Why are India’s communications satellites so heavy? | Explained

On November 2, the Indian Space Research Organisation (ISRO) launched the GSAT-7R satellite for the Indian Navy. The satellite’s launch mass was 4,410 kg — rendering the launch the heaviest of a communications satellite from Indian soil. For this mission, ISRO used its LVM-3 rocket, the most powerful yet in its stable of launch vehicles.

India’s communications satellites are heavy because they combine wide coverage, high power, and long service life in one spacecraft.

To serve the entire country and nearby seas, the communications payload needs to support many channels across multiple frequency bands. These are typically the C (4-8 GHz), the Ku (12-18 GHz), and sometimes the Ka (27-40 GHz) bands. This in turn requires several large deployable antennas, high-power amplifiers, waveguides, filters, switches, and either many analog transponders or flexible digital processors.

The antennas and pointing mechanisms also need to hold tight alignment in space, so the structure and thermal control systems are correspondingly robust and add more mass.

The satellites’ high throughput demands several kilowatt of electrical power. To supply this for 12–15 years, satellites carry large solar arrays, batteries large enough to account for daily eclipses, and power-conditioning units. These elements also increase mass, even as they must be built to withstand radiation and repeated heating and cooling cycles.

The spacecrafts’ long life demands redundancy, including in the form of duplicate computers, radios, and power units, so that they can continue working after failures. Redundancy improves reliability but increases weight.

Next, getting to the geostationary orbit (GTO) adds more mass in propellant. The GTO is a highly elliptical orbit that’s used to move satellites into the geostationary or geosynchronous orbits. A launch vehicle like ISRO’s LVM-3 will place the satellite into the GTO, and from there the satellite will use its own propulsion systems to move into the final intended orbit. In a GTO, the perigee, i.e. the point closest to the earth, can be a low-earth orbit (150-2,000 km above) while the apogee can be as high as the geostationary orbit (35,786 km).

When a communications satellite first enters the GTO, it must perform orbit-raising and station-keeping manoeuvres, as well as manage its momentum for more than a decade. The chemical propulsion systems that are still common on many Indian satellites need significant quantities of fuel for these tasks.

Finally, economic factors reinforce these choices. Launch opportunities are limited and operators prefer fewer, more capable satellites to cover specific national needs. As a result, it’s better for communications satellites to be designed to have high power, broad coverage, long lifetimes, and robust backups. In future electric propulsion systems can reduce the propellant mass, although the trade-off between satellite capability and satellite lifetime will still remain.