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RESEARCH ARTICLES
CURRENT SCIENCE, VOL. 117, NO. 7, 10 OCTOBER 2019
560
*For correspondence. (e-mail: arc@sac.isro.gov.in)
Orbiter High Resolution Camera onboard
Chandrayaan-2 Orbiter
Arup Roy Chowdhury*, Manish Saxena, Ankush Kumar, S. R. Joshi,
Amitabh, Aditya Dagar, Manish Mittal, Shweta Kirkire, Jalshri Desai,
Dhrupesh Shah, J. C. Karelia, Anand Kumar, Kailash Jha, Prasanta Das,
H. V. Bhagat, Jitendra Sharma, D. N. Ghonia, Meghal Desai, Gaurav Bansal
and Ashutosh Gupta
Space Applications Centre, Indian Space Research Organisation, Ahmedabad 380 015, India
Orbiter High Resolution Camera (OHRC) onboard
Chandrayaan-2 Orbiter-craft, is a very high spatial
resolution camera operating in visible panchromatic
band. OHRC’s primary goal is to image the landing-
site region prior to landing for characterization and
finding hazard-free zones. Post landing operation of
the OHRC will be for scientific studies of small-scale
features on the lunar surface. OHRC makes use of the
time delay integration detector to have good signal-to-
noise ratio under low illumination condition and less
integration time due to very high spatial resolution.
Ground sampling distance (GSD) and swath of OHRC
(in nadir view) are 0.25 m and 3 km respectively, from
100 km altitude. GSD is better than 0.32 m in oblique
view (25° pitch angle) during landing site imaging
from 100 km altitude in two stereo views in consecu-
tive orbits. This article includes the details of the con-
figuration, sub-systems, imaging modes, and optical,
spectral and radiometric characterization perfor-
mance.
Keywords: Ground sampling distance, orbiter high res-
olution camera, relative spectral response, square wave
response, time delay integration.
CHANDRAYAAN-2, India’s second lunar mission, has
orbiter-craft, lander-craft and rover as a composite mod-
ule. The orbiter-craft will have a circular polar orbit at an
altitude of 100 km. Orbiter High Resolution Camera
(OHRC) is one of the important instruments onboard the
orbiter-craft. OHRC has 0.25 m ground sampling distance
(GSD) and 3 km swath at nadir from 100 km. OHRC
makes use of the Ritchey–Chretien (RC)-telescope with
field correcting optics (FCO) and 12 K by 256 time delay
integration (TDI) detector. Stereo imaging is through the
spacecraft orientation and manoeuvring to have desired
view angles. Landing site is in highland region with typi-
cal reflectance of about 8% in the visible range. OHRC
can image lunar surface under low illumination condition
(Sun elevation angle about 5–6°). This is an important re-
quirement to have early landing in the lunar day-cycle
and maximize the number of days available for Lander-
craft and Rover instrument operation.
Objectives
There are broadly two type of objectives of OHRC
namely ‘mission objectives’ and ‘science objectives’.
Mission objective is prime for the safe landing of the
Chandrayaan-2 lander-craft by making use of the OHRC
images and derived datasets like crater catalogue and
hazard map (for boulders, craters, slopes and shadows).
Mission objectives
OHRC’s mission objective is to capture images of the
Moon surface with high spatial resolution to characterize
the landing site (prior to landing) with a GSD better than
0.32 m during two oblique views (0.25 m at nadir) from
100 km altitude. Nominal OHRC imaging region of land-
ing site region is about 3 km × 12 km area in two orbits
with different view angles (with spacecraft maneuvering)
to generate digital elevation model (DEM) information.
Major hazards for lander-craft are slopes, boulders, cra-
ters and shadows for which OHRC data is important.
Slope from DEM and shadows from brightness images
are required to generate the hazard map and safe-grid
location.
OHRC observation is important to update the landing
grid and the landing path (i.e. trajectory). OHRC data
processing is at ground to generate the required data sets
for lander-craft (e.g. updated crater catalogue, hazard
map or safe-grid map), required to assist other sensors
during various phases of the landing or descent trajectory
(e.g. land-mark identification, final safe-grid identifica-
tion during hovering). Locating and monitoring of the
Lander-craft and Rover by OHRC is the post-landing
OHRC imaging activity.
