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The high density extensive air shower array along with large area (560 m2) muon telescope of GRAPES-3 at Ooty is used to make precise measurements of cosmic ray properties by using the secondaries produced through cascades in the Earth’s atmosphere. The observed particle density at detector level is affected by atmospheric effects primarily due to variations in the pressure and temperature. Here, an attempt is made to simulate these phenomena that can provide a deeper understanding of these effects. In this study, the pressure dependence of the observed particle densities at GRAPES-3 is simulated by varying the atmospheric profiles available in CORSIKA for proton showers in the energy range of 1013–1016 eV. A comparative study of various combinations of hadronic interaction generators available in CORSIKA has been used to investigate the consistency of the results obtained and their broad implications have been intensely discussed.

The GRAPES-3 is a near-equatorial extensive air shower experiment, located in Ooty, India at an altitude of 2200 m above mean sea level. It consists of a dense array of 400 scintillator detectors of 1 m$^{2}$ area each with 8 m inter separation. The array triggers showers induced by cosmic ray and gamma ray primaries in the TeV-PeV energy range. An associated muon telescope of 560 m$^2$ area is designed to record muons above 1 GeV energy in the triggered showers. Additionally, it is designed to trigger on individual muons, providing measurement of muon flux with a statistics of $\sim$4$\times$10$^{9}$ muons per day and an average angular resolution of 4$^{\circ}$. In this paper, we summarize the recent results obtained using air shower data on cosmic ray energy spectrum and composition below the Knee, cosmic ray anisotropy, angular resolution, gamma ray source searches and thunderstorm phenomena using the muon flux data.

The GRAPES-3 extensive air shower (EAS) array located at Ooty is equipped with 400 plastic scintillator detectors spread over an area of 25000 m$^2$ and a muon telescope of area 560 m$^2$ built with 3,712 proportional counters. One of its principal objectives is to measure the primary cosmic ray energy spectrum in the TeV-PeV energy region. The response of the photo-multiplier tubes (PMTs) used in the plastic scintillator detectors becomes nonlinear at densities $>$ 50 particles per m$^2$ in large EAS. We describe a technique to correct for the nonlinearity of these PMTs, thereby extending the dynamic range of the detector for observed particle densities up to 5000 particles per m$^2$. The details of the technique will be presented.

A diffuse gamma-ray emission at $\sim$100 TeV can be expected as a result of the interactions of ultra-high-energy cosmic rays (UHECRs) with the cosmic microwave background (CMB) during their propagation. This radiation carries the information on the distribution of energetic sources and hence the cosmological evolution of the universe. The GRAPES-3 is an extensive air shower (EAS) array, located at Ooty in southern India. It consists of 400 plastic scintillators (each 1 m$^2$) and a large area (560 m$^2$) muon telescope. The muon telescope has the ability to differentiate the gamma-rays from charged cosmic rays through their muon content. We report on the study of isotropic diffuse gamma-ray flux from GRAPES-3 over 10-300 TeV.

The GRAPES-3 experiment located in Ooty consists of an array of 400 plastic scintillator detectors spread over an area of 25000𝑚 2 and a large area (560 𝑚 2 ) muon telescope. Every day, the array records about 3 million showers induced by the interaction of primary cosmic rays in the atmosphere. One of the primary objectives of the experiment is to measure the energy spectrum and composition of the cosmic rays in the TeV-PeV energy range. However, some of the detected showers have cores outside the array. This fraction increases with energy due to the higher lateral spread of shower particles at higher energies. Identifying these events is thus crucial for accurate measurement of the cosmic ray energy spectrum. This work will describe simple cut based as well as machine learning based strategies for identifying and excluding such events and their impact on the cosmic ray energy spectrum as measured by the Bayesian unfolding technique.

The Gamma Ray Astronomy at PeV EnergieS phase-3 (GRAPES-3) experiment is located at Ooty in India (11.4$^{\circ}$ N, 76.7$^{\circ}$ E and 2200 m above m.s.l.). It consists of a densely packed array of 400 plastic scintillator detectors and a large area muon telescope (560 m$^{2}$). It measures cosmic rays from several TeV to over 10 PeV energies providing a substantial overlap with direct experiments while covering the knee region. Shower parameters are reconstructed by fitting the observed particle densities with the NKG lateral distribution function (LDF). For this analysis, the QGSJET-II-04 hadronic interaction model is used to generate the air shower simulation data for proton, helium, nitrogen, aluminium, and iron primaries. Precise measurements of the average nuclear composition are obtained by fitting muon multiplicity distributions (MMDs) for all simulated primaries with the MMDs measured by the muon telescope. Details of the analysis and preliminary results for the extracted composition and elemental energy spectrum for proton and helium from a few tens of TeV to a few PeV will be presented.

