Satellite Systems (Oct. 1987)

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Sputnik's 30th anniversary October 4 this year is the 30th anniversary of the launching of the first artificial satellite of the Earth-Sputnik-1. The Soviet Un ion sent this small, spherical device into orbit at 1930 hours GMT on 4 October 1957. It was launched by a powerful, 27.5-m high rocket, with a lift-off thrust of 4.9 MN, from a missile test range a few miles north of the small town of Tyuratam in Kazakhstan. This town (about 63°E and 45°N) is just east of the Aral Sea and lies near the Syr Dania river flowing into that Sea.

Nowadays a new town built to the north of Tyuratam is one of the permanent launching centers for the Russian space program.

Our planet's first artificial satellite was an aluminum alloy sphere, only 580mm in diameter and weighing 83.6 kg. Much of this weight was made up by batteries for powering the two radio transmitters carried in the spacecraft, the metal sphere were four whip antennas, with lengths ranging from 2.4 to 2.9 meters.

The satellite was propelled into a low elliptical orbit with an apogee of 942 km, a perigee of 228 km and an inclination (to the equatorial plane) of 65°. It was spinning at a rate of about 7 revolutions per minute. The orbital period was approximately 90 minutes, and so, with the rotation of the Earth, its radio signals were soon heard all over the world.

These signals were on frequencies of 20.005 MHz and 40.002 MHz and were keyed to give pulses of carrier about 300 ms in duration. The p.r.f. was initially about 108 pulses/s and gradually increased to 150 pulses/s until October 7, when the keying stopped altogether.

Received signal strength was occasionally in the region of 35 V/m but much of the time was only just above the noise level.

With the 90-minute orbital period the reception time for each pass of the satellite was usually not more than about 5 minutes, though some observers recorded times as long as 30 minutes--possibly the result of ionospheric refraction.

This journal published a de tailed, five-page report (written by the undersigned) on the observations made, in the December 1957 issue, pp. 574-578.

In the UK these included measurements by the DSIR (now SERC), BBC, Post Office (now BT), RAE, RRE (now RSRE) and the Cambridge and Manchester universities' radio astronomy observatories. Using mainly Doppler and interferometric techniques, the observers were able to derive measurements of Sputnik's range, height and velocity and to calculate its track in terms of orbital period and inclination and the altitudes of its apogee and perigee.

When the radio transmissions ceased after 21 days, measurements were continued by means of radar. The spacecraft went on orbiting for 92 days. Atmospheric drag in the upper atmosphere reduced its orbital velocity and as a result it descended into denser air and burnt up on 4 January 1958.

The decision to launch Sputnik-1 on 4 October 1957 was determined by several events-a combination of a natural phenomenon, human ambitions and historical circumstances. First of all, it took place in the International Geophysical Year (actually 18 months from July 1957 to December 1958) when many countries collaborated in studying the effects of a peak of solar activity on the Earth. The ICY planning committee had already passed a resolution recommending the use of artificial satellites for this research if they could be launched. So Sputnik was to some extent a response to a scientific initiative. In fact the satellite provided measurements of temperature in the thermo sphere and exosphere, while its radio signals allowed ground observers to measure electron densities in the ionosphere.

But probably more influential in fixing this date was technological rivalry with the USA. The Russian team of experts, led by the aircraft and rocket engineer Sergei Korolev, were well aware that the Americans, under Wernher von Braun, were planning to launch a scientific satellite in September 1957. In the event von Braun's plan was turned down by the US authorities. Had it been accepted the USA might well have been the first country to go into space. As things turned out America be came No. 2, when its Explorer-1 satellite was sent into orbit on 31 January 1958.

Another important stimulus was national prestige, and this has several strands to it. October 1957 was the 40th anniversary of the Bolshevik revolution in Russia and thus an appropriate time to draw the world's attention to the scientific and technological achievements of the regime that came out of it. Secondly, 1957 was the centenary of the birth of Russia's greatly revered space theorist, Konstantin Tsiolkovsky (1857-1935). Korolev had visited him shortly before he died. And thirdly. Nikita Krushchev, then First Secretary of the Communist Party of the Soviet Union, badly needed to generate some national, and hence personal.

prestige to boost his declining position in the Politburo of that time. He had some powerful opponents. So it was Krushchev who was responsible for providing the necessary human and technical resources for the satellite project and giving approval for the launch.

Finally, the time at which Sputnik arrived can be seen as the result of a complicated interaction between engineering progress and historical accident.