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CURRENT SCIENCE, VOL. 118, NO. 4, 25 FEBRUARY 2020 561
Table 1. Parameters of Orbiter High Resolution Camera
Parameter Values
Orbit altitude (km) 100
GSD (m) at nadir 0.25
Swath (km) at nadir 3
Spectral range (nm) 450–800
Telescope diameter (mm) 300
Detector 12 K by 256 TDI
Quantization (bits) 10 (electronics) 8 (transmission)
Reference illumination condition 8% Albedo, 5–6° Sun elevation
Reference radiance (mW/cm2/sr/μm) 0.5
Saturation radiance (mW/cm2/sr/μm) 0.8 with 256 TDI
Signal-to-noise ratio (SNR) @ reference radiance 100 with 256 TDI (140 at saturation)
Stereo views Fore and Aft in two consecutive orbits
by spacecraft and maneuvering
Table 2. Optical system parameters
Parameter Value
Optical configuration Ritchey–Chretien telescope with field
correcting optics (RC + FCO)
Spectral range (nm) 450–800
Primary mirror diameter, D (mm) 300
Effective focal length (EFL; mm) 2046
Field-of-view (FOV, deg) ± 0.86
MTF (%) @ 96 lp/mm (Nyquist) 24
Table 3. Detector parameters
Parameter Value
Detector type TDI–CCD
Pixel size (μm) 5.2 × 5.2
Spectral range (nm) 450–800
TDI stages 256, 192, 128, 64 (selectable)
Full well capacity (ke–) 26.6
Read noise (e–) 40
Science objectives
OHRC’s science objectives are supported by its very high
spatial resolution imaging capability. Also low illumina-
tion imaging capability adds values to the observations in
the near polar regions. It can image small-scale features
in fine detail. The first objective of OHRC, after mission
objective, i.e. the landing-site imaging, is to image the
anthropogenic sites on lunar surface. The other scientific
objectives are: (i) to study the recent volcanism and other
small-scale volcanic sites; (ii) to study the central peaks
of young impact craters, and (iii) to understand tectonic
(seismic) processes (through boulder population determi-
nation, boulder rolling trails, seismicity related features,
etc.); impact cratering (small craters, ejecta material, melt
pools, etc.) and mass wasting (rock falls, debris flow,
etc.).
System configuration
OHRC has 0.25 m GSD and 3 km swath from 100 km
altitude. It operates in visible (450–800 nm) panchromatic
band. It makes use of the RC-telescope with FCO to col-
lect the signal and image onto a 256 stage TDI-detector
with 12,000 pixels. RC-telescope is a high focal length
and compact optical system. TDI detector is required to
collect sufficient signal under low illumination and less
integration time condition. TDI stages and integration
time can be selected through telecommand (TC). Table 1
gives the parameters of the OHRC.
Optical system
To collect adequate signal for meeting the SNR require-
ment, optical aperture diameter of 300 mm for primary
mirror is selected. For this aperture, reflective system has
less weight and is realizable. Considering pixel size of
the detector, effective focal length (EFL) requirement is
~2 m to have 0.25 m GSD from 100 km. To have 3 km
swath, field-of-view requirement is ±0.86°. Thus, Ritchey–
Chretien telescope with field correcting optics (i.e.
RC + FCO) configuration is selected to meet the large
focal length and field-of-view requirement. Figure 1
shows the optical configuration of OHRC. Table 2 pro-
vides the optical system parameters.
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Table 4. Camera electronics parameters
Parameter Value
Quantization (bit) 10 (electronics)
8 (transmission) by bit selection
Integration times (μs) 7 values (selectable by telecommand (TC)) 213.84, 209.58, 205.32, 181.74,
178.14, 174.87 (for landing site imaging catering to ±2 km altitude variation,
fore and aft view) and 162.10 μs (for Nadir view imaging)
TDI stages 256, 192, 128, 64 (selectable by TC) 256 (for landing site) 64 (post-landing)
Data volume (per view) 4.6 Gb for one 3 km × 12 km strip (for 8 bits)
Raw power (W) ~36
Figure 1. Optical configuration.
Figure 2. Camera electronics.
A trade-off study was carried out for selecting the
spectral range of the complete system considering the
SNR and modulation transfer function (MTF) perfor-
mance. Spectral range of 450–800 nm was found to be
optimum. Extending spectral range (specifically towards
higher wavelengths) was found to have negative impact
on the MTF performance without much gain in the signal
collection.
