|COSPAR-ID :||1960-Iota-1 (1960-009A)|
|Size:||30 meters in diameter|
|Begin:||August 12, 1960, 09:39 UTC|
|Starting place:||Cape Canaveral LC-17A|
|Launcher:||Thor Delta D2|
|Status:||burned up on May 24, 1968|
|Rotation time :||118.3 min|
|Orbit inclination :||47.2 °|
|Apogee height :||1684 km|
|Perigee height :||1524 km|
Echo 1 (actually Echo 1A ) is the name of a large balloon satellite in the United States that was launched on August 12, 1960 as the first news and geodetic satellite. Its COSPAR designation was 1960-009A (9th start of 1960, 1st component).
Takeoff, orbit and balloon
The launch took place as a second use of a newly developed launch rocket of the Delta type , which later became the most successful American rocket type in the medium power range. Her first flight (on May 13, 1960 with Echo 1 and a takeoff weight of around 60 kg) was a false start .
The thin aluminum coated thin balloon made of the polyester - plastic Mylar (only a fraction of a millimeter thick film ) was "blown up" only when the final height (km was achieved of about 1500 revolution time less than two hours). The initially 30 meter large, highly reflective sphere, which had already shrunk to a diameter of 18 meters after a year, could be seen as a bright star of the first magnitude for about eight years and served the passive transmission of signals in radio and radio traffic . Towards the end of his life - shortly before it burned up in 1968 - the orbit height had dropped from 1500 to about 1000 km.
Range of radio waves, visibility
A simple sketch or the application of the “ Pythagorean theorem ” shows that such an earth satellite is visible from a distance of more than 4600 km in a 1500 km high circular orbit with an earth radius of 6370 km. So if two radio stations are 9000 km apart and the satellite orbit passes between them, they can receive their mutual reflection if the radio waves are sufficiently strong .
The first long-distance telecommunications connection was established between two US earth stations in the east coast state of New Jersey and in western California (Crawsfords Hill and Goldstone ) over almost 4000 km. The aluminum-coated plastic cover turned out to be a sufficient reflector for the radio waves.
Such large missiles as a 30-meter balloon are not only visible with telescopes, but also with free eyes up to about 5,000 km . In general, however, the visual visibility of (smaller) earth satellites is more difficult than observing them using radio waves, because
- the satellite must be illuminated by the sun ,
- the observer must lie in the shade (i.e. on the twilight or night side ) of the earth,
- the brightness of a sphere depends on the angle between the incidence of light and the observer - see moon phases - and
- also drops sharply near the horizon due to atmospheric extinction .
Nevertheless, for precise purposes of satellite geodesy it is no problem to observe a missile like Echo 1 down to an altitude of 20 ° - which corresponds to a distance of 2900 km. This means that theoretically distances between surveying points of up to more than 5000 km can be “bridged”, and in practice at least 3000–4000 km.
Geodetic use of echo 1
Optically, satellites are best observed against the background of the starry sky . You don't need any special instruments - binoculars and a good star map are sufficient. In principle, at the beginning of space travel, the measurements were actually carried out this way, only the time measurement had to be correspondingly accurate .
If you measure the satellite track in front of the fixed stars with an accuracy of ± 0.01 ° (or 36 ″) and the satellite moves 0.3 ° per second, the time must be accurate to 1/30 of a second . But only very experienced observers in the " Moonwatch " program at the time could do that . For purposes of geodesy , an even higher accuracy would be necessary.
This higher quality was already available at that time with high-light photographic cameras with focal lengths from 20 cm (see satellite camera ). Although it can only be used to record satellites with a clear-eyed brightness, this was by far the case for bright balloon satellites of the echo type (due to their success, another three launches followed until 1966).
Photographic evaluation and stellar triangulation
If such photo recordings with satellite tracks plus short interruptions (closure with time markers) are available, the satellite orbit in the star coordinate system α, δ can be determined to about ± 2 ″ (see orbit determination ). Now you have two options:
- Analysis of the orbital disturbances and thus an exact determination of the earth's gravity field at satellite height, or
- simultaneous measurement of the satellite from two or more ground stations and the formation of large triangles between them. This method corresponds to the terrestrial triangulation with which the geodesists measured the earth over a large area between 1600 and 1950, and is therefore called " stellar triangulation ".
