PSLV C4’s GTO mission and METSAT

S.Arun^*

Introduction

PSLV-C4 launched on 12-Sept-2002 3.53 pm (IST), is the seventh flight of Polar Satellite Launch Vehicle (PSLV) and it is the first time PSLV launched a Geo-Stationary satellite (1060Kg METSAT satellite) in Geo-Transfer Orbit (GTO). By contrast all previous missions involved satellites (1000 to 1300Kg) in Sun-Synchronous Orbit (SSO) in North-South polar orientation at around 850 Km altitude. PSLV-C4 was launched at 3.55 pm Indian Standard Time from Satish Dhawan Space Center, Sriharikota in the easterly direction (~102° North) towards final inclination of 18° (with respect to the equator) to fully leverage earth's easterly rotational velocity for increased payload capability. This was unlike all previous PSLV launches that put remote sensing satellites in polar sun-synchronous orbit. These earlier launches required first launching in the easterly direction and then turning south to in a dog-leg maneuver to avoid flying over Sri-Lanka due to a range safety risk. If used for a polar launch, this PSLV-C4 could carry a satellite of about 1,500 kg[i].

Since its first flight PSLV has been progressively improved to launch heavier payload to a much higher orbit. However, based on the demand put forth for a minimum requirement of 1050 kg for an exclusive Meteorological Satellite (METSAT) to be deployed in GTO, several improvements were implemented on PSLV.  Some of the changes that have been made in PSLV since its previous launch on October 22, 2001 when it successfully launched three satellites, viz., India's Technology Experimental Satellite (TES), the German BIRD and the Belgian Proba, include the improvements brought about in the performance of the third stage solid propellant motor by optimizing the motor case, insulation, propellant loading, throat area ratio and flex nozzle (More powerful version called High Performance PS3, HPS3)[ii]. The HPS3 motor is an optimized version that is comparable in performance and mass ratio to contemporary motors of its class, globally.  Also, the propellant in the fourth stage liquid propellant motor has been increased from 2 metric tons to 2.5 metric tons. Besides, the PSLV-C4 employs a carbon composite payload adopter and improved electronic equipment bay resulting in substantial payload advantage.

To give an idea of the Indian space industry capabilities, it is worth remembering that the first stage of both GSLV and PSLV is the fourth most powerful solid fueled engine in the World, coming after the boosters of the Space Shuttle, Titan-IV and Ariane-5.

The previous GSLV-D1 mission (April 2001) was a successful GTO launch however the GRAMSAT satellite orbit raising from GTO to GSO was unsuccessful.

About the PSLV-C4 rocket  

In this configuration, the 44.4 meter tall, 295 metric ton PSLV has four stages using solid and liquid propulsion systems alternately. The first stage is amongst the largest solid propellant boosters in the world and carries 138 metric tons of Hydroxyl Terminated Poly Butadiene (HTPB) based propellant. It has a diameter of 2.8 m. Its motor case is made of maraging steel. The booster motor fires for 107 seconds and develops a maximum thrust of about 4,628 kilo Newton (kN).

Six strap-on motors, four of which are ignited on the ground, augment the first stage thrust. Each of these solid propellant strap-on motors carries nine tons of HTPB based propellant, burn for 45 seconds and produces 662 kN thrust. The remaining two strap-on motors were ignited 25 seconds after lift off.

The second stage employs indigenously built Vikas engine and carries 40 tons of liquid propellant, Unsymmetrical Di-Methyl Hydrazine (UDMH) as fuel and Nitrogen tetroxide (N2O4) as oxidizer. It generates a maximum thrust of about 725 kN.

Figure 1 PSLV-C4 and its predecessor

Unlike previous PSLV launches, the third stage of PSLV-C4 uses 7.6 tonne (4.1% more then PSLV-C3) of HTPB-based solid propellant and produces a maximum thrust of 260 kN. Also its motor case is lighter and made of polyaramide fiber. Compared to the previous launches this stage burns longer (31% more then PSLV-C3) and produces smaller thrust (20% less then PSLV-C3) thus expected to provide better ISP at better mass fraction.

The fourth and the terminal stage of PSLV has a twin engine configuration using liquid propellant. Unlike previous PSLV configurations the PSLV-C4 stage has increased propellant loading of 2.5 tons (Mono-methyl hydrazine as fuel and Mixed Oxides of Nitrogen as oxidizer) (instead of 2.0 tons in earlier models), each of these engines generates a maximum thrust of 7.4 kN. Again the increased fuel would also result in better stage mass fraction. A new lightweight carbon composite payload adopter also enables greater GTO payload capability.

