IEEE Geoscience and Remote Sensing Magazine - December 2014 - 23
In principle, one way to overcome the bandwidth
limitation is using the so-called interferometric GNSS-R
processing [74], or "iGNSS-R", in which the reflected signal is cross-correlated with the direct signal itself s D ^ t h
after proper Doppler frequency and delay adjustment as
sketched in Fig. 12, and formulated in (20) and (21). The
ultimate performance of the iGNSS-R will depend not only
on b, but on the SNR (as in (22)), the noise correlation, the
width of the tracking window, etc. as discussed in Section
VI. However, the value of b computed from the waveforms
shown in Fig. 15 (top) is b . 4.8 MHz, a much smaller
value than the receiver's and the signal's bandwidth, and
it will ultimately limit the achievable altimetry resolution
improvement by a factor of `4.8 MHz / 2.2 MHz (rms bandwidth of WF signal with composite signal / bandwidth of
C/A code) = 2.18 (1 10 ) [118], [119]. Similar results have
been obtained by [120]-[122]. The physical explanation of
this result is the smoother shape of the waveform resulting
from the convolution of the ACF with the scatterers on the
surface, as compared to the ACF itself. The best altimetry
performance could only be achieved for quasi-specular reflections, when the WF looks like the ACF.
1
Y i ^t, x, fd h = T
c
#t
t +TC
s R ^t lh s )D ^t l - x h e -j2r^Fc +fdht l dt l,
(20)
Ni
1
Y i ^x, fdh 2 . N / Y i ^t n, x, fd h 2.
i
n =1
(21)
The inter-comparison between cGNSS-R and iGNSS-R
is not straightforward since there are pros and cons for each
method. In cGNSS-R the code replica is generated locally:
it allows one to separate signals from different satellites by
their code; it inherently has an infinite SNR (small losses
can be expected from frequency responses mismatches);
smaller size antennas can be employed to track the reflected signals using frequency responses mismatches),
and smaller size (directivity) antennas can be used to track
the reflected signals. Use of currently available public C/A
codes for altimetry is not feasible because of their limited
bandwidth which leads to a limited range resolution. Also,
the delay and Doppler frequency dynamics for these codes
are larger, and these values must be adjusted more frequently for proper operation [123]. In iGNSS-R there is no need
to know the code, since the direct signal itself is used instead. It allows not only use of GNSS signals, but satellite
radio, satellite television, or any other sources of opportunity with larger transmitted power, larger bandwidth, and
better SNR, leading to potentially improved range resolution. In addition, the differential processing produced in
the cross-correlation leads to slower delay and Doppler frequency dynamics, which are - in principle- easier to track
[123]. The main drawbacks are the large antenna size (directivity) required for the up-looking antenna, even when
satellite television signals are used, which leads to the use
of beam-steering techniques, and eventually multi-beam
antennas if several reflection points are to be tracked, the
need to separate different satellites from their signature
december 2014
ieee Geoscience and remote sensing magazine
("location") in the delay-Doppler map, and the higher susceptibility to radio frequency interference.
To overcome some limitations of the previous techniques, newer approaches have been developed, namely,
the reconstructed GNSS-R (rGNSS-R) [124]-[126] and the
partial interferometric GNSS-R (piGNSS-R) [127]. The
rGNSS-R is similar to the cGNSS-R technique, but semicodeless techniques are used to reconstruct the P(Y) code
which is then correlated with the reflected signal. The
piGNSS-R is similar to the iGNSS-R technique, but the P
and M codes components of the direct signal are extracted
from the reference signal (direct signal) by coherent demodulation, and the interferometric approach is then applied to the reflected signal.
In Fig. 13 (top) the correlation approach used in the
down-looking channel (slave: shown) instrument provides P-code processing of encrypted GPS signals without
knowledge of the encrypted code, in addition to the C/A
code for cGNSS-R, while the up-looking channels (master: not shown) use a similar correlation approach and
feed the information to the down-looking channel (slave)
[119], [120]. In Fig. 13 (bottom) the direct L1-C/A signal
is processed with typical DLLs and PLLs. The locked C/A
code model is used to form a L1P model, which is then
applied to the direct signal (center left), and after integration over +0.5 MHz W-chips to estimate their signs, it
is combined with the P-code model to form a L1 Y-code
model which is used to correlate with the down-looking
channel. The advantages of this technique rely mainly on
the larger bandwidth of the P(Y) codes, as compared to
the C/A ones, and the large SNR, despite the losses of the
semi-codeless approach.
Fig. 14 shows the basic approach of the piGNSS-R technique. The advantage of this technique is an even better
range resolution as compared to the iGNSS-R one, but at
the expense of a 3 dB signal loss (C/A code has been removed), which needs to be compensated by a 3 dB larger
antenna directivity.
It is worth mentioning that relative altimetry or scatterometry observations can also be performed by applying the cGNSS-R techniques shown in Fig. 11 to the direct
signal as well. This approach is intrinsically more insensitive to errors than absolute measurements performed with
the basic scheme shown in Figs. 11 or 12. Alternatively, the
1 Tc
y ( ) dt
Tc 0
Figure 12. Basic concept of an interferometric GNSS-R instrument.
23
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