US3427623A - Communication satellite - Google Patents

Description

Feb. 11, 1989 J. c. YATER COMMUNICATION SATELLITE Filed April 22, 1965 R a om mm WC (K A .V 5 z ATTORNEYS United States Patent 3,427,623 COMMUNICATION SATELLITE Joseph C. Yater, 1706 St. Marks Place, Fairfax, Va. 22030 Filed Apr. 22, 1965, Ser. No. 449,934 U.S. Cl. 343-705 Int. Cl. H01q [/28, 15/20 7 Claims ABSTRACT OF THE DISCLOSURE the gravity gradient.

This invention relates to communication satellites and more particularly to a passive communication satellite. According to the invention, a highly directional antenna array is placed in orbit around the earth and is employed as a passive communication satellite, and functions as an eflicient reflector of transmitted electromagnetic signals such as communication signals.

The antenna array consists of spaced antenna elements, each of which is extended and stabilized in space, utilzing the gradient of the earths gravitational force field. This gradient or spatial change in direction and strength of the gravitational force provides forces for stabilizing an axis of the satellite with respect to the plane of the orbit of the satellite, and this gradient further provides a force used, in accordance with the discovery of this invention, in extending flexible filaments in space. The gravity gradient force extends and maintains in extension each antenna array in a direction normal to the surface of the earth, i.e., towards the center of the earth. This extension provides the major gain of the antenna array so that the antenna pattern of the complete antenna array will have its narrowest lobe in planes containing the major dimension of the antenna and its broadest lobe structure in the directions defining the surface of an imaginary cone having a constant angle with respect to the axis of the major dimension of the antenna array.

The antenna array of this invention exhibits the ability to steer or, synonimously, to direct, the reflected electromagnetic signals by simply varying the frequency of transmission. This steering or directing is derived from the regular spacing of reflecting elements positioned along the major or earth pointing axis of the satellite. Thus, by using a certain frequency of transmission, the signal can be reflected back, i.e., back towards the earth, at a desired angle with respect to the major axis of the satellite in order to be received at a desired locus of locations on the earths surface. The supporting structure of the elements of the array consists of a light-weight, flexible structure such as a thin filament made of plastic or glass. In a preferred embodiment, the passive satellite of this invention consists of a single, flexible filament of nonconducting material such as plastic or glass. Spaced regularly thereon, and functioning as reflectors of the transmitted electromagnetic energy, i.e., the signal, the surface is coated with a conducting material to thereby define a series of bands. The bands are spaced a distance apart coresponding to approximately seven-tenths of the wave length of the transmitting frequency, or multiples thereof. The antenna is made of very small diameter filament so that a 1000 foot long antenna weighs only a very small fraction of a pound.

In order to discuss the advantages available by using this invention over current approaches, it is helpful to discuss the present state of the art of communication satellites. Such satellites are under extensive study and experimentation, and large programs are under way using different approaches. Both passive and active approaches are being investigated and used. Passive satellites do not require power and merely reflect the transmitted electromagnetic energy back to earth. Active satellites, which require power, amplify the transmitted signal before relaying the communication signal back to earth.

An example of a passive satellite system that has shown good capabilities is called Project West Ford. In this case the individual satellites, which are merely dipoles weighing a few micrograms each, are placed in an orbit at an altitude of about 2,000 miles above the surface of the earth. Abuot 480 million of the dipoles, weighing altogether approximately 44 pounds, were actually placed into orbit using only one launching vehicle to dispense them into orbit. This orbiting dipole technique can provide continuous worldwide coverage with high reliability and high traffic handling ability, but the transmission loss of the system is large enough to thereby give rise to a low information rate, unless large ground equipment is used. In addition, special modulation-detection techniques are required to reduce the short term effects of the random motion of the dipoles in orbit. Also, such an orbiting belt of dipoles must be carefully observed and studied to insure that other radio services, scientific studies, and space exploration are not subjected to interference.

The active communication satellite system approach is also under intensive investigation and already hundreds of millions of dollars have been spent by governmental agencies alone on efforts to develop a system of this type. This approach has the advantage that the amplification of the signal in the satellite reduces the transmission loss over that of the passive approach, as in Project West Ford. But on the other hand the requirement for amplification reduces the capacity of the system beyond that of the Project West Ford approach, since the amount of communication signals that can be relayed through the satellite is limited by the bandwidth of the amplifier and also by the power available from the satellite. This makes the problem of sharing the satellite system among many users quite complex. The Comsat Corporation is considering the feasibility of either or both of a medium altitude (5,000 miles) or a synchronous altitude system (18,000 miles). NASA has already helped launch several medium altitude satellites (Telstars, Relays) and three synchronous altitude satellites (Syncoms). But all approaches so far considered require satellite systems costing many tens of millions of dollars with the potential cost to each user necessarily very high.

By the practice of the present invention the high capacity and reliability of the P roject West Ford is realized, but without the large transmission losses, special modulation-detection techniques, or diflicult noise problems. In addition, this is done at an extremely low cost, compared to the active satellite systems. These and other advantages, such as providing much higher jamming immunity of providing navigation signals to a wide class of users, result from the practice of this invention, making it feasible to construct high gain, highly directional antenna arrays in space at low cost and of low weight.

