1. Questions from industry to GARCA A
Questions from industry to GARCA A. Camps UPC & IEEC/CTE-UPC (extracted from Review of System Requirements GEROS-ISS-PhA-IEEC-UPC-TN-001, 28/1/2015)

2. Length of correlation window?
SRD
Altimetry observables (nadir):
Both SPU units shall be able to provide power versus delay waveforms of GNSS LHCP reflected signals at the two specified GNSS frequency bands, precisely calibrated in delay and amplitude, over a delay span, number of samples and sample resolution sufficient to fulfil the scientific requirements.
A detailed analysis performed in [1]. Main conclusions:
Increasing length of tracking window beyond width of auto-correlation function (i.e. 300 m) does improve significantly the range estimation.
Increasing sampling frequency above Nyquist one helps to decorrelate samples and to improve “slightly” altimetric performance.
[1] F. Martín, S. D’Addio, A. Camps, and M. Martín-Neira, “Modeling and analysis of GNSS-R waveforms sample-to-sample correlation,” IEEE J. Sel. Top. Appl. Earth Obs. Remote Sens., vol. 7, pp.1545–1559, May 2014, doi: /JSTARS

3. In PARIS-IOD Breadboarding the baseline sampling frequency was set to fs=80 MHz for two reasons:
1. To have some oversampling: 1 bit sampling, oversampling improves SNR.
2. Interferometric GNSS-R technique very sensitive to interferences.
Required to eliminate any harmonics in or close to the RF bands.
fs = 80 MHz leads to a sampling interval of 3.75 m.
Length of correlation window: 1500 m (400 lags)
- 900 m (figure) to get noise floor, leading edge, and beginning of trailing edge,
- ~ 200 m due to undulations of Earth (ellipsoid vs. geoid over the ocean, can be larger over land and ice), and
- ~ 200 m to account for misalignment errors.
In PARIS-IOD, the final sampling frequency a sampling frequency slightly different from 80 MHz was chosen due to technological issues, and it was tuned to MHz / 2 (for I&Q) = 76.8 MHz.

4. SRD
Scatterometry observables:
Same as altimetry observable. It is further desirable that the SPU units generate Delay Doppler Maps over an extension compatible with the required spatial resolution. It is also desirable that the SPU units generate power versus delay waveforms of GNSS RHCP reflected signals.
Scatterometric applications can be based on the position of the peak of the scattered waveform, its amplitude, or the decay of the trailing edge.
In the first two cases, a correlation window as in SRD is needed.
In the last case, according the Delay Doppler Maps extend much further away ( 4 KHz, 20 C/A chips).
If the complete DDMs are to be computed, then a much longer (x 4) correlation window is required: 6000 m or 1600 lags at 80 MHz. To easy this requirement, not all lags have to be computed (some can be decimated) implementing a variable time step, shorter in the sharp transitions and longer in the smooth parts of the waveform.