Hundreds of thousands of measurements were made between 1960 and 1968 - when the first balloon satellite burned up. The axes of the earth's ellipsoid , which until then were only known to about 100 meters (0.0016 percent of the earth's radius) due to the geodetically almost unbridgeable oceans , could be determined about ten times more precisely. With the advancement of technology, this accuracy increased fivefold by 1975 (to two to three meters); In 1980 one reached ± 1 m and today - however with microwave technology and GPS - one has reached a few centimeters to decimeters .
Development of satellite technology according to Echo-1
Active instead of passive radio satellites
For communications technology , however, the "passive principle" (reflection of the radio waves on the balloon skin ) was soon replaced by active systems :
- On the one hand, technical progress was reasonably rapidly in the early years of space travel and worked both at the start and control , so even with the built-in satellite technology from.
- On the other hand, it was soon recognized that an effective information transmission is only possible with actively working satellites and that the reflectivity of the balloon envelopes decreased too quickly.
The first active, commercial television satellite Telstar 1 was launched on July 10, 1962 and was already used in the summer for direct television transmission between the USA, Western Europe and Japan . It could send out several hundred audio channels at the same time .
Because of the success of the communications satellite, the communications satellite company COMSAT (Communications Satellite Corporation) was founded in the USA as early as 1963 . Intelsat (International Telecommunications Satellite Organization) followed a year later and COMSAT was a founding member. Their first communications satellite was called Early Bird (morning bird) and started in 1965. Early Bird (officially Intelsat 1 ) could transmit 240 telephone calls or one television channel.
Echo-2 and other balloon satellites
Because Echo-1 was quite successful despite its simplicity - also compared to its electronic successors mentioned above - a similar balloon satellite Echo 2 was launched in 1964. Echo 2 was even larger than Echo 1 at 41 meters in diameter. A higher gas pressure was used to inflate the balloon. Its orbit was slightly lower (initially about 1200 km) and ran very close to the pole ( orbit inclination now about 82 °, compared to Echo-1 with 47 °).
This made it possible to observe the satellite at higher geographical latitudes , where there was a great need for intercontinental connection measurements . At six or seven years, Echo-2 had a slightly shorter lifespan than Echo-1 (1960–1968). Both balloons lost their spherical shape only after a few years, although their gas filling was probably only available for a few hours due to micrometeorites . In the year before the crash, both balloons were already quite “dented”, and their rotation could be clearly recognized by their periodically changing light reflections. Its apparent brightness had decreased by almost one magnitude class from 1st magnitude (0.2 to 1.0) over the years.
To research the density of the upper atmosphere, NASA launched the smaller balloon satellites Explorer (No. 9, 19, 24 and 39), and in 1966 the 30-meter balloon PAGEOS followed . Its name means (German and English) "PAssive GEOdetic satellite".
PAGEOS and the world network
PAGEOS was started especially for the so-called " world network of satellite geodesy ", for which around 20 full-time observation teams were on the move around the world until 1973. With the tried and tested fully electronic BC-4 cameras (1: 3 / focal length 30 or 45 cm) they took a total of 3000 usable photo plates in 46 ground stations , from which the stations could be calculated three-dimensionally to an average of 4 m. The coordinator of these campaigns was Univ. Prof. Hellmut Schmid from ETH Zurich .
There were three stations in the world network in Europe: Catania in Sicily, Hohenpeissenberg in Bavaria and Tromsø in northern Norway. To supplement the pure directional network , precise route measurements were necessary, which were measured on four continents - and also across Europe - with accuracies of 0.5 mm per km.
- Eugene Nelson Hayes: Trackers of the skies: A history of the Smithsonian satellite tracking. Doyle, Cambridge MA 1968.
- Space.com: 1st Communication Satellite: A Giant Space Balloon 50 Years Ago (English)
- ↑ Echo 1 in the NSSDCA Master Catalog , accessed on October 8, 2012 (English).
- ^ A b Joel Strasser: New Look in This Year's Comsats ; Electronics Jul 19, 1963, p. 19
- ↑ This is the distance between the satellite and an observation point on the earth's surface (connected by a straight connecting line; this is around 4621 km long under the conditions mentioned). The distance between the vertical projection of the satellite onto the earth and the same observation point is about 4447 km. Under optimal conditions, two ground stations 8,894 km away could establish a connection via Echo 1.