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Remote Sensing of the Ocean
and Atmosphere:
John L. Wilkin
[email protected]
IMCS Building Room 214C
609-630-0559
Orbits and
Measurement Geometry (1)
The flow of information from land or sea surface to satellite to
user depends on features of the land/ocean phenomena, the
observing system, and the intervening atmosphere.
(4) resolution: FOV,
aperture, scan geometry
can depend on satellite
trajectory and altitude
(orbit), pointing
(5) Geolocation: position,
time, pointing. Orbit affects
repeat sample interval
(6) Position of the satellite,
or range to target may be
the actual data.
(3) Sensor: the
data is a
measurement or
image
(2) Water leaving signal:
WLR can depend on
relative position of sun and
satellite, time of day,
emissivity, reflectance
(1) Ocean phenomena:
color, temperature,
roughness, height
Johannes Kepler (1571 – 1630)
German mathematician,
astronomer and astrologer
1. Planets move in elliptical
orbits with the sun as one
focus
2. the radius vector from the
sun to the planet sweeps
out equals areas in equal
times
3. T2 : R3 ratio is constant for
all planets, where T is
orbital period and R is semimajor axis of the orbit
Galileo Galilei (1564 - 1642)
Italian physicist,
mathematician, astronomer,
and philosopher
Galileo's pupil Vincenzo Viviani stated that
Galileo had dropped balls of the same material,
but different masses, from the Leaning Tower of
Pisa to demonstrate that their time of descent
was independent of their mass.
This was contrary to what Aristotle
had taught: that heavy objects fall
faster than lighter ones, in direct
proportion to weight.
There is no account by Galileo
himself of such an experiment, and it
is generally accepted that it was at
most a thought experiment which did
not actually take place.
Isaac Newton (1643 - 1727)
English physicist,
mathematician, astronomer,
theologian
1. Newton discovered the laws
of gravitation and explained
planetary and satellite orbits
in terms of the balance of
forces:
2. Centripetal acceleration
F = ma = m
3. Gravity
Fgravity
dv
dt
GMm
= 2
r
v2
v2
change in velocity
(acceleration) is
perpendicular to
the direction of
travel
-v1
v1
Δv
Δθ
Δθ
Δs = rΔθ
Ds rDq
dq
=
=r
Dt
Dt
dt
dq v
Þ
=
dt
r
v =
Epoch (UTC):
10:07:53 AM, Monday, January 28, 2008
Eccentricity:
0.0005362
Inclination:
51.6418°
Perigee Height:
334 km
Apogee Height:
341 km
Right Ascension of
Ascending Node:
22.2238°
Orbit Number at Epoch:
52631
Revolutions per Day:
15.77540422
Nov 5, 2007 view from Space Shuttle
Stewart, R. H., 1985, Methods of Satellite Oceanography, University of California Press, 360 pp.
Right ascension - declination coordinate system
First Point of Aries: One of the two points on the Celestial Sphere where the
Ecliptic and the Celestial Equator cross one another.
Ecliptic: An imaginary line in the sky that approximates the plane in which
the planets orbit the Sun. From the perspective of an observer on Earth, the
planets and the Sun will always remain close to this line.
When the Sun reaches the First Point of Aries, as it does once each year, the
Northern hemisphere Vernal Equinox occurs.
The First Point of Aries, which is actually in Pisces, defines the zero-point for
Right Ascension.
Right ascension: the celestial equivalent of
longitude (starting at the First Point of Aries)
Declination: the celestial equivalent of latitude
The celestial north pole is at declination +90
http://spaceflight.nasa.gov/realdata/elements
See graphics of orbital elements and data for ISS
Jason-1 launch from Vandenburg Air Force Base, California
Launch: ** http://www.youtube.com/watch?v=Z4WHOSF2Ktg
Other launches:
Delta-II (Themis):
http://www.youtube.com/watch?v=p_pAhPecto0
Ariane launch failure:
http://www.youtube.com/watch?v=kYUrqdUyEpI
Jason-2 OSTM
http://www.youtube.com/watch?v=cf0loVEs_lo
Pegasus vehicle aircraft launch
http://www.youtube.com/watch?v=QRHiTvRHHd8
Space craft separation:
http://www.youtube.com/watch?v=1aBpcCVljt4
OSTM / Jason-2 Boost Profile
Space craft separation:
http://www.youtube.com/watch?v=1aBpcCVljt4
Remote Sensing of the Ocean and Atmosphere:
John L. Wilkin
[email protected]
IMCS Building Room 214C
609-630-0559 (g-voice)
Orbits and
Measurement Geometry (2)
Hohmann Transfer
Maneuver to different inclination
http://en.wikipedia.org/wiki/Hohmann_transfer
http://www.nytimes.com/2007/02/06/science/space/06orbi.html?emc=eta1
Collisions between existing debris
generate additional pieces of debris.
