Topics

Imaging the Surface

Joe WB9SBD
 

Watching the news this morning they had a thing on how they are using orbital satellites to monitor the Covid-19. It was amazing imagery!

I can understand Optics on getting the clear resolution. But how do they penetrate the blue haze?

The color balance was perfect in all the shots as if they were taken from a plane at 5000 feet say. Perfect crisp images and perfect color balance not a trace of Blue.

How?

Joe WB9SBD

L. Paul Verhage KD4STH
 

Interesting. I know near infrared penetrates the air like it wasn't there. But shorter wavelength light like blue is strongly scattered.

Perhaps because the satellite looks straight down, there's less air to scatter the light. When we look horizontally through the air, we are looking through miles of air at roughly the same density. When you look down, you are seeing throgh a column of air that increases the closer you get to the ground. 

One measurement of the atmosphere is its scale height. This is the distance one must climb before the air density (which closely matches air pressure*) decreases by a factor of e (2.718...), which means a decrease to 37%. When anything changes in value based on its amount, the number e always shows up just like pi shows up when circles are involved.

Scale height depends on things like the temperature, gravity, and gas molecule mass. So scale height can vary for different gasses in an atmosphere, but with the kind of mixing we have on Earth, the scale height of the different gases in our atmosphere is the same until you get above the stratosphere.

The scale height of our atmosphere is about five miles. Since the atmospheric density is decreasing by e for every scale height, all the air compressed to the surface and uniformly at standard atmospheric pressure is also five miles thick. So technically, the amount of scattering you observe looking hortizonally through five miles of air is the same amount you observing looking down from 200 miles up. 

So I guess we shouldn't be too surprised satellites can get clear images of the surface. Even though they more than 120 miles up, it only appears like they are looking through 5.1 miles of air at sea level air density. 

By the way, scale height also tells you how fast a balloon must expand in volume to remain buoyant. The scale height of Saturn's moon Titan is 30 miles. Keeping everything constant, a balloon should be able to climb about six times higher on Titan than Earth. Perhaps we should look at holding GPSL 2100 in Titan?

*I see there's about a 1% difference between air density and air pressure by the time a balloon climbs to 100,000 feet. By coincidence, Earth's gravity also decreases by 1% at 100,000 feet. Since pressure is related to force and force is due to gravity pulling on mass, I suspect that the difference between air density and air pressure is related to the decrease in gravity at 100,000 feet in altitude.

On Thu, Jun 11, 2020, 4:03 PM Joe WB9SBD <nss@...> wrote:
Watching the news this morning they had a thing on how they are using orbital satellites to monitor the Covid-19. It was amazing imagery!

I can understand Optics on getting the clear resolution. But how do they penetrate the blue haze?

The color balance was perfect in all the shots as if they were taken from a plane at 5000 feet say. Perfect crisp images and perfect color balance not a trace of Blue.

How?

Joe WB9SBD

Joe WB9SBD
 

I have yet to see a pic from say 60K and up from anyone's balloon, that you can say the image of the ground even when looking straight down is not color shifted way into the blue.

it's almost like a Black and white TV picture except it's Blue and white, everything is various shades of Blue.

Joe WB9SBD

On 6/11/2020 5:37 PM, L. Paul Verhage KD4STH wrote:
Interesting. I know near infrared penetrates the air like it wasn't there. But shorter wavelength light like blue is strongly scattered.

Perhaps because the satellite looks straight down, there's less air to scatter the light. When we look horizontally through the air, we are looking through miles of air at roughly the same density. When you look down, you are seeing throgh a column of air that increases the closer you get to the ground. 

One measurement of the atmosphere is its scale height. This is the distance one must climb before the air density (which closely matches air pressure*) decreases by a factor of e (2.718...), which means a decrease to 37%. When anything changes in value based on its amount, the number e always shows up just like pi shows up when circles are involved.

Scale height depends on things like the temperature, gravity, and gas molecule mass. So scale height can vary for different gasses in an atmosphere, but with the kind of mixing we have on Earth, the scale height of the different gases in our atmosphere is the same until you get above the stratosphere.

The scale height of our atmosphere is about five miles. Since the atmospheric density is decreasing by e for every scale height, all the air compressed to the surface and uniformly at standard atmospheric pressure is also five miles thick. So technically, the amount of scattering you observe looking hortizonally through five miles of air is the same amount you observing looking down from 200 miles up. 

So I guess we shouldn't be too surprised satellites can get clear images of the surface. Even though they more than 120 miles up, it only appears like they are looking through 5.1 miles of air at sea level air density. 

