APP - part 1 Flashcards
for the January 2021 exam
1
Q
Understand the basic sciences used for RS payload design. (Not formula)
A
-
Wave Theory Of Light
- Light is an Electromagnetic (EM) wave
- Wave theory: how light propagates
- Wavelength vs frequency
-
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)
-
Spectral ranges
- VIS, VNIR, SWIR, MWIR, TIR, MW
-
Reflected vs emitted radiation
- For EO, dividing line ~ 3 μm
- Below: reflected energy dominates
- Above: emitted IR energy dominates
2
Q
List the sequence of important steps in instrument design and development.
A
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
3
Q
List main elements/subsystems within an optical RS instrument.
A
Three target characteristics:
- Spatial
- Spectral
- Intensity
Corresponding sensors elements:
- optics
- scanner
- stabilization
- illuminator
- sensor/detector
- calibration
- processing
- mechanical
- thermal
- encryption
- comms
4
Q
Outline the important features of the illumination source inactive sensors and the associated limitations and problems in their operation.
A
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
5
Q
Explain why reflectors are generally preferred for radiation collection.
A
Reflectors generally preferred due to:
- large aperture at low mass
- long equivalent focal length
- wide wavelength coverage
- absence of chromatic aberration
- high transmission, lightweight
6
Q
List and explain key performance characteristics - spatial,
spectral, radiometric.
A
-
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
7
Q
List the parameters that determine the instrument data rate and the various factors that can limit the operating duty cycle in orbit.
A
-
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
8
Q
Discuss the methods of analog modulation.
A
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)
9
Q
Discuss the methods of Digital communication.
A
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)
10
Q
Discuss the relevance of S/N and BER to telecommunications.
A
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)
11
Q
Describe a Link Margin.
A
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.
12
Q
Perform a Link budget.
A
- 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
13
Q
Describe ways of changing the link margin.
A
- 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
14
Q
Identify the main components of a Geographic Information System (GIS)
and how they work together.
A
- “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.
15
Q
Distinguish the unique characteristics of GIS compared to other mapping
applications and information systems
A
-
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
16
Q
Describe the challenges of GIS data collection and integration with remote sensing information from satellite sensors
A
- A GIS can perform spatial operations because it links different data sets together
- View and analyze multiple layers of spatially related information associated with a geographic location
- Widespread collection and integration of imagery into GIS has been possible from a wide range of remote sensing sources
- Offers a consistent framework for analyzing geographic data!
- Data sources and quality are important considerations for any GIS project, including remote sensing data from Earth observation satellites.
17
Q
Discuss how GIS data and methods can be used for analyzing various geographic problems and Earth applications.
A
- change over time - Landscape
- classify urban growth areas
- change in green densities
- track oil slicks in oceans
- extreme weather events
- natural disasters
Spatial analysis functions distinguish a GIS from other information systems, transforming data into useful information for various applications and decision‐makers
18
Q
Describe advanced applications of satellite
communications
- Fixed services
- Mobile services for ships, trains, cars, and aircraft
- Satellite phones
- „Internet from Space“
A
- Fixed Mobile Communications
- Maritime, Aeronautical, Land-mobile
- INMARSAT, INMARSAT-4
- high bandwidth at relatively low prices
- Spot Beams – small comm beams
- More radio frequency power
- Can operate from operator terminals with more power
- Large coverage over land (urban/domestic use) and oceans (maritime use)
- User Terminal
- BGAN – Broadband Global Area Network
- For terrestrial applications
