Theory of GNSS Reflectometry - GNSS Remote Sensing: Theory, Methods and Applications - page 206

Table 8.4 Summary of the main differences between the code-replica approach and the PARIS
interferometric technique
Code-correlation

PIT

Correlation of the signals (either direct or
reflected) against well-known, publicly
available, modulation codes (C/A, L2C in
GPS)

Correlation of the total field received by an
up-looking antenna against the total field
received by a down-looking antenna

Two waveforms, direct and reflected, can be
generated separately

Direct and reflected signals generate the only
waveform

Only publicly available narrow bandwidth
modulations are used

The full power spectral density of the trans-
mitted GNSS signals is exploited: all mod-
ulations (both public/encrypted, narrow and
wide bandwidth) contribute to the waveform

Group-delay precision is limited by publicly
available narrow-bandwidth modulation
codes

Group-delay precision is not limited by pub-
licly available codes

The waveform presents noise introduced by
either the direct or the reflected signal

Noise from both direct and reflected radio-links
is present in the waveform

Relatively low-gain low-directivity antennas
are needed

High-gain highly-directive antennas are

required

The correlation process permits to separate and
identify the different visible GNSS
transmitters

The correlation process cannot separate and/or
identify the different GNSS transmitters,
unless they are distant along the delay-
Doppler space, or a high-gain, narrow-beam
antenna points to a single GNSS

Parallel multi-static performance easily
implementable

Parallel multi-static performance requires par-
allel beam-forming capabilities of the receiv-
ing antennas

altimetric mission ( Martín-Neira et al. 2011 ). Sometimes it is called PARIS-
approach (PAssive Reflectometric Interferometric System), although in some other
publications the PARIS concept might also be fully equivalent to GNSS-R. We will
use the term PARIS Interferometric Technique (PIT). Note that, unlike the replica-
correlation approach, in the PIT there is no need of knowing the code modulations,
meaning that also the encrypted wide bandwidth codes are being captured. In
fact, the PIT approach allows exploiting the full power spectral density of the
transmitted GNSS signals. Because of this, higher group-delay precision can be
obtained, making it attractive for altimetric applications. Other differences between
both approaches are summarized in Table 8.4 .
The PIT waveform is slightly different from the BPSK-codes' waveforms. It
contains the contributions from all the simultaneously transmitted codes from the
observed GNSS satellite. Figure 8.18 shows a real PIT waveform obtained from
an aircraft at 200 m altitude, from a reflection off estuary calm waters. The data
was acquired during the experimental campaign described in Rius et al. ( 2012 ).
The resulting PIT waveform almost corresponds to the theoretical one for specular
reflection ( Martín-Neira et al. 2011 , Fig. 6). The contribution of the C/A code within
the PIT waveform is shown (dashed ƒ 2

function). M-code (B.O.C.) features are