The deflection of cosmic rays (CRs) in the interstellar magnetic field results in an almost isotropic flux as observed on Earth. However, an anisotropy has been observed at the level of ∼ 10^{-4} − 10^{-3}. The GRAPES-3 experiment located at Ooty, India consists of an array of 400 plastic scintillator detectors. It measures the particle densities and their relative arrival times in extensive air showers produced by the CRs. This information collected is then reconstructed to obtain the energy and direction of the primary CRs. The near-equatorial location of GRAPES-3 provides an opportunity to study this anisotropy in both hemispheres of the celestial sphere in the TeV-PeV energy range. However, detector and atmospheric effects that induce a few per cent change in the primary CR flux are challenges to be addressed. This work describes the use of the time scrambling method to address some these systematics and observe anisotropy.

The Moon acts as a shield against the cosmic rays, preventing them from reaching the earth, which gives rise to a deficit in the flux along the direction of the Moon. The observed deficit can be used for obtaining the absolute calibration of the angular resolution and to verify the pointing accuracy of the array. GRAPES-3 is an extensive air shower experiment located at Ooty, India consisting of a dense array of scintillator detectors. It records $\sim$$10^9$ showers per year with a median energy of 10 TeV. With the precise determination of the arrival time of shower particles and an accurate correction for the shower front curvature, a major improvement in the angular resolution of the array has been achieved. This was done by the array division methods including the left-right and even-odd methods. Here, we present a verification of the angular resolution estimates and the pointing accuracy by observing shadow of the Moon in the cosmic ray flux.

The GRAPES-3 is a high-altitude, near-equator extensive air shower array at Ooty, India which is designed to observe cosmic and gamma-rays in TeV-PeV energy range. It consists of a dense array of 400 scintillator detectors operating in conjunction with a 560 $m^2$ area muon telescope. Due to recent improvements in the measurements of shower arrival time and corrections for shower front curvature based on shower size and age, the angular resolution of the array has been significantly improved. By leveraging the resultant improved angular resolution and an efficient rejection of the cosmic ray background using the muon content of the shower, a search for gamma-rays above 30 TeV from the Crab Nebula has been performed. The results will be presented during the conference.

The GRAPES-3 experiment reported the measurement of 1.3 GV potential across one of the massive thunderclouds recorded on 1 December 2014 by making use of the muon imaging technique. This measurement is ten times larger than the maximum potential reported previously by balloon and rocket sounding measurements, verifying the almost a century old prediction by C.T.R. Wilson. These measurements rely on the precise estimate of the change in the angular muon flux caused by the acceleration of muons during their passage through the charged layers of thunderstorms. The electric potential is estimated with the help of Monte Carlo simulations by using CORSIKA and other in-house tools. A study of the thunderstorms events recorded since April 2011 displays an asymmetry in their azimuthal distribution which can be understood to be caused by the ratio of $\mu^+$/$\mu^-$.

The large area 560\,m$^2$ directional muon telescope at Gamma Ray Astronomy at PeV Energies phase -3 (GRAPES-3) experiment in Ooty, India was designed primarily to study the extensive air showers (EAS) and made operational in year 1998. It has turned out to be an unique instrument to make fascinating study of exotic phenomenon by introduction of a new parallel data acquisition system (DAQ) in year 2000 to measure the muon directional flux. The recent discoveries of transient weakening of Earth’s magnetic shield probed by a Cosmic Ray Burst and measurement of the electrical properties of a thundercloud through muon imaging has demonstrated the capabilities of this instrument. The design of new triggerless muon data acquisition system (TM-DAQ) using Field-programmable gate array (FPGA) would enhance the present capabilities and open a new window on several physics fronts such as, a) precise measurement of the muon flux for thunderstorm studies, b) study of large angle EAS using the muon component, c) search for exotic particles characterized by early or delayed arrivals. We present here the key salient features of the TM-DAQ along with initial observations.