- Components: 1) internal antenna 2) compass 3) SIM card 4) battery 5) external power 6) USB 7) Indicators 8) ethernet
- BGAN – Broadband Global Area Network
- EDRS Data Relay System
- Transmit data from LEO satellites to GEO relay satellites that can transmit the data to a ground station on Earth
- Connect spacecraft together via satellites
- Maritime, Aeronautical, Train Applications
- Satellite very suitable
- In-flight entertainment (i.e. internet access, TV)
- Air traffic control
- High gain antenna, GPS pointing and tracking
- Fast moving needing constant time and location fix
- Auto-tracking for antenna
- Adjustment of azimuth, elevation, polarization angles
- Mechanical (bulky design, atmospheric drag)
- Electronic (phased array antenna, expensive)
- Frequency and timing changes due to movement of vehicle (Doppler effect)
- High gain antenna, GPS pointing and tracking
- Satellite very suitable
- Train Applications
- Most difficult
- High speeds moving relatively fast
- Most are electrically powered
- Signal interruptions due to tunnels and overhead arches
- Can be mitigated by having multiple antennas at different places on the trains
- Maritime Applications
- Relatively easy
- Slow motion (not demanding for synchronization)
- Large vessels are stable
- Compared to aeronautical and train systems, not very stringent requirements on tracking antennas
- Mobile Satellite TV and Radio
- Satellite Digital Media Broadcast
- Sat TV distribution still the most commercially important application
- Mobile TV successful in Korea, Japan, China; interest has vanished in Europe
- Satellite Radio – SIRIUS
- Originally 3 HEO sats covering US and Canada
- Parallel reception of 2 satellites and terrestrial repeaters for seamless coverage
- XM Satellite Radio
- GEO sats covering US and Canada
- 2008 – SIRIUS-XM merged
- Thuraya Satphone – on board spacecraft
- Can be used as terrestrial phone
- Used for remote area communications via satellite
- Not designed for the Americas
- GPS included (for synchronisation) and can be used for asset tracking
- Satellite Digital Media Broadcast
- Internet via Satellite
- Suitable for rural areas; Inadequate ground infrastructure
- Information transferred through downlink from satellite
- Star Network
- Transmitting information from terminal to terminal goes through Hub Stations
- Mesh Network
- No hub station needed to transfer between terminals
- Requires more power
- LEO One Web
- LEO constellation for global Internet Access
- 648 minisatellites planned in 18 polar orbit planes
- SpaceX Starlink
- LEO constellation for global Internet Access
- 40,000 cross-linked satellites
19
Q
Identify the advantages and limitations of Satcom systems.
A
- LEO constellations
- Lower delay
- Lower free-space loss
- Lower delay
- More satellites are needed to provide coverage
- Complete and complex handover from one satellite to another
- E.g. IRIDUM (66 sats), GLOBALSTAR (48 sats)
- Commercially not successful
- More satellites are needed to provide coverage
- Complex Tech
- On-board processing payloads
- Inter-satellite links
- Communication between adjacent satellites for better traffic routing
- Complex handover procedures
ADVANTAGES OF SATCOMS
- Wide coverage
- Broadcast capability
- High bandwidth
- Flexibility in network set-up
- Mobility
- Rapid deployment
- Reliability
- Economic solutions available
- Availability in areas without adequate terrestrial telecom Infrastructure
DISADVANTAGES
- Latency
- Atmospheric interference
- Remote areas may still not be covered
- Space debris
- LEO congestion
20
Q
What sensors and techniques can be applied to different types of disasters?
A
- Red Cross Estimates USD 220 billion damages per year caused by disasters:
- Drought is the most damaging: Landsat, SPOT, IRS, RADARSAT, Sentinel ALOS, and all high Res satellites
- Earthquakes: 1-2 spatial resolution, SAR interferometric techniques
- Fire: hotspot detections and trajectories, pay attention, it burns.
- Floods: detected with SAR and optical- high res IRS data, the greatest number of floods in Europe
- Volcanos and Landslides: SAR, optical satellites, InSAR = interferometric SAR
-
Application Gaps:
- integration space / non-space information
- rapid response of satellite information
21
Q
Explain what is radar and why it is used for remote sensing.
A
-
What does Radar stand for?
