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Chapter 11 Images from Space
While a satellite in continuous orbit passes over Earth, it will be imaging
the ground underneath it. However, during an orbit, a satellite can image only
a certain size of the ground at one time—it can’t see everything at once. The
swath width is a measurement of how much ground the satellite can image
during one pass. For instance, a Landsat satellite’s sensor has a swath width
of 185 kilometers, meaning that a 185-kilometer-wide area on the ground is
imaged as Landsat flies overhead. As satellites orbit, Earth is rotating beneath
them, meaning that several days may separate orbital paths (and thus, swaths
of the ground being imaged) over nearby geographic areas. For example, the
path of a Landsat satellite may pass over the middle of the United States on Day
1 of its orbit, pass over a swath of area hundreds of miles to the east on Day 2,
and not return to the path next to the first one until Day 8 (Figure 11.3).
After a certain amount of time the satellite will completely pass over
all areas on Earth and start again, back on the first path. For instance, the
Landsat series of satellites are set up in an orbital path such that they will pass
over the same swath on Earth’s surface every 16 days. So when Landsat’s orbit
carries it to the swath where it’s collecting imagery of your current location it
will be another 16 days before it’s able to image your location again. A Sunsynchronous orbit occurs when a satellite’s orbit takes it over the same area
on Earth at the same local time. Because of this, images collected from the satellite will have similar sun-lighting conditions and can be used for comparing
changes to an area over time. For instance, Landsat 7 has a Sun-synchronous
orbit, allowing it to cross the Equator at a constant time every day.
Think about all the jobs that satellites are
servicing—such as communication, satellite TV,
weather monitoring, military applications, and GPS—
then add remote sensing to that list. When you start
considering all these uses, you can see the need for
many, many satellites in orbit.
JSatTrak is a very cool program that you can use
to view (in real time) the locations of numerous
satellites (including remote sensing ones). JSatTrak
can be downloaded from http://www.gano.name/
shawn/JSatTrak/. When JSatTrak opens, you can access a list of satellites via the Windows pull-down
menu and by choosing Satellite Browser. A list of
available satellites will appear (many of the remote
sensing satellites described in this chapter and in
Chapter 12 can be found by expanding the Weather
and Earth Resources option, then selecting. Earth
Resources). When you drag the name of a satellite
from the list to the Object List window on the right
side of the screen, the orbit track and current location of the satellite will be shown on the display
window. Check out some satellites like Landsat 5,
Landsat 7, Terra, and others. JSatTrak has many other features to explore (such as the 3D Earth Window
for looking at satellite tracking).
NASA also provided an online satellite tracking
tool called J-Track (also J-Track 3D) that allowed users
to similarly track satellite positions in real time. Unfortunately, at the time of this writing, J-Track is currently unavailable, but hopefully will return one day.
Hands-on Application 11.1
Examining Satellite Orbits in Real Time
swath width the
width of the ground
area the satellite is
imaging.
Sun-synchronous
orbit an orbital path
set up so that the
satellite crosses the
same areas at the same
local time.
Chapter 11 Images from Space
While a satellite in continuous orbit passes over Earth, it will be imaging
the ground underneath it. However, during an orbit, a satellite can image only
a certain size of the ground at one time—it can’t see everything at once. The
swath width is a measurement of how much ground the satellite can image
during one pass. For instance, a Landsat satellite’s sensor has a swath width
of 185 kilometers, meaning that a 185-kilometer-wide area on the ground is
imaged as Landsat flies overhead. As satellites orbit, Earth is rotating beneath
them, meaning that several days may separate orbital paths (and thus, swaths
of the ground being imaged) over nearby geographic areas. For example, the
path of a Landsat satellite may pass over the middle of the United States on Day
1 of its orbit, pass over a swath of area hundreds of miles to the east on Day 2,
and not return to the path next to the first one until Day 8 (Figure 11.3).
After a certain amount of time the satellite will completely pass over
all areas on Earth and start again, back on the first path. For instance, the
Landsat series of satellites are set up in an orbital path such that they will pass
over the same swath on Earth’s surface every 16 days. So when Landsat’s orbit
carries it to the swath where it’s collecting imagery of your current location it
will be another 16 days before it’s able to image your location again. A Sunsynchronous orbit occurs when a satellite’s orbit takes it over the same area
on Earth at the same local time. Because of this, images collected from the satellite will have similar sun-lighting conditions and can be used for comparing
changes to an area over time. For instance, Landsat 7 has a Sun-synchronous
orbit, allowing it to cross the Equator at a constant time every day.
Think about all the jobs that satellites are
servicing—such as communication, satellite TV,
weather monitoring, military applications, and GPS—
then add remote sensing to that list. When you start
considering all these uses, you can see the need for
many, many satellites in orbit.
JSatTrak is a very cool program that you can use
to view (in real time) the locations of numerous
satellites (including remote sensing ones). JSatTrak
can be downloaded from http://www.gano.name/
shawn/JSatTrak/. When JSatTrak opens, you can access a list of satellites via the Windows pull-down
menu and by choosing Satellite Browser. A list of
available satellites will appear (many of the remote
sensing satellites described in this chapter and in
Chapter 12 can be found by expanding the Weather
and Earth Resources option, then selecting. Earth
Resources). When you drag the name of a satellite
from the list to the Object List window on the right
side of the screen, the orbit track and current location of the satellite will be shown on the display
window. Check out some satellites like Landsat 5,
Landsat 7, Terra, and others. JSatTrak has many other features to explore (such as the 3D Earth Window
for looking at satellite tracking).
NASA also provided an online satellite tracking
tool called J-Track (also J-Track 3D) that allowed users
to similarly track satellite positions in real time. Unfortunately, at the time of this writing, J-Track is currently unavailable, but hopefully will return one day.
Hands-on Application 11.1
Examining Satellite Orbits in Real Time
swath width the
width of the ground
area the satellite is
imaging.
Sun-synchronous
orbit an orbital path
set up so that the
satellite crosses the
same areas at the same
local time.