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This second in a series of International Geophysical Year (IGY) articles that appeared in Radio-Electronics magazine. The author covers basics of satellite configuration, launching, and tracking based on knowledge of the era. Keep in mind, though, that the U.S. had not actually launched its first satellite at the time. In fact, the two satellite models shown possess antennas suggesting active radio circuits within, but Echo, our first earth-orbiting satellite, was just a metallized plastic sphere that passively reflected radio signals. The Russian Sputnik, by comparison, did have electronic circuitry onboard.
|Electronics and the IGY - Part I, Electronics and the IGY - Part II, National Bureau of Standards' Role in IGY, How We Listen to Stars and Satellites, Radio Waves, Sunspots, and Planets|
Part II: - What part will earth satellites play in the International Geophysical Year?
By Jordan McQuay
In the search for scientific data during the IGY no single activity has stirred the imagination and interest of the world more than the earth satellites - the rocket-borne metal spheres sent into outer space to circle the earth, whirling free through an orbit in upper atmosphere. Long considered a theoretical possibility, it was not until recent development of rockets and missiles of tremendous size that this dream has become a reality.
In October, 1957, the first satellites were launched by Russia. A series of satellites will be launched by the United States during the late winter and spring of 1958.
Weighing about 185 pounds and about 2 feet in diameter, the Russian type of satellite is launched into space by multiple-stage rockets of tremendous size and thrust. Once it overcomes the gravitational pull of the earth at an altitude of several hundred miles, the satellite circles the globe in an orbit 200-400 miles above the earth at about 18,000 miles an hour. Altitude gradually drops over a period of several weeks or months, and the satellite eventually disintegrates due to air friction.
Although technical details of the Russian satellite have not been revealed, it contains a radio transmitter which broadcasts a coded signal (on 20 and 40 mc). Since it is powered by some sort of miniature storage battery, failure of the power supply after 4 or 5 weeks means the satellite moves silently through its orbit until it disintegrates. General theory behind the Russian satellite, however, is much the same as that of the several types of satellites soon to be launched by the United States.
High-altitude rockets, described last month, provided the first direct experimental observations of the upper atmosphere. These included data on air pressure, density, temperature, composition, wind fields, cosmic rays and other solar activities. A major limitation of these rockets is the brief period of time during which the measurements could be taken - usually for only 6 or 7 minutes. Rocket coverage is also restricted to a small part of the earth's atmosphere.
An artificial satellite, propelled into space and whirling in an orbit far above the earth, will provide weeks, months, even years of continuous, reliable data for scientific study. Also, the satellite traverses a vast amount of interplanetary space during each revolution around the earth and thus collects a great amount of geophysical information.
Once in its orbit, the satellite's velocity is such that its centrifugal force balances the earth's gravitational pull. Without additional propelling power, the satellite continues to circle the earth, making a complete revolution about once every 80 or 90 minutes.
The principal problem is launching the satellite and propelling it upward into its orbit. This is done by transporting the satellite in the nose of a multistage rocket which has sufficient power to carry the satellite into the upper atmosphere.
The United States uses a three-stage rocket (Fig. 1). It is 72 feet long and is launched vertically. Finless, it uses internal electronic controls for guidance. The trajectory of the rocket is shown in Fig. 2.
When fired, the first stage of the rocket thrusts the entire assembly upward almost vertically. It then tilts slightly until, at burnout, the rocket is inclined at about 35°. Then, its fuel exhausted, the first stage detaches itself from the rocket assembly. The second stage then drives the rocket to an altitude of about 140 miles, propelling it at a rapidly increasing speed to about 2,000 miles an hour, and - through electronic controls - diminishes the angle off inclination to only a few degrees. As the rocket levels off and coasts for some distance, the second stage detaches itself and ignites the third and final stage of the rocket. The third stage carries the satellite to its ultimate altitude of several hundred miles and to its top speed of about 20,000 miles an hour. The satellite separates from the third stage and, established in its orbit, continues under its own momentum - about 1,500 miles from the launching site and about 10 minutes after launching. Because of its extreme speed at time of separation, the third stage may continue to orbit somewhere in space behind the satellite. After some time, however, the third stage will drop in altitude until it disintegrates in more dense atmosphere.