The large area muon telescope of GRAPES-3 has been operating continuously for more than two decades with a DAQ which has several limitations. At present, this DAQ is in the process of being upgraded with a FPGA based system. The new DAQ system is designed to be triggerless and capable of recording every hit from the 3712 proportional counters along with a time-stamp (10 ns accuracy) which has significantly expanded the physics horizon of the experiment. This triggerless feature allows the detection of muons beyond the nominal zenith range of the current system ($\theta$$

A large area (560 m^2) muon telescope in the GRAPES-3 experiment at Ooty, India records muon intensity at high cutoff rigidities (Rc) varies from 14–32 GV along 169 independent directions spanning a field of view of 2.3 sr. The threshold energy of the recorded muons is sec(theta) GeV along a direction with a zenith angle (theta) and with the average angular accuracy of∼4 degrees. The directional capabilities of the muon telescope are exploited for studying the effect of atmospheric pressure on the muon flux as a function of Rc. It is observed that the barometric coefficient relationship with logarithmic Rc can be well described by a second order polynomial function with a high Spearman Rank correlation coefficient of 0.99

The GRAPES-3 experiment is designed to perform precision studies of gamma-ray sources in the TeV-PeV energy region. It consists of 400 plastic scintillator detectors spanning an effective area of 25000 m^2 and a large area (560 m^2) muon telescope which records ∼4 x 10^9 muons every day. With the recent installation of an improved triggerless data acquisition (DAQ) system, the information related to every muon is recorded with a timing resolution of 10 ns. The angular resolution and pointing accuracy of the upgraded muon telescope has been validated by characterizing the shadow of the moon among recorded muons. Here, the details of the analysis and results, as well as the simulation studies to account for the deflection of the particles in the Earth’s magnetic field will be presented.

Low energy ground-based cosmic ray air shower experiments generally have energy threshold in the range of a few tens to a few hundreds of TeV. The shower observables are measured indirectly with an array of detectors. The atmospheric absorption of low energy secondaries limits their detection frequencies at the Earth's surface. However, due to selection effects, a tiny fraction of low energy showers, which are produced in the lower atmosphere can reach the observational level. But, due to less information of shower observables, the reconstruction of these showers are arduous. Hence, it is believed that direct measurements by experiments aboard on satellites and balloon flights are more reliable at low energies. Despite having very small efficiency ($\sim$0.1%) at low energies, the large acceptance ($\sim$5 m$^2$sr) of GRAPES-3 experiment allows observing primary cosmic rays down below to $\sim$1 TeV and opens up the possibility to measure primary energy spectrum spanning from a few TeV to beyond cosmic ray knee (up to 10$^{16}$ eV), covering five orders of magnitude. The GRAPES-3 energy threshold for primary protons through Monte Carlo simulations are calculated, which gives reasonably good agreement with data. Furthermore, the total efficiencies and acceptance are also calculated for protons primaries. The ability of GRAPES-3 experiment to cover such a broader energy range may provide a unique handle to bridge the energy spectrum between direct measurements at low energies and indirect measurements at ultra-high energies., 15 pages, 6 figures

The large area (560 m 2 ) GRAPES-3 tracking muon telescope has been operating uninterruptedly at Ooty, India since 2001. Every day, it records 4 × 10 9 muons of ≥ 1 GeV with an angular resolution of ∼ 4°. The variation of atmospheric temperature affects the rate of decay of muons produced by the galactic cosmic rays (GCRs), which in turn modulates the muon intensity. By analyzing the GRAPES-3 data of six years (2005–2010), a small (amplitude ∼ 0.2%) seasonal variation (1 year (Yr) period) in the intensity of muons could be measured. The effective temperature ‘T eff ’ of the upper atmosphere also displays a periodic variation with an amplitude of ∼ 1 K which was responsible for the observed seasonal variation in the muon intensity. At GeV energies, the muons detected by the GRAPES-3 are expected to be anti-correlated with T eff . The anti-correlation between the seasonal variation of T eff , and the muon intensity was used to measure the temperature coefficient α T by fast Fourier transform (FFT) technique. The magnitude of α T was found to scale with the assumed attenuation length ‘ λ ’ of the hadrons in the range λ = 80–180 g cm − 2 . However, the magnitude of the correction in the muon intensity was found to be almost independent of the value of λ used. For λ = 120 g cm − 2 the value of temperature coefficient α T was found to be ( − 0.17 ± 0.02 )% K − 1 .

