Receiving U.S. Satellite Signals, March 1958 Radio News

In 1958, most people were not accustomed to seeing the now-familiar maps plotting sinusoidal courses of satellites across the face of the earth (a la Figure 1). It had only been in October of the previous year that any object other than the moon was in orbit around our home planet - that was U.S.S.R.'s Sputnik. Just as people of all ages and all backgrounds enthusiastically joined in the newfangled phenomenon of aeroplanes after the Wright Brothers flew their fragile craft at Kitty Hawk, electronics communications and scientists worldwide hopped aboard the satellite train (so to speak). This article from a 1958 issue of Radio & TV News magazine provided insight into the construction and flight characteristics of early U.S. satellites, and offered advice on how to participate in the ongoing International Geophysical Year (IGY) research effort by tuning in and reporting your signal reception characteristics. Activity was not just the domain of operators with sophisticated equipment, as the author outlines. To wit, "Signals from these vehicles can be received by radio amateurs, hi-fi enthusiasts, or electronic technicians without great difficulty." See "Magnetometer at Work in Outer Space."

Satellite with upper hemisphere removed is shown here. The complete electronic package with all its modules labeled is below.

The complete, authoritative story about the radio signals that the U. S. satellite will broadcast.

Electronic technicians and hobbyists have an opportunity to make valuable contributions to the scientific effort of the current International Geophysical Year. Plans for launching instrumented artificial earth satellites have been announced by the United States and the U.S.S.R. Signals from these vehicles can be received by radio amateurs, hi-fi enthusiasts, or electronic technicians without great difficulty. Participation of amateur observers would make available potential sources of valuable data which would be prohibitively costly by means of primary recording stations.

Editor's Note: As we go to press the Army, as was expected, has launched the "Explorer," the first U. S. satellite. The fully instrumented satellite described here and shown on our cover is part of the Navy's Vanguard project and is scheduled to go up in March. Much of the material contained in this article applies to any of the U. S. satellites.

Not all of the satellites to be launched by the United States will provide continuous transmission of scientific data. Early satellites may be placed in orbit as a part of the launching vehicle test program. These possible early satellites would carry unmodulated radio transmitting equipment intended primarily for radio tracking. While not regarded as true instrumented satellites, they would be capable of providing temperature information by accurate calibration and measurement of transmission frequencies. Several designs of fully instrumented scientific satellites are being prepared which will carry aloft, in succession, various combinations of scientific instruments. As presently planned, the first two instrumented satellites to be launched will provide continuous transmission of scientific data. It is these which would be of most interest to amateur observers. Designs for later launching will provide data transmission only when initiated by powerful ground transmitters. This technique is used where power required by scientific instruments prohibits continuous transmission or where it is necessary to play back and erase information recorded on magnetic tape. Amateur observers can participate in collecting the intermittently transmitted data only when located sufficiently close to a primary recording station to receive signals when data is being transmitted.

The U. S. satellite will be launched from the Patrick Air Force Base at Cape Canaveral in Florida. The satellite will circle the earth approximately once each 90 minutes. The plane of the orbit will make an angle of approximately 35° with respect to the equatorial plane of the earth. Due to the rotation of the earth, each successive orbit will receive an apparent shift westward of about 1200 miles. The general character of the path of the satellite over the surface of the earth is shown in Fig. 1. Eventually the satellite will rather thoroughly traverse the unshaded portion which represents nearly two thirds of the earth's surface area. Near overhead transits of the satellite would thus be available to observers between 35° N and 35° S latitudes. Reception of signals outside this zone would be determined by receiver sensitivity, antenna gain, and height of the satellite above the earth. The orbit of the satellite is expected to be elliptical with altitudes between 200 and 1500 miles.

Fig. 1 - Some typical paths of artificial earth satellite over the earth's surface.

