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Visible and Near-Infrared Imaging Spectrometer (VNIS) For In Situ Lunar
Surface Measurements
Zhiping He, Rui Xu, Chunlai Li, Gang Lv, Liyin Yuan, Binyong Wang, Rong Shu, Jianyu Wang
Key Laboratory of Space Active Opto-Electronics Technology, Shanghai Institute of Technical Physics, Chinese
Academy of Science, Shanghai, China
ABSTRACT:
The Visible and Near-Infrared Imaging Spectrometer (VNIS) onboard China’s Chang’E 3 lunar rover is capable of
simultaneously in situ acquiring full reflectance spectra for objects on the lunar surface and performing calibrations.
VNIS uses non-collinear acousto-optic tunable filters and consists of a VIS/NIR imaging spectrometer (0.45–0.95
μm), a shortwave IR spectrometer (0.9–2.4 μm), and a calibration unit with dust-proofing functionality. To bee n
underwent a full program of pre-flight ground tests, calibrations, and environmental simulation tests, VNIS entered
into orbit around the Moon on 6 December 2013 and landed on 14 December 2013 following Change’E 3. The first
operations of VNIS were conducted on 23 December 2013, and include several explorations and calibrations to
obtain several spectral images and spectral reflectance curves of the lunar soil in the Imbrium region. These
measurements include the first in situ spectral imaging detections on the lunar surface. This paper describes the
VNIS characteristics, lab calibration, in situ measurements and calibration on lunar surface.
Keywords: space vehicles: instruments, instrumentation: spectrographs, Moon, techniques: spectroscopic,
1. INTRODUCTION
Chang’E 3 operated by the China National Space Administration (CNSA) is the third in the Chang’E program
and consists of a lunar surface Lander and a lunar Rover, which is known as Yutu (meaning ‘jade rabbit’) 1–3.The
Visible and Near-infrared Imaging Spectrometer (VNIS) is one of the main scientific payloads on Yutu and consists
of an imaging spectrometer (0.45-0.95μm), a spectrometer (0.9-2.4μm), and a calibration unit. it is mounted on
the front of the rover to detect lunar surface objects with a 45° viewing angle and obtain the spectra and geometry
data at a height of 0.69 m4–7.
Minerals such as pyroxene, plagioclase, olivine, and ilmenite, which constitute most of lunar surface rocks with
varying size and shape, have distinctive spectral characteristics in the VNIR and SWIR regions, as shown in fig 14.
Morphological measurements and spectral measurements are the two main methods for analyzing rock structure and
composition. An imaging spectrometer has the ability to simultaneously obtain both the images and the spectral
signatures of the targets in the scene, and is widely used in terrestrial and space-based remote sensing applications.
The VNIS mostly addresses lunar surface material composition and available resource exploration. Mounted on the
platform of lunar rover Yutu, the VNIS detects the spectra and images of lunar objects in the roving area to provide
scientific data for determining the lunar surface mineral composition and performing comprehensive analysis of the
chemical composition5.
Sensors, Systems, and Next-Generation Satellites XIX, edited by Roland Meynart, Steven P. Neeck, Haruhisa Shimoda
Proc. of SPIE Vol. 9639, 96391S · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2194526
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08
02
- olivinediopside
M1vpersM1ene
umerele
00400 800 1200 1688
wavelength /nm 2000 2400
The calibration and
dustproof components
The non- worlung
status
Calibration
ode r5
Spectral
imaging mode
45° (the solar elevation angle)
The lunar surface
L
rCalibration
Unit
electronics
C
Collimating Lens VNHOiF
in.ño
C
Field
Diaphragm Imaging Lens
Area Array CMOS I Ci
VIS object! WW
Aelectronics ">
JW
IR obi iv T
mcaAS Detector COE
Field
Diaphragm [ Imaging Lens J
[ Collimating Lens > IRAOTF
RF Driver
electronics
The clothing
Board of base
The installation base
CPU
The calibration and
dust -proof components
Fig.1 The spectral reflectance curves of main single minerals of lunar surface (left) and the VNIS onboard Rover (right)
2. VNIS
The VNIS is designed for the scientific exploration mission of identifying the material composition and
available resources over the roving area of the lunar surface.As a passive optical instrument, the VNIS measures the
radiance diffusely reflected from solar illumination of the Moon’s surface. The VNIS use non-collinear AOTFs as
dispersive devices, and it has two detection channels: a VIS/NIR channel (0.45–0.95 μm) with a CMOS area array
detector, and a SWIR channel (0.9–2.4 μm) with an InGaAs single-element detector. It includes image-forming lens,
collimating lens, AOTF(acousto-optic tunable filter) light-splitting component, convergent lens, the detector
component, motor drive, RF(Radio Freqency) drive and main control circuit, as is shown in Fig24.