Following the GRAPES-3 discovery of a transient weakening of Earth's magnetic shield through observation of a cosmic ray burst on 22 June 2015, we have been involved in an effort to search for more such events in 20 years of archived data. An important step in analyzing the data is to simulate the cosmic ray trajectories in the geomagnetic field influenced by the interplanetary magnetic field during a geomagnetic storm. The simulation of 22 June 2015 burst required over two months of running time on the 1280 core GRAPES-3 computer cluster in Ooty. We have developed a fast, efficient technique which has allowed us to simulate the 22 June 2015 event in a fraction of a second with the same resolution as the earlier computational-intensive method. This has laid the foundation for quick analysis of a large number of such events in archival data, and of future events virtually in real-time. Details of this method will be presented during the conference.

The GRAPES-3 large area (560 m 2 ) tracking muon telescope is operating at Ooty in India since 2001. It records 4 × 10 9 muons of energy ≥ 1 GeV every day. These high statistics data have enabled extremely sensitive measurements of solar phenomena, including the solar anisotropies, Forbush decreases, coronal mass ejections etc. to be made. However, prior to such studies, the variation in observed muon rate caused by changes in atmospheric pressure needs to be corrected. Traditionally, the pressure coefficient ( β ) for the muon rate was derived from the observed data. But the influence of various solar effects makes the measurement of β somewhat difficult. In the present work, a different approach to circumvent this difficulty was used to measure β , almost independent of the solar activity. This approach exploits a small amplitude (∼1 hPa) periodic (12 h) variation of atmospheric pressure at Ooty that introduces a synchronous variation in the muon rate. By using the fast Fourier transform technique the spectral power distributions at 12 h from the atmospheric pressure, and muon rate were used to measure β . The value of pressure coefficient was found to be β = ( − 0.128 ± 0.005 ) % hPa − 1 .

The angular resolution of an extensive air shower (EAS) array plays a critical role in determining its sensitivity for the detection of point γ-ray sources in the multi-TeV energy range. The GRAPES-3, an EAS array located at Ooty in India (11.4oN, 76.7oE, 2200 m altitude) is designed to study γ-rays in the TeV-PeV energy range. It comprises of a dense array of 400 plastic scintillators deployed over an area of 25000 m2 and a large area (560 m2) muon telescope. The development of a new statistical method has allowed the real time determination of propagation delay of each detector in the GRAPES-3 array. The shape of the shower front is known to be curved and here the details of a new method developed for accurate measurement of the shower front curvature is presented. These two developments have led to a sizable improvement in the angular resolution of the GRAPES-3 array. It is shown that the curvature depends on the size and the age of an EAS. By employing two different techniques, namely, the odd-even and the left-right methods, independent estimates of the angular resolution are obtained. The odd-even method estimates the best achievable resolution of the array. For obtaining the angular resolution, the left-right method is used after implementing the size and age dependent curvature correction. A comparison of the angular resolution as a function of EAS energy by these two methods shows them be virtually indistinguishable. The angular resolution of the GRAPES-3 array is 47' for energies E>5 TeV and improves to 17' at E>100 TeV, eventually approaching 10' at E>500 TeV.

The GRAPES-3 muon telescope located in Ooty, India records rapid ($\sim$10 min) variations in the muon intensity during major thunderstorms. Out of a total of 184 thunderstorms recorded during the interval April 2011-December 2014, the one on 1 December 2014 produced a massive potential of 1.3 GV. The electric field measured by four well-separated (up to 6 km) monitors on the ground was used to help estimate some of the properties of this thundercloud including its altitude and area that were found to be 11.4 km above mean sea level (amsl) and $\geq$380 km$^2$, respectively. A charging time of 6 min to reach 1.3 GV implied the delivery of a power of $\geq$2 GW by this thundercloud that was moving at a speed of $\sim$60 km h$^{-1}$. This work possibly provides the first direct evidence for the generation of GV potentials in thunderclouds that could also possibly explain the production of highest energy (100 MeV) $\gamma$-rays in the terrestrial $\gamma$-ray flashes., Comment: Received 6 January 2019, Revised 21 January 2019, Published 15 March 2019

The Gamma Ray Astronomy at Pev EnergieS phase-3 (GRAPES-3) experiment, located at 11.4 ∘ N latitude, 76.7 ∘ E longitude and 2200 m altitude on the beautiful slopes of the Nilgiris hills at Ooty, India, consists of a world class indigenously developed detector system. The faithful operation and continuous improvement in the design of detectors, its associated electronics and the analysis software over a period of two decades has resulted in a better understanding of the universe at high energies. The GRAPES-3 is capable of studying the high energy cosmic ray composition in the knee region of the spectrum (Gupta, 2005) [1] solar flares , coronal mass ejection , and acceleration of particles in the atmospheric electric field. The core elements of the experiment are plastic scintillator (Sc) detectors and proportional counters (PRCs). The current status of the GRAPES-3 experimental system along with plans for the future upgrade are presented.