- Radio Detection and Ranging
- The radar system has three primary functions:
- Transmits microwave (radio) signals towards a scene
- Receives the portion of the transmitted energy backscattered from the scene
- Observes the strength (detection) and the time delay (ranging) of the return signals
-
Why use Radar for remote sensing?
- Controllable source of illumination (sees through cloud and rain and at night)
- Images can be high resolution (3 ‐ 10 m)
- Different features are portrayed or discriminated compared to visible sensors
- Some surface features can be better seen in radar images:
- Ice and ocean waves
- Soil moisture
- Vegetation mass
- Man‐made objects (e.g., buildings)
- Geological structures
22
Q
Distinguish the differences between optical and radar
imagery collected from space.
A
Advantage of radar compared to optical imagery:
- Frequency & Polarisation
- Penetrates clouds and can be an all‐weather remote sensing system
- Synoptic views of large areas (mapping at 1:25,000 to 1:400,000)
- Cloud‐shrouded countries may be imaged
- Coverage can be obtained at user‐specified times (e.g., night)
- Permits imaging at shallow look angles (different perspectives that cannot always be obtained using aerial photography)
- Senses in wavelengths outside visible and infrared regions of the electromagnetic spectrum, providing information on:
- Surface roughness
- Dielectric properties
- Moisture content
23
Q
Describe image signal processing and applications of an
active remote sensor, Synthetic Aperture Radar (SAR).
A
-
Based on the Doppler principle
- Doppler principle states that the frequency (pitch) of a sound changes if the listener and/or source are in motion relative to one another
-
Not dependent on the physical antenna size
- “synthesizes” a very broad antenna by sending multiple pulses to obtain finer spatial resolution
- Mounted on a moving platform and target scene is repeatedly illuminated with pulses of radio waves
- echo waveforms received successively and processed together
24
Q
Identify the best types of remote sensing data and analysis procedures for studying specific environmental problems or phenomena.
A
- Red Cross Estimates USD 220 billion damages per year caused by disasters:
- Drought is the most damaging: Landsat, SPOT, IRS, RADARSAT, Sentinel ALOS, and all high Res satellites
- Earthquakes: 1-2 spatial resolution, SAR interferometric techniques
- Fire: hotspot detections and trajectories, pay attention, it burns.
- Floods: detected with SAR and optical- high res IRS data, the greatest number of floods in Europe
- Volcanos and Landslides: SAR, optical satellites, InSAR = interferometric SAR
-
Application Gaps:
- integration space / non-space information
- rapid response of satellite information
25
Describe the **challenges** of collecting remote sensing data and how they
are **preprocessed and corrected.**
**What is Digital Image Processing (DIP)?**
* Technique involving manipulation of digital images to extract useful information
* Computer‐based processing of spatial data/imagery (from remote sensing sensors/platforms) into information for image interpretation and object identification
– Includes aerial photography, airborne data/imagery and satellite‐borne data/imagery
**_Pre-processing_**
• Preparation phase that improves image quality for later analysis that will extract meaningful information from the image
– Effort to remove the undesirable influence of atmospheric interference, system
noise, and sensor motion
• Two general categories:
**_a) Radiometric Correction_**
– Removal of sensor or atmospheric 'noise', to accurately represent the ground
\_ conditions: maybe to correct data loss, remove haze, enable mosaicing and comparison
**_b) Geometric correction_**
– Conversion of data to ground coordinates by removal of distortions from sensor geometry
**(a) Radiometric Correction**
• Two common sources of periodic noise:
_Striping (or banding)_
• Errors that occur in the sensor response and/or data recording and transmission, which results in a systematic error or shift of pixels between rows (esp. Landsat MSS sixth‐line striping due to detectors)
_Line dropouts_
• Errors that occur in the sensor response and/or data recording and transmission, which loses a row of pixels in the image. Unsystematic noise (possibly from orbital perturbations).
(b) Geometric Correction
• Possible causes of geometric distortions?