Fig.2 Overview of the VNIS system
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1=10.. -
11.=1,
The working principle of the VNIS is briefly described as follows5-7: reflected light from lunar objects arrives at
the AOTFs, where only narrow-wavelength-band data or imaging information passes through, depending on the
frequency that the RF driver generates. Then, the quasi-monochromatic light converges on the detector. The
spectrometer can realize flexible and rapid wavelength selection by altering the driving frequency exerted on the
AOTF, so we can acquire spectra or image data at different wavelengths over the available spectral coverage.
The VNIS adopted a splitting design that consists of a spectrometer probe located outside of the rover, and a
logical control and AOTF radio-frequency (RF) driver module, which is a unit called Remote Electric Control Box,
located inside the rover. These two parts are connected by cables and the configuration of the VNIS instrument is
shown in Fig.36.The major technical specifications of the VNIS are shown in Table 15–7.
Table 1 Main performance specifications of VNIS
Description
Specification
V-NIR SWIR
Spectral coverage (nm) 450-950 900-2400
Spectral resolution (nm) 2-7 3-12
FOV (degree) 8.5×8.5 Φ3.6
Effective pixels 256×256 1
Quantization (bits) 10 16
SNR(dB) ≥31 ≥32
Spectral sampling interval (nm) 5
Power consumption (W) 19.8
Weight(kg)
4.7 (Spectrometer probe)
0.7 (Logical control component in RECB)
Fig.3 Spectrometer probe of VNIS (left) and Remote Electrical Control Box (right).
The VNIS consists of a VNIR imaging channel and SWIR spectral channel. Their optical systems are both
composed of: an objective lens, field diaphragm, collimating lens, AOTF, imaging lens and detector, as shown in
Fig46.The VNIR channel and the bases of the SWIR channel’s image-forming mirrors lay on the main optical
backplane, while other parts of the SWIR channel are mounted on a lateral optical backplane.
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SWIR Objectiv VNIR Objective Mirror
Collimating Lens
Lens
'SWIR
AOTF
Single Element Detector
Detector Array
vN IRImaging Lens
The VNIS has the following functions6-7:
1) obtain spectral image data on specified objects in the visible and NIR (0.45–0.95 μm) and spectral data on
specified objects in the SWIR (0.9–2.4 μm)
2) obtain reflectance spectral images in the visible and NIR and reflectance spectra in the SWIR
3) perform on-orbit calibration
4) provide dust-proofing and insulation.
The calibration unit performs the calibration function and consists of an ultrasonic motor, framework, and an
internal diffuser panel. The inner surface of the calibration unit is the diffuser panel, which is located at the light
entrance. When the spectrometer is operated in detection mode, the calibration unit can be completely open at a 55°
angle to the mounting plane, which does not affect the light entrance. When the spectrometer operated in calibration
mode, the solar spectral irradiance was used as a calibration source, and the diffuser panel of calibration unit lay
parallel to the mounting plane to obtain calibration data. When it was not working, the calibration unit could be
closed up within the framework to prevent spectrometer damage from dust and other pollution and also to provide
good thermal insulation6-7.
Fig.4 Schematic diagram of the optical mudule
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W6 An.rn L CIozBC
Fig.5 Detection, calibration, and dust-proofing scheme of VNIS.
3. MAIN CHARACTERISTICS
The VNIS can obtain a spectral image in the VIS/NIR band and spectral data in the SWIR band simultaneously.
Its main characteristics include spectral characteristics, radiometric response characteristics，data characteristics, etc.
3.1 Spectral Characteristics
The spectral range and resolution of the VNIS are key characteristics and indicators for achieving the scientific
objectives and are directly related to the instrument’s ability to identify minerals by their spectra. AOTF is
dispersive component and has a direct impact on the final measurements of the spectrometer11. Its parameters, such
as the relation between diffraction wavelengths, RF driving frequency, diffraction efficiency, and spectral resolution,
should be tested before system integration and calibration. The AOTF performance analysis system is designed to
test the parameters of the AOTFs. Then, corresponding formulae for wavelength and RF driving frequency are fitted
using five order polynomials as the initial data for the instrument’s spectral calibration5, 11.
In the laboratory spectral calibration, the combination of a narrowband monochromatic source (including a
narrowband laser and monochromator) and a small passive integrating sphere was used as a light source, which is
shown in Fig.6. After calibration on the ground and confirmation of the calibration, the spectral range of the VNIS is
450–2400 nm, and the spectral resolution is 2–12 nm. The spectral resolutions are 2–7 nm in the VIS/NIR band and
3–12 nm in the SWIR band, as shown in Fig.74-7.
