The genesis of the Space Physics Laboratory (SPL) is closely entwined with the evolution of Space Sciences and Space Research in India and dates back to more than four decades. With the realization of the uniqueness of the magnetic equator in terms of the various geophysical processes, visionaries like Dr. Homi Baba and Dr. Vikram Sarabhai established the Thumba Equatorial Rocket Launching Station (TERLS) in 1963, right over the magnetic dip equator, to enable investigations of the equatorial ionospheric phenomena like the Equatorial Electrojet and Equatorial Spread F. On the one hand, Prof Sarabhai gave thrust to indigenous development of Space Technology by setting up Space Science and Technology Centre, while on the other he also envisioned the need to strengthen the space science component. Several ground-based experiments like, Ionosonde, Ionospheric drift and Magnetic field measurements, were initially set up by the Physical Research Laboratory (PRL) and Andhra University, in TERLS. In order to streamline these activities, Prof. Vikram Sarabhai started the "Space Physics Division" (SPD) during 1968. In the initial years, the SPD had literally been the equatorial laboratory of PRL, providing the necessary ground support for all the sounding rocket experiments from TERLS. Subsequently, with the addition of dedicated scientific faculty, independent research programs such as measurements of Ionospheric absorption, HF phase path, Relative Ionospheric Opacity (RIO) meter, HF Doppler radar, Telluric Current, Topside ionospheric sounder were taken up in SPD. The VHF Backscatter Radar, set up in Thumba for Electrojet studies, became a major research tool.
The first major scientific experiment taken up by SPD has been the ATS-6 (Application Technology Satellite) Radio Beacon Experiment project aiming at characterizing the equatorial ionospheric irregularities using multi-frequency scintillation measurements at high-frequency (HF) to ultra-high-frequency (UHF) and the measurement of the Total Electron Content (TEC) of the ionosphere and plasmasphere. This was facilitated by the Satellite Instructional Television Experiment (SITE) of Indian Space Research Organisation (ISRO) that made a scientific fiction into a reality through the effective use of the ATS-6 satellite (which also carried the coherent radio beacon payload onboard) lent for an year by the United States of America. The entire receiving system, having more than 22 channels to receive the amplitudes, phases and differential Doppler from the multi-frequency beacons, has been developed in-house to the required stability and sensitivity. This was among the "first of its kind" endeavours in indigenous experiment development. Several outstanding results emerged from this project on the science of equatorial ionosphere. It also provided insight into the fade margins needed to overcome the deleterious effects of ionospheric scintillations.
A major change in the constitution and outlook of SPD came about in the early eighties. Looking into the diversification of activities within the country and elsewhere, and keeping in mind the relevance and the need for such activities in ISRO, after an evaluation of the divisions program by an expert committee, ISRO formally elevated it in to an autonomous Laboratory on 11th April 1984. This marked the birth of the Space Physics Laboratory (SPL), with a clear mandate to carry out front-ranking research in Atmospheric and Space Sciences. SPL was conceived to emerge as a front-runner in many of the emerging areas of research in disciplines of atmospheric science, composition, radiation, dynamics, and electrodynamics. A bold initiative was taken to bring in niche areas like Laser probing of the atmospheric aerosols, acoustic sounders (SODARS) for atmospheric boundary layer studies, Meteor Wind Radar for studying middle atmospheric dynamics, in addition to the satellite beacon studies and rocket experiments. The programs of the laboratory (were) are to be overseen by a Scientific Advisory Committee (SAC), consisting of domain experts as its members. The recommendations of SAC-SPL (both programmatic and financial) (were) are put up to ISRO council. The VSSC (was) is to extend all the administrative and other supports needed for the functioning of SPL.
With the restructuring of the laboratory in place and with several new initiatives to its credit, SPL organized its Research and Developmental programs into different branches of activities, like Boundary Layer Physics, Aerosols and Atmospheric LIDARs, Atmospheric Dynamics, and Ionospheric-Magnetospheric Physics; all the experimental activities being supported by the Atmosphere Technology Division. Dr. C A Reddy became the first Director of SPL, with Dr. B V Krishna Murthy as the Assistant Director. Upon the superannuation of Dr. Reddy in 1991, Dr. B. V. Krishna Murthy became the director of SPL. After his retirement in 1998, Prof. R Sridharan took over as the Director, SPL. Dr. K. Krishnamoorthy lead the SPL team as its Director from 2010 to 2014. Subsequently, Dr. Anil Bhardwaj took over as the charge of Director in 2014 and continued till 2017. Presently, Dr. Radhika Ramachandran is leading the SPL team in capacity of the Director, SPL.
With the academic freedom and autonomy in place, and with resources becoming available, the program of the laboratory went through a rapid expansion phase, which continues even today. The emphasis on indigenous development of experiments has resulted in in-house development of experiments. A high-power pulsed Ruby LIDAR was built indigenously, for the first time in the country. Other developments included the Multi-Wavelength solar Radiometers (MWRs), CW (Continuous Wave) Lidar, Doppler Sodar, HF radar with beam swinging capability, Multi wavelength LIDAR, and a Partial Reflection Radar. Many of these have turned out to be the backbone for the research activities of the different branches of SPL.
