With this block you can access Sentinel 1 and PALSAR (Mosaic) data.
Sentinel 1 is a European C-band radar imaging system and provides global data every 12 days.
PALSAR is a Japanese L-band radar imaging system. Earth Blox uses its annual mosaic which is a continuous global map for an entire year.
The data are all georectified and analysis ready.
Using the Radar Block
TABLE OF CONTENTS
- Using the Radar Block
- When should you use radar imagery?
- Data Properties
- How to use the radar block
When should you use radar imagery?
- Radar imagers can see through clouds, so every image is guaranteed cloud free.
- Sentinel 1 works well for picking up objects on the ocean, including ships, wind turbines and icebergs, as well as artificial structures on the land surface (buildings, bridges, etc).
- Radar is also good for tracking changes in agricultural crops.
- Radar is particularly good at picking up areas of flooding. See this example of flood mapping in Mozambique.
- PALSAR, with its longer wavelengths, is also good at mapping forests and forest change. See this example of mapping deforestation in Liberia.
Sentinel 1 (S1)
- Sentinel 1 is a C-band radar imaging system that provides global data every 12 days. When both of the satellites are operational, there is a 6-day exact repeat cycle. However, the nature of the orbit allows overlapping images as frequently as every 3 days at the equator, less than 1 day at the Arctic, and 1-3 days over Europe, Canada and the main shipping routes.
- The first Sentinel 1 data is from 17th October 2014 until the present day.
- C-band means it has wavelengths of around 5cm long. This means it is good for tracking changes in agricultural crops, or other short vegetation.
- Since radars are active sensors, Sentinel 1 can collect data on both the day (ascending) and night (descending) sides of the orbit.
- All the S1 data in Earth Blox are derived from Interferometric Wide Swath mode. This means the spatial resolution is 20x20m. (Note: the grid spacing of the pixels is 10x10m, but the resolution is still 20x20m.)
- When you choose Sentinel 1, the radar block looks like this:
- PALSAR is a Japanese L-band radar imaging system.
- L-band has wavelengths of around 23cm and so is good for discriminating larger vegetation (i.e. trees, rather than crops). It will provide a higher forest/non-forest contrast than C-band.
- Earth Blox uses its annual mosaic which is a continuous global map for an entire year.
- Data is available from 2007-2010, and then annually from 2015.
- PALSAR Forest Change products are also visible under the FOREST->Other forest change datasets block.
How to use the radar block
- The block is available under SATELLITE IMAGERY in the toolbox.
- By default, it will include the radar visualisation block.
- Your first choice is between Sentinel 1 and PALSAR.
- The option to choose between dB and DN refers to the way the image is scaled. DN=digital number, which is the linear scaling of the radar data. dB=deciBels, which is a logarithmic scaling of the data and often makes it easier to see the features in the image.
- As with other data blocks, you need to select a time period and an area of interest.
- The Orbit option refers to which side of the orbit the image was collected on: ascending is going North (and looking East), whereas descending is going South (and looking West).
- If you choose Both, you get twice as many images. If you are looking at highly varied terrain, it is wise to choose only one or the other, not both.
- The options in the radar visualisation block refer to the polarisation channels.
- The HH, VV, VH and HV options in the RGB output refer to the polarisation channel. Radar signals can be transmitted or received as either horizontal (H) or vertical (V) waves. For instance, the HH channel is both transmitting and receiving in H, whereas the HV channel is transmitting H and receiving in V. (the "cross-polarisation").
- Sentinel 1 is a little more complicated. In the Polar regions, the satellite collects HH and HV. Everywhere else it collects VV and VH. This is because HH gives better contrast between sea ice and open water.
- When using Sentinel 1, for most uses either VV or VH will be the best option to select.
- PALSAR has HH and HV only. HV provides the best contrast between forests and non-forest.
- Finally, the Composite option allows you to combine multiple dates together to create a new image. If you select a time period that covers multiple image acquisitions, then using the Mean option create an image layer that is the average of those scenes. The other options would allocate to the final pixel value the minimum or maximum value, or the median, over all the images that have data over that pixel.
- Remember to add a Layer name.
- You are now ready to go. Click RUN WORKFLOW and view the results on the map.
You can use the Time series block to view a sequence of radar images. Connect the block within the radar block, as shown below. Then within the time series block, place the radar visualisation block.
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