By the way, scale height also tells you how fast a balloon must expand in volume to remain buoyant. The scale height of Saturn's moon Titan is 30 miles. Keeping everything constant, a balloon should be able to climb about six times higher on Titan than Earth. Perhaps we should look at holding GPSL 2100 in Titan?

*I see there's about a 1% difference between air density and air pressure by the time a balloon climbs to 100,000 feet. By coincidence, Earth's gravity also decreases by 1% at 100,000 feet. Since pressure is related to force and force is due to gravity pulling on mass, I suspect that the difference between air density and air pressure is related to the decrease in gravity at 100,000 feet in altitude.

On Thu, Jun 11, 2020, 4:03 PM Joe WB9SBD <nss@...> wrote:
Watching the news this morning they had a thing on how they are using orbital satellites to monitor the Covid-19. It was amazing imagery!

I can understand Optics on getting the clear resolution. But how do they penetrate the blue haze?

The color balance was perfect in all the shots as if they were taken from a plane at 5000 feet say. Perfect crisp images and perfect color balance not a trace of Blue.

How?

Joe WB9SBD


L. Paul Verhage KD4STH
 

Yeah, I always see blue unless I shoot black and white near infrared.


On Thu, Jun 11, 2020, 5:02 PM Joe WB9SBD <nss@...> wrote:
I have yet to see a pic from say 60K and up from anyone's balloon, that you can say the image of the ground even when looking straight down is not color shifted way into the blue.

it's almost like a Black and white TV picture except it's Blue and white, everything is various shades of Blue.

Joe WB9SBD

On 6/11/2020 5:37 PM, L. Paul Verhage KD4STH wrote:
Interesting. I know near infrared penetrates the air like it wasn't there. But shorter wavelength light like blue is strongly scattered.

Perhaps because the satellite looks straight down, there's less air to scatter the light. When we look horizontally through the air, we are looking through miles of air at roughly the same density. When you look down, you are seeing throgh a column of air that increases the closer you get to the ground. 

One measurement of the atmosphere is its scale height. This is the distance one must climb before the air density (which closely matches air pressure*) decreases by a factor of e (2.718...), which means a decrease to 37%. When anything changes in value based on its amount, the number e always shows up just like pi shows up when circles are involved.

Scale height depends on things like the temperature, gravity, and gas molecule mass. So scale height can vary for different gasses in an atmosphere, but with the kind of mixing we have on Earth, the scale height of the different gases in our atmosphere is the same until you get above the stratosphere.

The scale height of our atmosphere is about five miles. Since the atmospheric density is decreasing by e for every scale height, all the air compressed to the surface and uniformly at standard atmospheric pressure is also five miles thick. So technically, the amount of scattering you observe looking hortizonally through five miles of air is the same amount you observing looking down from 200 miles up. 

So I guess we shouldn't be too surprised satellites can get clear images of the surface. Even though they more than 120 miles up, it only appears like they are looking through 5.1 miles of air at sea level air density. 

By the way, scale height also tells you how fast a balloon must expand in volume to remain buoyant. The scale height of Saturn's moon Titan is 30 miles. Keeping everything constant, a balloon should be able to climb about six times higher on Titan than Earth. Perhaps we should look at holding GPSL 2100 in Titan?

*I see there's about a 1% difference between air density and air pressure by the time a balloon climbs to 100,000 feet. By coincidence, Earth's gravity also decreases by 1% at 100,000 feet. Since pressure is related to force and force is due to gravity pulling on mass, I suspect that the difference between air density and air pressure is related to the decrease in gravity at 100,000 feet in altitude.

On Thu, Jun 11, 2020, 4:03 PM Joe WB9SBD <nss@...> wrote:
Watching the news this morning they had a thing on how they are using orbital satellites to monitor the Covid-19. It was amazing imagery!

I can understand Optics on getting the clear resolution. But how do they penetrate the blue haze?

The color balance was perfect in all the shots as if they were taken from a plane at 5000 feet say. Perfect crisp images and perfect color balance not a trace of Blue.

How?

Joe WB9SBD


Joe WB9SBD
 

Thats what I was wondering how the Government from orbit has perfectly neutral colors.

Joe WB9SBD

On 6/11/2020 6:25 PM, L. Paul Verhage KD4STH wrote:
Yeah, I always see blue unless I shoot black and white near infrared.

On Thu, Jun 11, 2020, 5:02 PM Joe WB9SBD <nss@...> wrote:
I have yet to see a pic from say 60K and up from anyone's balloon, that you can say the image of the ground even when looking straight down is not color shifted way into the blue.

it's almost like a Black and white TV picture except it's Blue and white, everything is various shades of Blue.