The 3.2 m diameter metallic bulbous heat-shield of PSLV, which is made of isogrid construction, protects the spacecraft during the PSLV's passage through the dense atmosphere.

However more than brute force, precise navigation and control is required. A GTO launch from Sriharikota is quite an obstacle course. Once the Bay of Bengal is crossed, there are islands and landmasses speckled all over the place. When the spent stages of the PSLV are separated and abandoned, they have to fall into empty ocean. As India would be liable for any damage caused, it cannot afford to have the spent stages fall on land or in territorial waters. It is a measure of the confidence of ISRO's launch vehicle teams that they believe that the PSLV will be able to precisely fly the chosen trajectory[iii].

PSLV flight control system includes: 

a) First stage: Secondary Injection Thrust Vector Control (SITVC) for pitch and yaw, reaction control thrusters for roll and SITVC in two strap-on motors for roll control augmentation, 

b) Second stage: Engine gimbal for pitch and yaw and, hot gas reaction control for roll, 

c) Third stage: flex nozzle for pitch and yaw and PS-4 RCS for roll and 

d) Fourth stage: Engine gimbal for pitch, yaw & roll and on-off RCS for control during the coast phase.

PSLV's Inertial Navigation System (INS) is situated in its equipment bay, which is located on top of the vehicle's fourth stage. INS guides the vehicle from lift-off to spacecraft injection into orbit.

PSLV-C4 mission

PSLV-C4 launch cost of Rs. 75 crores (approx. US$ 16M) and resulted in a saving of about 50%. A typical PSLV has an injection velocity of 7.5 km/sec while the GSLV has 10 km/sec velocity. The velocity has to be increased to 10km/sec to overcome gravity and centrifugal force and take the satellite to 36,000 km orbit. At the same time care need to be taken not to exceed 11.2 km/sec velocity as the satellite would be out of earth's gravity and be lost forever.

The launch window was selected based on the spacecraft requirements like avoiding VHRR radiative cooler directly looking at the sun, and the favorable geometry for calibrating on-board gyros. The launch azimuth for PSLV-C4 was set at 102° compared to 140° for PSLV-C3. Since the launch pad azimuth is 135°, it required a roll maneuver to align the flight path immediately after lift-off. After METSAT separation from PSLV vehicle, the stage was maneuvered in yaw by 60°, and the passivation  function was initiated by sequentially opening the tank ullage to the ambient by firing pyro valves. This was implemented for the first time in PSLV-C4 to vent out and remove all energetics from the spent PS4 so as to avoid an explosion and generation of debris in orbit. This was clearly in response to the space debris field created by explosion of one of the previously expended PS4 stage that was not properly vented. The mission design of PSLV-C4 was thus aimed at meeting two major constraints of maximizing the payload capability and managing the impact of the spent stages, especially the PS3 stage within the safe corridor.[iv]

Figure 2: Typical Polar and GTO trajectory from Satish Dhawan Space Center, SHAR

The PSLV-C4 Stage-4 was shutoff 21 minutes after launch after reaching targeted velocity. It injected the METSAT payload into a 216km x 34,641km GTO orbit at 17.67° inclination against nominal target of 250 x 36,000 Km at 18° inclination and specification of Perigee > 180Km, Apogee=36,000Km[v]. At such high apogee the shortfall was quite minor that could be corrected by just using 10Kg of METSAT 560 Kg fuel. Of most of the 560 Kg fuel would be used for orbit raising from GTO to GSO, leaving around 100Kg for orbit control during its 7 year life. This variance from target velocity though within specification, indicates lack of full mastery in navigation and flight control that hinges on more refined sensors and control systems.

Figure 3 : PSLV-C4 and METSAT trajectory. GTO to GSO Orbit

The first orbit raising maneuver on 13^th September involved firing METSAT’s Liquid Apogee Motor (LAM)  on its second apogee for 31 minutes and 48 seconds to raise the orbit to 12,144 km x 34,492 Km and reduce the inclination to 4.7° at orbital period of 14Hr 08 minutes. The maneuver consumed approximately 280 Kg fuel resulting in change in orbital period from 10.5 Hrs to 14.16 Hrs. The second orbit raising was done on its forth apogee on 14^th September raising the orbit to 34,441 km x 34,535 km at 0.44° at orbital period of 22Hr 50 minutes. Finally the satellite was raised to GSO and parked at the intended slot on its fifth apogee orbit reaching 34,486 km x 35,676 km at –0.49° inclination[vi].