Fig. 2 - Satellite airborne telemetry encoder is at left. Center unit is the meteoritic collision electronics with its amplifier and three decimal digit counter. Unit at right is Lyman-alpha peak memory electronics including orbital switch for reset once each orbit. Circuits are printed and extensive use is made of transistors and magnetic amplifier coils. Units later covered with plastic foam.

Provision for an adequate number of official recording stations to permit continuous radio reception of satellite signals would be prohibitively costly. Primary recording station locations are also shown in Fig. 1. A chain of stations is located roughly along the 75th meridian down the east coast of the United States and the west coast of South America. These stations provide a "picket fence" coverage which the satellite must cross once each orbit. Two stations at Antigua, B.W.I. and San Diego, California provide data early in and before completion of the first orbit. One additional station at Woomera, Australia will provide frequent data at approximately half orbital periods from other stations for the benefit of scientific experimenters. Orbital parameters will be calculated from tracking data and made available to news wire services. Contact with local newspapers, television or radio stations should provide adequate information to permit amateur observers to know when to seek these signals, Possibilities of visual observation of the satellite are limited to times just before dawn or after dusk. The sun must be sufficiently below the horizon to provide a dark sky yet sufficiently near to illuminate the satellite.

All scientific data will be transmitted by means of the "Minitrack" radio tracking transmitter in the satellite. This transistorized, crystal-controlled transmitter operates on a carrier frequency of 108.00 megacycles and is amplitude modulated (AM) for data transmission. Peak operating power is about 100 milliwatts with 100% modulation. Batteries carried by the satellite are expected to provide between three and four weeks' active life. Airborne telemetry equipment is used to accept inputs from the various scientific instruments and combine them into a single coded electrical signal for transmission to the ground receiving stations.

Telemetry signal frequency components are in the audio frequency range and within the recording capability of many high-fidelity home magnetic tape recorders. Maximum frequencies of 15 kc. are used with signals rarely exceeding 12.5 kc. Two alternatives are. available to persons not now having equipment to receive 108-mega-cycle AM signals who wish to observe the satellite with their present equipment. Converters are available or can be built by owners of communications type receivers. This method offers the advantage of being able to use a beat-frequency oscillator to detect passage of a satellite transmitting an unmodulated carrier. The second method requires the addition of a simple AM detector circuit to a high-fidelity FM tuner. Commercially available antennas designed for fringe area reception of FM broadcast signals should provide ample gain for all but the most unfavorable reception conditions. Desirable mounting of such an antenna would be in a generally skyward direction for latitudes of 35° or less. Simple means could be provided for rotation of the antenna in a direction to permit adjustment in advance for best reception of the next anticipated transit.

This Month's Cover

The top half of the first fully instrumented U. S. satellite is carefully being put into place. The 20-inch, 21.5-pound shiny ball will then be mounted within the nose of the three-stage Vanguard rocket, shown blasting off on our cover. The rocket is then expected to rise vertically at first and then gently turn in the direction of its planned orbit. After successive burnout of the three stages, the tiny ball should be hurtling around the earth at about 18,000 miles per hour, (Photos: U. S. Naval Research Laboratory.)

Many of the scientific phenomena to be studied occur infrequently and at random. These very facts prevent their systematic study by high-altitude research rockets. Most desirable would be the occurrence of one of these events as the satellite signal was being recorded at a primary receiving station. Such an observation would provide a detailed record of instantaneous measurements as a function of absolute time. Although possible, unfortunately the chances for an official receiving station to record such an event are quite remote. An adequate number of properly located amateur observers would make certain the availability of the desired records. Airborne equipment will include memory devices for storing a limited amount of information concerning these events for later transmission to official ground stations. These signals will provide meager data on all such events viewed by satellite instruments but will be far less informative than a properly made amateur recording obtained during the event itself.