– Perspective of sensor optics (roll, pitch, yaw)
– Motion of the scanning system
– Satellite platform altitude, attitude, and velocity
– Terrain relief
– Curvature and rotation of the Earth
• Geometric corrections are intended to compensate for distortions, so that the geometric representation will be close to the real world
– Systematic/predictable Account by modeling and geometric relationships
– Unsystematic/random Cannot be modeled & requires geometric registration
- Can use an image that you know is really accurate and try to map your image on top. Must use ground control points (GCPs) then stretch your image out to fit onto the other.
- GCPs are collected from real groundwork.
26
Discuss how **digital processing techniques** can be used for extracting useful Earth observation information from satellite sensors.
* Image Enhancement
* Ability to enhance the view of an area by manipulating pixel values, thus making it easier for visual image interpretation
* Linear Contrast Stretch
* Increase brightness and contrast of the display image in order to better view dark or color-washed images
* Nonlinear Contrast Enhancement
* Better enhanced detail relative to original image
* Stretches out certain pixels disproportionately thereby enhancing the detail
* Spatial Filters
* Enhances or suppress spatial detail to improve visual interpretation in the final image
* Used for removing image noise
* Band Ratios
* Create new data sets that can highlight certain features in an image (e.g. a river)
* Digital Image Classification
* Spectral information is represented by digital numbers recorded as pixels in an image
* Basis for image classification
* Unsupervised Classification
* Automatic classification based on spectral similarity
* After classification, land cover types can be identified by each specific class
* Supervised Classification
* Statistical classification data inputted to determine areas that have similar spectral characteristics
* Land cover types are then identified based on specific search characteristics
27
Explain how satellite imagery and other spatial data products can be integrated into various **environmental applications.**
* * * Atmospheric chemistry and physics
* Ocean currents and temperature
* Ice-cover thickness and height
* (mapping), topography, and geographic information systems
* Geodesy, land use, and planning
* Geology and resource mapping
* Agriculture and forestry
* Environmental monitoring, natural hazards (fire, floods, drought)
* Military, government, and intelligence
* Pollution tracking
28
Describe different types of satellite ground stations
and ground station architectures.
* **Large stations** (operators’ head-ends, TV feeder links, hub stations)
* INTELSAT-A
* TV, telephony, data
* up to 32 m antenna diameter
* **Medium-sized stations**
* TV uplinks
* up to 9.5 m antenna diameter
* **VSATs** (very small aperture terminals)
* one-way
* interactive
* up to 3.7 m antennas
* typically 1.2...1.8 m
* **LOW-COST TERMINAL**
* **TRANSPORTABLE STATIONS**
* TV Receive-Only (**TVRO**)
* 35...120 cm (Ku)
* up to 3 m antenna diameter
* simple
* low-cost
* outdoor mount
* water-tight receiver unit
* single cable for intermediate frequency, power, control
29
Define the key elements of a ground station.
* Antenna System
* Horn antenna – waveguide transmission line for microwaves / electromagnetic energy
* Parabolic Antenna – primary focus feed w/ sub-reflector
* Offset Antenna
* Higher efficiency
* Steeper angle, less risk of snow/water pooling in dish
* Antenna Pointing
* Manual or motor-driven (for tracking)
* Beacon signal from satellite used to optimize pointing
* * 1) determine local position
* 2) calculate azimuth, elevation, polarization angles
* 3) adjust elevation angle
* 4) coarse alignment of azimuth
* 5) search for satellite beacon
* 6) fine adjustment of elevation/azimuth for maximum signal level
* 7) adjust polarization
* Transmitting & Receiving Equipment
* Telemetry, Tracking & Command (TT&C) Equipment
* Account for:
* Satellite movement
* Compensation of wind force
* Compensation of beamwidth
* * Program Track – employ orbit calculations and enter s/c orbital elements
* Use data to control azimuth/elevation actuators
* Step Track
* Simple algorithm; antenna moved a discrete step
* Satisfactory for most applications
30
Compare the frequency ranges.