Although a satellite may continue to circle the earth for protracted periods of time - several weeks, months or longer - ultimately atmospheric drag will bring its orbit closer and closer to the earth. When it enters the denser atmosphere of lower altitudes, the satellite (due to air friction) will burn out far above the earth's surface. Both it and burned-out stages of the rocket will drift to earth as indistinguishable dust and ashes.
While in flight, the tiny satellite broadcasts a periodic signal giving the specific data it measures - such as air density, pressure, temperature .or solar activities. The radio transmitter in each US satellite weighs about 13 ounces and has a 10-mw output at a fixed frequency of 108 mc. It is crystal-controlled and completely transistorized. Some types of satellites may have transmitters powered by seven 1.2-volt miniature batteries. Others will be powered by solar batteries, which give the transmitter a continuous life until the satellite eventually disintegrates.
The type of data transmitted by a satellite depends upon the instruments contained within its spherical metal shell. Measurements are fed to electronic telemetering equipment, which translates them into coded signals. Then the radio transmitter broadcasts these signals to ground tracking and observing stations.
Tracking the Satellites
Once established in its orbit, a satellite must be tracked - both optically and electronically - to provide position and path information to correlate with other readings and measurements. With previous knowledge of the probable path of a satellite gained through electronics, observers at ground stations can use photo-theodolites for optical tracking. This method of tracking, however, depends upon fair visibility for good accuracy.
A more reliable method of tracking is the Minitrack system, which utilizes radio receiving equipment. The radio transmitter within the satellite produces a periodic signal at 108 mc, which is radiated by small antennas outside the metal sphere. On the ground, the signal can be detected by highly sensitive receiving equipment whenever the satellite passes in the general vicinity of a receiving station.
Orbits of all US satellites are expected to have a latitude range of about 35° above and below the equator. Within this broad belt, Minitrack stations have been erected by many governments at strategic points around the world. Although development and launching of the US satellite is primarily a contribution of this country to the IGY, all countries are participating in observing and measuring data obtained by each of the US satellites.
Each ground station of the Minitrack system is equipped with several sets of two specially designed and highly balanced receiving antennas, a frequency converter, a high-gain amplifier and a visual recording device. When tuned to the 108-mc frequency and with the satellite within receiving range, there will be an indication on the output recording device - the amount depending on the proximity of the satellite to the station. The satellite can be located in its orbit by comparing the signal from one antenna with the signal from the second antenna of each set. This is equivalent to comparing the path length of the signal from the satellite transmitter to one receiving antenna with the path length of the signal to the second antenna of each set of matched antennas. Similar measurements with other sets of matched antennas at the receiving station will fix the satellite even more accurately.
A simplified version of the Minitrack system can be used by radio amateurs residing in the region to be covered by each US satellite. As shown in Fig. 3, as few as two balanced antennas are connected via a frequency converter, to the input of a conventional communications receiver. An S-meter or other visual indicating device is used at the receiver's output. As the satellite passes over the vicinity of the station, the receiver output varies from a minimum to a peak. The maximum reading determines the general position of the satellite. It recurs about every 90 minutes.
The equipment at each Minitrack station is considerably more complex and provides a high degree of precision measurement. In addition, the satellite signal is continuously recorded at each Minitrack station. In event of failure of the satellite's radio transmitter, ground-based radar equipment tracks the satellite.
All tracking information is transmitted to key or central stations. There, using instantly available data from all reporting sources, electronic computers calculate orbital information and predict the exact path of any satellite for each successive revolution. This prediction includes the time and place of meridian passage, the zenith angle and the angular velocity of a satellite in its orbit. With each successive revolution, these data are reevaluated and recomputed, and new predictions are made by electronic data processing and computing equipment.
