Ocean Remote Sensing Using GNSS-R - GNSS Remote Sensing: Theory, Methods and Applications - page 215

bandwidth, directivity of the antennas, etc. GNSS altimetry, with its higher spatial
and temporal sampling, could make contributions to the following research topics:

Eddies and fronts are essential to understand Ocean circulation on all space
and time scales, but a large fraction of their kinetic energy is associated with
spatial scales that cannot be resolved with a single radar altimeter. Mesoscale
variability is key to understand large-scale circulation and climate variability
through eddy transport of momentum in interactions with the mean flow and
meridional heat transport. GNSS altimetry, with its relatively large number of
simultaneous measurements, can provide greater spatial and temporal resolution,
and these can be traded against each other for a particular application.
Tsunamis at open waters have a relatively small vertical signal (typically a few
decimeters) but very large horizontal pattern, corresponding to ocean waves of
long periods (5-60 min) and long wavelengths (up to 800 km). Its detection and
warning must be as quick as possible, preferably 15-25 min after its origin. The
synoptic-view capabilities of the GNSS-R system might help detecting them
timely. Stosius et al. ( 2010 ) shown that strong tsunamis with magnitudes 8.5
can be detected with certainty from any orbit altitude within 15-25 min by a 48/8
or 81/9 Walker constellation of GNSS-R satellites if tsunami waves of 20 cm or
higher can be detected by the technique.
Geostrophic Velocities and Gravity Anomalies are estimated based on mea-
surements of the surface slope. Radar altimeters can measure surface slopes
in only one direction-along track, except at widely time-spaced track crossover
points. With GNSS altimetry, the slope measurements could be done along the
paths of the multiple simultaneous specular points on the surface. Over time,
a single resolution cell would be crossed by several such paths, in different
orientations. This potentially allows 2-D surface slopes to be estimated for larger
set of resolution cells.
Tides, in particular higher-order tidal components, might be better identified
than in standard RA because of the non-repeating nature of the GNSS concept.
Another possible advantage of the GNSS-R altimetry could be better sampling of
internal tides, of wavelengths below about 100 km. These tides play an important
role in large-scale circulation and vertical mixing in the deep ocean.
Physical-Chemical-Biological Interactions mostly occur at scales not resolved
with radar altimeters. One of the more important of these interactions is the
oceans connection to the carbon cycle, which is crucial to understanding long-
term climate variations.

The GNSS-R can measure the electromagnetic slant delay between the trans-
mitter and the receiver, after the signal rebounds off the sea surface, .This
electromagnetic delay contains a geometric component geo due to the total distance
travelled by the signal; it also includes atmospheric induced delays atm (ionospheric
and tropospheric effects); instrumental components ins (clock errors, sub-system
delays, antenna offsets); and noise n:

D geo C atm C ins C n (9.1)