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Originally posted by VeniVidi
Probably satellites that are out in space.
Originally posted by gariac
reply to post by Shadowhawk
Is it me, or does everyone else get those links blocked? They exist, but I can't access them.
[5.3] INTERPLANETARY RADAR
* With the introduction of large, powerful dish radio telescopes, astronomers began to think of perform radar observations of the planets. Radar involves transmitting radio pulses and then measuring the time it takes for the pulses to return from a target. Obviously, this requires a powerful transmitter and a sensitive receiver since even the nearest planets are far away.
Radar pulses had been reflected from the Moon not long after the end of World War II, but this was basically a stunt. In fact, when the big US "Ballistic Missile Early Warning System (BMEWS)" military radars were set up in Greenland and Alaska in the early 1960s, the first time the Moon rose in the line of sight of one of the radars it set off alarms; the designers hadn't factored the Moon into their considerations, and some changes had to be made to ignore the returns from the Moon.
In 1961 astronomers actually performed the first useful radar observations of another world, bouncing radar pulses off of Venus during a conjunction of Earth and Venus. The pulses were in the form of "pseudorandom noise" trains that provided long and distinctive patterns, allowing the astronomers to sort out the returns. The radar studies showed Venus had a slow rotation rate, and that it rotated in the reverse direction of other planets.
In the 1970s, improved radar observations using the Arecibo instrument actually permitted some mapping of gross surface detail on the planet as well, with the radar penetrating the unending thick cloud cover of the planet. This was just a prelude for much more detailed and comprehensive radar maps of the planet, produced by US and Soviet Venus orbiting spacecraft in the 1980s.
By that time, radar was being used to observe asteroids passing by the Earth, leading to the imaging of the asteroid Castalia in 1989. A number of other asteroids have been imaged by radar since that time, and in fact radar observations of large asteroids have been performed into the asteroid belt itself, beyond the orbit of Mars.
[5.6] VERY LONG BASELINE INTERFEROMETRY / THE VLBA
* Along with the development of large radio interferometry arrays like the VLA, techniques were developed to link radio telescopes spanning continents or oceans into interferometry arrays, a technique known as "very long baseline interferometry (VLBI)".
Even before the construction of the VLA, radio astronomers were considering VLBI. The VLA is a "connected element" interferometer, with direct signal connections between the dishes, but for longer baselines direct connections are not practical. Observations had to be recorded on magnetic tape, along with the necessary precise timing information, which required accuracies better than a microsecond. This meant that any observatory participating in a VLBI network had to have recording equipment linked to a precise local atomic clock.
The first experimental VLBI measurements were made in 1967 by several Canadian and American research teams. A year later, a Swedish group joined the effort, providing an intercontinental baseline for observations. By the early 1970s, VLBI observations were providing details of the cosmic jets emitted by distant quasars.
The VLBI pioneers managed to perform some significant observations of radio objects, but the logistics of conducting VLBI were burdensome and quickly exceeded the capability of informal groups of researchers. In particular, trying to schedule observations on multiple radio telescopes at the same time was troublesome, and argued for a set of radio telescopes dedicated strictly to VLBI.
VLBI advocates joined together in the "Network Users Group", which lobbied for a formal and properly funded VLBI effort in 1980 report on priorities for astronomical research. The result was the American "Very Long Baseline Array (VLBA)", an array of ten identical radio telescopes, each with a diameter of 25 meters, scattered across the North American continent. The VLBA was funded by the NSF at a cost of $84 million US in 1989 dollars, and was formally dedicated in 1993. The VLBA provides the highest resolution of any astronomical observatory ever built, up to a tenth of an arc-millisecond.
The VLBA is controlled by the NRAO Array Operations Center (AOC) in Socorro, New Mexico. The ten radio dishes, or network "nodes", are located at:
Kitt Peak, Arizona.
Pie Town, New Mexico.
Mauna Kea, Hawaii.
Owens Valley, California.
Los Alamos, New Mexico.
Fort Davis, Texas.
North Liberty, Iowa.
Hancock, New Hampshire.
Saint Croix, Virgin Islands.
The baselines between the ten nodes range from 200 to 8,000 kilometers (125 to 5,000 miles). Each node site has a small local staff to handle observations and routine maintenance. The AOC provides central control over the entire network and a dedicated VLBI processor, or "correlator", that crunches the observations from the ten nodes. The AOC also maintains a spares stockpile, and an engineering staff to provide high level repairs or upgrades for the nodes.