Altimetry Flashcards
Spaceborne altimeter
- System consisting of: Satellite, active nadir-facing sensor, orbit determination system, and data processing system
Radar altimeter
- Radar pulse
- Ku-band (13.575 GHz) or Ka-band (35.75 GHz)
Laser altimeters
- Laser pulse
- Infrared (1064nm) or green (532nm) light
- ICESat GLAS (2003-2010): both
MOLA
- Mars Orbiter Laser Altimeter
- 1997-2006
Current missions
- Sentinel-3 ESA
- Jason-2 and 3, NASA/CNES/EUMETSAT/NOAA
- Saral-AltKA, ISRO
- HY-2, China Academy of Space Technology
- Cryosat-2, ESA
- All radar altimeters, ICESat-2, a laser altimeter will launch in 2017
Altimetry
- Technique for measuring height
- Measures time taken by pulse to travel from satellite antenna to the surface and back to satellite receiver, elapsed time is proportional to the altimeter height
Satellite altimeters provide height estimates accurate to within?
- A few cm
- Complex surfaces w/ a single footprint (tens to hundreds of meters for satellite) make height estimates more difficult
Time and Range
- Time: Distance from satellite to a target surface found by measuring the satellite-to-surface round-trip time of a radar pulse
- Range: Time is converted to satellite-to-surface range
Altimeter Height, halt
- halt = (speed of light*time delay)/2
Surface height measurement
- Difference btwn sat’s position on orbit w/ respect to an arbitrary reference surface and the range
- Reference surface can be the Earth’s centre, or a rough approx. of the Earth’s surface (reference ellipsoid)
What are 2 examples of other measurements besides surface height that can be captured with an altimeter?
- Signal backscatter
- Waveform
What can signal backscatter be used to measure?
- Windspeed over ocean
Dual-frequency altimeters emit in how many frequencies and what does this allow for?
- 2 frequencies
- Allows for comparison btwn return signals, which generates interesting results (ex. atm effects like rain rate over ocean)
Altimetry requires lots of information to be able to use data. What are some error sources in altimetry?
- 2 key factors in data processing:
- Satellite position (exact lat, long, and altitude relative to geoid)
- Atm effects: interface by water vapour and particles that changes signal speed
How can atm effects be determined?
- By using supporting instruments flown on same mission
Sea Level/Ocean height is measured relative to what?
- A reference ellipsoid
- Variations in height used to infer processes about currents and weather (short-term) or overall rises in sea-level (long-term)
What is the Globally averaged sea level change (MSL) from 1901-2010
- Increased 19cm overall, 3mm/yr since 1993
What are the 2 major contributors to sea level rise?
- Melting of perennial ice (glaciers, ice sheets, and permafrost), which drains water from land to the oceans (approx. 50%)
- Thermal expansion of the oceans (approx. 50%)
What is the major ice type that is currently the largest contributor to sea level rise?
- Glaciers and small ice caps, followed by Greenland ice sheet and Antarctic Ice sheet
What are factors that must be considered in context of ice sheets melting?
- Land subsidence
- Ocean dynamics
- Gravitational and rotational changes
What will play a key role on the observation and mitigation of the predicted large rise in sea level in the coming decades?
- Altimetry
Which satellite missions have been used to create elevation models of major ice sheets>
- Altimeter data from ICESat (2003-2010) and Cryosat-2 (2010-present)
What platform type is sea ice thickness measured from?
- Not space
- Instead it is estimated using archimedes principle where basically the height at which ice is floating above water line is related to thickness
- Satellite altimetry is used
Freeboard measurement
- Detection of ice and open water allows for freeboard
- Freeboard is floating height above water
- Then freeboard used to estimate thickness
Radar freeboard, and factors
- Not sam as ice freeboard due to differences in penetration depth of radar signal
- Factors: Snow wetness/melt, snow salinity (brine from sea ice), snow layering
- Error in ice thickness due to salinity can be 25-80%
In addition to radar penetration depth, what are other sources of error in sea ice thickness estimates?
- Assumptions of density of sea ice (not pure ice so variations in ice, air, brine volume fractions affect density spatially and temporally)
- Variations in sea surface height due to geoid and ocean dynamics mean regular calibration needed
- Snow loading depresses the sea ice freeboard, making it lower than if volume were devoid of snow
What are other applications to altimetry?
- Geodesy and gravity anomalies
- Tsunamis
- Large scale ocean circulation patterns, tides, cyclones
- Climate, el niño
- Lake and river level monitoring
- etc.
Cyro-Vex 2017 and Can/UK research program
- Dedicated ice remote sensing and validation experiment april-june 2017
- Validation of cryosat, saral-alitka, and other satellites w/ focus on resolution of sea ice thickness retrieval errors
