METEOROLOGY SATELLITES

THE WAY WEATHER SYSTEMS DEVELOP and move around the globe can be seen by meteorology satellites. They record the images that appear nightly on our television screens, show cloud cover, and monitor hurricanes growing and moving across the oceans. Meteorology satellites also carry instruments that take readings, which are converted to the temperatures, pressures and humidities needed for weather forecasting. These, together with information from sources such as weather buoys, balloons, and ships, help forecasters to improve their predictions.

HURRICANE FORECASTING

Before weather satellites existed, hurricanes would develop unseen over oceans and strike land with very little warning. One notorious hurricane killed 6,000 people in Texas in 1906. hurricanes are extreme tropical storms with the wind speed persistently in excess of 120km/hr. in tropical storms, winds circle a calm eye of low air pressure. Now weather satellites constantly view the oceans where such storms gather strength. People need no longer die for lack of warning.

HURRICANE CENTRE

During the tropical storm season between May and November, the US National Hurricane Center in Miami keeps a 24hr watch of all satellite data. As storms develop, satellites track their paths across the oceans. The centre distributes storm and hurricane warnings for the Caribbean, all the coasts of the USA, and the Gulf of Mexico.

SCANNING THE GLOBE

Geostationary satellites scan the region beneath them every 30 minutes. If a tropical storm develops, they scan that region in more detail every 15 minutes. The satellites also measure temperature, which helps forecasters predict hurricane strength.

HOMING IN

As the tropical storm becomes a hurricane and nears land, the US Air force scrambles its Weather Squadron – the Hurricane Hunters – which flies into the storm and adds the measurements to those of coastal radar and satellites.

WEATHER ORBITS

Weather satellites occupy geostationary and polar orbits. Geostationary satellites, such as GOES, stay above the same place on the Equator and record changes continually. Each one can see a third of the globe, but they have a poor view on northern regions. Polar orbit satellites, such as NOAA 10, do not have a constant view of the same region, but they do see the poles and more detail than is possible from geostationary orbit.


PREDICTING LANDFALL

It is very difficult to predict the track of a hurricane, but for each year during the past 20 years satellites have contributed to improvements of between 0.5 and 1 per cent in the accuracy of forecasts. The place where a hurricane will make land – known as landfall – can now be predicted to within less than 160 km.

SATELLITES AND COMPUTING

Computers are essential for scientists to turn satellite measurements into the temperatures, pressures, humidities, and wind speeds needed for a weather report. The computers also combine data from radar, ships, buoys, planes, and satellites to give timely and accurate forecasts.

EL NINO

During El Nino, warm water replaces the usually cold water off South America, which appears to affect weather throughout the world. These satellite pictures show the warm current as red/white area, moving eastwards near the Equator. Black areas are land while other colors represent cooler water surrounding the warm current. By analyzing such images, scientists hope to understand the links between El Nino and changes in the world’s weather.

NAVIGATION SATELLITES

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.

COMMUNICATIONS SATELLITES

COMMUNICATIONS LINK
Antennas on the ground and on satellites send and receive radio waves that carry telephone calls, television signals, or data. A telephone call from the Europe to the USA, for example, might pass through the public telephone network to a nearby Earth station, which transmits the radio waves to a satellite in GEO. The satellite would then amplify and retransmit the radio waves to an antenna in the USA, where the signal is routed over the telephone network to its destination.

TRANSPONDERS
Devices called the transponders are at the heart of communications satellites. They contain a chain of electronic components. These components clean up radio signals, which can be distorted after traveling through the atmosphere, and convert them to the frequency necessary for transmission back to earth. They also amplify the signals before retransmitting them.

EARTH STATIONS
The antennas and other equipments needed on the ground to transmit and receive signals to and from satellites are known as the Earth station. Earth stations can be housed in large buildings. Their antennas act as gateway through which, for example, thousands of telephone calls are transmitted to and from the satellite. Earth stations can also be small units, designed to fit on ships or planes.

SATELLITE FOOT PRINT
Just as the beams of spotlights have different shapes and sizes, so radio waves transmitted by a satellite fall on Earth with a particular pattern. This pattern is known as satellite foot print. Antennas within the foot print can transmit and receive signals to and from the satellite.

GEOSTATIONARY SATELLITES
Satellites in GEO above the Equator always seem to stay over the same spot on Earth. They appear stationary because a satellite 36,000 km above Earth takes the same time to complete one orbit as Earth takes to spin on its axis. They remain in sight of the same Earth station.

FREQUENCY
Radio waves are part of the electromagnetic spectrum. Communications satellites transmit radio waves at frequencies that passes through the atmosphere without being absorbed by water vapour.

SATELLITES AND ORBITS

TYPES OF ORBIT
Most satellites are launched into one of four main orbits. A nearly circular low-Earth orbit is up to about 250 km above Earth. Polar orbits are often 800 km high. A highly elliptical orbit has a much lower altitude at its closest approach to Earth (its perigee) than when it is most distant (its apogee). A geostationary orbit is 36,000 km above the equator.

TELEMETRY, TRACKING AND COMMAND
Telemetry – literally, measuring from far away – allows people on the ground to receive measurements from satellites in orbit. The measurements, sent as radio signals, might include information that allows operators to pinpoint the satellite’s position. This allows people to track the satellite, and to send command signals that can change its position. Telemetry also includes data that allow ground controllers to check that the satellite is operating correctly.

