multispectral imager design

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Multispectral Imager Design. for Nanosatellites. V.H.R.I. Doedee, R. Deerenberg, E.Dokter – Faculties 3mE, AE & EEMCS. 1. Introduction. Nanosatellite missions so far. Education Technology Demonstration No serious remote sensing Are nano-satellites capable for remote sensing jobs?. - PowerPoint PPT Presentation

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PowerPoint-presentatieDelft University of Technology
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1.
Introduction
Nanosatellite missions so far
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Major Constraints
volume less than 10 cm x 10 cm x 15 cm
power consumption less than 3.0 W
mass less than 1.5 kg
imaging of preferably R, G, B, NIR, MIR, TIR bands
system should survive space environment, including ‘accidental’ sun exposures
operational life time should be at least five years
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sensors with better quantum efficiencies and lower noise
deployable instead of rigid optical systems
in-orbit calibration instead of on-ground
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Potential Applications
Google Earth-like applications
When using a constellation of nanosatellites
q.e. the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair
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Contents
2.
Basics of Remote Imaging
Multispectral: multiple bandwiths
NIR; the near infrared range, 0.7-1.1 μm
SWIR; the short wave infrared range, 1.1-2.5 μm
MWIR; the midwave infrared range, 2.5-7.5 μm
LWIR; the long wave infrared range, 7.5-15 μm
Source: http://www.antonine-education.co.uk/physics_gcse/Unit_1/Topic_5/em_spectrum.jpg
Resolution
Spatial
Spectral
Radiometric
Temporal
Spatial Resolution
Smallest measure of seperation between two objects that can be resolved by the system [T.A. Warner et al, 2009]
Rayleigh Criterion
Minimal Spatial Resolution
Spectral Resolution
Unitless Ratio:
Source: http://www.cas.sc.edu/geog/rslab/rscc/mod1/specres.gif
Radiometric Resolution
How fine a difference in incident spectral radiance can be measured by the sensor
[T.A. Warner, M. Duane Nellis, G.M. Foody, 2009]
Quantization of incoming radiation
Temporal Resolution
EM Propagation and Sensors
Energy needed (bandgap):
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Materials
Indium Antimonide (InSb), 0.3-5.5 μm
Mercury Cadmium Telluride (HgCdTe), 0.7-15 μm
Wavelengths absorption of different InGaAs alloys.
Source: http://www.sensorsinc.com/GaAs.html
Beam splitters
Cube, Plate, Pellicle
Pellicle: average transmission 50% (375–2400 nm), light, little ghosting, sensitive to vibrations
Source: http://www.newport.com/store/genproduct.aspx?id=141118&lang=1033&Section=Detail
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3.
CCD
(Charged-coupled devices)
q.e. the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair
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CMOS
CCD vs CMOS
100 times more power + consume little power
Complex + easy to manufacture
4.
Orbit
Orbit
Same illumination condition surface Earth.
Easy data collection.
Altitude = 400 Km:
Lower altitude also means more drag.
Design lifetime of 5 years achievable.
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In-Orbit Calibration
Normally the lens subsystem is calibrated on ground and designed such that it can withstand the launch without losing focus.
This has major disadvantages
Increased risk.
More mass.
Why not perform the calibration in-orbit?
Advantages:
Can use the same mechanism of a possible deployable lens
Disadvantages:
In-Orbit Calibration
How it works:
The camera makes an image, adjusts the focal length slightly and takes another image. The two images are then compared. And if the process is beneficial to the quality of the image then the process is repeated.
Images are compared either on:
The satellite itself – More dedicated electronics.
or
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5.
Noise
Multispectral Imager Design for Nanosatellites
Signal to Noise Ratio - SNR
The SNR is a measure of the quality of the taken image.
There are two ways to increase the SNR:
Decrease noise as much as possible
Increase Integration time to decrease single shot noise such as
Shot Noise
Thermal Noise
Thermal Noise is one of the easiest ways to decrease noise.
Materials emit a certain amount of electrons according to their temperature.
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Thermal Control
In order to decrease Thermal Noise, the sensor must be cooled.
But:
Thermal control of a nanosatellite is difficult due to it’s limited size
No Active control can be applied
Only Passive control is an option:
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Further decreasing SNR
Other types of noise are dependent on the sensor and electronics.
Such as:
Quantum Efficiency (QE) – Measure of efficiency of the sensor, this value should be as high as possible within the required spectrum.
Amplifying noise
1/f noise
Etc…
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Integration Time
Some values of noise are a single event values, these do not increase with measurement time.
When the time in which we view an object (Integration Time) is increased, the SNR goes up.
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Blurring Effect?
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Motion Compensation
Two ways in doing so:
Mechanical movement of the lens subsystem to track a point on the ground.
Time Delay Integration - TDI.
Mechanical movement method.
In order to view the same point on the ground a tilting mirror can be placed in front of the lens subsystem to reflect the rays of light.
The motion of this mirror has to be synchronized with the movement of the S/C w.r.t the ground.
This increases complexity since moving parts are necessary
Increase in power consumption
Increase in development cost
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Time Delay Integration - TDI
Time Delay Integration is a method which uses no moving parts. Instead it uses an array of pixels.
When the satellite passes a point on the ground, the first pixel takes a measurement, and the pixel is read out.
Due to the motion of the spacecraft, the same point can be seen by the next pixel in the direction of flight after a small time step. This holds for the entire pixel array.
This method is also called push broom scanning
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Time Delay Integration - TDI
Much lower mass
Questions?