Detector
To meet the mission requirements in terms of GSD, swath
and SNR; a commercial of the shelf (COTS) TDI–CCD
detector was considered. This detector was qualified
in-house for flight use. It has pixel size of 5.2 μm, format
of 12,000 pixels and up to 256 TDI stages with four
selectable values. System level measurement shows that
readout noise is about 40 e– and full-well capacity is
26.6 ke–. Table 3 provides the detector parameters.
Camera electronics
To operate detector and acquire data, camera electronics
(CE) generates required clocks, biases and control sig-
nals. CE consists of detector proximity electronics (DPE),
front end camera electronics (FECE), logic and control
electronics (LCE) and power supply electronics (PSE).
CE block diagram is shown in Figure 2.
DPE provides interface of clocks, biases and output
video data from detector to FECE.
FECE provides low noise DC biases, clocks (of suita-
ble voltage level) for detector operation, acquiring video
data and data preprocessing. FECE also do analog to
digital conversion, serialization and data interface to the
spacecraft’s baseband data handling (BDH) system.
LCE generates the clocks for FECE and interfaces with
BDH for clocks and data. LCE also interfaces with the
spacecraft’s telecommand and telemetry (TC&TM)
system.
PSE draws power from the spacecraft and caters to the
power requirements of DPE, FECE and LCE. Table 4
provides the CE parameters.
Mechanical and thermal system
To house the optical module, detector head assembly
(DHA) and CE, a mechanical system is required which
includes structure, packages and other components of
electro-optical module (EOM) like baffles, fixtures, coup-
lers, alignment cube, etc. (Figure 3). Thermal control in
the instrument ensures that all sensitive elements remain
within the specified temperature limits, considering about
300°C temperature difference to be viewed by the
instrument over the Moon’s surface. The thermal control
is achieved by passive thermal control techniques
augmented with auto/commandable heaters.
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CURRENT SCIENCE, VOL. 118, NO. 4, 25 FEBRUARY 2020 563
OHRC is mounted on the negative pitch (-P) panel of
the orbiter-craft. Table 5 shows the physical parameters
of OHRC. Figure 3 shows the CAD model for EOM of
OHRC. It shows various components like mirrors,
cylinder, FCO, DHA, baffles and alignment cube.
Imaging modes
Earth bound phase imaging mode for Earth and
deep space observation
This mode is planned to observe Earth during the Earth
bound orbital phase for functional testing. Suitable
spacecraft maneuvering and scan rate is required for
this operation. Deep space observation for dark data is
required for dark noise performance study.
Lunar bound phase test-imaging mode for lunar
observation
This mode is for ‘readiness for landing site imaging’. It is
required to observe Lunar surface few orbits before the
nominal imaging sessions for the landing site. This is
required for functional test and initial processing verifica-
tion and readiness at ground for actual landing site obser-
vation and processing.
Pre-landing nominal-imaging mode for lunar
observation
This mode is to meet the ‘mission objective’. This is the
nominal observation of the actual landing site for its
characterization with respect to hazards and safe zones.
Landing site observation with two view angles in two
consecutive orbits is configured by spacecraft maneuver-
ing (Figure 4). Two stereo views are +5° and –25° about
the pitch axis. Also roll tilt 23° (orbit# –4) and 17.6°
(orbit# –3) respectively is required to ensure proper side
view of the landing site few orbits prior to landing orbit.
Time period of the satellite is about 2 h. Time taken for
imaging the landing site (i.e. 3 km × 12 km strip) is about
8 sec per stereo view, every orbit.
Post-landing imaging mode for lunar observation
This mode is to meet the ‘science objective’. After land-
ing of lander, OHRC is to be used for nominal high reso-
lution imaging of the lunar surface. It can also be used for
imaging lander and rover after landing. Nominal integra-
tion time for Nadir view imaging is 162.1 μs for nominal
spacecraft altitude of 100 km. Due to higher illumination
during post-landing imaging scenario, lower TDI stages
(e.g. 64) can be selected among four options (64, 128,
192 and 256 TDI stages) by TC.
Post-landing imaging mode for Earth and deep
space observation
This mode is planned for observing Earth during the
Moon bound orbital phase after landing operation for
scientific interest and outreach. Suitable spacecraft
maneuvering and scan rate is required for this operation.
Deep space observation for dark data is required for dark
noise performance study.
Payload characterization
Payload characterization results are discussed in this
section. Figure 5 shows the realized OHRC payload.
Figure 3. CAD model of electro-optical module of OHRC.