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AOTF
Imaging
Spectrometer
Narrowband
Monochromatic
Source
Response curve of 487.0 nm Laser source
1000
900
800
700
600
ó500
400
300
200
100
Peak wavelength @ 487.1nm
X: 487.1
Y: 505
0475 480 485 490 495 500
wavelength nm
Fig.6 Configuration of laboratory spectral calibration and calibration curve
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
0
2
4
6
8
10
12
wavelength (nm)
FWHM (nm)
V-NIR c hannel of VNIS
SWIR channel of VNIS
Fig.7 Spectral resolution of VNIS
3.2 Radiometric Characteristics
Radiometric calibration is a procedure that associates the value measured by the instrument [e.g., the data
number (DN) of a CMOS image pixel or single-element detector] with absolute physical quantities (such as the
spectral radiance). The inversion model for the radiance of a target is established using a standard radiation source.
The configuration of the laboratory radiometric calibration system is shown in Fig.8; the radiance level of the
integrating sphere can be adjusted by switching the calibrated lamps6.
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H910ä6 lnua21
Pwk
vonllAJiilll
?bps
Fig.8 Configuration of laboratory radiometric calibration system
The spectrometer probe of VNIS instrument is aimed at the central area of the integrating sphere port, where
the uniformity is better than 1‰. The DN curves, (, )
D
NClass
λ
, were acquired by VNIS under different integrating
sphere radiance levels calibrated as (, )IsRad level
λ
. The radiance inversion model is shown in Eq. (1)6.
,, 0,
,, 0,
(, ) ( (, )) (, )
(, ) ( (, ) (, ))
ij ij ij
ij ij ij
DN Class f IsRad level DN Class
I
sRad level g DN Class DN Class
λ
λλ
λλλ
=+
=−
(1)
Where pixel (, )ijcan be ignored in the SWIR channel since the SWIR detector has only a single element.
,(, )
ij
D
NClass
λ
is the DN response curve of the SWIR single element detector or each pixel (, )ijon the CMOS
detector under certain operating modes, while 0, (, )
ij
D
NClass
λ
is the corresponding dark level curve. The response
matrix of available signal of the detectors is ,()
ij
f, while ,()
ij
g is the corresponding inverse matrix. For VNIS, the
radiance inversion model is calculated based on the radiometric calibration results through a linear model6.
In radiometric calibration, the spectral response data are acquired under different integrating sphere radiance
levels. Therefore, the radiance inversion model is finally obtained by linear fitting to the DN of each pixel in each
band with the trend of the variation in the light source. The uncertainty of the inversion results is 3.69% in the
VIS/NIR channel and 5.39% in the SWIR channel5-7.
The signal-to-noise ratio (SNR) is an important indicator of the radiometric response characteristics of the
VNIS and changes with the input signal when the noise is fixed. The SNR of the VNIS was tested and analyzed by
ground radiometric calibration, and the SNR was obtained by calculation as shown in Fig.9.When the albedo is 0.09
and the solar elevation angle is 45° in the VIS/NIR band, and the albedo is 0.09 and the solar elevation angle is 15°
in the SWIR band, the SNR must be greater than 30 dB7.
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400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
0
50
100
150
200
wavelength / nm
S/N
S/N of VIS/NIR channel
S/N of SWIR channel
40 dB
30 dB
Fig.9 SNR of VNIS
While operating on the lunar surface, the VNIS is required to obtain reliable data over a large temperature range,
from –20° to 55°. Furthermore, due to the splitting design of VNIS instrument, the spectrometer probe and RECB
are in environments with different temperatures. A series of tests were designed in order to determine the influence
of temperature fluctuations on the SWIR channel that is more temperature sensitive. First of all, the standard
temperature curves of the RF amplifier and the InGaAs detector, with the thermoelectric cooler (TEC) at room
temperature (23±1°C), were measured as a reference. Then, a detection system was set up, using an integrating
sphere as an active radiation source and a vacuum tank as the temperature regulator. The temperature characteristics
of the spectrometer probe and RECB were analyzed independently and the temperature effect correction models for
the RF amplifier and the detector were modeled6.
3.3 DATA CHARACTERISTICS
The VNIS is equipped with a VIS/NIR spectral imaging channel and SWIR spectral detecting channel with an
18 mm separation between the optical axes; these systems can detect lunar surface objects simultaneously. With a
spectral sampling interval of 5 nm, the VNIS detected the lunar surface default automatically, and it sequentially
sampled 100 frames of spectral images in the VIS/NIR band and 300 frames of spectral data in the SWIR band. In
addition, two spectral bands each obtained an extra 20 frames of dark level for data processing. The geometrical
characteristics of imaging in the VIS/NIR band and spectral detection in the SWIR, as obtained by testing and
calibration on the ground, are shown in Figure 10. The circle represents the SWIR channel’s field of view (FOV),
which has a diameter of 107 pixels and is centered at the coordinate (96, 128) of the VIS/NIR channel4.
