The Sentinels are the satellites of European Space Agency (ESA), designed to deliver a vast amount of data and imagery for Europe’s Copernicus program.
The Sentinel-1 is equipped with twin polar orbiting satellites designed to provide a spatial data for environment and security warranting, global economic and business growth.
The satellites are to operate day-and-night and perform a synthetic aperture with radar imaging. Sentinel-1 bands allow to get imagery in all weather conditions. It works in a pre-programmed conflict-free operation mode allowing to reach a high reliability of service and create a long term consistent data archives to be used in the application based on long time series.
The Sentinel-1 Mission Guide contributes a detailed description of the satellite type itself, mission goals, Ground Segment. It also presents heritage missions, thematic areas, orbit characteristics and coverage along with instrument payload.
|Design life||7 Years (consumables for 12 years)|
|Mean Local/Solar Time||18:00 at Ascending Node|
|Orbital Period||98.6 Minutes|
|Max Eclipse Duration||19 Minutes|
|Attitude Stabilisation||3 Axis Stabilised|
|Attitude Accuracy||0.01 deg (each axis)|
|Nominal Flight Attitude||
|Attitude Profile||Geocentric and Geodetic|
|Operative Autonomy||96 hours|
|Launch Weight||2,300 kg|
|Dimensions (stowed)||3,900 x 2,600 x 2,500 mm^3|
|Solar Array Average Power||5,900 W (End-of-Life)|
|Battery Capacity||324 Ah|
|Science Data/Storage Capacity||1,410 Gigabits (End-of-Life)|
|S-Band TT&C Data Rates||4 kbps TC; 16/128/512 kbps ™ (programmable)|
|X-Band Downlink/Data Rate||2 x 260 Mbps|
|Launcher||Soyuz from Kourou|
Copyright © www.esa.int
The C-SAR instrument and supporting software were developed by EADS Astrium GMBH of Germany. The instrument was designed to provide remote sensing imagery under all weather conditions and time periods. It’s able to capture exact measurement data at high and medium resolutions, applicable for land, coastal and ice zones observations.
C-SAR is an active phased array antenna created to provide a faster scanning in different elevations and in azimuth. It allows to cover bigger areas of incidence angle, to support the ScanSAR operation, to use TOPS techniques and helps to fit the required imaging performance. The instrument has been dual channelled to transmit and receive modules which use an H/V polarised pair of waveguides making it possible to meet polarization demands.
The C-SAR’s advantage appears to be an ability to operate at C-band wavelength that cannot be blocked by cloud cover or insufficient light. It is equipped with an internal calibration scheme which allows to transmit signals then being routed into the receiver to record information. This process helps to monitor amplitude and phase to facilitate high radiometric stability.
C-SAR also includes reinforced carbon fibre plastic radiating metalized waveguides which can help ensure good radiometric stability. There are many types of elements which aren’t covered by the usual internal calibration scheme. To cover this, there is the digital chirp generator and selectable receive filter for specific band number widths to increase an onboard storage. This process takes into consideration the ground resolution dependence on incidence angle.
|Centre Frequency||5.405 GHz|
|Polarisation||HH+HV, VV+VH, VV, HH|
|Incidence angle range||20”-46”|
|Antenna type||Slotted waveguide radiators|
|Antenna size||12.3 m x 0.821 m|
|Antenna mass||880 kg|
|Azimuth beam width||0.23”|
|Elevation beam steering range||-0.9” to +0.9”|
|RF peak power||3.43”|
|Pulse width||-13.0” to +12.3”|
|Transmit duty cycle||4.368 kW, -4.075 kW|
|Receiver noise figure at model input||3 dB|
|Maximum range bandwidth||100 MHz|
|PRF (pulse repetition frequency)||1000-3000 Hz|
|ADC sampling frequency||300 MHz|
|Data quantisation||10 bit|
|Total instrument mass (including antenna)||945 kg|
|Attitude steering||Zero-Doppler steering and roll steering|
Copyright © sentinel.esa.int/web/sentinel/home
There are four exclusive acquisition modes produced using satellite bands, unique to the Sentinel-1 A & 1 B:
- Stripmap (SM)
- Interferometric wide swath (IW)
- Extra wide swath (EW)
- Wave (WV)
Here is a quick overview of the following Sensor modes:
Stripmap, interferometric wide swath, and extra wide swath modes function on a 25 minute cycle per each orbit, and the wave mode functions on a 75 minute cycle per each orbit.
The stripmap, interferometric wide swath, and extra wide swath modes use single polarisation which is either in HH or VV, and dual polarisation which can be HH+HV and VV+VH. These modes are possible due to implementation of a transmitting chain and switch between two parallel receiving chains with both H and V polarisations. The WV modes are only supported by single polarisations in either HH or VV.
