APP - part 1 Flashcards
for the January 2021 exam
Understand the basic sciences used for RS payload design. (Not formula)
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Wave Theory Of Light
- Light is an Electromagnetic (EM) wave
- Wave theory: how light propagates
- Wavelength vs frequency
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Particle Theory Of Light
- How EM energy interacts with matter
- Longer l have lower energy
-
Black body radiation
- Any body above 0 K emits radiation
- Amount of energy not uniform with wavelength
- Boltzman’s law - (amount of energy radiated)
- Planck’s Law - not uniform w/ wavelength
- Wein’s Law- (black body temp)
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Spectral ranges
- VIS, VNIR, SWIR, MWIR, TIR, MW
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Reflected vs emitted radiation
- For EO, dividing line ~ 3 μm
- Below: reflected energy dominates
- Above: emitted IR energy dominates
List the sequence of important steps in instrument design and development.
1. Select P/L objectives
* Strongly related to mission objectives
* But more specific: what the P/L must do (output) 2. _Conduct **subject trades**_
* Determine the subject (fire IR rad., visible smoke)
* Determine performance thresholds (temp. gradient) 3. _Develop the P/L **operational concept**_
* How will the end-user receive and act on the data?
* à Impact on costs 4. _Determine the **required P/L capability**_
* Throughput & performance required to meet the performance threshold? (resolution, accuracy) 5. _Identify **candidate P/L and their specifications.**_ 6. _Estimate **candidates and select a baseline**_
* Determine performance characteristics, cost, impact on S/C bus & ground segment à cost vs. perf.
* Mass, size, power, pointing, data rate, thermal, orbit, commanding, processing, structural support, etc. 7. _**Evaluate candidates** and select a baseline_
* Compare then make a preliminary selection 8. _Assess **life-cycle cost & operability**_
* Iteration of requirements with end-user 9. _Define P/L- **derived requirements**_
* Detailed definition of selected P/L’s impact
* Power, pointing, data storage, thermal stability,… 10. **_Document and iterate_**
* What has been decided and why à useful for future system trades
List main elements/subsystems within an optical RS instrument.
Three target characteristics:
- Spatial
- Spectral
- Intensity
Corresponding sensors elements:
- optics
- scanner
- stabilization
- illuminator
- sensor/detector
- calibration
- processing
- mechanical
- thermal
- encryption
- comms
Outline the important features of the illumination source inactive sensors and the associated limitations and problems in their operation.
Internal sources:
- Lamps in VIS/NIR region
- Black body (BB) radiator at a known temperature in Thermal infrared (TIR/MW)
External sources:
- Sun via diffuser in VIS/NIR
- Space in IR/MW
Limitation for active sensors:
- need power to drive it
- need high data rates
- need a cooling system
Explain why reflectors are generally preferred for radiation collection.
Reflectors generally preferred due to:
- large aperture at low mass
- long equivalent focal length
- wide wavelength coverage
- absence of chromatic aberration
- high transmission, lightweight
List and explain key performance characteristics - spatial,
spectral, radiometric.
-
Spatial
- A point source is not focused on a perfect point but spread by diffraction.
- Linear resolution at the ground, x ~ h* lambda/D
- D – Aperture diameter (size of the object)
- GSD – Ground Sample Distance = P* h/f
- P – Pixel size (micron)
- h – Flight height
- f – Focal length
-
Factors limiting resolution
- detector size,
- aberrations of the optical system,
- platform motion,
- atmosphere (in the limit)
-
Spectral
- Most optical RS imagers to date have used 5 -10 VIS/IR bands plus a broad ‘panchromatic’ channel.
- Bands (EM payload focus)
- Panchromatic channel
- The trend towards hyperspectral imaging ( > 30 bands or more) to give higher discrimination - vegetation, soil.
-
Radiometric (intensity)
- Dynamic range
- the ratio between max and min values of the sensor measurement range.
- Linearity
- the measure of how well the instrument signal is proportional to received radiation over this range
- Noise Equivalent Power (NEP)
- the value of the incident power which
gives an output signal equal to the noise level
- the value of the incident power which
- Noise Equivalent Reflectance or Temperature
- changes in scene roh or T which gives an output signal equal to the noise i.e. SNR = 1
-
Influencing factors
- Reflectivity, ρ
- Temperature, T
- Emissivity, ε
- Absorptivity, α
- Dynamic range
List the parameters that determine the instrument data rate and the various factors that can limit the operating duty cycle in orbit.
