emissivity, f, must sum to unity;
f + C = 1. GNSS, like most satellite transmissions, uses
circular polarization. Reflectivity has been extracted from
both polarization components, and combined observables
are under study for improved soil moisture retrieval with
less sensitivity to biomass [10].
GNSS operates in L-band, very close to the spectrum
allocation protected for radiometry and radio astronomy
(1.4 GHz). Signals at these frequencies, however, only penetrate the first +5cm of soil. Radiometry at lower frequencies, allowing deeper penetration, would be difficult due to
the abundance of radio frequency interference (RFI) and the
required antenna size. In this regard, it should be pointed out
that another key advantage of signals of opportunity in that
they allow measurements to be made in bands that are not
protected. In fact, SoOp measurements can only be made in
bands allocated for other purposes (communications or navigation) due to the requirement for an existing signal. We will
describe some recent work in this area in the next section.
Finally, we would like to reiterate that the research examples that we have chosen for the limited space of this article
were selected based upon our assessment of technical maturity as determined by the progress towards a satellite demonstration. A number of other Earth science variables are
being observed with GNSS-R, using a variety of innovative
instrument designs and signal processing methods, includ-

FIGURE 1. Installation of S-band SoOp experiment on NOAA P-3

"Hurricane Hunter" aircraft. Left: Purdue University graduate student
Nick Rainville performing tests of the receiver following installation.
Right: Ocean-view (nadir) S-band antenna for reflected signal reception.
56

ing snow and ice, glaciers and permafrost. Additional references are provided in the FURTHER READING section.
SIGNALS OF OPPORTUNITY (SoOp)
The success of GNSS-R is due to the presence of a PRN code,
generated by a known algorithm, with well-understood
autocorrelation properties. Communication signals, in contrast, do not contain such a pre-determined signal. Digital
communications, however, usually require encryption and
compression, which would optimally reduce the transmission to a noise-like signal, filling the available bandwidth.
Under this assumption, and the high SNR required in satellite communications, it has been found that the direct
signal from the transmitter, received through a sky-view
antenna, can be used as a reference for cross-correlation
with the reflected signal. This is essentially the iGNSS-R
method described earlier.
SoOp measurements have been collected in S-band, using
transmissions from a commercial Satellite Digital Radio Service (SDARS) available in North America. Observations have
been made from a fixed location on the Harvest Platform, off
the coast of Port Arguello, CA [11]. Those experiments have
demonstrated retrievals of significant wave height (SWH)
and compared them to a local buoy, lidar measurements
from the platform, and overflights of the Jason-2 altimeter.
X-band signals transmitted from Direct Broadcast TV Satellites have also been used to verify their potential altimetric
performance [12]. S-band SoOp data has been collected on
several aircraft flights, including the NOAA P-3 during the
2014 hurricane season. Figure 1 shows the installation of
this equipment on the P-3, illustrating the small size of the
reflected signal antenna. Strong signals were observed during fights through several hurricanes, but the data are still
being processed at the present time.
Another recent initiative to extend SoOp methods to
lower frequencies, for deeper penetration of the soil, is the
Signals of Opportunity Airborne Demonstrator (SoOpAD). A 2013 selection under NASA's Instrument Incubator
Program, SoOp-AD will be an airborne prototype instrument for making reflectivity measurements in P-band
(257 MHz). This three-year development activity, begun in
April 2014, will produce a TRL-5 instrument and conduct
calibration and validation experiments.
SUMMARY OF SATELLITE MISSIONS
The first dedicated collection of GNSS-R data from orbit
was reported from an opportunistic experiment on the
UK-DMC satellite in 2003. UK-DMC was placed in a 680
km Sun-Synchronous orbit and was used to make a total
of 60 data collections over the ocean, land and ice surfaces
with a 12 dBi LHCP antenna. 20 seconds of sampled data
were recorded in each collection and then post-processed
on the ground to generate delay-Doppler maps. Built and
operated by Surrey Satellite Technology Ltd. (SSTL), UKDMC was a milestone experiment in the development of
GNSS-R as a viable approach to Earth remote sensing [13].
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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - December 2014