3.1.3. Signals of Opportunity (SoOp): GNSS-R, and Receiver of SoOp

The utmost sensors used for oceanography (SARs and radar altimeters) have features that make them difficult to board on nano-satellites, most notably the power requirements, and the antenna size. An attractive option to explore the sea surface topography is the use of reflected Global Navigation Satellite Systems (GNSS) signals [34,35]. GNSS reflectometry is a favourable technique to perform some ocean measurements with small satellites [36]. The advantage of this technique is the capability to operate in all-weather conditions with a spatial resolution of ∼25 km. In the last two decades, a big effort has been made to develop models that prove the feasibility of using GNSS signals, proving to be successful for sea surface, altimetry measurements [37,38], wind speed [39,40], soil moisture [41–45], ice thickness [46], ice cover [47], and others. A few characteristics of GNSS-R missions have been identified and summarized in Table A6.

The current and planned missions using GNSS-R technology are presented in Table A6, such as TechDemosat-1 (TDS-1) [48], the Cyclone Global Navigation Satellite System (CYGNSS) [36], and FSSCAT [49,50].

TDS-1 was launched in June 2014 and it includes a GNSS-R payload with a mass of around 1.5 kg and approximately 10 W power consumption . It demonstrated the capabilities of GNSS-R for low power, low cost, and low mass. This payload measures complete delay-Doppler Maps (DDM) providing scientific-quality data [51]. The CYGNSS mission takes advantage of a constellation of eight microsatellites (weighting 17.6 kg) that provide nearly gap-free Earth coverage over Equatorial regions, with an average revisit time of seven hours and a median revisit time of three hours. CYGNSS was launched on December 2016. FFSCAT is a tandem mission of two 6U Cubesats (3Cat-5/A and 3Cat-5/B) featuring a hybrid microwave radiometer/GNSS- Reflectometer and a hyperspectral imager. FSSCAT will be the first nanosatellite mission to complement the Copernicus program [49]. Its main focus is over Polar Regions, and it will be launched in 2019.

The European Space Agency (ESA) conducted the studies of a space-borne demonstrator called Passive Reflectometry and Interferometry System In-Orbit Demonstrator (PARIS IoD) [52–54]. PARIS IoD was later reincarnated into the GEROS experiment on board the International Space Station [55], but it was never implemented.

Novel techniques using signals of opportunity, such as from Direct Broadcast Satellite (DBS) television at Ku- or X-bands, can be used to measure precipitation and winds over the sea surface [56], and these signals are sensitive to detect fluctuations of the sea surface roughness.

In this regard, the SGR-ReSI [57] payload onboard TDS-1 is selected as a possible candidate to cover the measurements with gaps such as wind speed over the sea surface (horizontal), sea ice cover, sea ice thickness, and soil moisture [6].

#### 3.1.4. Receiver: Automatic Identification System (AIS)

Although not an EO technique, Automatic identification systems (AIS) could also be a potential technology for emergency and management for the Copernicus services. AIS is an automatic tracking system used by ships and vessel traffic services. The AIS is a standardized receiver using two channels in the maritime VHF band. It has a positioning system with electronic navigation sensors such as a gyrocompass or rate of turn indicator. The main advantages of this system are the accuraccy of the position, course, and speed information. Additionally, the International Maritime Organization (IMO) has normative guidelines to put AIS on board for all passenger ships larger than 300 GT. Additionally, the latency can be reduced thanks to an update rate of ∼3 min. In addition, it is suitable for nano-satellites [58] (low size, low power, low weight, and these can be translated into low system cost) (Table A7).
