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This condensation of an article that appeared in the June, 1957, issue of the U.S.S.R. publication Radio is timely in view of the wide interest expressed by amateurs in picking up the signals from the first satellite. It covers the general aspects of satellite travel and offers suggestions for participation by radio amateurs in the experiment. The translation is one distributed to members of the IGY technical panel on ionospheric physics.
During the International Geophysical Year the U.S.S.R. proposes to launch several artificial Earth satellites equipped with radio transmitting apparatus. Radio observation of the signals of these satellites will make it possible to obtain new data on the structure of the ionosphere, to establish with precision the size, shape, and position of the satellite's orbit, and to obtain information on the processes taking place in the satellite during its flight. Since radio amateur observations will be of a mass character they can secure extremely important data on the satellite's flight and the state of ionosphere.
The success of radio amateur observations and the value of the data they obtain will depend largely on how well the amateurs take into consideration those characteristics of reception which are associated with the unusually high altitude, unusually high speed, and other characteristics of the flight of a satellite.
Orbits of artificial earth satellites
The artificial Earth satellite will be launched with the aid of rockets which will raise it to an altitude of several hundred kilometers and then accelerate it in the horizontal direction to a speed of about 8000 m/sec. (Fig. 1), after which the rocket motors will cut off, the satellite will be separated from the rocket, and the former will move around the Earth, making one revolution in approximately one hour and a half. The satellite's orbit will be approximately elliptical in shape; the center of the Earth will be the position of one of the foci of the ellipse (Fig. 2).
Fig. 1. Diagram of satellite launching.
Fig. 2. Orbit of satellite.
After launching, the satellite will experience a slight braking action due to friction in the upper atmosphere and, as a result, its flight speed will gradually decrease; thus the flight altitude will also decrease. After several days or weeks the flight altitude will be so reduced that the satellite will enter the denser layers of the atmosphere, be greatly slowed down, and be heated by friction with the atmosphere and burn up. The braking force and, consequently, the length of the life of the satellite will depend on the density of the upper layers of the atmosphere, which is known only in the most approximate terms at the present time; therefore the data on how rapidly the satellite is braked and burns up are of considerable scientific interest.
Of particularly high value will be amateur observations at the end of the satellite's flight, to the extent that the process of entry of the satellite into the denser layers of the atmosphere may take place in regions where there are no professional receiving sets.
Region of observations of the artificial satellite
The diagram of the relative motion of the satellite and observers is shown in Fig. 3. The plane of the satellite's orbit does not participate in the rotation of the Earth, while observers on the Earth's surface move with the earth's rotation from west to east on lines shown in Fig. 3 by dotted lines. During one revolution of the satellite (approximately 1.5 hours) an observer on the equator has moved 2500 km. to the east, an observer at 45% latitude has moved 1760 km and one at 60% latitude has moved 1000 km. The northern and southern limits of observation are determined by the inclination of the orbit; the more steeply the orbital plane is inclined, the further north and south the satellite will pass in its motion. In 24 hours the satellite will make a full 16 revolutions,(1) as a result of which the Earth's surface will have been covered with an almost uniform "grid." A satellite launched in the U.S.S.R. will in its flight cover practically all the populated area of the Earth.
Fig. 3. Motion of satellite and observers. 1-Position of observer during radio pickup of rising part of orbit, 2-position of observer during second pick-up period (on ascending part of next orbit), 3-position of observer during pickup PXX on descending part of orbit, 4-observer located close to northern limit of observation.
Any observer located between the northern and southern limits of the region of observation will be able to observe the satellite, no matter what the longitude of his position; as a result of intersect its plane. At any point on the Earth south of the northern limit of the "orbital grid" and north of its southern limit the satellite will be observable twice in twenty-four hours: On the "rising" and "declining" branches of the orbit (Fig. 3). In the most northern and southern regions both observations will be combined into one.
The time during which the radio signal will be audible on one revolution will'be determined by the speed of the satellite (8 km./sec), the range of the radio facilities, and the distance of the path of the given revolution of the orbit from the observation point (Fig. 4). The average duration of one reception period will be several minutes.
Fig. 4. Duration of audibility of satellite during overhead and lateral passages.
Rotational motion of a satellite and its influence on radio reception
The highest rate of rotation of the satellite will not exceed several turns per minute. The influence of rotational motion of the satellite on radio reception is determined first of all by the design of the satellite's antennas: Sufficiently low fading results if the antenna on the satellite is so constructed that it radiates a wave with circular polarization while the antenna of the ground station is designed for reception of linear polarization. In this case reception of signals is guaranteed for almost any rotation of the satellite.
