Remote Sensing involves four basic inputs: 1. The Target; 2. The Platform; 3. The Sensor(s); and 4. The Signal (usually electromagnetic radiation or acoustical waves). The target is comprised of the features or materials being sensed. Sensors and signals (including the concept of signatures) are treated later in this Section. On this page we will concentrate on the topic of Platforms, the structures that house or support the sensors. In most instances, the platforms will be in motion; by moving they automatically proceed to new positions from whence they sense new targets. A satellite orbiting the Earth is a typical platform. Most of the platforms involved are pictured in this diagram:
Those platforms above Earth's atmosphere and beyond include satellites, spacecraft, space stations, all in orbit around Earth, and planetary satellite orbiters, flyby satellites, probes, and landers/rovers that are used to explore planets, moons, and asteroids/comets.
We concentrate on those that function within the atmosphere, in orbit around Earth (including ones that look back at Earth and ones that peer outward at planets, stars, and galaxies), and from systems that are sent to explore other regions of the Solar System. We mention here only aircraft (planes and helicopters), blimps, and balloons, on which remote sensors have been mounted; the principles of operation are the same as those on platforms in Earth orbit and beyond.
The next few paragraphs will describe how rockets are used to get platforms into space. (It is assumed that the reader already understands how balloons are launched (they contain gases that are lighter in density than air, and thus rise, and they can be propelled by motors; and how airplanes fly aerodynamically by getting differential lift from air flowing above and below wings and forward motion from propellers or jets)
What we wish to consider briefly now is the physics and mechanics of launching platforms into space. The key ideas are found in these three websites: Orbital mechanics, How satellites are launched and Principles of rockets After reading through these Internet sources, take into account the next comments.
Satellites and probes are launched by rockets (as is the Space Shuttle, which uses rockets for initial boost [these are dropped off] and then its own rocket engine for further propulsion). Rockets are propelled by the burning (combustion) of either solid or liquid (or both) fuels whose gaseous end products are allowed to escape into air or space, providing a thrust. The thrust is a force that propels the rocket forward; the amount of thrust is calculated by this equation:
Two other factors are the amount of fuel and the time duration of the combustion. At any instant there is a specific velocity; the velocity increases systematically, causing the rocket vehicle to accelerate. At the time of engine combustion turnoff, some terminal velocity will have been achieved. Depending on that velocity, the rocket and its payload (such as a satellite) will either fall back to Earth, or go into Earth orbit, or eventually escape Earth's gravity and go into outer space.
To achieve a circular orbit, the payload must travel at 17,000 mph (27,359 kph). To leave Earth, the payload must accelerate to at least 25,039 mph (40,320 kph). It the desired orbit is to be geosynchronous, i.e., remain in orbit such that it seems to hover over the same local area on the Earth's surface, it must reach altitude of about 22,223 miles (~ 36000 km). The escape velocity from any body (such as a planet or moon) is calculated with this formula:
Since the opening of the Space Age in 1957, thousands of satellites have been orbited around the Earth. This is a general summary diagram:
Many of these satellites are described on this website: Remote Sensing satellites.. One trend in recent years has been to make satellites (as sensor platforms) smaller so as to cut the costs of launching. These microsatelites, as they are called, are discussed at this site:types of orbits
Next, we turn to the topic of "rockets" used to launch satellites and probes. Tthere is a wide variety and range of rockets - some small, being used for suborbital flights (as sounders), others much larger, and some reserved for manned missions. An example of the evolution of launch vehicles in the NASA program, note this diagram that depicts rockets developed during the early days of the space program:
Several U.S. manufacturers have produced the bulk of the NASA and military launch vehicles. One is Lockheed-Martin; a family of their rocket products appears in this diagram:
We will call attention to six more modern launch vehicles, since there is no other page in this Tutorial that focuses primarily on this topic. The first is the mightiest rocket of them all - The Saturn V - which remains Wernher von Braun's greatest triumph. The first scene shows this rocket (which traces its lineage back to the V-2) on its conveyor vehicle enroute to a launch pad at Cape Canaveral. The second photo portrays a "surplus" Saturn-V (never used after the Apollo program ended) vehicle on display at the Johnson Space Center in Houston; its massive size is evident from the people near it. The bottom image captures the famous moment when Apollo 11 lifted off for its historic journey to the Moon and back. Saturn V was 33 stories tall (113 m or 363 ft) and could deliver a thrust of 7.5 million pounds.
The two workhorse launch vehicles in the U.S. space program have been the Atlas and Delta rockets, each having been upgraded over the last 20 years. Both satellites orbiting Earth and spacecraft leaving for outer space have utilized these rockets.
The most recent of the Delta rockets built by Boeing is an advanced Delta 4 which, after several years of delay, successfully put a spy satellite in orbit on June 27, 2006:
The U.S. Air Force's primary launch vehicle is the Titan.
Other nations have built their own vehicles for varied purposes. The French Ariane rocket, operating out of Guiana in northern South America, is frequently chosen by companies launching commercial satellites. It is shown schematically below, along with two Russian rockets:
The Ariane 5 is the launch vehicle of choice for European Space Agency (ESA) launches. It is quite powerful, being able to put two large astronomical observatories (Herschel and Planck) into deep space orbit in May of 2009:
The Soviet/Russian rockets come in a wide variety of powerful vehicles. One group is the Soyuz family:
The Chinese have come more recently into the space program. But they have developed a broad range of launch vehicles, as illustrated here:
The main launch vehicle for the NASA manned program is the Space Transport System (STS), more commonly known as the Space Shuttle. It uses two recoverable external fuel tanks plus its own central engine. The Soviets copied this vehicle (calling it Buran) but they use an Energia rocket (expendible) to launch it; very few flights have occurred, especially since it is now carried over into a Russian space program that has more limited funding. The Russians also use a Proton rocket to launch unmanned spacecraft.
As we move through the 21st century, the variety of launch vehicles proliferates. Launch sites include several in Asia, one in South America, and Cape Canaveral (Florida) and Vandenberg Air Force Base (Lompoc, California) as primary locations. A novel "portable" site, Sea Launch, is just starting operation. Based out of Long Beach, CA, and operated by a private consortium, this converted Norwegian drill platform can go almost anywhere in the open Pacific from which to launch. Then it returns to port to be refit with another rocket for the next event.
For those who may want to know more about space travel in general and launching in particular, we recommend this website: Rocket and Space Technology, which also includes other aspects of space history. More can be learned about launch sites, and launch vehicles themselves, by visiting this Space Today Internet site - it's loaded with information. A briefer review of launch procedures is presented at this Wikepedia site. A useful summary of the variety of rockets sent into space is found at this List of launch vehicles website maintained by Wikipedia.