The specific order of data detection by the VNIS is5-6:
1) sampling 10 frames of dark level data in the VIS/NIR band (CMOS array detector);
2) sampling 10 frames of dark level data in the SWIR band (InGaAs single-element detector);
3) obtaining 300 frames of scientific detection data from the lunar surface (in detection mode) or the calibration
diffuse reflection plate (in calibration mode) in the SWIR band (InGaAs single-element detector);
4) sampling 10 frames of dark level in the SWIR band (InGaAs single-element detector);
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o, o255, 0
0, 255
The pixel in the center of
SWIR channel FOV is
( 96, 128), and the
diameter is 107 pixels.
255, 255
CMOS Detector
Level 1 Data
Subtracti
Flat Field
VNIS Raw
Data
SWIR Detector
Level 1 Data
Dark
Current
Subtraction
Temperature
Calibration
CMOS Detector
Level 2A Data
tic
Calibration
Radiometri
Calibratio
Geometric
Cal ibratioli
SWIR Detector
Level 2A Data
Geometric
Calibration
CMOS Detector
Level 2B Data
SWIR Detector
Level 2B Data
5) sampling 100 frames of scientific detection data in detection mode or calibration mode in the VIS/NIR band
(CMOS array detector);
6) sampling 10 frames of dark level in the VIS/NIR band (CMOS array detector).
Fig.10 Geometrical relationships in VNIS detection
Fig.11 Preprocessing pipeline and data products for VNIS data
The images and spectral data obtained in detection and calibration modes were the original light response
signals, which should be used after data preprocessing and scientific data processing were completed. The data
preprocessing applied to the raw data included dark current deduction and temperature, radiometric, and geometric
corrections. As VNIS is the first AOTF type imaging spectrometer to be used for lunar exploration, its instrument
preprocessing is shown in Fig.118. The instrument data preprocessing pipeline mainly contains channel data
preprocessing and SWIR channel data preprocessing for the two different detectors.
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19`30'48"W 19`3047"1N 19°3046"W 19°30'45"W 19°30'
andér
* Finished Exploration Points
Navigation Points
Rover Rotite
19 °30'48"W
N0209(17,61)
I.
208(16,N)
90207(15,N)
)NO2C4(12,N)
19`30'43"W 19`30'42"W
N01039,801
0104(4,0) 1
14,
I
N0108(8.E2/83/H)
,;N0201(9,N)
'1_^-1,E1)
/19°30'4]"W 19°30'46"W 19°30'45"W 19°30'44"W 19'30'43"W 19°30'42"W
,
0.15
0.12 -
0.09 -
0.06 -
0.03 -
- Node_E_Reflectance
- Node_S3_Reflectance
- Node_N203_Reflectance
- Node N205 Reflectance
tit t
500 1000 1500 2000 2500
Wavelength(nm)
4. Data Acquisitions
The VNIS has two operating modes for working on the lunar surface: lunar surface detection, and in-orbit
calibration. In detection mode, the spectrometer begins to collect data of lunar surface; in calibration mode, the
spectrometer uses solar radiation as its calibration source. Here, the diffuser panel of the calibration unit is set to the
horizontal position to detect the solar radiance. By injecting instruction codes, the VNIS can shift the operating
modes to detection or calibration4.
Fig. 12 Map of the path traversed by the Yutu rover and the distribution of detection points
Fig.13 Data from VNIS: false color picture (500 nm, 550 nm, 645 nm, right) and spectral reflectance curves
After the Chang’E 3 mission began the scientific exploration stage, the VNIS instrument successfully
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completed the first lunar surface spectral acquisition at BTC 10:10 on Dec. 23rd, 2013. After the first two lunar days,
VNIS made measurements at four different points (E, S3, N203 and N205 in Fig. 12), and obtained data in detection
mode four times and calibration mode three times. The total size of data is 350 MB. All these data have been
released to the scientific community8.
5. CONCLUSION
As the main scientific payload of the Chang’E 3 rover, the VNIS completed a ground test, calibration, and
environmental simulation test. After landing successfully on the Moon, the VNIS performed several explorations and
calibrations, and obtained several spectral images and spectral reflectance curves of the lunar soil in the Imbrium
region following its first successful operation on the Moon on December 23, 2013. The VNIS performed the first
in-situ spectral imaging detection on the lunar surface; the high-resolution and informative spectral imaging data
obtained by the VNIS can provide a more valuable reference for scientific applications4-7.
Acknowledgments
This paper was supported by the Chinese lunar exploration program’s special funds for the second phase and the National Natural
Science Foundation (No. 21105109). The authors thank the Science and Application Center for Moon and Deep Space
Exploration of the Chinese Academy of Sciences for the ground test and data preprocessing, and the National Space Science
Center of the Chinese Academy of Sciences for its contributions to the development and testing of the instruments.
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