Category Science

Extra-solar planets are very difficult to see because they are outshone by the light from their parent stars. It can be deter-mined whether or not a star has a planetary system by observing whether or not the star’s- light “wobbles”. As a planet orbits a star, its gravitational pull will cause the star’s light to bend slightly, and thus to change colour. This technique only works for giant planets, however, because an Earth-sized world would have little effect on its parent.

It is not easy to detect another planet so far away from Earth. Unlike stars which are fueled by nuclear reactions, planets only reflect the optical light of their stellar companion. In our solar system, for example, the Sun outshines its planets about one billion times in visible light. Because of the distant planets’ faintness near the brightness of the nearby star, astronomers have had to devise clever methods to detect them. Currently, the most successful approach is based on the fact that a nearby planet will cause the star to wobble back and forth just a bit as the planet revolves around it. Astronomers can detect this tiny wobble and then calculate the orbit and mass of the object which is causing it. Even using this technique, however, it is still not easy to detect planets around other stars. Consider this: someone looking at our Sun from 30 light-years away would see it wobbling in a circle whose size would be about as big as a quarter viewed from 10,000 kilometers away!

During the past few years, researchers have detected over a dozen planets orbiting sunlike stars. The first was reported in October 1995 by Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland. While observing the star 51 Pegasi, they noticed a change in the light from the star – its light repeatedly shifted back and forth between the blue and red ends of the electromagnetic spectrum. The timing of this Doppler shift implied that the star was “wobbling” a little because of a closely orbiting planet. In fact, the planet appeared to be revolving around the star every 4.2 days. Shortly thereafter, a survey of over a hundred other sunlike stars performed by the team of Geoff Marcy and Paul Butler at San Francisco State University and the University of California at Berkeley, turned up six more such planets. Of those, one planet circling the star 16 Cygni B was independently discovered by astronomers William D. Cochran and Artie P. Hatzes of the University of Texas McDonald Observatory. Since 1996, the announcement of the detection of new planets has become fairly routine….but always exciting!

Global positioning satellites beam signals to special receivers on Earth. These receivers, which are not much larger than mobile phones, know the difference between when the satellite signal was sent and when it was received. This allows the receiver to work out the distance between each of the satellites and itself, and there-fore calculate its position.

GPS is accurate and handy to use, so much so that we rely on it more and more every day. It’s not often we take the time to learn how it works. The idea of GPS refers to a Global Positioning System; a collection of satellites in orbit above the Earth that transmit location data down to our devices. As hobbyists, we can get GPS modules that will read and interpret this data for us! They’re known as GPS receivers, and they are used everywhere, like your phone, tablet, and other electronic devices. GPS receivers will relay a satellite’s location data directly to a microcontroller in the form of serial data strings, which we can break down into relevant bite-sized chunks of data about where we are and how we are moving!

Firstly, the signal of time is sent from a GPS satellite at a given point. Subsequently, the time difference between GPS time and the point of time clock which GPS receiver receives the time signal will be calculated to generate the distance from the receiver to the satellite. The same process will be done with three other available satellites. It is possible to calculate the position of the GPS receiver from distance from the GPS receiver to three satellites. However, the position generated by means of this method is not accurate, for there is an error in calculated distance between satellites and a GPS receiver, which arises from a time error on the clock incorporated into a GPS receiver. For a satellite, an atomic clock is incorporated to generate on-the-spot time information, but the time generated by clocks incorporated into GPS receivers is not as precise as the time generated by atomic clocks on satellites. Here, the fourth satellite comes to play its role: the distance from the fourth satellite to the receiver can be used to compute the position in relations to the position data generated by distance between three satellites and the receiver, hence reducing the margin of error in position accuracy.

The Fig 1-3 below illustrates an example of positioning by two dimensions (position acquisition by using two given points). We can compute where we are at by calculating distance from two given points, and the GPS is the system that can be illustrated by multiplying given points and replacing them with GPS satellites on this figure.