Recordings of amateur observers offer many possibilities of obtaining valuable data other than observation of specific events. This is particularly true of data obtained during the early life of a satellite. Obviously such data would be of utmost value in case of a short active life of a satellite or any of its scientific experiments. Such records would also be of value in case of errors in the predicted rate at which phenomena being studied are expected to occur. Details regarding potential value of amateur recordings will be presented later in the discussion of each experiment.

One of the first fully instrumented scientific satellites planned for launching by the United States will be devoted to two general types of measurements. One group of measurements will study ultraviolet radiation from the sun in the region of the solar spectrum known as Lyman-alpha. The second group of experiments designed to study the satellite environment includes measurement of temperatures and various types of information relative to collision between the satellite and small particles (micrometeorites) in space. Equipment in the satellite will include a telemetry encoder system. This equipment converts measurements made by the various airborne instruments into a single signal for transmission to ground receiving stations. The desired scientific information may be extracted from the received signal by appropriate decoding.

Fig. 3 - Here the entire electronics package is being installed in the satellite. The electronic circuits are encased in a foam-in-place plastic material.

Fig. 4 - Typical Lyman-alpha satellite scientific telemetry signal with all components of signal properly labeled.

All information will be transmitted in the form of a series of high-frequency (5 to 15 kc.) bursts carrying information on the frequency of these bursts, time duration of each burst, and the time interval between bursts. Thus three information channels are associated with each burst. Each frame or scan of all information channels will consist of a sequence of sixteen such bursts for a basically 48-channel telemetry system. The airborne telemetry encoder system used to produce these signals is shown in Fig. 2. This unit is 5 1/2 inches in diameter, 3/4,-inch high, and weighs about 3 1/2 ounces. It operates on 2.7 volts with a current drain of 3 to 4 milliamperes. A new frame will begin immediately upon completion of each scan. Thus the exact scanning rate will be a variable quantity determined by the values of input signals which control burst duration and spacings. Signal values are expected to lie in a range which provides three to four frames of telemetered data per second.

Of the measurements being made, some will undergo only insignificant short time changes while others will be changing rapidly with time. For example, outer shell temperatures will rise slowly as the satellite heats up when in sunlight and slowly drop as the shell cools when in the earth's shadow. Thus, temperature changes during one telemetry frame are of no great importance. On the other hand, measurements of inputs such as the Lyman-alpha radiation do undergo significant changes during a single telemetry frame. For this reason, several channels in each frame are devoted to frequent repetition of these rapidly changing signals. Such signals are also transmitted as a burst frequency to permit measurement of changes during the burst interval.

An oscillogram of a complete typical telemetry frame is shown in Fig. 4. Photographic resolution at the scanning rate used to present a full frame does not permit distinction between cycles in each high-frequency burst. This oscillogram, together with Table 1, will permit ample description of channel assignments to permit amateur observers to seek out any desired signal for study. It will be noted that all channels carrying information in the form of the burst frequency are identified by a single letter. Channels whose data is presented as a burst duration are designated by a letter followed by the numeral 1. Similarly information contained in a burst spacing carries a letter identification followed by the numeral 2.

Each telemetry frame may be conveniently regarded as being divided into two sequences of eight high-frequency bursts each. Each such half frame is distinguished by six initial bursts containing an alternate rhythmic repetition of long and short bursts carrying data on instantaneous solar Lyman-alpha radiation and solar aspect (orientation of the satellite with respect to the sun). Thus each half frame can be seen to start with the burst channel sequence A-B-A-B-A-B in Fig. 4 or Table 1. Identification of these channels is relatively easy in any display medium whether it be audible listening, oscillographic presentation, or photographic reproduction. The remaining two high-frequency bursts in each half-frame scan will carry data readout of satellite memory units. The two final bursts of successive half-frames will thus alternately present information on (1) units digit and tens digit of the cumulative meteoritic collision count (channels C and D) or (2) hundreds digit of the meteoritic collision count and orbital peak value of solar Lyman-alpha radiation.

Table 1 - Telemetry channel assignments for Lyman-alpha-environmental satellite.