* **L-Band** – 1-2 GHz
* **S-Band** – 2-3 GHz
* Satellite multimedia systems
* **C-Band** – 4-6 GHz
* First band that was exploited/oldest used
* Terrestrial radio link systems
* Less effected by propagation effects – very useful for tropical areas
* **X-Band** – 8-10 GHz
* Deep space communications, EO, military use,
* Heavily crowded
* **KU-Band** – 11/14. 12/14 GHz
* **Ka-Band** – 20-30 GHz
* Applications for high speed internet access
* **Q-V Band** – 40-50 GHz
* Experimental for TV satellites
31
Identify the applications of different ground station types.
* VSATs – Very Small Aperture Terminals
* Front end cost-optimized
* Small outdoor unit, directly mounted on or near antenna feed
* Low noise downconverter block (LNB)
* Upconverter and high-power amplifier
* Low Noise Amplifier
* Solid State
* High Gain
* Low Noise Power
* High Power Amplifier
* Solid State Power Amplifier
* Higher Power
* TVRO – TV Receive-Only
* Simple & low-cost
* Outdoor Mount
* Water-Tight Receiver Unit
* Single Cable for Intermediate Frequency, Power, Control
32
Describe different applications of satellite communications systems (fixed, mobile, and nomadic).
* * 1) TV Distribution
* An oldest but most important application
* The exploitation of broadcast capability of satellite
* STAR network architecture
* 2) Satellite Internet Access
* Transport of IP packets via forwarding link
* Datacasting (one-way) or Interactive (two-way)
* Return link via terrestrial network or via satellites
* Improvement of telecom infrastructure in rural areas
* * STAR network architecture
* * Business Communications – All IP Network
* LAN interconnection
* Fast internet and guaranteed quality of service
* Supports all standard local computer applications
* Ability to send voice, video conference, high-quality video, and transfer of large files
* Fully Meshed network
* One terminal has the power for capacity and network management sending to and receiving from the satellite
* Emergency Communications / Disaster Management
* Communication satellites
* Ideal for natural disasters and areas w/ inadequate infrastructure
* Satellite comms suitable for re-establishment of temporary infrastructure if normal telecom infrastructure is disrupted
* Transportable terminals – operational within 20-30 min
* Relay between temporary base station & fly-in terminal via satellite
* * Transfer patient vital data at high speed
* e.g. x-ray, CT images, ultrasound
* provision of video and telephony services between remote hospital and expert center
* Interconnection of hospitals and medical centers via satellite
* Tele-training of medical personnel
* * Satcom systems attached to cars usually in remote areas
* Provides fast file transfer
* Voice/video conference service between test personnel
* Access to intranet in remote areas
* Similar set up STAR, but nomadic (moving) terminal
33
Compare network architectures and topologies of
satellite communications systems.
* Point-to-point
* Communications leave one source, relay from satellite, and arrive at another source
* Star
* Central Hub Station responsible for managing capacity and network
* Mesh
* Any terminal can communicate with any other terminal without a hub
**STAR / MESH comparison**
* Star topology suitable if the data flow is always to/from the central site (e.g. Internet via satellite)
* If satellite terminals need to communicate with each other, traffic has to be routed via central hub station
* Double satellite hop (ca. 500 ms) prohibits interactive applications (voice, video conference,…)
* In mesh mode only a single hop (250 ms)
* Any terminal can communicate
34
Identify the advantages/disadvantages with respect
to terrestrial systems.
**Advantages:**
* Normal telecom infrastructure often disrupted
* Satellite communications suitable for re-establishment of temporary infrastructure
* Rapid information for decision-makers (emergency centers)
* Provision of services in remote areas
* Rapid deployment
* Reliable systems
* Low-cost solutions available using DVB technology
* Satellite communications indispensable tool for
– High-speed data collection & dissemination
– Voice/video/data when other communications links are disrupted
– Integrated decision support systems
– Tele-medicine
– Tele-education