GPS, or the Global Positioning System, is designed to aid navigation around the planet. It consists of 24 satellites in six different orbits around Earth. Their position in these orbits means that any receiver, anywhere on Earth, can always receive a signal from four satellites or more. Using data from these signals, a GPS receiver can work out its position, including altitude, to within a few metres.

The Global Positioning System (GPS), originally NAVSTAR GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite systems (GNSS) that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. Obstacles such as mountains and buildings block the relatively weak GPS signals.

The GPS does not require the user to transmit any data, and it operates independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the GPS positioning information. The GPS provides critical positioning capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver.

The GPS project was started by the U.S. Department of Defense in 1973, with the first prototype spacecraft launched in 1978 and the full constellation of 24 satellites operational in 1993. Originally limited to use by the United States military, civilian use was allowed from the 1980s following an executive order from President Ronald Reagan. Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System (OCX). Announcements from Vice President AL Gore and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the modernization effort, GPS III. During the 1990s, GPS quality was degraded by the United States government in a program called “Selective Availability”; this was discontinued in May 2000 by a law signed by President Bill Clinton.

When selective availability was lifted in 2000, GPS had about five-meter (16 ft.) accuracy. The latest stage of accuracy enhancement uses the L5 band and is now fully deployed. GPS receivers released in 2018 that use the L5 band can have much higher accuracy, pinpointing to within 30 centimeters or 11.8 inches.

A satellite cannot do its job properly if it is not stable. A satellite dish must always point towards its location, or signals will be lost in space. In order to keep satellites from flying out of control, some are deliberately designed to spin. In the same way that a spinning top remains stable if it is spinning quickly, a satellite that is spinning will not deviate from its course. Some satellites have small, spinning wheels at various points on their frame. These wheels can be used to realign the satellite if it moves off course.

If you throw a ball into the air, the ball comes right back down. That’s because of gravity—the same force that holds us on Earth and keeps us all from floating away. To get into orbit, satellites first have to launch on a rocket. A rocket can go 25,000 miles per hour! That’s fast enough to overcome the strong pull of gravity and leave Earth’s atmosphere. Once the rocket reaches the right location above Earth, it lets go of the satellite.

The satellite uses the energy it picked up from the rocket to stay in motion. That motion is called momentum. But how does the satellite stay in orbit? Wouldn’t it just fly off in a straight line out into space? Not quite. You see, even when a satellite is thousands of miles away, Earth’s gravity is still tugging on it. That tug toward Earth–combined with the momentum from the rocket… …causes the satellite to follow a circular path around Earth: an orbit. When a satellite is in orbit, it has a perfect balance between its momentum and Earth’s gravity. But finding this balance is sort of tricky.

Gravity is stronger the closer you are to Earth. And satellites that orbit close to Earth must travel at very high speeds to stay in orbit.

For example, the satellite NOAA-20 orbits just a few hundred miles above Earth. It has to travel at 17,000 miles per hour to stay in orbit. On the other hand, NOAA’s GOES-East satellite orbits 22,000 miles above Earth. It only has to travel about 6,700 miles per hour to overcome gravity and stay in orbit. Satellites can stay in an orbit for hundreds of years like this, so we don’t have to worry about them falling down to Earth.

Satellites can help scientists learn a great deal more about the planet than instruments on aircraft and ships can. They use Earth-resources satellites to monitor every part of the world in order to find out information about the planet’s condition. Satellites can detect things such as the amount of water in a field of crops, which will give early warning of a harvest failure. They can also detect large areas of deforestation, showing changes over large periods of time.

ERS (Earth Resources Satellite) are the first two remote sensing satellites launched by ESA (European Space Agency). Their primary mission was to monitor Earth’s oceans, ice caps, and coastal regions.