In addition to these self-generated programs, SPL also played key roles in several scientific programs with ISRO on the lead. The ISRO-led multi-agency program IMAP (Indian Middle Atmospheric Program) of the 1980s, the Indian Ocean Experiment (INDOEX - India) program of 1996-1999; the Indian Remote sensing Satellite (IRS-P3) validation experiments, are some of the examples where SPL contributed extensively. Some of these activities also paved the way for indigenous development of instrumentation, installing networks for collecting data from different environments following a common protocol and also capacity development in areas where the scientific expertise in the country was at large. In-house development of MWRs for aerosol measurements and deploying them at selected locations in collaboration with research institutions and universities during the IMAP is an excellent example. However, the pinnacle was SPLs involvement in the ISROs Geosphere Biosphere Program (I-GBP) since early 1990s in the areas of Aerosol Characterization and Impact assessment, as well as the Atmospheric Boundary-layer Characterization, that took the science out of the laboratory into the national canvas with much wider objectives and applications. With these activities emerging as national benchmarks and maturing into projects on par with the current international scenario, these have been formulated into Projects namely; ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget), ARFI (Aerosol Radiative Forcing over India) and ABL N&C (Currently renamed as NOBLE (Network of Observatories for Boundary Layer Experiment)) and entrusted to SPL. The ICARB (conducted in 2006 and 2008-09) remains as the biggest multi-instrumented, multi-platform field experiment ever conducted in Asia, with network observatories, fully equipped research ship cruise over Bay of Bengal, Indian Ocean, and Arabian Sea and research aircrafts all running in tandem. Under ARFI, SPL runs a national network of 36 aerosol observatories across the Indian mainland and islands; from Trivandrum to Hanle, Naliya to Dibrugarh and in the islands of Minicoy and Port Blair. SPLs network has spread also to the poles - the Arctic, the Antarctic, and the Himalayas (also called the "third pole").
By the end of the last century, SPL also expanded its intrinsic activities in to new forays by carefully blending in-house developed instruments and commercial ones. Optical Aeronomy experiments; Coherent Radio Beacon Experiment (CRABEX); Middle Atmospheric dynamics program (MIDAS) through coordinated rocket, Balloon, and ground-based measurements; investigations of atmospheric trace gases; chemical characterization of aerosols; Atmospheric modeling and operation of - state of the art round the clock Meteor Wind Radar; Micro pulsed LIDAR; Digital Ionosonde, Partial reflection Radar, Doppler SODAR, Micro Rain Radar, along with complementary experiments. This period also marked the beginning, in a modest way, of investigations of the planetary environments, microwave studies of atmosphere and surface, and the use of satellite data for atmospheric studies.
By the beginning of the present century, the planetary science research emerged into a full-fledged research discipline, entwining explorations, observations, modeling, and instrumentations. Development of instruments for planetary exploration started with the two experiments (viz., SARA and ChACE/MIP) in Chandrayaan-1 - Indias first mission to the Moon. These payloads brought out several new "discovery" class findings on the lunar atmosphere, regolith and the interactions with solar wind. Besides, SPL also collaborated with several international institutions in observational and modeling studies of planetary and cometary environments using state of the art international instruments leading to several new findings on Jupiter and Saturn. Use of satellite data and development of satellite payloads have been on the increase, with SPL having experiments onboard the satellite GSAT and YouthSAT, built with the help of different entities of VSSC and other ISRO centres. Satellite (Indian and other nations) data are being used extensively in all the disciplines of SPL. Currently the development of instruments for planetary studies includes neutral mass spectrometer-based payloads, viz., CHACE-2 for the Chandrayaan-2 and MENCA for the Mars-2013 mission - first Indian mission to Mars. Expanding the facilities for payload development, a High Vacuum Space Simulation Facility (HVSSF) has been established for development and calibration of space-borne instruments in the space-simulated conditions, which also include plasma analyzers, PLEX, LEIMA.
SPL's atmospheric science activities grew into extensive use of satellite (Indian and foreign) data for investigations of aerosols and clouds, climate modeling, formulating operational/forecasting models for both atmospheric and space weather conditions, weather forecast support for ISROs rocket launches, in-house development of innovative sounding rocket experiments, viz., ENWi and EACE. Installation of a hyperspectral microwave radiometer profiler provided the much needed thrust to the microwave remote sensing and propagation studies. SPL played key roles in developing microwave emissivity maps of India, operational models of GAGAN (both ionospheric and tropospheric correction models), model based radiation corrections to indigenously built radiosondes, and regional models for aerosol chemistry. SPL led from the front in organizing the science theme, carrying out co-ordinated measurements, analyzing the data during the noon-time annular solar eclipse of January 2010 (the first such event in the history of SPL). The synthesized scientific outputs from this campaign, brought out several "first time" findings and deeper insights into the eclipse manifestation of the atmosphere and ionosphere. SPL is actively associated in the operational forecasting of atmospheric weather systems, near-Earth space weather conditions, and also in some of the major initiatives of ISRO like the Climate and Weather of the Sun Earth System (CAWSES-INDIA).
Making a humble beginning as a ground support provider for the sounding rocket experiments, getting an identity as Space Physics Division located in Flora Villa, the former residence of the headman of the Pallithura village in TERLS area, later moving on to the main campus of VSSC on Veli hill, (and now poised to move into its own building), and with the expansion of the horizon covering the whole gamut of atmospheric and space sciences, the Space Physics Laboratory, with its autonomy in science programs, has traveled a long way. Today, it has attained the status of a vibrant academic institution with front ranking research areas and problems, and a strong and vibrant research fellows program and a leading Atmospheric, Space, and Planetary Research Laboratory in the country, with international repute. It interacts very closely with the academia and other research institutions in India and abroad. Recognition, in the form of awards and accolades, has been coming-in along with the Laboratorys evolution and growth. SPL has ambitious programs for the years to come, with the extensive support from ISRO.