16: Remote Sensing - Satellites Flashcards
SATELLITE advantages and disadvantages
Advantages:
- high resolution
- sensors collect multiple EM bands
- wide image coverage (swaths)
- temporal resolution
- rapid data acquisition
- once in orbit they can work for a long time
Disadvantages:
- cost
- development time
- satellite launch failure (rocket explosion)
ORBITS TYPES (3)
Geostationary
Intermediate circular orbit
Polar/Near Polar
KEY PLATFORMS
AVHRR - Advanced very high resolution radiometer
2399km swath, 1.1km resolution, 2x NOAA satellites, 20 year climate record
MODIS - Moderate resolution imaging spectrometer
2330km swath, 250, 500 and 1000m resolution, complete coverage 1-2 days
ASTER - Advanced spaceborne thermal emission and reflection radiometer
60km swath, 15-90m resolution, coverage 16 days, no blue band, so can’t create true colour image
Landsat 8 (LDCM)
185km swath
15, 30 and 100m res
16 days
Worldview-2
Panchromatic with 0.46m res
Multispectral with 8 bands and 1.2m res
26 hour coverage with 18km swath
Worldview-3 Panchromatic at 0.3m res Multispectral at 1.2m res with 8 bands SWIR at 3.7m res with 4 bands CAVIS 30m res with 12 bands
CLASSIFICATION METHODS
Supervised classification
led by analyst
Unsupervised classification
led by computer and then categories are defined by analyst at the end
History of remote sensing
1858 - first aerial photos from hot air balloons, taken from several 100s of meters high
1903 - pigeon camera with timing mechanism
WW1 and WW2 photographs from air planes to record enemy positions
1962 airborne intelligence informed the Cuban Missile Crisis, U2 aircrafts very high up
1960s Spy satellites, Corona system (US), first satellite images for military purpose, film photographs, resolution 1.5x1.5m
1964 meteorological satellites, Nasa’s Nimbus-1, multispectral (water vapour, liquid water, cloud temperatures)
1979 satellites detected the ozone hole
1970s Landsat program, records repeated images of land around the globe
Swath
Satellite’s field of view
- width of data collected
- ‘mowing the lawn’
- wide swath typically has lower resolution, can monitor earth in a short amount of time
- narrow swath possible to have very wide resolution, generally monitor a small area
INSTRUMENTS
multispectral scanner (MSS): most commonly used scanning system for collecting data over different wavelength ranges
information from a narrow wavelength range is gathered and stored in a channel / band
Multispectral scanner instrument types
Panchromatic: single channel/wide band
Multispectral: multi channel (<10) narrow bands
Superspectral: multi channel (>10) even narrower bands
Hyperspectral
- imaging spectrometer
- HySpex (visible and near infrared)
- 100+ bands
DATA ACQUISITION
humans: using visual interpretation to classify features in an image
computers: digital classification? we have to be aware of how the computer sees the world… numbers
- spectral pattern recognition
overall aim: assign all image pixels to particular classes/themes
result: classified image comprised of pixel mosaic, each belonging to a particular class, essentially a thematic map of the original image which we can then use for analysis
How do satellites acquire data?
Scanning - 2 main scanning modes/methods to acquire multispectral image data
Across track scanners (ATS)
- scans at tangent to flight path
- 1 pixel at a time
- “whisk broom” (has a mirror which is rotating)
- during forward movement the satellite scans across 1 line, needs some correction for the forward movement of the satellite
Along track scanners (AlongTS)
- scans parallel to flight path
- multiple pixels at a time in strips
- “push broom”
Geostationary orbit
- high earth orbit (~36000km = exact rotation time of the earth over 24 hours)
- generally fixed on 1 region (follow a set point on planet)
- weather/communication satellites
Intermediate circular orbit (ICO)
- medium earth orbit (~20000km)
- orbital periods of 2-24 hours (travel across the sky)
- GPS/GLONASS
- communication satellites
Polar/Near Polar orbit
- low earth orbit (LEO) (400-2000km)
- passes above or nearly above both poles
- passes over many regions in fixed period
- landsat / cryosat
- sun-synchronous
AVHRR
Advanced very high resolution radiometer
one of the first multispectral images that could map the earth’s surface completely, ground resolution is not very high (1.1km) with 2399km swath, can scan the earth’s surface in a short time
Supervised classification
- analyst first decides which categories / classes to look for
- identifies in imagery homogenous representative samples of surface cover types of interest (training areas)
- computer use this information to process and classify the image into the areas of interest
Process:
- form images of data
- choose training pixels for each category
- calculate statistical descriptors