News Excerpt:

The Kalpana 1 and INSATs 3A, 3D, and 3DR satellites have bolstered India’s weather monitoring and warning services.

How do satellites obtain the images of fog from space?

• According to a paper published by IMD scientists, the INSAT 3D satellite has a red-green-blue, or RGB, imager whose images’ colours are determined by two factors: solar reflectance and brightness temperature.
• Solar reflectance is a ratio of the amount of solar energy reflected by a surface and the amount of solar energy incident on it. 
• Brightness temperature means the relationship between the temperature of an object and the corresponding brightness of its surface. 
• It is different from temperature as we usually understand it – like the temperature we ‘feel’ when we touch a glass of hot tea – because brightness temperature also has to do with how the tea glass emits the thermal radiation, which is at different frequencies in different directions.

Day Microphysics:

• INSAT 3D’s ‘day microphysics’ data component studies solar reflectance at three wavelengths: 0.5 µm (visible radiation), 1.6 µm (shortwave infrared radiation) and 10.8 µm (thermal infrared radiation). 
  • The strength of the 0.5-µm visible signal determines the amount of green colour; 
  • The strength of the 1.6-µm shortwave infrared signal determines the amount of red colour; 
  • The strength of the 10.8-µm thermal infrared signal determines the amount of blue colour. 
  • This way, the INSAT 3D computer determines the colour on each point of the image.
• How does the satellite track snow?: 
  • While the solar reflectance of snow and that of clouds is similar in the visible part of the spectrum, snow strongly absorbs radiation of wavelength 1.6 µm, i.e. shortwave infrared. 
  • As a result, when the satellite tracks snow, the red component of the colour scheme becomes very weak.

Night Microphysics:

• Here, two colours are determined not by a single signal but by the strength of the difference between two signals. 
• The computer determines the amount of red colour according to the difference between two thermal infrared signals: 12 µm and 10 µm. 
• The amount of green colour varies according to the difference between a thermal infrared and a middle infrared signal: 10.8 µm and 3.9 µm.
• The amount of blue colour is not a difference but is determined by the strength of a thermal infrared signal of wavelength 10.8 µm.

• For example, in the image above, the data indicates three kinds of clouds. (‘K’ denotes the temperature differences in kelvin.) 
• A mature cumulonimbus (“CB”) cell, possibly part of a tropical storm, hangs over West Bengal and is visible mostly in red, but whose blue component indicates it is also very cold. 
• Further north of Delhi, flecks of green dominate, indicating a preponderance of lower clouds. 
• The sky even further north is dominated by a high and heavy cloud system that encompasses lower clouds as well.

Combining Day and Night Microphysics:

• By combining day and night microphysics data, atmospheric scientists can elucidate the presence of moisture droplets of different shapes and temperature differences over time, and in turn track the formation, evolution and depletion of cyclones and other weather events.
  • For example, taking advantage of the fact that INSAT 3D can produce images based on signals of multiple wavelengths.
• Both INSAT 3D and INSAT 3DR use radiometers to make their spectral measurements. 
  • A radiometer is a device that measures various useful properties of radiation, typically by taking advantage of radiation’s interaction with matter, for example in the form of temperature or electrical activity). 
• Both satellites also carry atmospheric sounders. 
  • Sounders measure temperature and humidity, and study water vapour as a function of their heights from the ground.
• Scientists combine the radiometer and sounder measurements to understand various atmospheric characteristics.

Additional Information:

INSAT 3D:

• INSAT-3D is a dedicated meteorological spacecraft designed for enhanced meteorological observation and monitoring of land and ocean surfaces of weather forecasting and disaster warning.
• It was launched on July 26, 2013 with a lift of mass of about 2100 kg by ARIANE-5/GSLV-MK II /Soyuz launcher.
• The satellite has 3 payloads -
• Meteorological (MET) - IMAGER and SOUNDER
• Data Relay Transponder (DRT)
• Satellite Aided Search and Rescue (SAS&R)
• The geophysical parameters that will be extracted from INSAT3D are -
• Outgoing Longwave Radiation (OLR), 
• Quantitative Precipitation Estimation (QPE), 
• Sea Surface Temperature (SST), 
• Snow Cover, Snow Depth, 
• Fire, Smoke, Aerosol, 
• Cloud Motion Vector (CMV), Water Vapour Wind (WVW), 
• Upper Tropospheric Humidity (UTH), Temperature, Humidity Profile and Total Ozone, 
• Fog, Visible Wind Vector and other value added parameters from Imager and Sounder.

INSAT 3DR:

• INSAT-3DR similar to INSAT-3D, is an advanced meteorological satellite of India configured with an imaging System and an Atmospheric Sounder. 
• The significant improvements incorporated in INSAT-3DR are:
  • Imaging in Middle Infrared band to provide night time pictures of low clouds and fog;
  • Imaging in two Thermal Infrared bands for estimation of Sea Surface Temperature (SST) with better accuracy;
  • Higher Spatial Resolution in the Visible and Thermal Infrared bands.
• It carries a Data Relay Transponder as well as a Search and Rescue Transponder. 
• It has a lift-off mass of 2211 kg, which includes about 1255 kg of propellant. 
  • The propellant carried by INSAT-3DR is mainly required to raise the satellite from the Geosynchronous Transfer Orbit (GTO) to its final Geostationary Orbit and to maintain the satellite in its orbital slot during its life. 
  • The satellite has a solar array generating 1700 Watts of power
• Payloads of INSAT-3DR:
• It carries a multi spectral Imager, 19 channel Sounder, Data Relay Transponder and Search and Rescue Transponder.

Kalpana 1:

• METSAT (renamed as Kalpana - 1 on February 5, 2003 after the Indian born American Astronaut Dr. Kalpana Chawla, who died on February 1, 2003 in the US Space Shuttle Columbia disaster) is the first in the series of exclusive meteorological satellites built by ISRO.
• It provides the first set to transponders for Swaran Jayanti Vidya Vikas Antariksh Upagraha Yojana (Vidya Vahini) for interactive training and developmental communication giving fillip to the training and developmental Communication channel of INSAT.
• Spacecraft Mass: 1060 kg mass (at Lift – off)
• Payload: Very High Resolution Radiometer (VHRR), Data Relay Transponder (DRT)
• Launch date: 12 September 2002
• Launch site: SHAR, Sriharikota
• Launch vehicle: PSLV – C4
• Orbit: Geostationary

INSAT 3DS (Upcoming):

• In February 2024, the Indian Space Research Organisation is expected to launch the INSAT 3DS meteorological satellite onboard its GSLV Mk II launch vehicle, with a launch mass of two tonnes.
• The rocket uses cryogenic liquid propellants for all its three stages - this use of liquid fuel makes for a more complicated engineering, but it allows for much higher lift-off weight capacity.
• This mission was initiated as a part of a collaboration between ISRO and IMD to improve the network of climate services.
• While “3DR” stood for “3D repeat”, “3DS” stands for “3D second repeat”.