After the orbit raising to GSO orbit was completed, the spacecraft which had a propellant of 560 kg at the time of its injection in GTO, is still left with about 105 kg of propellant, sufficient for its station keeping operations during its designed mission life of 7 years.

METSAT

The Rs. 70 crore (USD 15 Million) METSAT is India’s first dedicated meteorological satellite. Till now the meteorological component formed part of the multipurpose INSAT series of satellites with meteorological and communications components, a configuration that ISRO pioneered. It was prudent to continue to build these multi-purpose satellites even in the 1990s because they were cost-effective then. As the demand for communications satellites with more transponders and signal power has gone up significantly, so much so that the tradeoff involved in meeting the often conflicting design constrains and requirements between metrology instruments and communication payload limits the scope of synergy. For instance, INSAT-3A is a massive satellite that weighs almost 3,000 kg in the launch pad. It has 24 transponders that generate lots of heat, however the meteorological payload called VHRR (Very High Resolution Radiometer) and a charge-coupled device camera for infra-red imaging require cryogenic operating temperature. Similarly the demand for attitude control for communication antenna pointing and the undisturbed scanning of metrological payload create challenging situations. Not to mention that communication and metrology payload compete to be located on the same earth facing side of the satellite. So it was logical to break up these multi-purpose satellites into separate missions, one for meteorology and another for communications. It is interesting to note that last few INSAT series were dedicated to communications payload and did not carry any metrological payload, but due to scheduling constrain of PSLV launch, METSAT could not be launched. For over a year the country suffered the consequences of severe storms with limited weather forecasting due to the absence of metrological satellite. Another benefit of separate meteorological satellite is that unlike communication satellites, that require complicated international coordination to avoid interference with the radio signals from other satellites, whereas METSAT can be placed more easily in an advantageous position in the geostationary orbit

Payload

METSAT has a Very High Resolution Radiometer (VHRR) for imaging the earth in visible, infrared and water vapor bands. The VHRR has three bands comprising visible operating in 0.55–0.75 mm, water vapor (WV) in 5.7–7.1 mm and thermal infrared (TIR) in 10.5–12.5 mm, to provide both day and night coverage. The radiometer employing a bi-axial scan mechanism for coverage in the east–west and north–south direction is designed to operate from body-stabilized Geostationary satellite. The ground resolution of VHRR 8-inch aperture telescope at the sub-satellite point is nominally 2 km × 2 km in the visible and 8 km × 8 km in the WV/TIR bands[vii].

The water vapor band is a new capability that is particularly useful for countries with tropical climate. It would allow the meteorological department to measure the water content in the atmosphere and arrive at meaningful and more accurate prediction of rainfall. Compared to INSAT series the resolution in the visible band has been improved from 8 x 8 km to 2 x 2 km. This has become possible by the use of better optics and sensors. The use of lightweight planar array antenna gives better ground communication and craft control.  METSAT will give pictures of clouds and warn about approaching cyclones. Its images can help in mapping the water vapor content in the atmosphere. The dedicated METSAT has the freedom to scan a particular zone of interest. Though this was technologically feasible earlier in the INSAT series but it was not put to use, since it would lead to scheduling conflict with communication and broadcasting operation. METSAT has a data relay transponder that will collect weather data from various unattended platforms in the country and relay them to a central station in New Delhi.

METSAT was launched in GTO, instead of LEO, because at that height it could give view the same spot on the globe once every 30 minutes. Thus, changes in the movement of clouds and cyclones could be monitored. On the other hand in low earth orbit, a satellite can view a particular region only periodically as it visited the same region only once in a few days. It can however take higher resolution picture of say, five meters or 2.5 meters, depending on its capability. But a VHRR in METSAT in the GTO could give a synoptic view of one-third of the global region at a resolution of 2 km every half an hour thus changes in the movement of clouds and cyclones could be observed for better weather forecasting.

Operational applications of METSAT data

The foremost application of data from METSAT will be towards operational services in terms of the following:

1. Watch and monitor the growth of weather phenomena like cumulonimbus cells, thunderstorm, fog, etc. and their decay.

2. Track movements of migrating systems such as tropical cyclones, monsoon depressions, western disturbance, etc.

3. Identify and locate primary synoptic systems like surface lows, troughs/ridges, jet streams, regions of intensive convection, inter-tropical convergence

4. zones, etc.