Positive identification of alternate half-frames is provided by appropriate control of burst and spacing durations. Thus the duration of the meteoritic collision count units digit burst (channel C) is used to present the long calibration channel (C1). This burst duration remains essentially unchanged and is always of greater duration than any other burst. This long calibration channel is followed immediately by the shortest burst spacing in the frame (channel C2). This shortest burst spacing monitors the telemetry battery voltage. This identification of the telemetry signal half-frames is evident from an examination of Fig. 4. The long calibration channel C1 in the first half-frame is obviously longer than its counterpart channel E1 in the second half-frame. Similarly the succeeding channel C2 is much shorter than is channel E2.

Methods used by amateur observers to study records made will be determined, to a great extent, by the equipment available. Experience has shown that, with practice, a simple listening test can reveal valuable information. Listening tests are greatly enhanced when original recordings are made at a high tape speed and played back at a lower speed. This technique reduces the high frequencies for improved audibility and at the same time increases burst durations and spacing for easier resolution.

Approximate calibration curves for the various information channels are shown in Fig. 5. These charts should be regarded as general guides only since variations can be expected between various telemetry assemblies. Precise measurements can only be made by use of individual calibration for the specific equipment flown in any satellite. Obviously this cannot be known until a satellite is launched since last minute equipment substitution may be required. The curves shown should however prove adequate for most needs of amateur observers.

Solar Lyman-Alpha Radiation: Lyman-alpha radiation from the sun refers to a very limited portion of the ultraviolet region of the solar spectrum. Measuring devices sensitive only to this radiation are carried. The earth's atmosphere is quite opaque to such radiation and thus prohibits its measurement from ground based stations. Satellite or rocket vehicles must therefore be used in any such studies. Much has been learned by means of research rockets but this technique has many limitations. Time required for rockets to reach altitude and the short time they remain aloft have effectively hampered study of transitory Lyman-alpha phenomena.

Telemetry signals will contain two types of presentation of data relative to solar Lyman-alpha radiation. The first type of presentation will transmit an instantaneous value of this radiation at all times. Study of these signals will permit study of this radiation from a quiescent sun. Short time variations in this signal are of great importance. Not only may short term changes take place in the signal from the sun but will additionally be varying due to spin of the satellite itself. Thus the detectors will spend approximately two thirds of each spin period in the shadow of the satellite and the telemetry signal will be that indicating no input to the detector. Adequate study thus requires a great portion of the telemetry time to be devoted to instantaneous values of solar Lyman-alpha data. From Fig. 4 it can be seen that this is accomplished in two ways. First, this signal (channel A) is repeated six times in each telemetry frame. Second, each presentation is continued for a comparatively long period of time. Any single such display is normally only exceeded in length by the long calibration and identification channel C1.

Fig. 5 - Some typical Lyman-alpha satellite calibration curves are shown here.

The second type of solar Lyman-alpha radiation signal transmitted represents an orbital peak or maximum value (channel F). Marked increases in solar Lyman-alpha radiation are expected to take place during periods of solar flares. Solar flares take place infrequently and at random and are of relatively short duration. While possible, it is quite unlikely that a solar flare will take place as the satellite is passing over a primary recording station. While leaving much to be desired, correlation between visually observed solar flares and orbital peak values of Lyman-alpha signals will permit collection of valuable data. Two magnetic memory units will be used in storing and transmitting peak orbital values of these signals. One memory unit will be storing information on the current orbit while the second memory unit is transmitting the peak value stored during the immediately preceding orbit. Once each orbit, on transition from darkness to daylight, a light-sensitive device is used to interchange the function of the two memory units. The unit which has been transmitting has its previous memory cancelled and begins to store data during the next orbit. Simultaneously the unit which has been storing information starts to transmit its stored data. Since the technique used permits the transmitted signal to change only once each complete orbit, adequate utilization of this data is expected from official recording stations. Two calibration curves are shown for peak Lyman-alpha data in Fig. 5. Different frequency ranges are used for readout of the two memory units. This permits individual calibration as well as a check on proper functioning.

