Uncle Roger:
A satellite is something that moves around something else. Satellites follow a fixed path, known as an orbit.

Scientists have sent many satellites into outer space that revolve around the Earth. These satellites continuously take pictures of the Earth. We can use these satellite images to study the Earth.

The science of acquiring and studying these images is called satellite remote sensing.

Julie:
The moon also moves around the Earth. Is the moon a satellite?

Uncle Roger:
That is a very good question. The moon is indeed a satellite. It is a natural satellite. What I was talking about are satellites made by people.

The Earth has one moon. Some other planets in our solar system have many moons.

Steve:
But why do we want satellites to take pictures of the Earth? I can take pictures of anything with my camera.

Uncle Roger:
This is because you are very close to the Earth's surface. Remember, when you are very close, you see a small area. To take pictures of the whole Earth, you have to be very far away. Satellites are very far away from the Earth's surface. They see a large part of the Earth in one satellite image.

Steve:
I still think my camera is better. It has a big flash and I can take pictures in the dark. I am sure that satellites can not take pictures when it is dark.

Uncle Roger:
Some satellites need sunlight to take images. There are other satellites that work like your flash camera. Instead of a bright flash they send out a wave-like signal. The signal is known as a radar signal or a microwave signal. The signal hits the Earth's surface and goes back to the satellite. The satellite uses the radar signal to make an image. These satellites are known as SAR satellites. SAR stands for synthetic aperture radar.

Tim:
Can the satellites see through clouds?

Steve:
Are the microwave signals like the microwave in our kitchen?

Uncle Roger:
The microwave in the kitchen is used to cook food. Microwaves can pass through the surface of the food and heat it up.

Similarly, the microwave signals sent by the SAR satellite can pass the clouds and see through them. This makes SAR satellites special. We can get SAR images of the Earth even when there are clouds in the sky. Because SAR satellites do not depend on sunlight, they can produce SAR images day and night.

Steve:
I think SAR satellites are cool! But what do SAR images look like? How can we use them to find our treasure?

Julie:
How large is the area I can see on a SAR image?

Uncle Roger:
One standard SAR image covers an area of about 100 times 100 kilometers.

Julie:
You said one image was approximately 10,000 square kilometers. Imagine that great-grandfather lost the treasure somewhere between Anchorage and Nome. This is about 1,800 kilometers. Let us say he could have gone about 100 kilometers away on either side of the main track. Then we will need to search an area of 360,000 square kilometers. We need to see at least 36 images.

Tim:
Wow! That was great. You should be a mathematician.

Julie:
But, I already am.

A satellite is launched (lifted) from the Earth's surface by a rocket. Once the satellite reaches its orbit, it continues to move in its orbit and does not require the push from the rocket any more. Satellite orbits can be at different distances from the Earth. Two important types of orbits are:

 are in the same plane as the equator and are about 35000 km away from the Earth.

 pass very close to the north and south poles (also known as near-polar orbits). They are about 700 to 800 km away from the Earth.

Depending on their use there are different kinds of satellites.

 help to provide radio, television and phone coverage. These satellites always look at the same point on the Earth as they revolve around the Earth (geostationary).

Some  are geostationary. Others are in polar orbit. They all provide information that is used for weather forecast.

 are special satellites that help us to find our exact location on the Earth. GPS (Global Positioning Systems) receivers that you can now buy at many stores use these satellites to show us our location.

 point away from the Earth and are used to study outer space and other planets.

 are specially designed to study the processes on Earth. Many of these satellites occupy near-polar orbits. Some also operate in other orbit types.

Natural satellites that orbit planets are often called moons.

• There are at least 140 moons within Earth's solar system.
• Jupiter has more than 60 moons and Saturn has more than 40 moons.
• Mercury and Venus have no moons at all.
• Earth has one large moon ("The Moon").
• Mars has two tiny moons.
• Pluto has a very large moon named Charon (sometimes considered a double planet).
• Visit related resources at NASA for further reading.

Satellites carry instruments called sensors. Most satellites have multiple sensors.

One good way of categorizing satellite sensors and satellite remote sensing is by the energy required to collect data.

When an external energy source is required to carry out remote sensing it is called . Active remote sensing can be done day and night. The SAR images shown later in the story are all from active remote sensing.

When the sensors rely on the energy coming from natural sources, it is called . The natural source could be the sun. In this type of passive remote sensing the sun's energy is reflected by the Earth's surface. And this kind of remote sensing can be done only during the day.

In other types of passive remote sensing the energy coming out of the Earth's surface is recorded. This type can be carried out at day and night.

Julie believes that Samuel may have traveled from Nome to Fairbanks to Anchorage. This total route is about 1800 km long.

Julie thinks that Samuel could have gone 100 km away on either side of the main track between Nome, Fairbanks and Anchorage. 100 km on either side means a total distance of 200 km across the main track.

You can imagine this as a 200 km broad zone where the track is in the middle. Scientists refer to such zones as "buffer zones." This buffer zone is the children's search zone.

The total area of the search zone = total length of the track multiplied by the total thickness of the buffer zone. The total search area = 1800 km * 200 km = 360,000 sq km.

Each SAR image is about 100 km long and 100 km wide. Therefore total area covered by a SAR image is 100 km * 100 km = 10,000 sq km.

The minimum number of SAR images required to cover the total search area = total search area divided by total area of a SAR image = 360000/10000 = 36 images.

Now calculate at least how many images the children would need to see if Samuel could have gone

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