Joe WB9SBD

On 6/11/2020 5:37 PM, L. Paul Verhage KD4STH wrote:
Interesting. I know near infrared penetrates the air like it wasn't there. But shorter wavelength light like blue is strongly scattered.

Perhaps because the satellite looks straight down, there's less air to scatter the light. When we look horizontally through the air, we are looking through miles of air at roughly the same density. When you look down, you are seeing throgh a column of air that increases the closer you get to the ground. 

One measurement of the atmosphere is its scale height. This is the distance one must climb before the air density (which closely matches air pressure*) decreases by a factor of e (2.718...), which means a decrease to 37%. When anything changes in value based on its amount, the number e always shows up just like pi shows up when circles are involved.

Scale height depends on things like the temperature, gravity, and gas molecule mass. So scale height can vary for different gasses in an atmosphere, but with the kind of mixing we have on Earth, the scale height of the different gases in our atmosphere is the same until you get above the stratosphere.

The scale height of our atmosphere is about five miles. Since the atmospheric density is decreasing by e for every scale height, all the air compressed to the surface and uniformly at standard atmospheric pressure is also five miles thick. So technically, the amount of scattering you observe looking hortizonally through five miles of air is the same amount you observing looking down from 200 miles up. 

So I guess we shouldn't be too surprised satellites can get clear images of the surface. Even though they more than 120 miles up, it only appears like they are looking through 5.1 miles of air at sea level air density. 

By the way, scale height also tells you how fast a balloon must expand in volume to remain buoyant. The scale height of Saturn's moon Titan is 30 miles. Keeping everything constant, a balloon should be able to climb about six times higher on Titan than Earth. Perhaps we should look at holding GPSL 2100 in Titan?

*I see there's about a 1% difference between air density and air pressure by the time a balloon climbs to 100,000 feet. By coincidence, Earth's gravity also decreases by 1% at 100,000 feet. Since pressure is related to force and force is due to gravity pulling on mass, I suspect that the difference between air density and air pressure is related to the decrease in gravity at 100,000 feet in altitude.

On Thu, Jun 11, 2020, 4:03 PM Joe WB9SBD <nss@...> wrote:
Watching the news this morning they had a thing on how they are using orbital satellites to monitor the Covid-19. It was amazing imagery!

I can understand Optics on getting the clear resolution. But how do they penetrate the blue haze?

The color balance was perfect in all the shots as if they were taken from a plane at 5000 feet say. Perfect crisp images and perfect color balance not a trace of Blue.

How?

Joe WB9SBD



Hank Riley
 

Rayleigh scattering.    

Notice how the big cloudless areas of land as seen from far out in space (reference 1; looking "straight down") are not very blue tinted at all.  Conversely the balloon pictures from in or near the altitudes of the clouds, and including some look angles from the camera's point of view that are oblique and looking at the clouds, are remarkably bluish.
______________________________________________________________________




Creager, Gerald J - Capt, CAP - Health Services Officer - (SWR-OK-001)
 

The big satellites use sensors on a bunch of discrete wavelengths, and the bandwidth of the sensors isn't too broad. They bring all the wavelengths down and post-process them into false-color images for almost everything released to the public, but there are some sensors that are "true color". These require significant pixel counts, but again, the image mechanism is different: Most are still line-scanners. Very fast line scanners. This allows them to keep the focus and parallax in control, and, their imagery is attitude stabilized, so they are always looking at the same angle, depending on mission.

Weather satellites are like this, although they might be looking with 20 different instruments at the same scene. The bandwidth is significant.

73
Gerry N5JXS



CAP E-Mail Logo

Capt Gerald J Creager, CAP
Health Services Officer - (SWR-OK-001)
(H) 979.229.5301
(C) 979.229.5301
U.S. Air Force Auxiliary
https://www.GoCivilAirPatrol.com/
https://www.okwgcap.org/
https://www.okwgcap.org/wingstaff




From: GPSL@groups.io <GPSL@groups.io> on behalf of Joe WB9SBD via groups.io <nss@...>
Sent: Thursday, June 11, 2020 21:42
To: GPSL@groups.io <GPSL@groups.io>
Subject: Re: [GPSL] Imaging the Surface
 
Thats what I was wondering how the Government from orbit has perfectly neutral colors.

Joe WB9SBD

On 6/11/2020 6:25 PM, L. Paul Verhage KD4STH wrote:
Yeah, I always see blue unless I shoot black and white near infrared.

