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Sentinel 2 satellites for remote sensing in agriculture

Following in the vein of the articles on precision agriculture and the possibilities of vegetation indices , today we are going to talk about one of the most current and modern series of publicly financed satellites that will allow us to do remote sensing at a cost of 0 in the acquisition of images. Let’s see what the Copernicus program consists of and especially the Sentinel 2 satellites .

WHAT ABOUT THE ESA COPERNICUS PROGRAM?

Before talking about satellite remote sensing and Sentinel 2 satellites and their application for agriculture, we will start by talking about ESA. A European agency that is surprisingly little known by the citizens themselves. ESA (European Space Agengy) founded in 1975 by 10 EU countries, already has a historical journey in world space exploration that we will not mention in this article. Today there are already 22 EU states funding and collaborating to create very ambitious space programs. One of them, that of exploration and terrestrial observation at all levels is Copernicus in which other world institutions also collaborate. A program to put a series of satellites with a multitude of sensors into orbitand to carry out studies in a large number of scientific disciplines. They are basically divided into 6 large groups:

  • Soil management (where water, forest and agriculture resources come in (which is what we are dealing with in this bog).
  • The marine environment
  • Response to emergency situations (fires, for example).
  • Safety
  • The atmosphere
  • Climate change

For this, the program will put 6 cutting-edge satellites called Sentinel, which will capture images and information with different sensors embedded in each of them. Briefly summarizing:

  • Sentinel-1 : Pair of satellites 1A and 1B in polar orbit providing radar images that allow monitoring both day and night weather conditions both on land and in the oceans.
  • Sentinel-2 : Another pair of satellites 2A and 2B in polar orbit also to provide high resolution optical (multispectral) images that will allow monitoring of vegetation among other studies.
  • Sentinel-3: It also provides high precision radar and optical data but this time with a focus more related to the topography of continents and oceans, measuring for example the temperature of the land and sea surface and altimetry. The first one (3-A) was launched in 2016 and the 3-B is scheduled to launch in 2017
  • Sentinel-4 : This mission is to monitor the atmospheric composition and especially the air quality by measuring parameters such as nitrogen oxides, ozone, sulfur dioxide, formaldehyde … The great asset of this series of satellites is its frequency of revision, which will be every hour! Providing time series with great temporal precision.
  • Sentinel-5: It has other sensors than the Sentinel-4 but they will work together since this series will also monitor the atmospheric composition with a high revision frequency.
  • Sentinel-6: Its purpose is the study of sea level. That is its great task.

Sentinel 4 is planned to launch in 2019. Sentinel 5 and 6 are planned to launch in 2020.

 

 

AND WHAT DOES THIS HAVE TO DO WITH AGRICULTURE?

Of the satellites presented here, the one that really interests us for the cutives is the Sentinel-2 series whose satellites are already orbiting us, providing us with multispectral images every 5 days for free. They are two twin satellites in an orbit of 10 days each.

Of course, they are not the only satellites in orbit that provide this information, but they will be the most accurate and modern in providing it for free . Other programs that also provide multispectral imaging today are:

LANDSAT series: Specifically the LandSat 7 and 8 satellites. Also free but with 3 times lower resolutions than the Sentinels. They are also public and the images are free to purchase.

From here on, the Worldview 1-4, SPOT, DEIMOS etc satellites are all private initiatives and the images have a cost . The great advantage of these is their resolution that even being “older” than the Sentinel 2 give much higher resolutions, up to 0.3 m / pixel in the case of the Worldview-4 for example.

Returning to the Sentinel 2, we can say that thanks to their images, from now on we will be able to do interesting studies on remote sensing in agriculture from home and at no cost (except for your time and the consumption of your computer). Let’s see a little more in depth what their sensors offer.

LET’S TALK ABOUT SENTINEL 2 BANDS

The two satellites in the Sentinel 2 series capture multiple layers of the same image. Its multispectral sensor is capable of collecting information in 13 bands of the electromagnetic spectrum at a certain resolution that we will see later. If you are still not sure what a spectrum band is and this multispectral sounds like Chinese to you, we recommend that you read the article we recently published on vegetation indices used in precision agriculture.

A band is nothing more than a range or interval of wavelengths. The Sentinel 2 MSI sensor captures 13 bands or 13 wavelength ranges that we specify in the following table.

 

BandsSpatial resolution (m)Spectral resolution (nm)
Band 1 (Aerosol)60443
Band 2 (Blue)10490
Band 3 (Green)10560
Band 4 (Red)10665
Band 5 (NIR)20705
Band 6 (NIR)20740
Band 7 (NIR)20783
Band 8 (NIR)10842
Band 8a20865
Band 9 (Water vapor)609945
Band 10 (Cirrus)601375
Band 11 (SWIR)201610
Band 12 (SWIR)202190
Showing from 1 to 13 of 13 records

 

Within the bands provided by the satellite, in principle in agriculture those that interest us are those of near infrared, red, near red and green . Blue is also interesting for RGB compositions that give us the real color of the image and because in some indexes such as the EVI (Enhanced Vegetation Index), the blue band is also used.

[alert style = »green»] Do you want to know what NDVI consists of, all its fundamentals and how to apply it? Here you have an extensive article about this index. [/ Alert]

LET’S TALK ABOUT RESOLUTIONS

In remote sensing, whatever the type (satellite, RPAs or manned aircraft) there are a series of resolutions that define the precision of the multispectral image.

SPATIAL RESOLUTION

It is the precision that the sensor can achieve per pixel, that is, the real land area represented in a digital level of information (pixel). Sentinel 2, according to which bands (see table above) achieves resolutions of 10m / pixel. This means that a pixel represents 10 × 10 = 100 m 2 of land.

SPECTRAL RESOLUTION

This resolution refers to the bands or ranges of the spectrum that the sensor is capable of capturing. In the previous section we have already talked about the Sentinel 2 bands.

RADIOMETRIC RESOLUTION

The measurement of a multispectral image is expressed in reflectance, that is, the amount of electromagnetic radiation that a body reflects when radiation from the sun strikes. Each digital level or pixel will have a reflectance or radiance value (they are not the same but they are equivalent). Radiometric resolution is, therefore, the sensor’s ability to discriminate ranges of radiance . For example, Landsat 8 has a radiometric resolution of 256 levels (8 bits). Sentinel 2 for example achieves a value of 4096 intensity levels (12 bits) .

TEMPORAL RESOLUTION

And finally we will talk about its temporal resolution, a factor of great importance for studies in agriculture. Until now, (except paying) you could get free Landsat images on a specific piece of land every 16 days. This, depending on which crops and times of cultivation, may be insufficient for a specific study.

Sentinel 2 with its 2 twin satellites, provide a temporal resolution of 5 days, considerably increasing its frequency of review by the same point.

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