The satellites provided systematic, repetitive global measurements of wind speed and direction, wave height, surface temperature, surface altitude, cloud cover, and atmospheric water vapor level. Data from ERS-1 were shared with NASA under a reciprocal agreement for Seasat and Nimbus 7 data. ERS-2 carries the same suite of instruments as ERS-1 with the addition of the Global Ozone Measuring Equipment (GOME) which measures ozone distribution in the outer atmosphere. Having performed well for nine years – more than three times its planned lifetime – the ERS-1 mission was ended on March 10, 2000, by a failure in the onboard attitude control system.

The length of its operation enabled scientists to track several El Nino episodes through combined observations of surface currents, topography, temperatures, and winds. The measurements of sea surface temperatures, critical to the understanding of climate change by the ERS-1 Along-Track Scanning Radiometer were the most accurate ever made from space. All these important measurements are being continued by ERS-

Meteorology satellites, which orbit in geostationary and polar orbits, can keep a constant watch over the weather systems at work around the planet. They record data, such as cloud formation and movement, pressures, wind speeds and humidities, and send them to Earth, where scientists can use them to predict weather in preparation for weather forecasts. Satellites are also used to detect hurricanes — fierce tropical storms with wind speeds of over 130km/h (80mph). These storms can strike with very little warning, but satellites can detect them before they hit land, warning people of danger in time for them to take cover.

Weather satellites carry instruments called radiometers (not cameras) that scan the Earth to form images. These instruments usually have some sort of small telescope or antenna, a scanning mechanism, and one or more detectors that detect either visible, infrared, or microwave radiation for the purpose of monitoring weather systems around the world.

The measurements these instruments make are in the form of electrical voltages, which are digitized and then transmitted to receiving stations on the ground. The data are then relayed to various weather forecast centers around the world, and are made available over the internet in the form of images. Because weather changes quickly, the time from satellite measurement to image availability can be less than a minute.

Most of the satellites and instruments they carry are designed to operate for 3 to 7 years, although many of them last much longer than that. Weather satellites are put into one of two kinds of orbits around the Earth, each of which has advantages (and disadvantages) for weather monitoring. The first is a “geostationary” orbit, with the satellite at a very high altitude (about 22,500 miles) and orbiting over the equator at the same rate that the Earth turns. This allows the satellite to view the same geographic area continuously, and is used to provide most of the satellite imagery you see on TV or the internet.

For instance, GOES-East and GOES-West provide coverage of much of the Western Hemisphere, from the western coast of Africa to the West Pacific, and the Arctic to the Antarctic. The European Space Agency’s Meteosat satellite provides coverage of Europe and Africa. The disadvantages of a geostationary orbit are (1) its very high altitude, which requires elaborate telescopes and precise scanning mechanisisms in order to image the Earth at high resolution (currently, 1 km at best); and (2) only a portion of the Earth can be viewed.

The other orbit type is called near-polar, sun-synchronous (or just “polar”), where the satellite is put into a relatively low altitude orbit (around 500 miles) that carries the satellite near the North Pole and the South Pole approximately every 100 minutes. Unlike the geostationary orbit, the polar orbit allows complete Earth coverage as the Earth turns beneath it.

These orbits are “sun-synchronous”, allowing the satellite to measure the same location on the Earth twice each day at the same local time. Of course, the diadvantage of this orbit is that the satellite can image a particular location only every 12 hours, rather than continuously as in the case of the geostationary satellite. To offset this disadvantage, two satellites put into orbits at different sun-synchronous times have allowed up to 6 hourly monitoring.

But because of the lower altitude (500 miles rather than 22,000 miles), the instruments the polar-orbiting satellite carries to image the Earth do not have to be as elaborate in order to achieve the same ground resolution. Also, the lower orbit allows microwave radiometers to be used, which must have relatively large antennas in order to achieve ground resolutions fine enough to be useful. The advantage of microwave radiometers is their ability to measure through clouds to sense precipitation, temperature in different layers of the atmosphere, and surface characteristics like ocean surface winds.