The GRAPES-3 experiment, a high-density extensive air shower (EAS) array located at Ooty, India is designed for high precision measurements of cosmic ray energy spectrum and nuclear composition in energy range of 10^13 - 10^16 eV . It consists of tightly packed 1 m 2 area 400 plastic scintillation detectors covering an effective area of 25,000 m^2. The trigger rate and particle density measured by EAS array shows strong correlation with the atmospheric parameters such as the pressure and temperature. By employing linear regression, and Fast Fourier Transform techniques, the pressure and temperature coefficients for the dependence of trigger rate and particle density, respectively were obtained. Thereafter, by applying corrections for these effects the contribution of atmospheric effects was largely eliminated in the EAS data. These corrections are expected to result in a better measurement of EAS parameters which will be presented during the conference.

The GRAPES-3 experiment is a unique facility to study cosmic ray energy spectrum and composition with high precision. It consists an array of 400 plastic scinitillation detectors and a muon telescope of 3712 proportional counters to study extensive air shower (EAS) phenomenon around knee ($\gtrsim$ 10$^{15}$ eV). Study of energy spectrum and composition of primary cosmic rays (PCRs) can improve the understanding about the nature of the sources accelerating PCRs to energies \(\gtrsim 10^{15}\) eV which may be studied by using high statistics data produced by GRAPES-3. Measurement of particle densities ($\gtrsim$ 5000 m$^{-2}$) is achieved by viewing each scintillator with two photomultiplier tubes (PMTs). The results obtained till date will be presented at the conference.

The GRAPES-3 is a high-density extensive air shower (EAS) array located at Ooty India. The array is presently operating with 400 plastic scintillation detectors (1 m^2 area each) with the inter-detector separation of 8 m spread over 25,000 m^2 and a large area (560 m^2) tracking muon detector. The variation in atmospheric pressure can significantly affect the observed cosmic ray flux measured by these detectors. We have simulated this effect by using the CORSIKA package after folding the detector response using GEANT4. The results of this work will be discussed during the conference.

The large area ($ \rm 560 \, m^2$) GRAPES-3 tracking muon telescope located at Ooty, India has been operating uninterruptedly since 2001. Everyday it records $ \rm 4\times 10^9$ muons of energy $\rm > 1 \, GeV$ with an angular resolution of $\rm \sim 4^{\circ}$. Atmospheric temperature variation affects the rate of decay of these GeV muons produced by the galactic cosmic rays (GCRs), which in turn modulates the intensity of detected muons. Since the daily temperature induced variation combines with the diurnal modulation of the GCRs by the magnetized solar wind, it becomes rather difficult to segregate the respective contributions of these two phenomena. A small seasonal variation in the intensity of cosmic ray muon ($ \rm \sim 0.4\%$) with periodicity $\rm \sim $ 1 year (Yr) was measured by analysing the GRAPES-3 data of six years (2005-2010). The effective temperature `$\rm T_{eff}$' of the upper atmosphere above Ooty also displayed a similar periodic variation with an amplitude of $\rm \sim 1 K$, which was responsible for the observed seasonal variation in the muon intensity. At GeV energies, the muons detected by the GRAPES-3 show an anti-correlation with $\rm T_{eff}$ calculated by using a hadronic attenuation length $\lambda$. Using the fast Fourier transform (FFT) technique and making use of the anti-correlation between the seasonal variation of $\rm T_{eff}$ with the muon intensity, we calculated the temperature coefficient $\rm \alpha_T$. The magnitude of $\rm \alpha_T$ was found to scale with the assumed attenuation length $\rm \lambda$, which we varied within a range of 80-180 $\rm g cm^{-2}$. However, the actual magnitude of the correction was found to be independent of the value of $\rm \lambda$.