Opportunity does exist for amateur observers to make a major contribution to the solar Lyman-alpha radiation experiment. Valuable indeed would be a properly prepared amateur recording showing instantaneous values of Lyman-alpha radiation during the progress of a solar flare. Recordings of interest would be any showing an electrometer current in excess of 200 microamperes. By use of the calibration curve for channel A of Fig. 5, this can be translated into a telemetry signal frequency. Thus, it can be seen that recordings of value would be those in which the frequency in any Lyman-alpha burst momentarily drops below 10 kc.

Precise measurement of these signals requires elaborate equipment. Amateur observers can make reasonably accurate determinations from simple listening tests. A little practice will permit rather easy identification of the Lyman-alpha signal burst channel. This information is distinguished as the first and longest of the often repeated rhythmical alternation between channels A and B. It should be pointed out that recordings played back at reduced speed will have the desired signal frequencies correspondingly reduced. Thus a tape played back at one half of the recording speed would be of interest if the Lyman-alpha burst frequency dropped below 5 kc. It is suggested that a comparison test tape be prepared for ease in identifying valuable signals. An audio oscillator would be used to record a 10 kc. signal at the same tape speed planned for recording satellite signals. This tape would then be played back at the same speed used for listening tests. This procedure provides automatic compensation for changes in tape speeds.

Solar Aspect: A silicon solar cell located on the satellite equator will provide information on the orientation of the satellite with respect to the sun. This data is required by the Lyman-alpha experimenters for detector sensitivity calibration. It will also provide accurate information on satellite spin and spin damping. This signal is presented six times each telemetry frame on channel B. Calibration is shown in Fig. 5.

Fig. 6 - This is a portion of the telemetry record obtained from a rocket flight. The record shows the collision of the rocket with a micrometeorite.

Erosion of the surface of the satellite will be studied by means of small erosion gages attached to the outer shell. These gages consist of small glass plates on which a thin-film metallic resistor has been deposited. The telemetry system will detect the wearing away of this metallic film by monitoring changes in resistance during this process. Reference to Fig. 5 shows two types of gages are used for different sensitivity ranges. Thus a polar erosion gage providing information on channel A1, consists of a very thin film having an initial resistance of 20,000 ohms. Thicker films on gages represented by channels E1 and F1 provide an initial resistance of 2000 ohms. Resistance of all gages will increase as erosion takes place. At anticipated erosion rates, ample information is expected from primary receiving stations. Thus the resistance values are expected to undergo only minute changes during intervals between official observations. Properly spaced amateur recordings could provide vital information in the event of early complete breaks in the metallic film. Amateur recordings could, under these circumstances, provide the only source of information to distinguish between an abnormally high but uniform erosion or collision with a single particle (s) of sufficient size to result in destruction of gages on first impact.

Another type of detector for meteoritic collision experiments uses a cadmium-sulphide cell covered by an opaque layer. The cadmium sulphide is photoresistive in that its electrical resistance decreases when exposed to light. This cell is initially protected from sunlight by the opaque cover. As

meteorites puncture the protective layer, sunlight will be permitted to reach the sensitive element. When the satellite is in sunlight, the resistance change will reflect the amount of the opaque layer removed. The photoresistive element is of a selected type having a long time constant. Thus the resistance changes will take place only slowly so as to minimize effect of satellite spin. This information is displayed on telemetry channel F2. No calibration for this channel is given in Fig. 5. This channel will initially be at its high resistance value presenting a burst spacing of about 11 milliseconds. As the opaque layer is punctured this interval will become progressively shorter during the sunlit portion of each orbit.

Amateur recordings could provide much valuable information for this experiment. Adequate coverage could provide accurate timing of collisions detected. Such recordings could also permit the distinction between single collisions with large particles and multiple collisions with small particles removing the same area of cell cover material.