On Thu, Jun 11, 2020, 5:02 PM Joe WB9SBD <nss@...> wrote:
I have yet to see a pic from say 60K and up from anyone's balloon, that you can say the image of the ground even when looking straight down is not color shifted way into the blue.

it's almost like a Black and white TV picture except it's Blue and white, everything is various shades of Blue.

Joe WB9SBD

On 6/11/2020 5:37 PM, L. Paul Verhage KD4STH wrote:
Interesting. I know near infrared penetrates the air like it wasn't there. But shorter wavelength light like blue is strongly scattered.

Perhaps because the satellite looks straight down, there's less air to scatter the light. When we look horizontally through the air, we are looking through miles of air at roughly the same density. When you look down, you are seeing throgh a column of air that increases the closer you get to the ground. 

One measurement of the atmosphere is its scale height. This is the distance one must climb before the air density (which closely matches air pressure*) decreases by a factor of e (2.718...), which means a decrease to 37%. When anything changes in value based on its amount, the number e always shows up just like pi shows up when circles are involved.

Scale height depends on things like the temperature, gravity, and gas molecule mass. So scale height can vary for different gasses in an atmosphere, but with the kind of mixing we have on Earth, the scale height of the different gases in our atmosphere is the same until you get above the stratosphere.

The scale height of our atmosphere is about five miles. Since the atmospheric density is decreasing by e for every scale height, all the air compressed to the surface and uniformly at standard atmospheric pressure is also five miles thick. So technically, the amount of scattering you observe looking hortizonally through five miles of air is the same amount you observing looking down from 200 miles up. 

So I guess we shouldn't be too surprised satellites can get clear images of the surface. Even though they more than 120 miles up, it only appears like they are looking through 5.1 miles of air at sea level air density. 

By the way, scale height also tells you how fast a balloon must expand in volume to remain buoyant. The scale height of Saturn's moon Titan is 30 miles. Keeping everything constant, a balloon should be able to climb about six times higher on Titan than Earth. Perhaps we should look at holding GPSL 2100 in Titan?

*I see there's about a 1% difference between air density and air pressure by the time a balloon climbs to 100,000 feet. By coincidence, Earth's gravity also decreases by 1% at 100,000 feet. Since pressure is related to force and force is due to gravity pulling on mass, I suspect that the difference between air density and air pressure is related to the decrease in gravity at 100,000 feet in altitude.

On Thu, Jun 11, 2020, 4:03 PM Joe WB9SBD <nss@...> wrote:
Watching the news this morning they had a thing on how they are using orbital satellites to monitor the Covid-19. It was amazing imagery!

I can understand Optics on getting the clear resolution. But how do they penetrate the blue haze?

The color balance was perfect in all the shots as if they were taken from a plane at 5000 feet say. Perfect crisp images and perfect color balance not a trace of Blue.

How?

Joe WB9SBD



L. Paul Verhage KD4STH
 

A ling time ago I use to receive APT images from some NOAA weather satellites. Those were black and white raster images that built up line by line. 

Some time I'll need to find out if I can do that again. It was a fun project back in 1992 and 1993.

If I remember correctly, I got images from NOAA-9 through 12.

On Sat, Jun 13, 2020, 9:04 AM Creager, Gerald J - Capt, CAP - Health Services Officer - (SWR-OK-001) <gerald.j.creager@...> wrote:
The big satellites use sensors on a bunch of discrete wavelengths, and the bandwidth of the sensors isn't too broad. They bring all the wavelengths down and post-process them into false-color images for almost everything released to the public, but there are some sensors that are "true color". These require significant pixel counts, but again, the image mechanism is different: Most are still line-scanners. Very fast line scanners. This allows them to keep the focus and parallax in control, and, their imagery is attitude stabilized, so they are always looking at the same angle, depending on mission.

Weather satellites are like this, although they might be looking with 20 different instruments at the same scene. The bandwidth is significant.

73
Gerry N5JXS



CAP E-Mail Logo

Capt Gerald J Creager, CAP
Health Services Officer - (SWR-OK-001)
(H) 979.229.5301
(C) 979.229.5301
U.S. Air Force Auxiliary
https://www.GoCivilAirPatrol.com/
https://www.okwgcap.org/
https://www.okwgcap.org/wingstaff




From: GPSL@groups.io <GPSL@groups.io> on behalf of Joe WB9SBD via groups.io <nss=mwt.net@groups.io>
Sent: Thursday, June 11, 2020 21:42
To: GPSL@groups.io <GPSL@groups.io>
Subject: Re: [GPSL] Imaging the Surface
 
Thats what I was wondering how the Government from orbit has perfectly neutral colors.

Joe WB9SBD

On 6/11/2020 6:25 PM, L. Paul Verhage KD4STH wrote:
Yeah, I always see blue unless I shoot black and white near infrared.