STABILIZING SATELLITES
If a satellite is not stable – if it swings about in an unpredictable way – it cannot do its job. For example, the dish of a communications satellite must always point towards its receiving station or towards the right country if it is transmitting television signals. Two techniques commonly used to maintain stability are spin and three axis stabilization.

SPIN STABILIZATION
Things that spin are naturally stable. A spinning op remains stable if it is spun fast enough, and the turning of its wheels helps to keep bicycle upright. In the early days of satellites, designers decided to exploit this principle. The result is spin-stabilized satellites. These are often cylindrical in shape, and make about one revolution every second. The antenna dish must always point to Earth, so it does not spin. Designers must take care that the dish does not destabilize the satellite.

THREE-AXIS STABILIZATION
Three-axis stabilized satellites contain small spinning wheels that rotate in such a way that they always keep the satellite in the same orientation to the Earth and Sun. if the satellite’s sensors detect a deviation on any of the three axes of the cube, a signal is sent to the wheels to spin faster or slower. These changes restore the satellite to the correct orientation.

ROCKET PROPULSION

SPACE SHUTTLE FUEL
At liftoff, propellant accounts for nearly 90 percent of the weight of the Space Shuttle System. Both solid and liquid propellants are used. The external tank carries liquid hydrogen and, separately, the liquid oxygen needed for combustion. About 470 kg of propellant are delivered to each of the three main engines every second. The solid fuel is in the boosters on either side of the orbiter. Each booster weighs 83 tones, and can hold 504 tones of propellant.

SOLID ROCKET FUEL
The propellant in solid – fuel rockets is shaped into pellets that contain both oxidant and fuel. The pellets also contain substances to prevent them decomposing in storage. The way the propellant is packed into the casing determines how the energy is released. If it is packed so that the surface burns at a constant rate (neutral burn), it provides an even thrust. If the pellets are packed so that the surface area where burning occurs increases gradually, thrust increases gradually (progressive burn). If the burning surface area decreases, the thrust decreases gradually (regressive burn).

LIQUID ROCKET FUEL
The boiling point of liquid oxygen is -183degree Celsius, cold enough to crack metal or shatter rubber. Liquid hydrogen boils at -253degree Celsius. Such low temperatures make both difficult to handle, but they make an efficient propellant.


SPECIFIC IMPULSE
The efficiency of a propellant, known as specific impulse, is defined as the time for which 1 kg of propellant can deliver 1 kg of thrust. So, 1 kg of propellant with a specific impulse of 262 seconds, such as that in the Space shuttle’s solid rocket boosters, can produce 1 kg of thrust for 262 seconds. The higher the specific impulse, the more effective the mix. Liquid propellants have higher specific impulses than solid fuels have.

HOW ROCKETS WORK

MASS AND WEIGHT
The mass of an object is a measure of how much matter it consists of. Mass is the same everywhere. The weight of an object is the result of the force of gravity acting on the object’s mass. Gravity (and therefore weight) decrease with distance from Earth.

THRUST AND ACCELERATION
A launcher needs sufficient thrust to lift its own mass and to overcome gravity. As fuel burns during the ascent, mass is reduced. With increased distance from Earth, both mass and the pull of gravity lessen, and the rocket picks up speed and accelerates to space.

ACTION AND REACTION
The thrust that lifts the launcher comes from burning fuel in its combustion chamber. If the chamber were sealed, it would explode. Gases are allowed to escape through a nozzle. Because they cannot escape upwards, the gases exert an upward force (reaction) that is equal and opposite to the force (action) of the escaping exhaust.

SATELLITE PAYLOAD
The cargo a launcher carries is known as a payload. All the fierce combustion and powerful forces are harnessed to lift a few tones of payload from the Earth’s Surface. Some launchers carry a heavier payload to space than others.

ESCAPING GRAVITY
At an altitude of 200km, a launch vehicle must give a satellite enough horizontal force to reach 7.8km/s, if it is to enter orbit. If it reached a little over 11km/s, the satellite would escape Earth’s gravity, and head off into space. This speed is called escape velocity.

ORBITAL PHYSICS
Imagine a bullet fired horizontally from a gun. Gravity pulls it vertically towards the Earth. If a bullet could be fired with sufficient horizontal force, it would never reach the ground: the bullet would be in the orbit. In the same way, launch vehicles carry satellites above the atmosphere and release them with enough horizontal force to remain in orbit.

EXPLORING SPACE

The biggest revolution in the history of the human race has taken place in the past 50 years: we have been able to leave our planet and explore space. It has totally changed our lives – in fact, many of us would not recognize the world as it was before the launch of SPUTNIK 1 in 1957. Now, flotillas of satellites circle Earth, beaming a cacophony of communications into our homes, while weather, resources, and even wars were surveyed from space. The attendant breakthroughs in miniaturization and computer power can be appreciated by anyone with a personal computer. Space is also a human frontier. Hundreds of people have now flown in space and even walked on the Moon – and with the forthcoming International Space Station, thousands more will join them. Further afield, sophisticated craft has explored all the planets of our Solar System except Pluto – and the next step will be to set off for the stars themselves.