Stirmap mode is used for cohesion with ERS and Envisat missions. When the satellite is put into this mode, the instrument provides constant coverage which uses a geometric resolution of 5 x 5 m with a swath width of 80 km. There are six swaths and each one has an antenna capable of generating a beam with fixed azimuth and elevation pointing. This kind of elevation beamforming is used to suppress range ambiguity. There is an option to select one of six beams by simple changing the incidence angle and the elevation beamwidth. Altogether these six overlapping swaths cover the total range of 375 kms.
Interferometric Wide Swath
IW mode, short for interferometric wide swath mode is the next one to consider. It combines swath width of 250 km with a moderate resolution of 5 m by 20 m, so appears to be the most used over land as helps to spot a target on the ground. This mode implements a newly formulated ScanSAR mode named “Terrain Observation with Progressive Scans”, TOPS shortened. This scan helps reduce the drawbacks amount of using a ScanSAR mode by shrinking the azimuth antenna pattern while going along track direction. The result is being achieved by guiding the antenna in the opposite direction of the Spotlight support.
TOPSAR targeted use is to get the same coverage and resolution as a ScanSAR, but to have it with approximately uniform SNR (Signal-to-Noise Ratio) and DTAR (Distributed Target Ambiguity Ratio). TOPS uses a rotation antenna in the azimuth direction. It’s very similar to the ScanSAR, usb-swath is switching from burst to burst to acquire several sub-swaths pseudo-simultaneously. At the same time reducing azimuth resolution as in the ScanSAR helps increase the swath coverage.
To reduce IW resolution and scalloping effects on the image, TOPS shrinks the effects of the antenna footprint onto a ground target instead of slicing the antenna pattern. In IW GRD products the interferometry is achieved by sufficient overlap of the Doppler spectrum (in the azimuth domain) and the wave number spectrum (in the elevation domain) so we get the homogeneous image quality throughout the swath.
The Sentinel-1 C-SAR system has been enabled to use TOPS burst synchronization which uses repeat-pass data-takes to help support the generation of TOPS interferograms and coherent radar change detection. For the the IW and EW modes the TOPS burst duration ranges from 0.82 secs to 0.54 secs are the worst case scenario. However, the requirement for achieving fast synchronization of less than 5ms will only happen between corresponding bursts.
TOPSAR requires that there is high accuracy within the image co-registrations. When a small co-registration error occurs in the azimuth, it will introduce an azimuth phase ramp due to the SAR antenna beam sweeping which causes the Doppler centroid overpass frequency variations of 5.5 kHz.
Extra Wide Swath Mode
This mode is similar to the IW mode, except it uses TOPSAR techniques which help cover a wider area of up to 400 kms by using five sub-swaths. This unit only takes photo at a medium resolution up to 20 by 40 m away from the ground. EW mode is mostly applicable for maritime, over ice and polar zones where it is necessary to have wide coverage and short revisit time.
WV mode resembles ERS and Evnisat mode, but with better spatial resolution, larger vignettes and a ‘leap frog’ acquisition pattern. In this mode the stripmap image is taken by using alternating elevation beams at a fixed on/off duty cycle, to get vignettes 20 by 20 km every 100 kms along the orbit. The result of this mode usage consists of several vignettes exclusively in VV or HH polarisation and each one is a separate image. Same incidence angles vignettes are separated by 200 kms per swath. Swath alternatives incidence angles will be taken between near range and far range angles at 23 degrees and 36.5 degrees. WV mode is used over ocean, giving a result of long wave mode data-takes per orbit.
Listed below are the product definitions:
Provides compressed, unprocessed condensed source packets which have additional annotations and lists auxiliary information that supports processing. Includes manifest.safe file, measurement and index data components.
Level-1 Slant-range single-look complex products (SLC):
Uses focused data in the slant-range by azimuth imaging plane, single looks, which contain phase and amplitude information. Each image pixel is represented by I and Q magnitude values. These products are done for all imagery modes.
Level-1 ground range Detected geo-referenced products (GRD):
Uses focused data which projected to ground range, then detected and multi-looked. All data is projected to ground range using an Earth ellipsoid model corrected with specified terrain height which maintains the original satellite path direction and includes complete georeferencing information. The result has nearly square spatial resolution and square pixel spacing with reduced speckle.
Level 2 products consist of geo-located geophysical products copied from Level-1.
Level-2 Ocean (OCN) products applicable for wind, wave and currents can have geophysical components obtained from the SAR data like: Ocean Wind field (OWI), Ocean Swell spectral range (OSW), Surface Radial Velocity (RVL), Availability of each one highly depends on mode of acquisition.