On Thu, Jun 11, 2020, 5:02 PM Joe WB9SBD <nss@...> wrote:
I have yet to see a pic from say 60K and up from anyone's balloon, that you can say the image of the ground even when looking straight down is not color shifted way into the blue.

it's almost like a Black and white TV picture except it's Blue and white, everything is various shades of Blue.

Joe WB9SBD

On 6/11/2020 5:37 PM, L. Paul Verhage KD4STH wrote:
Interesting. I know near infrared penetrates the air like it wasn't there. But shorter wavelength light like blue is strongly scattered.

Perhaps because the satellite looks straight down, there's less air to scatter the light. When we look horizontally through the air, we are looking through miles of air at roughly the same density. When you look down, you are seeing throgh a column of air that increases the closer you get to the ground. 

One measurement of the atmosphere is its scale height. This is the distance one must climb before the air density (which closely matches air pressure*) decreases by a factor of e (2.718...), which means a decrease to 37%. When anything changes in value based on its amount, the number e always shows up just like pi shows up when circles are involved.

Scale height depends on things like the temperature, gravity, and gas molecule mass. So scale height can vary for different gasses in an atmosphere, but with the kind of mixing we have on Earth, the scale height of the different gases in our atmosphere is the same until you get above the stratosphere.

The scale height of our atmosphere is about five miles. Since the atmospheric density is decreasing by e for every scale height, all the air compressed to the surface and uniformly at standard atmospheric pressure is also five miles thick. So technically, the amount of scattering you observe looking hortizonally through five miles of air is the same amount you observing looking down from 200 miles up. 

So I guess we shouldn't be too surprised satellites can get clear images of the surface. Even though they more than 120 miles up, it only appears like they are looking through 5.1 miles of air at sea level air density. 

By the way, scale height also tells you how fast a balloon must expand in volume to remain buoyant. The scale height of Saturn's moon Titan is 30 miles. Keeping everything constant, a balloon should be able to climb about six times higher on Titan than Earth. Perhaps we should look at holding GPSL 2100 in Titan?

*I see there's about a 1% difference between air density and air pressure by the time a balloon climbs to 100,000 feet. By coincidence, Earth's gravity also decreases by 1% at 100,000 feet. Since pressure is related to force and force is due to gravity pulling on mass, I suspect that the difference between air density and air pressure is related to the decrease in gravity at 100,000 feet in altitude.

On Thu, Jun 11, 2020, 4:03 PM Joe WB9SBD <nss@...> wrote:
Watching the news this morning they had a thing on how they are using orbital satellites to monitor the Covid-19. It was amazing imagery!

I can understand Optics on getting the clear resolution. But how do they penetrate the blue haze?

The color balance was perfect in all the shots as if they were taken from a plane at 5000 feet say. Perfect crisp images and perfect color balance not a trace of Blue.

How?

Joe WB9SBD



James Hannon
 

On 6/13/2020 10:27 AM, L. Paul Verhage KD4STH wrote:
A ling time ago I use to receive APT images from some NOAA weather satellites. Those were black and white raster images that built up line by line.
Some time I'll need to find out if I can do that again. It was a fun project back in 1992 and 1993.
If I remember correctly, I got images from NOAA-9 through 12.
--
WB0TXL
WEB Page http://www.ocrslc.net/
Blog http://jimhannon.wordpress.com
CoCoRaHS station IA-LN-7
42,11.90N,91,39.26W

L. Paul Verhage KD4STH
 

Whoa, that's better than I was getting. I'm about 6 degrees further north than San Francisco, so I may need a slightly larger antenna.

$200 for a GOES station is about what I paid for a APT black and white station. Back then, the receiver was a cheap short wave receiver with its IF filter removed so it could handle the Doppler shift. 

Let me start collecting some pennies.

On Sat, Jun 13, 2020, 9:44 AM James Hannon <jmhannon@...> wrote:
Paul Try this.

https://pietern.github.io/goestools/guides/minimal_receiver.html

Jim H

On 6/13/2020 10:27 AM, L. Paul Verhage KD4STH wrote:
> A ling time ago I use to receive APT images from some NOAA weather
> satellites. Those were black and white raster images that built up line
> by line.
>
> Some time I'll need to find out if I can do that again. It was a fun
> project back in 1992 and 1993.
>
> If I remember correctly, I got images from NOAA-9 through 12.
>

--
WB0TXL
WEB Page http://www.ocrslc.net/
Blog http://jimhannon.wordpress.com
CoCoRaHS station IA-LN-7
42,11.90N,91,39.26W