5. Monitor onset and progress of monsoon systems.

6. Detect genesis and growth of tropical cyclones and monitor their intensification and movement till land-fall.

In this context, the capability of METSAT payload to provide data in three imaging modes is found to be of relevance. These special imaging modes permit scanning of intense weather systems frequently, to get insights into the dynamics of the atmosphere. The higher resolution of 2 km in visible and 8 km in thermal IR permits study of mesoscale events. Besides the monitoring capability through imageries, the METSAT payloads are capable of providing several operational parameters for assimilation in weather pre-diction models and atmospheric research. Imaging in the WV channel greatly enhances insight into atmospheric circulation and humidity in the middle atmosphere with improved forecasts. [viii]

Spacecraft systems

According to V.R. Katti, Programme Director, GEOSAT, METSAT has several novel features. ISRO's satellite builders have used carbon fiber reinforced plastic (CFRP) in the structure of the spacecraft instead of aluminum monocoque. He said: "We want a structure that is light and meets various requirements. In addition, we prefer to have a surface that is a fairly good conductor of electricity. All this can be met by the CFRP." The satellite builders developed a new propellant tank for METSAT. INSAT-2A, 2E and others had a solar array on the one side and a boom and a sail on the other.  "It uses a specially designed magnetic `torquer' to take care of the imbalances resulting from radiation pressure exerted on the solar panel," Dr. Katti said.

Another new feature of METSAT is that there will only be 12 thrusters aboard to control its altitude compared to 16 used in INSATs. The solar panel of METSAT with was deployed immediately after its injection into the GTO to take advantage of full power generation in the GTO itself. The solar panel is 2.15 m × 1.8 m in size using GaAs/Ge technology and generates 640 W of power at BOL (Beginning Of Life) equinox season. Power available at EOL (End Of Life) summer solstice is about 500 W against a requirement of 460 W. The Solar Array Driver Assembly (SADA) slip rings and drive mechanism are modified to meet the power transfer and drive requirements, and it is mass-optimized[ix]. In order to de-couple the uncertainty in VHRR radiative cooler performance associated with the solar sail/boom configuration in earlier missions, METSAT configuration has been simplified with no sail/boom the north side of the spacecraft and with one solar array deployed on other side of the spacecraft.

One of the new elements used in METSAT is the light weight “planar array antenna” which transmits the data from the Very High Resolution Radiometer (VHRR) and Data Relay Transponder (DRT).  METSAT also has been designed using a new spacecraft bus.[x].

METSAT weighs 1060 kg, which includes about 560 kg of propellant. The propellant carried by METSAT was mainly required to raise the satellite from the Geo-synchronous Transfer Orbit to its final Geostationary orbit.

The basic structure of the METSAT satellite can be used to create a small 1,000 kg class communication satellite carrying four to six transponders, observes P.S. Goel, Director of the ISRO Satellite Center in Bangalore. Such a satellite, combined with the low cost of a PSLV launch, could provide an attractive option that ISRO could offer on the international market[xi].

Future meteorological missions

Several satellite missions have been planned globally as well as by India to support the operational needs and on-going research efforts. The future METSAT missions will carry improved VHRR and vertical sounders for temperature/humidity profiles. The Megha-Tropiques mission slated for launch in 2004 will be a joint project by ISRO and CNES, France with the objective of studying the water cycle and energy exchanges in the tropics. With an equatorial inclined orbit, the satellite will have high repetitivity over tropical areas, thus providing frequent sampling.

The Megha-Tropiques payloads consist of Sca Rab for radiation budget measurements in short and long wavelengths, SAPHIR with six channels for atmospheric WV distribution and MADRAS operating in microwave region (89 and 157 GHz) for study of convective systems. The data from this mission are expected to provide better insights into the convective processes in the tropical regions[xii]

PSLV-C4 Rocket Configuration

Launch Date: 12-September 2002. SSO Orbit: 1500Kg to 820Km Orbit (limited to 1500Kg due to  down range safety). GTO Payload: 1060 kg at 18° inclination. Liftoff Thrust: 540,000 kgf. Total Mass: 295,000 kg. Core Diameter: 2.8 m. Total Length: 44.4 m. Flyaway Unit Cost $: Rs. 75 crores (approx. US$ 16M).

PSLV-C4 Configuration

Future PSLV capabilities

ISRO scientists are optimistic about increasing the payload of the PSLV launched into the GTO from 1055 kg to 1200 kg. 

The ability to maintain the METSAT flight to a predetermined path in conjunction with altitude and orientation has given a great boost to ISRO's lunar mission.  In the case of lunar mission, the precision of the launch vehicle path is very demanding. A difference of more than 0.01 degree can prove fatal. In the case of METSAT launch, the precision required was 0.2 degree. "There are many challenges to be met but this launch has made us more confident of a lunar mission," said ISRO spokesperson.