There is concern that eventually a chain
reaction will ensue (the Kessler
syndrome) littering the low earth orbit
(LEO) with debris making it almost
impenetrable to new satellite launches for
fear of collision.
http://www.nytimes.com/2007/02/06/science/20070206_ORBIT_GRAPHIC.html
Kessler, D. J., and Burton G. Cour-Palais. "Collision frequency of artificial satellites – The creation of a debris
belt." Journal of Geophysical Research Vol. 83 (June 1978): 2637-2646.
ostm-topex-5days-orbit animation
Semi-major axis
= Rearth + orbit height
= 6371 + 1366 km
= 7737 km
i = 66o
3122 km
245 km = 316 cos (39)
Jason altimeter
satellite
10-day exact repeat.
Equatorial separation
of Jason ground-tracks
is 316 km
ENVISAT
35-day repeat
Sidereal day = time it takes for Earth to rotate through 360o
This is slightly shorter than a solar day (24 hours)
There are 365.25 solar days in a year but 366.25 sidereal days
Sidereal day = 365.25/366.25*24 = 23.93 hours
solar
sidereal
Sun synchronous orbit exploits intentional precession of orbital plane
http://en.wikipedia.org/wiki/Sidereal_time
SeaWiFS daily
coverage
seawifs_daily_coverage animation
Sun-synchronous orbit
altitude = 7228 km
Semi-major axis
= Rearth + alt
= 7228 km
Is this orbit prograde or retrograde?
Terra satellite, MODIS instrument, coverage of all instrument
swaths during a single day ( 01/28/2006 )
http://aquarius.nasa.gov/
Sun synchronous terminator orbit
The sun is a significant source
of L-band radiation, so Aquarius
antenna points toward
night-time side of orbit
to avoid reflection
from the ocean surface
Molniya orbit
•Highly elliptical: eccentricity = 0.72
•Semi-major axis = 26,554 km
•Spends a long time over one hemisphere
•Used for communications at high latitudes
(Russia in particular) because equatorial
geostationary has poor coverage at high latitude.
•Used for spy satellites and ICBM early warning
surveillance with an orbit that dwells over the US
•With inclination 63.4 will precess (like sunsynchronous) to maintain apogee over a desired
ground point
•Molniya orbits require more rocket power to
achieve than geosynchronous
•Period is 12 hours so there are two regions on
the earth surface that receive repeated visits
•The related tundra orbit has a period of 24
hours so its apogee point is unique on the earth
surface – this tundra orbit is used by Sirius
Satellite Radio
In Orbit Architect:
e = 0.72
Semi-maj axis = 26,500
Arg. perigee = 296
Adjust RAAN to place orbit
over region of interest
Scan geometry
• zenith angle w.r.t. satellite nadir
• look angle and solar zenith
angle w.r.t local vertical
• Fixed solid angle Field of View
(FOV) such as for an optical lens,
gives elliptical footprint off-nadir
and varying spatial resolution in
the scan
Sun glint shows
atmospheric gravity
waves modifying ocean
surface roughness
On its descent into the
wave trough, the air
roughens the water
surface and the
surface appears dark.
The brighter regions
show the crests of the
atmospheric waves
where the water is
calm and reflects light
directly back towards
the sensor.
Clouds commonly
form at the crests of
the waves and are
visible throughout this
scene.
30
Panoramic distortion
NOAA AVHRR
Pixels 2.89 times bigger
if earth flat
Panoramic distortion
NOAA AVHRR
Pixels 2.89 times bigger
if earth flat
Pixels 4.94 times bigger
if earth round
Rotating mirror continuous scan e.g. AVHRR
“Whisk-broom” e.g. MODIS
“Push-broom”
http://maracoos.org/irene/wpcontent/uploads/2011/08/2011.08.29.1513.Terra_.RGB_.mab
_.jp
MODIS is a 55 deg +/- whisk broom scanner
Mirror side stripe (banding) is a sudden change of bias
level of all detectors. The change occurs during the
scan mirror’s turnaround, and the amount of change is
quite constant. The image appearance is slightly
brighter and darker scans (clearly seen in
homogeneous areas such as oceans)
2011-Aug-19
MODIS/Aqua
Internet resources on orbits and satellites
Unusual orbits (especially ground track)
•
•
•
•
•
•
Chandra
IMAGE
CRRES (in a geosynchronous transfer orbit)
Tundra
Molniya
http://heavens-above.com
Shows predicted orbits and visibility magnitudes and star charts of pass
trajectories for all satellites
–See Homework 1
•
http://tinyurl.com/11-670-451-homework-1
http://spaceflight.nasa.gov/realdata/elements
–See graphics of orbital elements and data for ISS
•
Real-time satellite tracking at http://www.n2yo.com