Theory of GNSS Reflectometry - GNSS Remote Sensing: Theory, Methods and Applications - page 176

8.2 Multi-static System: Geometry and Coverage

The GNSS-Reflectometry (GNSS-R) technique seeks to take the most of the large
amount of GNSS signals reaching any spot on Earth simultaneously, in a wide
range of incidence angles. The amount of reflected radio-links can be as large as
the amount of direct radio-links. Currently, with more than 55 fully operational
GNSS transmitters (GPS and GLONASS), it is easy to capture signals from 15 to
25 GNSS sources simultaneously using a receiver on the Earth surface (Fig. 8.3 ).
This number increases with the altitude of the receiver. Moreover, this number
will steadily increase with the deployment of new constellations: the European
GALILEO (two prototypes in orbit); the Chinese COMPASS/Beidou2 (14 usable
transmitters, 4 of them in Mid Earth Orbit, as of December 2012); and regional
constellations (Chinese Beidou1, Indian IRNSS, and Japanese QZSS). All together
will provide more than 130 GNSS transmitters globally distributed.
As a consequence of this large amount of simultaneously received signals, the
spatial coverage obtained with a nadir-looking GNSS-R antenna corresponds to a
gapped and irregularly sampled wide swath image of the surface beneath (Fig. 8.2 ).
This image is compound by the mosaic of glistening zones produced by the reflected
GNSS signals. In spite of the distance between glistening zones, the coverage
potentially achievable from a space-based GNSS-R receiver gives a synoptic view
of many hundreds of kilometers across-track. The time evolving pattern of the
observed areas reminds an irregular comb.

30

20

Fig. 8.3 Frequency percent
of simultaneously visible
GNSS transmitters from a
receiver located at the
surface-level, and mid
latitudes (45 ı ). Statistics
performed with real orbits
corresponding to the GPS and
GLONASS constellations as
in March 18 2012. These
numbers would increase for
reflected signals from a
receiver at higher altitude

10

0

15 20 25

Number of visible GPS+GLONASS