TO STEER AN ACCURATE COURSE between two places, a navigator needs to know his or her exact position. For thousands of years, sailors monitors their position using the moon, stars, and sun. When clouds obscure the sky, however, it is easy to go far off course. Satellite navigation systems have solved this problem. Satellites transmit radio waves that can be detected on Earth even when it is cloudy. As a result, navigation is now possible in any weather. By the late 1990’s, the Global Positioning System (GPS) developed in the USA had become the most reliable and accurate navigation system ever.
HOW GPS WORK
GPS consists of 24 satellites as well as equipment on the ground. The satellites broadcast their positions and the time. They are spaced in orbits so that a receiver any where on earth can always receive signals from at least four satellites. The GPS receiver knows precisely when the signal was sent and when it arrived, and so can calculate the distance between itself and each of the satellites. With this information, it works out its own position, including altitude.
GPS GROUND CONTROL
The US air force monitors the speed, position and altitude of GPS satellites. Tracking stations send this information to the Master Control Center. Using this, the centre predicts the satellite’s positions in orbit for the next 12 hours. Ground antennas transmit these positions to the satellites for broadcasting to the earth. The tracking data enable the Control Center to update constantly predictions of the satellites’ positions.
GPS SATELLITE
Each GPS satellite has a mass of 844kg, about the same as a small car. When the solar panels are fully open, the satellites are 5.3 cm wide. Each satellite carries atomic clocks to give time accurately. The satellites are designed to last for seven and a half years, and their orbit is at an altitude of 20,200 km.
GPS RECEIVERS
Early receivers displayed the user’s position as latitude and longitude, which had to be plotted on a map. Modern ones display a map marking the user’s position to within a few meters. As well as position, the receivers calculate speed and direction of travel.
GLONASS
The Global Orbiting Navigation Satellite system (glonass) is owned by Russia. Glonass allows users to work out their positions to between 20m and 100m. when needed, special techniques permit greater precision. Glonass satellites give world wide coverage. The European Space Agency is improving coverage of Europe by building equipment designed to receive signals from both Glonass and GPS.
AIR NAVIGATION
Until the early 1990’s, pilots of locust-spraying aircraft in the Sahara desert had only a map and compass to guide them. Given that the Sahara has few outstanding features visible, navigating was difficult. By 1991, small GPS receivers were available , and pest-spraying aircraft could pinpoint their positions to within 30m.
ATOMIC CLOCKS
Atomic clocks keep time with spectacular accuracy: caesium clocks lose only a second every million years. Smaller atomic clocks on GPS and Glonass satellites keep time to within 1 second every 300,000years, enabling accurate time signals to be transmitted to earth.
CAR NAVIGATION
Manufactures of cars from France to Japan are installing GPS receivers to air route planning – more than half a million Japanese cars are already equipped with the system. Some emergency vehicles also use GPS signals to pinpoint their locations. By linking the GPS receiver with a computer map, paramedics, police, of fire fighters can quickly see the fastest route to the scene of an emergency.
HOW GPS WORK
GPS consists of 24 satellites as well as equipment on the ground. The satellites broadcast their positions and the time. They are spaced in orbits so that a receiver any where on earth can always receive signals from at least four satellites. The GPS receiver knows precisely when the signal was sent and when it arrived, and so can calculate the distance between itself and each of the satellites. With this information, it works out its own position, including altitude.
GPS GROUND CONTROL
The US air force monitors the speed, position and altitude of GPS satellites. Tracking stations send this information to the Master Control Center. Using this, the centre predicts the satellite’s positions in orbit for the next 12 hours. Ground antennas transmit these positions to the satellites for broadcasting to the earth. The tracking data enable the Control Center to update constantly predictions of the satellites’ positions.
GPS SATELLITE
Each GPS satellite has a mass of 844kg, about the same as a small car. When the solar panels are fully open, the satellites are 5.3 cm wide. Each satellite carries atomic clocks to give time accurately. The satellites are designed to last for seven and a half years, and their orbit is at an altitude of 20,200 km.
GPS RECEIVERS
Early receivers displayed the user’s position as latitude and longitude, which had to be plotted on a map. Modern ones display a map marking the user’s position to within a few meters. As well as position, the receivers calculate speed and direction of travel.
GLONASS
The Global Orbiting Navigation Satellite system (glonass) is owned by Russia. Glonass allows users to work out their positions to between 20m and 100m. when needed, special techniques permit greater precision. Glonass satellites give world wide coverage. The European Space Agency is improving coverage of Europe by building equipment designed to receive signals from both Glonass and GPS.
AIR NAVIGATION
Until the early 1990’s, pilots of locust-spraying aircraft in the Sahara desert had only a map and compass to guide them. Given that the Sahara has few outstanding features visible, navigating was difficult. By 1991, small GPS receivers were available , and pest-spraying aircraft could pinpoint their positions to within 30m.
ATOMIC CLOCKS
Atomic clocks keep time with spectacular accuracy: caesium clocks lose only a second every million years. Smaller atomic clocks on GPS and Glonass satellites keep time to within 1 second every 300,000years, enabling accurate time signals to be transmitted to earth.
CAR NAVIGATION
Manufactures of cars from France to Japan are installing GPS receivers to air route planning – more than half a million Japanese cars are already equipped with the system. Some emergency vehicles also use GPS signals to pinpoint their locations. By linking the GPS receiver with a computer map, paramedics, police, of fire fighters can quickly see the fastest route to the scene of an emergency.
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