Figure 4. Typical schematic of tilted view observation with tilt about
pitch axis.
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CURRENT SCIENCE, VOL. 118, NO. 4, 25 FEBRUARY 2020
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Optical characterization – SWR measurement
Square wave response (SWR) is the important parameter
of the image quality of a camera. SWR indicates the
sharpness or contrast of the features in the image. SWR
is measured for the OHRC’s central as well extreme
fields at Nyquist (Nq, i.e. 97 lp/mm) as well as Nq/2
(48 lp/mm) spatial frequency (Table 6).
TDI mode characterization
The TDI mode imaging is verified in laboratory by imag-
ing a rotating cylinder with targets printed on its body.
The rotation of the cylinder is in synchronization with the
vertical charge transfer rate of the TDI detector. Figure 6
shows the image captured by OHRC.
Figure 5. Realized OHRC payload.
Table 5. Camera mass, size and operating temperature
Parameter Values
Mass (kg) 17.5
Size (mm3) 486 × 463 × 700
Operating temperature (°C) 10–30
Spectral characterization
Relative spectral response (RSR) is measured using a
double monochromator-based spectro-radiometer system
during spectral characterization activity. System level
measured RSR is shown in Figure 7.
Figure 6. Image captured by OHRC in time delay integration mode
of operation in laboratory.
Figure 7. Relative spectral response of OHRC.
Figure 8. Linearity of the radiometric response of the OHRC (input
radiance in mW/cm2/sr/μm on x-axis versus output counts (10 bit) on
y-axis).
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Table 6. Square wave response (SWR) of the OHRC
Extreme field (R1) Centre field (R2) Extreme field (R3)
SWR (%) @ Nq/2 28 34 30
SWR (%) @ Nq 13 16 14
Table 7. Radiometric performance of OHRC, saturation radiance
(SR) and SNR
Parameter Values
Reference radiance (mW/cm2/sr/μm) ~0.5
SR (mW/cm2/sr/μm) 0.8 @256 TDI 3.2 @64 TDI
SNR ~100 (reference radiance)
(~140 at SR)
Figure 9. Radiometric performance in terms of the signal-to-noise ratio
(on y-axis) at various input radiances in mW/cm2/sr/μm (on x-axis).
Radiometric characterization
Camera’s radiometric response linearity (Figure 8), count
(10 bit) to radiance coefficients and SNR (Figure 9) are
measured during radiometric characterization activity
using integrating sphere as uniform light source. This
activity also known as light transfer characteristic and the
performance is tabulated in Table 7.
Data product
Data processing for OHRC is planned for two different
scenarios, one is for pre-landing decision support system
(DSS) and other for regular operations. In DSS, two
consecutive orbit datasets will be acquired and processed
for the DEM and corresponding hazard map generation.
A software pipeline has been developed to process OHRC
DSS images (~3 km × 12 km), which is capable of gene-
rating DEM in 21 min after receiving the second orbit da-
ta. Regular operations products are according to defined
levels. There are three levels (0, 1 and 2) defined for
OHRC data products.
Level-0 dataset consists of payload raw data (as it is
collected from payload), ancillary data (mainly SPICE
kernels generated from the mission) and housekeeping
data (health of the spacecraft). Level-1 data will be radi-
ometrically corrected and geometrically tagged data.
Radiometrically corrected datasets are free of photo-
response non-uniformity. Level-2 datasets include both
radiometric and geometric correction. DEM generation
and Ortho correction using Lunar control points or refer-
ence data are planned for stereo acquisitions.
Summary
OHRC flight model with 0.25 m GSD from 100 km
is developed and characterized for optical, spectral and
radiometric performance. This instrument provides
mission-critical information with respect to hazard avoid-
ance during landing, for which it has been tested and
qualified through various tests.
ACKNOWLEDGEMENTS. We thank the contributions of all
colleagues involved in the realization of the instrument. Several units
of ISRO are involved in the development of the OHRC. The Sensors
Development Area at Space Applications Centre is the lead area
responsible for overall design, development, testing, qualification and
delivery of the instrument. The optical elements are fabricated at
LEOS, Bengaluru. The structure was developed at CMSE, Thiruvanan-
thapuram. We gratefully acknowledge the support and guidance
received from various areas and are thankful to the Director, Space
Applications Centre for the constant encouragement.
Received and accepted 27 August 2019
doi: 10.18520/cs/v118/i4/560-565
