SCISAT-1

Observing the Atmosphere

Observing the Atmosphere

The rising Sun being observed by SCISAT-1. SCISAT-1 records the solar spectrum as the sunlight passes through the Earth's atmosphere.

SCISAT-1 is designed to make observations of the Earth's atmosphere. The primary instrument onboard is an optical spectrometer which records the spectrum of the Sun, as sunlight passes through the Earth's atmosphere.

The process whereby the Sun is eclipsed by the Earth is called an occultation.

Astronomers use planetary occultations (which occur when a planet passes in front of a star) to determine the structure of a planet's atmosphere.

SCISAT-1 will use the occultation of the Sun (by the Earth) to make detailed determinations of the structure and chemistry of that part of the Earth's atmosphere which is too high to reach by balloon (or airplane) and too low to be accessed directly by orbiting satellites.

The orbit of SCISAT-1

The orbit of SCISAT-1, as seen looking down on the Earth's North Pole. The scales are exaggerated to illustrate how SCISAT-1 "sees" sunlight which is transmitted through the Earth's atmosphere.
Transparency Master

The diagram above illustrates the method whereby SCISAT-1 observes the Sun as the sunlight passes through the Earth's atmosphere

In low Earth orbit (about 650 km for SCISAT-1) the satellite passes through the Earth's shadow once per orbit.

In this diagram the "halo" around the earth is meant to represent it's atmosphere (which is exaggerated in scale for purposes of illustration). The incident sunlight is indicated by arrows on the right hand margin of the diagram. Because of the Sun's very great distance from the Earth, the Sun's rays are essentially parallel, making the Sun visible from both sides of the Earth during the satellite's orbit.

As the satellite passes into the Earth's shadow the Sun appears to "set," during which time the Sun's light "grazes" the Earth's surface and passes through the Earth's atmosphere as it does so.

When the satellite emerges from the Earth's shadow the Sun appears to "rise" through the Earth's atmosphere.

The orbit of SCISAT-1

The orbit of SCISAT-1, as seen looking down on the Earth's North Pole. The scales are exaggerated to illustrate how SCISAT-1 "sees" sunlight which is transmitted through the Earth's atmosphere.
Transparency Master

During these sunrise and sunset phases SCISAT-1 records the spectrum of the sunlight as the Sun's rays pass through the Earth's atmosphere.

The sunlight appears to change colour (as observed at sunset) as the path of the sunlight through the atmosphere increase in length. In addition, very specific wavelengths (colours) are removed from the transmitted beams of sunlight by atoms and molecules of various chemical substances which occur in the atmosphere.

Two main processes are responsible for changing the spectrum of the transmitted sunlight.

The first process is molecular scattering, whereby the shortest wavelengths are much more strongly scattered by air molecules than the longer wavelengths. The result is that longer wavelengths (red) are more easily transmitted through long optical paths in the Earth's atmosphere (hence a red sunset).

The second process, called absorption, results when atoms (and molecules) absorb very specific wavelengths. These absorbed wavelengths are unique to each kind of atom (or molecule).

Analysis (of the spectrum) of the transmitted light gives scientists clues about the chemical composition and chemical abundance of the molecules that make up the Earth's atmosphere.

The orbit of SCISAT-1

The orbit of SCISAT-1, as seen looking down on the Earth's nightside equator. As the satellite orbits around the Earth it "sees" the sun through the Earth's atmosphere twice on each orbit giving it a "sunrise" view and a "sunset" view.
Transparency Master

Recall that, because of the Sun's huge distance from the Earth, it is "seen" by the orbiting satellite from both sides of the Earth. Although the diagram may seem to show two Suns, astute readers will recognize that it the same Sun, simply drawn twice for illustrative purposes.

Orbiting the Earth 15 times a day provides the satellite an opportunity to observe sunlight which has passed through the Earth's atmosphere during 15 brief "sunrises" and "sunsets", allowing 30 sets of observations per day.

During each sunrise and sunset the satellite records the intensity and wavelength of the transmitted sunlight. The observed light which interests scientists is the light which passes through layers of the Earth's atmosphere in heights ranging from 4 to 100 kilometres (above the Earth's surface).

The orbit of SCISAT-1

The plane of SCISAT-1's orbit has been rotated so that the satellite "sees" sections of the Earth's atmosphere at higher latitudes.
Transparency Master

To observe a greater range of the Earth's atmosphere over its surface the plane of a satellite's orbit can be inclined at an angle (with respect to the Earth's equator).

The combined rotation of the Earth (which moves its atmosphere with it) and the orbital motion of the satellite allow a major fraction of the Earth's atmosphere to be scanned over a period of several weeks.

In this way a three dimensional picture of the Earth's atmosphere can gradually be constructed.

The orbit of SCISAT-1

The plane of SCISAT-1's orbit has been rotated so that the satellite "sees" sections of the Earth's atmosphere at higher latitudes.
Transparency Master

If the plane of the satellite's orbit is also oriented so that it "faces" towards the Sun, the time the Sun takes to rise and set increases. This occurs because the Sun (as seen from the satellite) rises and sets at an angle to the Earth's horizon, in exactly the same way that long lingering sunsets occur on Earth during the Canadian summer.

You may have noticed that the summer Sun does not set at right angles to the horizon, but rather at a more gentle angle...in fact above the arctic circle, the angle of the setting Sun is sometimes so shallow, the Sun fails to set at all!

A satellite is rarely "moved" into a new orbit because it requires a lot of (expensive) energy to do so. Furthermore, few satellites are equipped with rocket engines that are suitable for more than minor orbital adjustments.

The plane of SCISAT-1's orbit

The plane of SCISAT-1's orbit has been rotated so that the satellite "sees" sections of the Earth's atmosphere at higher latitudes.
Transparency Master

A common method used to ensure that the satellite "sees" the entire planet (without invoking large orbital manoeuvres) in suitable detail, in a reasonable amount of time, and at a reasonable cost, is to allow the satellite's orbit to undergo a process called precession whereby the satellites' orbital plane is allowed to rotate very slowly with respect to a frame of reference defined by the fixed stars.

This precession, combined with the Earth's rotation, allows the satellite to make detailed, systematic observations which cover the entire Earth.