The occurrence of strong signal fading accompanying the rotation of the satellite is of low probability; more probable are some (moderate) fluctuations of the signal strength.
Radio signal fading
In addition to the above described phenomena associated with rotation of the satellite, there may occur ordinary signal fading caused by the addition of radio waves arriving at the receiving antenna by different paths (Fig. 5).
Fig. 5. Passage of signals from satellite.
The character of fading can be somewhat unusual: Since the satellite moves at a high speed, the path followed by radio waves will change rapidly. Therefore the moments when waves passing from different directions cancel each other out and the moments when the waves are additive can alternate extremely rapidly, thus fading will not be the slow oscillations in signal strength to which radio amateurs are accustomed, but instead rapid modulation of the signal with a frequency of tens or even hundreds of cycles per second.(2)
Doppler effect
The Doppler effect is such that in the case when the radio receiver and transmitter are moved closer together or farther apart the frequency of the signal arriving at the radio receiver varies in proportion to the rate of movement together or apart.
When the movement is together the frequency of the signal increases, and when apart the frequency decreases. An approximate graph of the variation of the radio signal frequency with time is shown in Fig. 6.
Fig. 6. Graph of frequency variation (Doppler effect) depending on distance along Earth's surface between observation point and plane of orbit.
The rate or variation of the frequency in the period of flight past the receiving point depends on the distance at which the satellite passes; the closer the satellite approaches the receiver, the more rapidly the frequency varies from maximum to minimum (see curves in Fig. 6).
The whole period of frequency variation occupies only two or three minutes; if the heterodyne of the receiver is sufficiently stable(3) and during the time of reception does not become detuned, the Doppler effect can be easily detected and recorded. This provides important data on the position of the orbit relative to the receiving point. At the beginning of reception of radio signals from the satellite the heterodyne must be tuned so as to take into account the fact that the frequency of the tone at the middle of the reception period varies by approximately 2000 c.p.s. (for 40 Mc.) and approximately 1000 c.p.s. (for 20 Mc.); subsequently, during the remainder of the listening period the tuning of the heterodyne should not be changed. (Note: It should be remembered that, if the heterodyne is tuned below the carrier frequency, then the frequency of the audible tone will be decreased while, if the heterodyne is tuned above the carrier frequency, the frequency of the tone will rise).
Use of radio amateur observations for precise establishment of the orbit
The task of precise determination of the orbit is distinguished from the task of determining the trajectory of an aircraft by radar, for example, principally the fact that in our case we know ahead of time that the satellite is unable to perform arbitrary motions in space, and for given initial data it can move only along a completely predictable trajectory. This circumstance permits the use of more simple measurements than in the case of radar. For example, if the position of the satellite has been accurately found by bearings from five or six points and the accurate time of these bearings has been established then the position of this orbit can be calculated with an accuracy sufficient for practical purposes.
For determination of the orbit, Doppler effect recordings can also be used (Fig. 6). With these it is possible to determine the distance at which the satellite passes and the moment of time when it is at the minimum distance.(4)
Therefore, in order to use radio amateur observations, it is extremely important to have recordings of signals on magnetic tape which can be used for, in the first place, measurement of the Doppler effect and, in the second place, "tying down" of the recording obtained to the exact time. From the duration of tones and pauses information can also be obtained on some processes taking place in the satellite itself.
Highly qualified radio amateurs and radio clubs can also build apparatus with which to take direction bearings on the satellite. The moment of direction finding also must be "tied down" to the exact time.
It must be noted that, in order to check the orbit, the signal with the frequency of 40 mc. is of greatest value, since it is less distorted by passing through the ionosphere.
Notes
- The actual figure for the first satellite turned out to be almost exactly 15, the period being a few seconds over 96 minutes. - Editor. the Earth's rotation he will sooner or later approach the orbit and
- A type of fading similar to that known to amateurs as "auroral flutter" when associated with v.h.f. auroral propagation and, on lower frequencies, magnetic disturbances. - Editor.
- I.e., over-all stability of the order of 10 cycles or less during the period while the signal is audible. Crystal-controlled oscillators are desirable. - Editor.
- For additional discussion on Doppler effect see letter from Paul E. Wilkins, W4SBA, in "Technical Correspondence," QST, October, 1956, page 46. - Editor.
V. Vakhnin.