This experiment is designed to detect collision with particles of sufficient collision energy to puncture the outer shell. Two pressure-tight zones are incorporated into the satellite outer shell. Prior to launching, these two zones are initially pressurized to significantly different pressures. A single differential pressure measurement made between the two zones can indicate the puncture of either or both zones. With both zones intact, the differential pressure gage would indicate the initial value. When either zone becomes punctured its internal pressure will equalize with the near perfect vacuum of the satellite environment. Thus with either one of the two zones punctured, the differential pressure measurement will display the pressure of the zone remaining intact. When both zones become punctured, the differential pressure will be zero since both zones will be evacuated to ambient atmospheric conditions. Amateur recordings could assist in more precisely determining the time at which punctures occurred. If both zones were punctured in an interval between primary recordings, amateur observations would be the only potential source of information to permit determining whether separate punctures occurred or if a single particle punctured both zones. This data is presented on telemetry channel B1.

Sensitive microphones attached to the outer shell of the satellite are used to detect collision with small particles in space. Magnetic memory units in the satellite are used to store and continuously transmit information on the cumulative number of such counts detected. Three channels of the telemetry system serve to provide counting data in the form of three decimal digits. Consider, for example, the telemetry channel C which provides units-count data. As can be seen from Fig. 5, the frequency in this burst increases from about 5 kc. to 12.5 kc. in discrete intervals. One such frequency change is made for each input count up to nine. Upon arrival of the tenth count, channel C returns to 5 kc. ("zero" units count) while the frequency in channel D makes the appropriate increase to indicate the tens count; i.e., the number of times the unit count has returned to zero. Similarly, channel E frequency will present hundreds digit information by advancing one "count" for each reset of the tens digit counter. This can be seen to be similar to the operation of the mileage indicator of an automobile speedometer. The units counter advances one digit for each mile traveled. For each ten miles the units digit returns from nine to zero while one count is added to the tens digit. The nature of these signals can be further illustrated by reference to Fig. 6. The two oscillograms show similar portions of two successive telemetry frames. Both display the final portion of the units count and the complete tens count burst. The signals shown were extracted from records made during a rocket flight in which satellite instrumentation equipment was being test flown. The brief interruption of the tens burst is the equipment "dead time" which occurs whenever a count is being received. This interruption is many times shorter than the shortest burst spacing and would not be confused by a practiced observer. The upper trace thus shows a time interval including an actual collision of the rocket in flight with a meteoritic particle. The unit count burst in this upper trace is at its highest (12.5 kc.) frequency representing a count of "nine," while the tens count starts at a slightly lower (12 kc.) frequency representing a count of "eight," thus indicating a cumulative count of 89. The hundreds count is not shown but is unimportant in this illustrative discussion. Following this collision the frequency of the tens count burst has increased to 12.5 kc. representing a count of "nine" in the tens digit. The bottom oscillographic trace shows indeed that the units count has returned to its low (5 kc.) frequency to indicate a "zero" units count while the tens count remains unchanged at its "nine" count for a cumulative count of 90.

No really satisfactory estimates are available as to the number of such collisions to expect from a satellite in orbit around the earth. Counts of a few hundred or less per orbit can be studied by means of the difference in cumulative count between recording periods. The information transmitted presents total counts only up to 999 before returning to 000 to begin a new sequence. Therefore, high counting rates extending into thousands of counts per orbit cannot be studied in this way. Such large counting rates would be studied by change in total count over shorter time intervals, such as the signal reception interval for a single receiving station or occasional satellite transits which provide less than full orbit intervals between recordings.

