# A New Orbiting Deployable System for Small Satellite Observations for Ecology and Earth Observation
## 3 Study Cases
### Application 3: Coastal Area Monitoring
#### 3.3.4 Altimetry: Sea Surface Wind and Height  
Altimetry has been used to retrieve surface topography (including sea level and wave height), ocean currents, and bathymetry (submarine topography) (e.g., [168; 169]). Additionally, it is one of the most reliable ways to observe mesoscale eddies, detected through the small displacements of the sea surface elevation, and scalar wind speeds (e.g., [170]). The most common instruments for altimetry are nadir pointing radar altimeters, sampling along the ground track (e.g., [168; 171]). The radar signal is assembled in a waveform of 20 Hz frequency (\(\sim\)300 m along track), where the peaked waveforms correspond to a low sea state condition, whereas broad waveforms correspond to high sea state conditions.  
The sea surface wind (SSW) field is a key parameter affecting oceanic and atmospheric processes. Satellite-derived sea surface wind observations are currently routinely used in a wide variety of oceanic, atmospheric, and climate applications. Within this context, real aperture microwave active (i.e., scatterometers [172]) and passive (i.e., radiometers [173]) sensors are widely employed to provide sea surface wind observations to the oceanic and atmospheric operational communities. Although not specifically designed to estimate sea surface wind field, the SAR has been shown to result in reliable enough estimates of sea surface wind field on a finer spatial resolution scale and, therefore, has potential for coastal area applications [174; 175].  
Sea surface height (SSH) represents the difference of the ocean surface from a constant geopotential surface of reference (level of no motion). SSH is, therefore, key to measuring the thermal expansion and contraction occurring through the different seasons, or to providing information on tides and wind, e.g., [176]. The wave height is estimated from the extent of the leading edge, and typically averaged every 7 km along the tracks (the so-called 1-Hz sampling rate) (e.g., [177; 178]). Initially designed for observations on the high seas, such as the sea level change, ocean circulation, and ocean tides [179], the satellite radar has recently shown its relevance for observations of the coastal regions too [180; 181].  
Large satellites, such as TOPEX/Poseidon and Sentinel-6, can provide altimetric measurements of the global ocean, but they have been also cooperating with the small satellite series Jason (Jason-1, 2, and 3), equipped with a dual frequencies (C and Ku-band) radar altimeter [182; 183], for monitoring the global ocean surface topography and thus measuring variations in the height above the sea surface [184].  
However, altimeter-derived SSH measurements can come with large errors due to the rapid variation of the coastal topographic surfaces, leading to complex waveforms that are not accurately analyzed by classical re-tracking algorithms [185]. Mesoscale processes instead require altimetric measurements with high spatial and temporal resolutions. At the same time, measuring satellite altitude requires the ability to accurately and independently determine a satellite's orbital trajectory, e.g., its exact longitude, latitude, and altitude. In more recent years, the Global Navigation Satellite System reflectometry (GNSS-R) technique was introduced to measure ocean parameters such as SSW and SSH [186]. In this case, altimetry calculation relies on the position and timing information obtained from several simultaneous measurements of the entire GPS constellation, distributed over an area of thousands of km across-track [187; 188].