- Describe various types of satellite images and the information that each provides.
- Explain how a Global Positioning System (GPS) works.
- Explain how computers can be used to make maps.
- Geographic Information System (GIS)
- geostationary orbit
- polar orbit
If you look at the surface of the Earth from your yard or street, you can only see a short distance. If you climb a tree or go to the top floor of your apartment building, you can see further. If you flew over your neighborhood in a plane, you could see still further. Finally, if you orbited the Earth, you would be able to see a very large portion of the planet. This is why scientists use satellites to get a good view of Earth. To see things on a large scale, you need to get the highest view.
What Satellites Can Do
To understand what satellites can do, let’s look at an example. One of the deadliest hurricanes in United States history hit Galveston, Texas in 1900. The storm was first spotted at sea on Monday, August 27th, 1900. It was a tropical storm when it hit Cuba on September 3rd. By September 8th, it had intensified to a hurricane over the Gulf of Mexico. It came ashore at Galveston (Figure below). Because there was not advanced warning, more than 8000 people lost their lives.
Left: Track of hurricane that hit Galveston, Texas on Sept. 8, 1900. Right: Galveston in the aftermath.
Today, we have satellites with many different types of instruments that orbit the Earth. With these satellites, satellites can see hurricanes form at sea. They can follow hurricanes as they move from far out in the oceans to shore. Weather forecasters can warn people who live along the coasts. These advanced warning give people time to prepare for the storm. They can find a safe place or even evacuate the area, which helps save lives.
Satellites orbit high above the Earth in several ways. Different orbits are important for viewing different things about the planet.
A satellite in a geostationary orbit flies above the planet at a distance of 36,000 km. It takes 24 hours to complete one orbit. The satellite and the Earth both complete one rotation in 24 hours. This means that the satellite stays over the same spot. Weather satellites use this type of orbit to observe changing weather conditions over a region. Communications satellites, like satellite TV, use this type of orbit to keep communications going full time.
Another useful orbit is the polar orbit (Figure below). The satellite orbits at a distance of several hundred kilometers. It makes one complete orbit around the Earth from the North Pole to the South Pole about every 90 minutes. In this same amount of time, the Earth rotates only slightly underneath the satellite. So in less than a day, the satellite can see the entire surface of the Earth. Some weather satellites use a polar orbit to see how the weather is changing globally. Also, some satellites that observe the land and oceans use a polar orbit.
Satellite in a polar orbit.
The National Aeronautics and Space Administration (NASA) has launched a fleet of satellites to study the Earth (Figure above). The satellites are operated by several government agencies, including NASA, the National Oceanographic and Atmospheric Administration (NOAA), and the United States Geological Survey (USGS). By using different types of scientific instruments, satellites make many kinds of measurements of the Earth.
NASA’s fleet of satellites to study the Earth.
- Some satellites measure the temperatures of the land and oceans.
- Some record amounts of gases in the atmosphere, such as water vapor and carbon dioxide.
- Some measure their height above the oceans very precisely. From this information, they can measure sea level.
- Some measure the ability of the surface to reflect various colors of light. This information tells us about plant life.
Some examples of the images from these types of satellites are shown in Figure below.
Various satellite images: (a) water vapor in atmosphere, (b) ocean surface temperatures, (c) global vegetation.
Global Positioning System
In order to locate your position on a map, you must know your latitude and your longitude. But you need several instruments to measure latitude and longitude. What if you could do the same thing with only one instrument? Satellites can also help you locate your position on the Earth’s surface.
By 1993, the United States military had launched 24 satellites to help soldiers locate their positions on battlefields. This system of satellites was called the Global Positioning System (GPS). Later, the United States government allowed the public to use this system. Here’s how it works.
(a) You need a GPS receiver to use the GPS system. (b) It takes signals from 4 GPS satellites to find your location precisely on the surface
You must have a GPS receiver to use the system (Figure above). You can buy many types of these in stores. The GPS receiver detects radio signals from nearby GPS satellites. There are precise clocks on each satellite and in the receiver. The receiver measures the time for radio signals from satellite to reach it. The receiver uses the time and the speed of radio signals to calculate the distance between the receiver and the satellite. The receiver does this with at least four different satellites to locate its position on the Earth’s surface (Figure above). GPS receivers are now being built into many items, such as cell phones and cars.
Prior to the late 20th and early 21st centuries, mapmakers sent people out in the field to determine the boundaries and locations for various features for maps. State or county borders were used to mark geological features. Today, people in the field use GPS receivers to mark the locations of features. Map-makers also use various satellite images and computers to draw maps. Computers are able to break apart the fine details of a satellite image, store the pieces of information, and put them back together to make a map. In some instances, computers can make 3-D images of the map and even animate them. For example, scientists used computers and satellite images from Mars to create a 3-D image of Mars' ice cap (Figure below). The image makes you feel as if you are looking at the ice cap from the surface of Mars.
This three-dimensional image of Mars' north pole was made from satellite images and computers.
When you link any type of information to a location, you can put together incredibly useful maps and images. The information could be numbers of people living in an area, types of plants or soil, locations of groundwater or levels of rainfall. As long as you can link the information to a position with a GPS receiver, you can store it in a computer for later processing and map-making. This type of mapping is called a Geographic Information System (GIS). Geologists can use GIS to make maps of natural resources. City leaders might link these resources to where people live and help plan the growth of cities or communities. Other types of data can be linked by GIS. For example, Figure below shows a map of the counties where farmers made insurance claims for crop damage in 2008.
Map of insurance filings for crop damage in 2008.
Computers have improved how maps are made. They have also increased the amount of information that can be displayed. During the 21st century, computers will be used more and more in mapping.
- Satellites give a larger view of the Earth’s surface from high above. They carry instruments that make many types of measurements for Earth scientists.
- Satellites can enter different types of Earth orbits to gather different types of information.
- A group of specialized satellites called Global Positioning Satellites help people to pinpoint their location.
- Location information, satellite views, and other information can be linked together in Geographical Information Systems (GIS).
Lesson Review Questions
1. What is the use of each of these types of satellites?
- weather satellite
- communications satellite
- global positioning satellite
- climate satellite
2. What is Geographical Information System, or GIS, used for?
3. Explain the difference between geostationary orbits and polar orbits.
4. What if you had a GPS that could track only one satellite? Two satellites? How many satellites do you need for a good estimate of your location? Why that number?
5. What would have happened if there had been satellites during the time of the 1900 Galveston earthquake?
6. What would have happened if there had been no satellites when hurricane Katrina struck the Gulf of Mexico coast in 2005?
Points to Consider
- How is tracking a hurricane different from trying to predict where a tornado will strike?
- People have GPS units in their cars. What skills are they no longer using if they use a GPS?
- What do images of objects in space do for our view of humans and of the universe?