-
Data rate = n*N*V*W/x2
- n - bits/pixel*,
- N - spectral bands,
- x - pixel dimension
- W - swath width :
- transmission efficiency & bandwidth
- data size
- processing
- legal
-
Limitations or interruptions can arise due to:
- Initial commissioning
- Scene illumination (e.g. passive VIS-MIR - sunlight only)
- Cloud cover (VIS/IR)
- Power availability or thermal control (e.g. for SAR)
- Data storage/ transmission (hi-res instruments)
- Orbit/attitude manoeuvres
- Calibration cycles
Discuss the methods of analog modulation.
1) Signal (Carrier wave) with no information content.
2) Information can only be transmitted by changing (modulating that signal)
3) The change can be in steps (digital) or continuous (analog).
• Amplitude Modulation
- (simple but high level of noise)
- volt or power level of info signal changes amplitude of carrier
• Frequency Modulation
- (resilience to noise, poor spectral efficiency)
- higher amplitude of info signal = greater frequency change
• Phase Modulation
(noise immunity but needs two signals)
Discuss the methods of Digital communication.
Similarly to analog transmission, a carrier is required.
- Only two levels need to be imposed (“0” and “1”)
- These levels are produced by “shifting” the signal between two levels:
- Amplitude in AM ⇒ Amplitude Shift Keying (ASK)
- no TT&C due to noise
- Frequency in FM ⇒ Frequency Shift Keying (FSK)
- low bit rate
- simple detectors
- Higher bit rates lead to higher subcarrier frequencies and broader bandwidths
- Phase in PM ⇒ Phase Shift Keying (PSK = BPSK (Binary PSK)):
- high-speed telemetry
- less bandwidth than FSK
- Amplitude in AM ⇒ Amplitude Shift Keying (ASK)
- Telemetry Tracking & Control systems.
- (BER) Bit Error Rate becomes central to the transmission process.
- In the digital data stream, (C/N) Carrier to Noise ratio can be linked to the (Eb) Energy per bit
- The clarity of the signal will depend on the ratio of this energy against background noise: (Eb/N)
Discuss the relevance of S/N and BER to telecommunications.
BER used in TT&C:
Should be high enough to provide an unambiguous interpretation of the signal.
- “Nyquist’s Law” states that the data rate should be at least twice the highest frequency of the signal to be sampled accurately.
- Trade-off between sampling resolution and maximum rate avail
- Telemetry Tracking & Control systems.
- In the digital data stream, (C/N) Carrier to Noise ratio can be linked to the (Eb) Energy per bit.
- The clarity of the signal will depend on the ratio of this energy against background noise: (Eb/N)
Describe a Link Margin.
The Link Margin (LKM), measured in dB, is the ratio of received signal strength Eb/N0 to required signal strength Eb/N0, measured in power or error rate. A
positive margin indicates a good link.
Perform a Link budget.
- Link budget is an accounting of all the gains and losses between the transmitter and the receiver.
- A simple Link Budget:
- Received Power = Transmitted Power + Gains – Losses
- It should be performed separately for uplink and downlink (freq/wavelengths differ)
- Received power level Pr proportional to Pt, Gt, Gr and inversely proportional to d.
- The received power level Pr is typically in the order of 10-5 W
- Received Power = Transmitted Power + Gains – Losses
Describe ways of changing the link margin.
- Increase size/shape of antenna and pointing accuracy
- Increase power transmitted (EIRP)
- Increase wavelength
- Reduce distance
- Losses can be caused by:
- Losses affect quality of trans
- Free space loss (distance and antenna aperture lead to loss of radio energy)
- Weather
- Antenna pointing errors
- Polarization mismatch in transmission
- RF hardware
Identify the main components of a Geographic Information System (GIS)
and how they work together.
- “GIS is a collection of computer hardware, software, geographic data, methods, and personnel designed to efficiently capture, store, update, manipulate, analyze, and display all forms of geographically referenced information”
- A Geographic Information System (GIS) allows the viewing and analysis of multiple layers of spatially related information associated with a geographic location.
- A geographic information system links locational (spatial) and database (tabular) information and enables a person to visualize patterns, relationships, and trends.
The five components of a GIS system are:
1) hardware,
2) software,
3) data,
4) user and
5) analysis/methods.
Distinguish the unique characteristics of GIS compared to other mapping
applications and information systems
-
Use of spatially referenced data
* data whose location is known in a specific coordinate system - Graphical and attribute data input and editing
- Selective spatial and attribute query
- Specialized spatial analysis tools
- e.g., map overlay, buffer zones, spatial search, network analysis, terrain analysis
5. Map and report generation -
Spatial analysis functions distinguish a GIS from other information systems, transforming data into useful information for various applications
and decision‐makers- Analysis functions use spatial and non‐spatial attributes in the database to answer questions about the real world
