After a geostationary satellite like INSAT is placed in its orbit, it might experience orbital perturbations because of the combined gravitational attractions of the Sun and Moon. A satellite must remain within its prescribed boundaries to satisfy its mission requirements. To keep a satellite within its equatorial and longitude planes, “North-South" and "East-West" station-keeping maneuvers are performed weekly. "North-South" maneuvers accounts for 95% of total station-keeping propellant consumption.
A satellite's orientation can be maintained by momentum wheels supplemented by magnetic torques and thrusters. Ion propulsion systems, are being used increasingly for station-keeping. For the first time, ISRO, had used electric propulsion for its GSAT-4 satellite. 2 indigenously developed and 2 imported SPT (stationary plasma thruster) has flown on board GSAT-4 to cater for "North South" station keeping operations. Since less than 5 m/s per year delta velocity needs to be imparted for east–west station keeping (EWSK), EPS system usage is not advantageous for EWSK, as overheads will negate the benefits. Similarly, use of EPS systems for orbit raising involves months of continuous operation and a very long wait to reach GSO, nullifying the advantage. However, this could be a backup option for conventional chemical propulsion.
Electric propulsion (EP) offers a cost effective and sound engineering solution for space applications. Use of high performance electric propulsion system (EPS) will result into reduced chemical propellant and tankage requirements, in exchange for significant usage of power. Chemical rocket engines, like those on the lower stages of GSLV and PSLV, work by burning two gases to create heat, which causes the gases to expand and exit the engine through a nozzle. These exiting gases produce thrust which lifts the rocket. Instead of relying only on the energy stored in the propellants, if we add external energy using electricity, we can increase the temperature of the gases and thus create more thrust per pound of fuel. This is the basic concept of an electric propulsion or EP. EP provides much lower thrust compared to a chemical rockets but they provide very high specific impulse. This in effect means that though EP must burn for longer durations compared to a chemical rocket to achieve desired thrust, it consumes very less fuel because of higher specific impulse.
EP systems fall into three major categories: (a) electrostatic propulsion, (b) electro thermal propulsion, and (c) electromagnetic propulsion. GSAT-4 employs electromagnetic propulsion and uses Hall Effect thrusters or stationary plasma thruster (SPT) in particular. Soviet Union has done pioneering research work on Hall thrusters. Soviet Union developed two types of Hall thrusters; stationary plasma thruster (SPT) and the anode layer thruster (ALT). They have been using SPT's on their satellites since 1972. A Hall Effect thruster was also used by the European SMART-1 probe.
Four components are needed to make a complete electric propulsion system: a power source, a power processing unit (PPU), a propellant management system (PMS), and a control computer. The power source can be any source of electrical power, but solar and nuclear are the primary options. A solar electric propulsion system (SEP) uses sunlight and solar cells for power generation. A nuclear electric propulsion system (NEP) uses a nuclear heat source coupled to an electric generator. The PPU converts the electrical power generated by the power source into the power required by each component of the Hall thruster. It generates the high voltages required by the Hall thruster channel and the high currents required for the hollow cathode. The PMS controls the propellant flow from the propellant tank to the thruster and hollow cathode. Modern PMS units have evolved to a level of sophisticated design that no longer requires moving parts. The control computer controls and monitors system performance. The Hall thruster then processes the propellant and power to perform work. Hall thrusters use inert gas as propellant. The thrust is generated from the force that the propellant ions impart to the electron cloud inside the thruster.
GSAT-4, envisaged as a technology demonstrator, carries a communication payload consisting of a multi-beam Ka-band bent pipe, regenerative transponder and a navigation payload in C, L1 and L5 bands. GSAT-4 having propulsion system with four stationary plasma thrusters, Bus Management Unit (BMU), miniaturized dynamically tuned gyros, 36 AH Lithium ion battery, 70 V bus for Ka band TWTAs, on-board structural dynamic vibration beam accelerometer, are some of the new technologies developed for the mission. The satellite weighs around 2200 kg and was a payload power of 1600W.