LASTEC Delhi in a joint collaborative activity with LEOS, Bangalore is developing a space qualified diode array
pumped Nd:YAG laser transmitter delivering 30 mJ @ 10 pps of 10 ns duration. For space applications laser diodes are
preferred because of their excellent reliability with lifetimes exceeding 100,000 hours. However, they are extremely
sensitive to electro-static discharge, excessive current levels, and current spikes and transients. Small variations in bias
voltage may produce large fluctuations in the current causing instability and damage to the device. Hence instead of the
traditional power supplies a current controlled laser diode driver is required. This paper presents the design of a Q-CW
laser diode driver based on closed loop current regulator, capable of driving 24 QCW laser diode bars each with 75W
peak power at 70 A. The driver can generate up to 100 Amp peak current and 200μsec pulse width operating at 10 Hz.
The current source design includes special circuits for low noise operation, slow turn-on and turn-off, circuits for over
voltage and transient current protection; and good regulation. Space qualified and radiation hardened components are
required to be used to sustain stringent space environment requirements during mission life of two years.
Present day remote sensing satellites orbiting in low earth orbit (LEO) have increasingly sophisticated and high
resolution onboard sensors. Their frequency and area of observation is also increasing. This generates large volume of
data which needs to be communicated. However their visibility to ground station is limited. Free space optical
communication between remote sensing satellite in LEO and communication satellite in geostationary earth orbit (GEO)
can be favorable approach. Subsequently GEO satellite relays the data to ground station. To demonstrate this, a concept
model operating at data rates greater than 1 Gbps is under development at LEOS. The system consisting of laser
transmitter with 20cm diameter telescope and receiver with 30cm telescope is planned. It uses commercially available
optical and optoelectronic components. This concept model will demonstrate and verify link margins available as against
expected. Subsequent to this, it is planned to concentrate on design and other issues involved in acquisition, tracking and
pointing (ATP) due to highly narrow laser beam.
Development of a laser transmitter for space applications is a highly challenging task. The laser must be rugged, reliable, lightweight, compact and energy efficient. Most of these features are inherently achieved by diode pumping of solid state lasers. Overall system reliability can further be improved by appropriate optical design of the laser resonator besides selection of suitable electro-optical and opto-mechanical components. This paper presents the design details and the theoretically estimated performance of a crossed-porro prism based, folded Z-shaped laser resonator. A symmetrically pumped Nd: YAG laser rod of 3 mm diameter and 60 mm length is placed in the gain arm with total input peak power of 1800 W from laser diode arrays. Electro-optical Q-switching is achieved through a combination of a polarizer, a fractional waveplate and LiNbO<sub>3</sub> Q-switch crystal (9 x 9 x 25 mm) placed in the feedback arm. Polarization coupled output is obtained by optimizing azimuth angle of quarter wave plate placed in the gain arm. Theoretical estimation of laser output energy and pulse width has been carried out by varying input power levels and resonator length to analyse the performance tolerances. The designed system is capable of meeting the objective of generating laser pulses of 10 ns duration and 30 mJ energy @ 10 Hz.
The first two indigenously developed communication satellites of India at Geosynchronous orbit (GEO) Insat-2A and Insat-2B have an oscillatory type earth sensor for providing the pitch and roll reference to the momentum biased, 3 axis stabilized control system. The sensor consists of an oscillatory resonant scan mechanism which provides a East-West scan field of +/- 25 degrees. The excellent on orbit performance of the above sensor and also its advantages such as simplicity and long life have given rise to explore the possibility of using such a sensor for low earth orbit (LEO) mission where the subtended angle is large, i.e. nearly 120 degrees. This paper presents a design approach and test results of a germanium wedge prism adapter which separates the scan in the North-South direction by +/- 45 degrees from the existing +/- 6.1 degrees in the GEO sensor and also extends the scan field in the East-West direction from +/- 25 degrees to beyond +/- 60 degrees providing the four horizon cross over points for the computation of pitch and roll errors of the spacecraft. Thus with a marginal change in the optics and a marginal reduction in the signal, all the advantages of the GEO sensor such as long life, higher update frequency, lower weight can be made use of for the LEO mission. A comparative study is also made with the conical type scanning horizon sensor using a motor driven mechanism with the above sensor and the advantages outweight the disadvantages. The development model results show the technical feasibility and confirm the advantages of the sensor for LEO application.
INSAT-2 series satellites are second generation, multipurpose satellites developed by Indian Space Research Organisation for telecommunication and meteorological remote sensing. INSAT-2A was launched successfully in July 1992. The solar sail mounted on the north face of the satellite balances the differential solar radiations torque on the solar panel mounted on the south face. The sail, being lightweight, during its deployment causes very little attitude disturbance on the satellite to sense the deployment. A novel utilization of an earth sensor (ES) mounted on the same face of the satellite gave a clear indication of the deployment initiation and the progress of the boom extension during this period. This paper briefly describes the observations made using the earth sensor during the sail deployment and analyzes the data received in terms of the sail extension in this period.
The second-generation Indian communication satellite INSAT-II is scheduled for launch in 1991-92. The primary attitude sensor in this satellite is a two-axis scanning infrared earth sensor. The satellite carries three earth sensors, one being used exclusively in transfer orbit. The earth sensor contains an oscillatory scan mechanism which has been qualified for 10 years life in orbit. A mirror is made to oscillate on a taut band at low frequency by a sensor and torquer system. Two sensors nominally scan in the east-west direction while the third sensor scans in the north-south direction. The sensor has features to work in presence of sun/moon interference. The paper describes the elaborate qualification tests and characterization carried out in thermo-vacuum conditions. The sensor has a thermal design which was verified by a thermo-vacuum solar simulation test.
The Indian Remote Sensing Satellite IRS-1A, launched in March 1988, is a three-axis stabilized, polar sun synchronous satellite orbiting at an altitude of 904 km. Two types of earth sensors are used for pointing and control of the satellite. One is a pair of conical scanning sensors using a rotating germanium wedge prism. The other is a static horizon sensor operating on the principle of radiation balancing. The latter sensor used novel normalization technique for removing the effects due to radiation gradients, which is one of the main problems of this type of sensor. The in-flight performance of the sensor is quite satisfactory with very low noise behavior. However, there were certain problems noticed in the acquisition mode of operation of the sensor which were traced to the heating of the IR filter due to direct sun viewing by the sensor near the poles. Based upon this experience, the configuration for IRS-1B to be launched in 1991 was modified. This paper briefly describes the configuration, the flight performance, and the modification carried out in the future models of the sensor.