IEEE Geoscience and Remote Sensing Magazine - June 2013 - 56

Delay Doppler Map
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fIGure 5. Example delay-Doppler maps from UK-DMC GPS-R experiment. (a) Ocean reflection, (b) sea ice/water reflection.

post processing on the ground. The raw data were processed
on the ground into DDMs using software receiver techniques to allow analysis of signal returns off ocean, land
and ice. Two example DDMs are shown in Fig. 5; they measure the spread in energy away from the specular point, and
the spread grows as the surface becomes rougher.
A substantial effort into the modeling of signal returns
has been undertaken using data from the first UK-DMC
experiment with the intention to assess inversion of sea
state parameters [4], [5] and the retrieval of directional
roughness information [7], [8]. Although severely bandlimited, the collection of reflected Galileo signals (from
GIOVE-A) was also demonstrated. Moreover, the collection of signals over mixed sea and ice indicates the
potential of GNSS reflectometry for ice edge mapping [9].
The UK-DMC experiment demonstrated the feasibility
for many remote sensing applications but limited spacebased data is available for robust assessment of the geophysical retrieval accuracy of GNSS-R.
3.3. The sPace gnss receiVer-reMoTe
sensing insTruMenT (sgr-resi)
and delay doPPler MaPs
The UK-DMC experiment demonstrated that a microsatellite-compatible passive instrument was able to make
scientifically relevant geophysical measurements using
GPS reflectometry.
Satellite Technology Ltd. (SSTL) teamed with the
National Oceanographic Centre in Southampton and other
partners to develop a new GNSS-R instrument for this purpose, the Space GNSS Receiver-Remote Sensing Instrument
(SGR-ReSI).
A schematic of the SGR-ReSI [10] is shown in
Fig. 6. The SGR-ReSI in effect fulfils in one module what
56

might be handled by three separate units on previous
spacecraft.
a) It performs all the core functions of a space GNSS
receiver, with front-ends supporting up to 8 single or 4
dual frequency antenna ports.
b) It is able to store a quantity of raw sampled data from
multiple front-ends or processed data in its 1 GByte solid
state data recorder
c) It has a dedicated reprogrammable FPGA co-processor
(Virtex 4).
The co-processor was specifically included for the
real-time processing of the raw reflected GNSS data into
DDMs. However, it has flexibility to be programmed in
orbit as required for different purposes, for example to
track new GNSS signals, or to apply spectral analysis to
received signals.
For the co-processor to generate DDMs of the sampled
reflected data, it needs to be primed with the PRN (pseudorandom noise) code of the transmitting GPS satellite, and
the estimated time delay and Doppler of the reflection as
seen from the satellite. These are calculated by the processor in conjunction with the main navigation solution-the
data flow for this is shown in Fig. 7. Direct signals (received
by the zenith antenna) are used to acquire and track GNSS
signals. From the broadcast ephemerides, the GNSS satellite positions are known. Then, from the geometry of the
position of the transmit and receive satellites, the reflectometion geometry can be calculated.
The processing of the Delay Doppler Map is performed on the coprocessor using data directly sampled
from the nadir antenna. In common with a standard
GNSS receiver, the local PRN is generated on-board the
co-processor. As an alternative to synchronizing and
decoding the reflected signal in a standalone manner, the
ieee Geoscience and remote sensing magazine

JUNE 2013



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