Data from the Satellites
As each satellite travels through space, specialized types of scientific data are also obtained and measured by instruments within the sphere and then transmitted to ground stations for collation and record.
These specialized data may relate to the sun's ultraviolet rays, meteor particles, air density, cosmic rays, the ionosphere or any of many other fields of scientific endeavor during the IGY. The type of data obtained and telemetered to earth by each satellite depends entirely upon the type of instruments within the satellite. The instrumentation is usually different for each of the US satellites, depending upon its mission. By nature of their movement in space, however, all satellites provide important scientific data concerning air density, the shape of the earth, the ionosphere and other scientific fields.
Since the orbit of a satellite is influenced by local non uniformities in the gravitational field, observations of the orbit at ground tracking stations make possible calculations of mass distribution of the earth. This, in turn, yields information about the composition of the earth's crust. Similar information, after electronic analysis, also provides data about the oblateness or flatness of the earth near the poles. Orbit observations also make possible precise determinations of latitude and longitude, particularly for isolated islands, many of which in the Pacific have never been located and mapped accurately.
Since radio signals from a satellite are affected as they pass through the ionosphere, this phenomenon permits measurement of refraction as well a other characteristics of the ionosphere. Such measurements are important to the study and prediction of radio-wave propagation.
All types of data collected and recorded during the IGY - by Minitrack, optical and other tracking stations as well as scientific observing stations - are transmitted to key or central stations. There the various data are fed to electronic data processing equipment for immediate or future reference.
From this mass of accumulated and correlated information, detailed an accurate scientific data can be compiled electronically and almost instantly months, even years, after a satellite has completed its flight through interplanetary space.
During this winter and spring, the United States will launch four miniature satellites. These are trial flights primarily to test the Minitrack and rocket-launching systems. Each of the test satellites is about 6 inches in diameter - the size of a grapefruit. Each has six protruding antennas and contains a tiny radio transmitter powered by solar batteries to convert energy from the sun into electricity. (See Fig. 4.) Each will be launched by a conventional high-power three-stage rocket.
Larger satellites, to be launched during this summer, will be equipped to obtain specialized scientific data. These are about 20 inches in diameter and weigh 21 pounds. Each is equipped with a transmitter and has four protruding antennas to radiate data to the ground. (See Fig. 5.)
The first of the large satellites will carry instruments to study the sun's ultraviolet rays and obtain environmental data. Succeeding satellites will record erosion of meteor particles in space, measure air density and composition, the earth's magnetic field, cosmic rays, and obtain other scientific data.
There are numerous other studies of scientific significance during the IGY. Simultaneous studies in oceanography and glaciology are exploring the heat and water interrelationships that also affect the earth's weather and climate. A study of seismology leads to new knowledge of the earth's core and crust. Gravity measurements and related studies are also part of the activities of all participating countries. But electronics assists to only a very small degree in these international ventures.
Electronics, however, is widely utilized in most of these studies for recording, filing and storing the wealth of data obtained.
At key control centers throughout the world, the latest types of electronic data-processing equipment handle, record and store the vast amount of data collected continuously during the IGY. Electronic computers are utilized for fast computation and analysis. Data is recorded on punched cards or on metallic tape, then filed in electronic storage memories for future reference. This makes the handling of billions of items of measurement and observation largely automatic - through electronic processing.
The IGY is destined to yield unprecedented knowledge about the mysteries of the earth and the atmosphere and their relationship to the sun. In geophysics, the universe itself performs the experiments in which mankind is interested. The events that determine our physical environment are therefore worldwide in nature, and only through the cooperative efforts of all countries can their secrets be discovered.
Thus, through the joint effort of many nations, the International Geophysical Year is not only an expression of the scientific interests of various countries, but the scientific community of the world as a whole.
March 1958 Radio-Electronics[Table of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio-Electronics articles I have already posted. All copyrights are hereby acknowledged.
Posted February 27, 2014