Sentinel-1 Mission Overview
The Sentinel-1 mission (Torres et al., 2012) objectives are to provide Synthetic Aperture Radar (SAR) observations for various operational applications, including: * sea ice zone and arctic environment monitoring * marine environment surveillance * land surface motion and associated risk monitoring * land surface mapping (forest, water and soil, agriculture) * aid in humanitarian crisis situations
The Sentinel-1 mission comprises a constellation of two satellites (Sentinel-1A, launched on 3 April 2014 and Sentinel-1B, launched on 25 April 2016) that fly in a polar orbit at 693 km altitude, with a local overpass time of ~18:00 h. Both satellites and the on-board instrument have a designed 7-year lifetime, extendable to 12 years. The two satellites enable a 6-day global coverage and a 3-day revisit frequency, the latter improving to ~1 day at arctic latitudes. See Table 1 for an overview of the mission’s main characteristics.
Table 1: Sentinel-1 main flight and instrument characteristics.
| Characteristic | Description |
|---|---|
| Design life | 7 Years |
| Orbit altitude | 693 km |
| Repeat cycle | 6 days (2 satellites) |
| Mean Local Overpass Time | 18:00 at Ascending Node |
| Orbital Period | 98.6 minutes |
| Centre Frequency | 5.405 GHz |
| Bandwidth | 0-100 MHz |
| Polarization | HH+HV, VV+VH, VV, HH |
| Incidence angle range | 20o-46o |
| Look direction ` | right |
| Antenna size | 12.3 m \(\times\) 0.821 m |
C-Band Synthetic Aperture Radar (C-SAR)
The instrument on-board Sentinel-1 is an actively imaging C-band Synthetic Aperture Radar (C-SAR), which enables observations during both day and night (no dependency on reflected sunlight), and under all weather conditions, thanks to the radar beam penetrating through clouds. Sentinel-1’s C-SAR transmits microwave pulses with a frequency of 5.405 GHz (\(\lambda\)=5.55 cm). Below, a short explanation on radar imaging and SAR is given.
Synthetic Aperture Radar (SAR) principle
A radar system generates its own radio waves and transmits them from its antenna towards a target at a certain angle, the so-called incidence angle. Depending on the target properties and the imaging geometry, the antenna will receive a certain degree of the transmitted pulses. The elapsed time between pulse transmittance and reception (backscatter) is proportional to the target’s distance (range) from the antenna. The second dimension is the along-track (azimuth) direction. In the azimuth dimension, the surface is scanned by the beam moving at a rate equal to the satellite speed. An image is built from the backscattered signals in both dimensions, see Figure 1 for a schematic.
Figure 1: Radar imaging geometry. (Credit: NASA)
The radar image spatial resolution differs between the range and azimuth (along-track) direction. The latter is determined by the radar beam’s footprint at the surface, which in turn is inversely proportional to the antenna length. This implies that finer azimuth resolutions require a longer antenna, which of course has its physical limitations. To mitigate this physical size limitation, the antenna’s motion along the azimuth direction is used to synthesize a longer antenna, which forms the SAR basis. Such a synthesis is possible because a suface target remains within the radar beam for many consecutive pulses (see Figure 2 for a conceptual image). Adding up these backscattered signals appropriately allows to synthesize a large antenna with a much narrower beam width, resulting in an enhanced azimuthal spatial resolution.
Figure 2: Synthetic Aperture Radar concept. (Credit: NASA)
Acquistion modes
Sentinel-1 has four different acquisition modes, see Table 2, which are used for various land surface and sea/ocean applications, see Table 3.
Table 2: Sentinel-1 acquisition modes.
| Acquisition mode | spatial resolution [m] | swath width [km] |
|---|---|---|
| SM | 5 \(\times\) 5 | 80 |
| IW | 5 \(\times\) 20 | 250 |
| EW | 25 \(\times\) 100 | 400 |
| WV | 5 \(\times\) 20 | 20 @100 km intervals |
Table 3: Sentinel-1 acquisition modes and applications. The Acquisition modes: SM = Strip Map, IW = Interferometric Wide Swath, EW = Extra Wide Swath, WV = Wave
| Application | SM | IW | EW | WV |
|---|---|---|---|---|
| Arctic and sea-ice | X | X | ||
| Open ocean ship surveillance | X | X | ||
| Oil pollution monitoring | X | X | ||
| Marine winds | X | X | X | |
| Forestry | X | |||
| Agriculture | X | |||
| Urban deformation mapping | X | |||
| Flood monitoring | X | X | ||
| Earthquake analysis | X | X | ||
| Landslide and volcano monitoring | X | X |
More information is available on the acquisition modes’ techniques and Sentinel-1 data and products.