Here an important factor was the career and character of the project leader himself. Sergei Korolev (1906-1966). As well as being an aeronautical engineer, Korolev was privately a life-long enthusiast for rocketry and space flight. To begin with there was no official outlet in the USSR for this interest and Korolev pursued it through a small, private group of like-minded scientists and engineers who had got together to study rocket propulsion. Later this group was taken up and given government support by the then armaments minister Mikhail Tukhachevsky, and in 1933 the first Russian liquid-fuel rocket was built and flown.

But in 1937 Tukhachevsky fell victim to one of Stalin's notorious political purges, and with him his whole rocket team including Korolev. From then on Korolev was held in various grades of prison, including the special ones for scientists and engineers (as described in Solzhenitsyn's novel The First Circle), until Stalin died in 1953. He was then rehabilitated and soon afterwards elected to the Soviet Academy of Sciences as a reward for his work. Krushchev was impressed with Korolev's achievements in rocket research and promoted him under the new post-Stalin regime. As a result Korolev was now, in the mid-1950s, in a strong enough position to press for permission for a satellite project, and this was eventually obtained in the summer of 1957.

From the successful debut of Sputnik-1 followed the whole Soviet space program. Recently the USSR has been launching satellites at an average rate of about two per week.

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above: The microwave imaging sensor shown here being adjusted at Hughes Aircraft is now orbiting the Earth at an altitude of 850 km in a US military weather satellite. Among other things it is designed to detect where rain is falling over land. With visible and infra-red imaging from meteorological satellites this is not normally possible because the rain-clouds themselves block the e.m. emissions at such wavelengths coming from below them. But the pals of the clouds not producing rain can be penetrated by the longer wavelengths in the microwave region. (In meteorology this normally means a range of frequencies at e.h.f. between about 20 GHz aid 200 GHz.) The underlying rain structure thus observed may prove useful for forecasting hurricanes and typhoons. Operated by tie US Air Force, the new metsat is also being used to measure wind speeds over the sea, moisture content in the ground and mountain snow packs, and the extent and thickness of ice coverage. Information obtained is being shared with international civilian weather agencies.

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Anglo-Japanese X-ray astronomy

The largest X-ray sensor to be carried in a satellite has just started a five-year program of celestial observations. Called a large-area counter, it is the result of a collaborative research project in X-ray astronomy by British and Japanese scientists.

In the UK, Leicester University and the SERC's Rutherford Appleton Laboratory built the detector proper, which has a sensitive area of 0.5 m^2, and its analog signal-processing electronics. In Japan the Institute of Space and Astronautical Science (ISAS) and Nagoya University provided the advanced, digital data-processing unit and the interfaces between the detector payload and the satellite. The UK work was funded by the British National Space Center.

Japan also provided the satellite itself, called Astro-C, and launched it, in February this year, from ISAS's Kagoshima Space Center at the southern-most tip of the country's four main islands. Astro-C has a fairly low elliptical orbit with an apogee of 670 km and perigee of 505 km.

X-ray astronomy started about 25 years ago, using balloons and rockets to take the detectors as far as possible beyond the shielding effect of the Earth's atmosphere. It is one of several modern branches of astronomy which have extended our knowledge of the universe by making observations at wavelengths beyond the visible part of the electromagnetic spectrum. At the long-wavelength end of the spectrum there are infra-red and radio astronomy; at the short wavelength end, ultra-violet, X ray and gamma-ray astronomy.

Satellites were a boon for X-ray observations and started to be used in 1970. Britain's Ariel-5 X-ray satellite was launched as early as 1974 and contributed to the general mapping of X-ray sources.

The significance of the large-area counter in Astro-C is that it is very sensitive and is being used for a kind of observation that has not been done before. It measuring the intensity variations of celestial X-ray sources over different intervals of time, ranging from under a second to several months. These variations provide information on the size of the sources. The size information is important in studying those X-ray sources which are associated with black holes or neutron stars. Here the X-rays are thought to be the result of dust or gas accelerating spirally into the intense gravitational fields of these centers and be coming enormously hot in the process. Temperatures of 1088 or 108 K are estimated and such high energy would result in the emission of X-rays.

Astro-C also carries two other astronomical instruments, an all-sky X-ray monitor from Osaka University, Japan, and a gamma-ray burst detector from Los Alamos, USA.

Satellite Systems is compiled by Tom Ivall

Satellite Systems Correction

To those readers who might have been worried by a reference, on p.832 of the August issue, to the apparent transference of French Guiana from South America to Africa, an apology is due.

French Guiana is quite stably based in South America. The error was not Torn Ivall's, but entirely ours.

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(adapted from: Wireless World , Oct. 1987)

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