Many potential contributions could be made by amateur observers in the study of collision between the satellite and micrometeorites. For example, in a single orbit, a series of amateur records from properly distributed geographical locations could establish whether particles are distributed at random or clustered in clouds. Amateur recordings would also be of great value in the event of unanticipated high counting rates. Ample data could also provide clues as to the possible origin of such particles. Study of micrometeorites by means of satellites is a new field with most meager background information upon which to base estimates and plan experimental techniques. The subject is expected to be much better understood after the present satellite experiment has been completed. Extensive useable records from a single successful satellite would provide a sound basis for an exhaustive scientific study and analysis.

Measurements of the temperature of the outer shell of the satellite are made at two points. One temperature sensing element is located near one satellite pole; i.e., near the spin axis. The second sensing element is located near the equator of the satellite; i.e., the plane perpendicular to the spin axis passing through the center of the sphere. Data on these two measurements are transmitted as burst spacings on channels D2 and E2 respectively.

Virtually all heat transfer to and from the satellite in orbit will take place by the process of radiation. Ambient atmosphere at satellite altitudes is so rarefied as to make negligible any aerodynamic heating or heat transfer to or from this atmosphere. The satellite will be heated by radiant energy received primarily from the sun and the earth. Some heating will also be derived from power dissipation of internal instrumentation equipment. The satellite will be cooled by radiating energy into space. Satellite temperatures can be controlled to a certain extent by means of coatings applied to the outer surface. These coatings permit selective control of the ability of the satellite to radiate heat into space and its ability to absorb radiant energy. Thus a satellite which was an efficient radiator and a poor absorber would tend to run cold and vice versa. Precise temperature control is not possible since coatings are applied prior to launching on the basis of assumed values of many factors which affect the satellite temperature. For example, heating of the satellite due to radiation from the sun will depend upon the length of time the satellite remains in sunlight in each orbit. Variations in orbit cap. cause a variation of nearly two to one in the absolute time per orbit which the satellite remains in sunlight. As one further example, the energy radiated by the earth will undergo radical changes with the amount of cloud cover. In spite of the many unpredictable factors, it is expected that mean orbital temperatures of the outer shell will lie between 0° C and 60° C (32° F and 140° F) under most unfavorable conditions. Outer shell temperatures during any orbit will vary from ±5° C to ±15° C from the orbital mean value as the satellite heats up when in sunlight and cools when in. the earth's shadow.

Ample information on temperatures are expected from primary recording stations for any satellites of a normal active life. Recordings made by amateur observers would be of great value in the event of unexpected temperature extremes which could result in early equipment or battery failures.

Airborne electronic equipment is located in an internal instrumentation compartment. This inner compartment will be thermally isolated from the outer shell so that heat exchange will be a very slow interchange by radiation. This tends to isolate the internal equipment from orbital temperature variations. The instrument compartment temperature will thus remain relatively stable at a value a very few degrees above the mean orbital temperature of the outer shell.

It can be seen that many potential contributions are possible from recordings of satellite scientific signals prepared by amateur observers. The necessary equipment is available to many now and could be prepared by others by inexpensive modification of present equipment. Preferable tapes would be made on "stereophonic" tape recorders with the satellite signal on one track and a reliable time base on the second track. Suitable time base would be obtained from a stable oscillator or recording of signals from WWV including a time announcement.

To be useable, any tapes must be accompanied by certain basic information as follows:

(1) Name and address of the observer if tapes and their interpretation are to be returned.

(6) Exact time at which recording was made including a.m. or p.m., time zone (or Greenwich time), and whether daylight or standard time.

(7) Exact location at which recording was made, preferably including latitude and longitude.

All correspondence. relative to programs for receiving satellite signals or available tapes should be addressed to U. S. Naval Research Laboratory, Washington 25, D. C., Attention: Code 4105. Recorded tapes should not be mailed without further instructions.

Any tapes prepared early in the life of a satellite should be carefully preserved until the fate of the satellite and its scientific experiments have been determined. Such recordings could be of great value.

Receiving U.S. Satellite SignalsMarch 1958 Radio News

Receiving U.S. Satellite Signals
March 1958 Radio News