GPS Global Positioning System

Global Positioning System is a constellation of 24 satellites which orbit at an altitudeof 20,200 km above the earth’s surface in an inclined elliptical orbit. GPS providesvery precise latitude and longitude details which will be in centimetres.

GPS coversthe whole earth and any point on the earth surface can be pin-pointed. The reasonfor choosing GPS satellite is because of its continuous availability throughout theyear.

If the future GPS satellites like GLONASS, GALILEO, etc., are equipped withirradiation measuring instruments like Total Irradiance Monitor [SORCE], etc., andmoreover, if the measurements are made continuously over the earth’s atmospherethen the amount of irradiation data available are enormous which will lead to betterresults in irradiance monitoring.

During any particular year at least a minimum of 12satellites will be facing the Sun and monitoring the irradiance over that region and theorbital paths of the satellite will be well-known.

One more added advantage of usingthe GPS satellite is the repetitive coverage over an area on a single day, so the variationin the same area over a short period of time can also be calculated.

Space- and ground-based measurement and monitoring

At any instant nearly 12 satellites will be facing the Sun and they will measure theirradiance. The orbital path of the satellites are predefined and well-known. To measurethe irradiation precisely the measurement should be made at ground also at thesame instant of time when the satellite is passing over a certain area.

The ground measurementsare performed to reveal the solar irradiation available on the ground afterseveral atmospheric path losses. The reasons to measure the irradiation both on topof the atmosphere and on the ground are to understand the changes occurring in theirradiation during their travel to the surface, etc. Ground instruments are required atseveral clearly available places for measurement.

For example, if the satellite is at a specific location in space and their co-ordinates interms of latitude and longitude are knownby pre-calculations, e.g., 13◦00 N80◦00 E,the satellite measures the irradiation at that particular position in space and if themeasurements are made in the ground at the same place then we can compare thechanges in the values.

If more number of ground stations are established then for eachground station data we will obtain the top of the atmosphere data from the satellite.More stations will lead to better results, because we can compare more ground datawith the satellite data.

For a particular instant of time and at a particular point we will get the irradiation atthe top of the atmosphere, at the ground, the difference in irradiation, the amount of loss during that time and the variation in irradiation when compared to the correspondingprevious readings at the same point of observation.

Though the irradiation variationon the top of the atmosphere is very small, monitoring those small variations will alsohelp us to understand the influence of solar radiation on earth. The ground monitoringis difficult also because of the multiple scattering of radiation from the ground whichmakes the reading difficult.

Though there are some difficulties, the data obtained fromthese calculations will be better than the interpolated or extrapolated values which arewidely used.