Sentinel 1
Sentinel-1 is a Synthetic Aperture Radar (SAR) satellite system operated by the European Space Agency (ESA) that provides high-resolution and persistent radar-based observation of the Earth. It was the first of five missions developed by the European Space Agency under the Copernicus programme. The Sentinel-1 mission is currently actively involved in the activities of two satellites. Sentinel-1A was launched on April 3, 2014, and the latest Sentinel-1C was launched on December 5, 2024 by the VEGA-C rocket. Sentinel-1B was a prior mission to Sentinel-1C having launched on April 25, 2016, but its mission ended in 2022. Sentinel-1 images can be downloaded for processing, for example, from the satellite data portal.
Video 1. Introduction to Sentinel-1 mission (source: ESA)
Aperture is a term that expresses the light-transmitting opening in an optical system. For example, in photography, aperture is the opening in the lens through which light enters the camera's sensor or film. Synthetic aperture means that an artificially enlarged aperture is used, rather than a physically larger antenna. This is achieved through special computational methods and signal processing that allow the radar to produce very sharp images and increase resolution as if it were a larger antenna, when in fact the antenna used is relatively small.
Unlike passive optical sensors that require sunlight, SAR (Synthetic Aperture Radar) works actively by sending a microwave signal itself to illuminate the Earth's surface from a certain angle. This radar sends microwave signals towards Earth and receives some of the reflected energy. While optical imaging, as in the case of Sentinel-2, is similar to the interpretation of photography, SAR requires a different way of thinking, as the signal reacts with surface material properties such as soil structure and moisture. Unlike optical satellites, Sentinel-1 can penetrate clouds to collect data about the ground.
Interpreting a SAR image can be tricky because it does not usually represent the landscape in the colour or optical format we are used to seeing in photographs, but is traditionally displayed in grey tones and has a kind of rough and grainy appearance (Figure 1). SAR measures reflected radar signals, and different ground structures such as roads, buildings, forests and bodies of water reflect radar signals differently. Read more about it on NASA's website.
Figure 1: Sentinel-1 radar image of Pärnu Bay and its surroundings (date 04.03.2024).
In general, as surface roughness and structure increases, backscatter also increases. A rough, uneven surface scatters the microwave energy, returning much of it to the radar antenna, causing a brighter object. Flat, even, and smooth surfaces, on the other hand, reflect away to a large extent, creating a darker object. Also, if an area or object has a complex structure, such as a forest, it will appear brighter as the signal interacts with leaves, branches and trunks, causing more of the signal to return to the sensor.
Radar measurements are also influenced by the Earth's dielectric constant. The dielectric constant determines the reflectivity and electrical conductivity of materials. Water has a significantly higher dielectric constant compared to other dry natural surfaces. Therefore, soil and vegetation moisture causes substantial reflectivity.
These characteristics make SAR technology very suitable for various applications, such as geology and geomorphology, soil moisture assessment, land cover analysis, oceanography, and the marine domain. The polarimetric and interferometric properties of SAR prove to be particularly valuable, enabling accurate mapping of soil and surface structure and monitoring of changes.
Polarisation is an important concept for making sense of the information collected by radar. The direction of vibration of electromagnetic waves can be random (non-specific) or fixed (polarised). Sentinel-1 SAR technology allows electromagnetic radiation to be transmitted and received in different polarizations (Figure 2). In turn, the ground and objects have features or structures that cause the radar signal to be reflected in a vertical or horizontal polarization. The purpose of polarization is to measure the direction of the vibrations of the reflected signal, which helps the radar to understand the features and structure of the object and then generate the radar image. In polarisation combinations, the first letter indicates the transmit polarisation and the second letter the receive polarisation. Read more about it on NASA's website.
Figure 2. Visual example of polarization combinations; focus on the plane of line movement and the arrows (source: EARTHDATA).
VV: vertical transmission, vertical reception – the radar transmits a vertically polarized signal and measures the backscatter with the same vertical polarization. For example, imagine a vertically standing pole, such as a streetlight or a tall tree. VV polarization reflects strongly from such vertical objects. Therefore, VV is well-suited for analyzing vegetation and buildings, as they contain many vertical elements. For instance, it is used for assessing forest height and density.
VH: vertical transmission, horizontal reception – the radar transmits a vertically polarized signal but receives the backscatter in horizontal polarization. Figuratively speaking, VH polarization tracks how vertical structures, such as tree trunks, alter the direction of the signal reflection so that it scatters horizontally. Therefore, VH is useful for studying landscape and surface details. For example, it can be used to assess forest density or the condition of agricultural land. In dense forests with many branches and leaves, the radar wave experiences greater horizontal scattering to various surfaces. In areas of the forest where there has been significant logging, there is less scattering, or fewer backscattered signals. Using the same logic, VH also shows differences between tall trees (vertical) and shrubs or low plants (a combination of vertical and horizontal elements). Additionally, VH can detect corner reflections (for instance, when the radar signal first bounces off the ground and then from a tree trunk), which helps distinguish more complex terrain forms. VH is also a classic choice for detecting floods and analyzing the land surface, as it is more sensitive to backscattered signals from water surfaces.
HV: horizontal transmission, vertical reception – the radar transmits a horizontally polarized signal and receives the backscatter in vertical polarization. HV polarization helps analyze how surface-level horizontal structures, such as roads, fields, or uneven areas, affect the signal's backscatter. Imagine observing a surface covered with rocks and uneven terrain. The radar transmits a horizontal signal (H). When the signal bounces off rocks and uneven surfaces, it changes direction because the surface is not perfectly flat. Therefore, HV is well-suited for studying uneven terrain, such as plains and open areas with horizontal patterns. For example, it can be used to detect roads or agricultural field boundaries.
HH: horizontal transmission, horizontal reception – the radar transmits a horizontally polarized signal and measures the backscatter with the same horizontal polarization. HH tracks how the general horizontal characteristics of the surface, such as flatness or large horizontal areas, reflect the signal. Therefore, HH is good for general surface exploration, especially for analyzing large open areas, such as flat fields or wetlands. It can also be used for assessing vegetation structure, such as in crop analysis.
Useful reading and references:
Sentinel-1 | SentiVista explore the satellite's structure and read more about the mission and its greatest successes.
ARSET - Fundamentals of Remote Sensing | NASA Applied Sciences
ARSET - Disaster Assessment Using Synthetic Aperture Radar | NASA Applied Sciences