The ISRO's, Polar Satellite Launch Vehicle launches off onboard DRDO's imaging satellite 'Microsat R' along with student satellite 'Kalamsat' at Satish Dhawan Space centre in Sriharikota

“Launch time depends on the requirement of the satellite. Here, they wanted the satellite to cross south to north of equator around noon, maybe due to sunlight condition," K Sivan, ISRO chairman said. "Our launcher goes from north to south of equator. To meet the requirement of the satellite, we needed to have a launch during the night,” ISRO chief said

CHENNAI: As PSLV lifted off from Satish Dhawan Space Centre in Sriharikota soaring through the night skies for the second time in five months, Microsat-R, the main payload, played a key role in deciding the launch time. The 740 kg imaging satellite was programmed to cross the equator every day around noon local time when the sun illuminates the Indian region. At 11.37pm on Thursday, PSLV-C44 lifted off with Microsat-R and Kalamsat from the first launch pad in the Indian spaceport at Sriharikota.

“Launch time depends on the requirement of the satellite. Here, they wanted the satellite to cross south to north of equator around noon, maybe due to sunlight condition. Our launcher goes from north to south of equator. To meet the requirement of the satellite, we needed to have a night launch,” said K Sivan, ISRO chairman. “It is the satellite team that decides when they want the satellite payload to take photographs. It could be during maximum sunlight condition or minimum cloud condition. Launch time depends on that.”

Microsat-R, an imaging satellite designed for military applications by DRDO laboratories, will circle the earth at an altitude of about 277km sweeping pole to pole, as the planet turns on its axis and around the sun. The space agency had its previous night launch in September 2018 when PSLV C-42 lifted off at 10.08 pm with two earth observation satellites into a low earth orbit of around 583 km.

While meeting the satellite requirements is the primary goal, ISRO officials said several other factors – including the type of rocket, flight’s target and the desired trajectory where the launch vehicle uses minimum fuel to inject the satellite in its intended orbit, without having the need to spend the satellite’s fuel to move it to the precise orbit – too influence the launch time during missions. However, the launch time could be postponed or advanced by a minute or two, without affecting the launcher’s trajectory, if a space debris, which is catalogued, is predicted to come in the way.

“We will also consider the factor that the sun should be available at a particular angle for the satellite to get solar power once it is injected,” said the ISRO chairman.

The satellite’s exposure to the sun is a deciding factor for GSLV launches too. The launch time for GSLV rockets, which carry geostationary satellites meant for communication purposes that hover in orbits at an altitude of 35,000 km, is planned in such a way – mostly in the evening hours – that the satellite gets maximum sun exposure for its solar panels to function while it is in an elliptical orbit.

GSLV rockets routinely inject satellites in a geosynchronous transfer orbit, which is an elliptical orbit where the apogee – the satellite’s farthest point from the earth – is high, while the perigee – the satellite’s nearest point to the earth – is less. Typically, after the satellite is injected into the elliptical orbit, ISRO scientists fire the satellite’s onboard thrusters to move the satellite into a circular orbit with zero inclination.

“When we launch, the apogee of the satellite will be high, while the perigee will be low. If the apogee comes in the way of an eclipse, the satellite’s duration without exposure to sun may become longer. That is why the launches take place in the night, so that it is the perigee that crosses an eclipse,” explained Mylsamy Annadurai, retired ISRO scientist.