Large scale anisotropy of cosmic rays in the TeV-PeV energy region has been reported by several experiments located in the Northern Hemisphere as well as by the IceCube, and its surface array IceTop at the South Pole. The GRAPES-3 experiment in Ooty, India is designed to observe cosmic rays in the energy range from a few TeV to over 10 PeV. Its near-equatorial location (11.4$^{\circ}$N) provides a unique advantage of its observations on cosmic ray anisotropy, overlapping with experiments in both the hemispheres. The GRAPES-3 consists of a dense array of 400 plastic scintillator detectors spread over an area of 25000 m$^2$. It also contains a large area (560 m$^2$) tracking muon detector which provides an excellent capability to discriminate $\gamma$-rays against cosmic rays. It has recorded more than 10 billion showers since its operation began in 2000. Measurements of cosmic ray anisotropy with a subset of this data is presented here.

The GRAPES-3 experiment in Ooty, India has been operating a large area (560m$^2$) tracking muon telescope since 2000. It consists of 16 identical modules and each one is designed to measure the flux of muons in 13 $\times$ 13 directions covering 2.3 sr field of view. The high statistics data has enabled to probe transient space weather events on time scale of minutes. Due to independent nature of operation of the modules, despite intermittent failure of individual modules, a continuity in rate could still be achieved. By correcting for transient instrumental problems and gradual efficiency variations, an uninterrupted muon record is being assembled which may prove to be a valuable database for probing both transient and long-term solar phenomena. Details of the efficiency correction technique will be presented during the conference.

The GRAPES-3 (Gamma Ray Astronomy at PeV EnergieS-3) experiment has been designed to study the composition and energy spectrum of primary cosmic rays (PCRs) over TeV - PeV range. It is equipped with 400 plastic scintillators covering an area of 25000 m$^2$ and a muon detector spread in an area of 560 m$^2$. The precise measurement of particle densities from each detector is very important for any air shower experiment. Photo-multiplier tubes (PMT) are widely used as a transducer to collect the light produced in detector medium to electric signals. These signals are then processed to measure the particle densities. However, these PMTs tend to show non-linearity and saturation at higher signals due to the large shower core landing near the detector. The GRAPES-3 detectors record $\sim$ (50 -70) particles in the linear range. This results in improper shower size estimation near the knee energy. In order to overcome this situation and to estimate the correct shower size, a systematic approach has been devised and demonstrated to correct the particle density beyond the linear range. It is seen from the method allows us to estimate particle densities $\sim$ (500 - 800) which can improve the measurements beyond knee.

Precise measurements of the nuclear composition and energy spectrum of primary cosmic rays at the ’knee’ and beyond are essential to understand their astrophysical origin, acceleration, and properties of the interstellar medium. The GRAPES-3 experiment located at Ooty in India is designed with a densely packed array of scintillator detectors. It measures cosmic rays from several TeV to over 10 PeV while providing a substantial overlap with direct experiments. The muon multiplicity distribution measured by the large area tracking muon detector associated with the array is sensitive to composition. Recently, we have attempted to measure the energy spectrum and composition from sub-TeV to over 10 PeV. The results obtained so far will be presented during the conference.

The thunderstorm events observed by GRAPES-3 muon telescope can be studied with aid of Monte Carlo simulations. One such event observed on 1 December 2014 was analysed and its electrical properties of thundercloud were derived using muon imaging. This recent result showed production of Giga-Volt potential in thunderclouds, possibly responsible for production of 100 MeV $\gamma$-rays in terrestrial $\gamma$-ray flashes. However, these properties derived from simulations rely on choice of low and high energy hadronic interaction models used in CORSIKA. So, the derived properties are model dependent which makes this study very important. The comparison of various combinations using low and high energy hadronic models and its impact on electric potential estimation will be discussed in this work.

Extensive air showers spread laterally mainly when electrons undergo multiple coulumb scattering in the atmosphere.The low energy secondaries and tertiaries will scatter further and will end up farther from the shower core. The particles which are scattered at larger angles during the shower development have higher path lengths and they arrive at the observational level slightly later. Moreover, the particles farther from the core, being lower in energy travel slower giving rise to additional time delays. The time delays, so produced, give rise to a shower front which is non planar in shape. The shape of the shower front has been determined to be conical by various air shower experiments. Here, we present the study of shower front curvature with the data from GRAPES-3 array. We observed that the shower front can be well approximated with a conical shape. However, our detail study has shown that the slope of the conical front has a strong dependence on both shower size and age. The correction of these dependence has led to an improvement in the angular resolution by a factor of two. This would provide efficient observation of multi-TeV gamma ray sources with GRAPES-3