In the high gain, directive antenna of this invention, the signal reflected from each reflector along the axis of the antenna is in phase, at the receiver, with the transmitted signal. This increases the reflected power in the desired direction proportional to the square of the numbers of reflectors, i.e., their second power. For

randomly spaced reflectors, such as those employed in Project West Ford, the power reflected in the desired direction is only proportional to the number of reflectors, i.e., their first power. So, for a given number of dipoles in a stabilized directive array as compared to the same number of randomly oriented and randomly spaced dipoles as in Project West Ford, the reflected power in the desired direction according to the present invention is increased by a factor that is larger than the number of dipoles used. As an example, one filament of this invention containing 10 bands, i.e., dipoles, has the ability to return a signal of strength as could be returned from all 10 dipoles of the Project West Ford, if the latter dipoles were all concentrated in one region and could be illuminated in one antenna beam width. Or, to state the comparison in other terms, the 10 dipoles of the antenna element of this invention can return a signal approximately 10 times stronger than the reflected signal from the Project West Ford dipoles if the 60 foot transmitting antennas of Project West FOPd are used in both cases at ranges to the dipoles of 3,000 miles.

As far as the active satellites so far launched are concerned, all have very little antenna gain, so that the signal that can be returned from an .antenna array of this invention is much larger for the above example, where the 20 kw. 8 kmc., Project West Ford transmitter and the 60 foot dish antenna are used. This signal strength is 100 or more times larger than the current and programmed active satellite systems can provide.

In addition, this passive directional antenna array can handle large numbers of users, as the number of users that can employ the system on a non-interfernig basis is a function of the gain of the antenna array, and if the users are appropriately spaced, hundreds of thousands of users can transmit and receive via the same array. This high capacity of the system is further enhanced by the lightweight and inexpensive nature of the antenna arrays. Separate groups of the filaments may be made to handle separate frequency bands in order that a wide class of users can be handled.

In the drawings:

FIGURE '1 is a schematic representation of a single antenna array of this invention in orbit around the earth.

FIGURE 2 is a view of a single antenna array.

FIGURE 3 is a schematic showing of the relationship between an imaginary cone of transmission and its corresponding imaginary cone of reception for the antenna array shown in FIGURE 1.

FIGURE 4 is a view, similar to FIGURE 2, of a modification.

FIGURE 5 is a view of a further modification.

FIGURE 6 is a view of a further modification.

Referring now to FIGURE 1 of the drawings, the numeral 10 denotes a single filament of glass or plastic forming a directional antenna array by coating the surface with a conducting material 1 2 at periodic intervals denoted by D. This periodic requirement is imposed in order to enable the reflected signals trom each reflector 12 to be in phase at the receiver. The required spacing D of the reflectors 12, which may be considered dipoles at the signal frequency, and the relative directions from the reflectors 12 to the transmitter and receiver, is given by the equation cos a+cos 8 wherein D is the spacing along the filament between the center of the reflectors 12 as shown in FIGURE 2, and

n is an integer A is the signal wavelength at, B are the angles between the straight orbiting filament 10 and the directions to the transmitter and the receiver,

respectively, as shown at FIGURES 1 and 3.

Referring now to FIGURE 3 of the drawings, the zone Z represents the locus of points on the surface of the earth (sh-own hat for ease in representation) for which a transmitter defines the angle at of FIGURE 1 with respect to the filament (antenna array) 10. The zone Z then represents the locus of points on the surface of the earth of all receivers which define the angle ,6 of FIGURE 1. The zones are illlustrated as having appreciable width for purposes of illustration.

From a consideration of the above explanations, it will be seen that a transmitter placed anywhere within zone Z will obey the relation n)\=lD (c0s al+cos ,9) with respect to a receiver located anywhere within zone Z The ability to steer or direct the transmitted signals is derived from the regular spacing of the reflecting bands or dipoles of the antenna array. From the expression n)\=D(c0s a-j-cos 8), it is seen that for a fixed angle a (a fixed transmitter), a change in wavelength of the transmitted signal will cause a change in 8, and hence a change along the earths surface for those points Z on which constructive reinforcement of the signal will occur. Assuming a case for the first order, i.e., 11: 1, for an orbital altitude of 3,600 miles for the antenna array and an initial angle 3 of 30 degrees, a change of 50 megacycles from an 8 ki-lo-megacycle transmission frequency will cause a change of radius on the zone Z of the order of magnitude of miles.

In practice a ring of filaments is placed in orbit or a Syncom type orbit may be employed. In the former case, when one filament passes out of the position shown in FIGURE l with respect to the transmitter and receiver, another will move into its place. In the latter case, the filament remains fixed, as shown in FIGURE 1.

The length of each coated band and the spacing therebetween is made to correspond with the desired altitude and operating frequency.

This relationship holds for microwave antenna lengths of less than several thousand feet in low altitude orbits, and for longer arrays at high altitudes or lower frequencies. For longer arrays than these limits, the curvature of the phase front of the transmitted electromagnetic wave at the larger values of 0; causes deterioration in the theoretical gain of a microwave array.

This limit on the antenna length for any given a and p can be removed by changing the spacing of the reflecting elements 12 so that the array is focused to receive or transmit using the given pair of angles, a and 5. A several fold increase in allowable length can be obtained for a useful range of values of a and B by adjusting the spacing for a midpoint value of a and p3 of the given range of values of a and 3. But this adjusting of the spacing of the reflectors will be necessary only for extremely specialized applications, since the gain of a 1,000 foot microwave antenna is already large enough to meet nearly all of the usual communication or signal relaying requirements.

This limitation on length only applies with respect to maintaining an in phase relation in a given direction for signals of a given frequency. For many applications, it will be feasible to lengthen the filament to allow the same filament to be the supporting structure to the required number of arrays with each array having a different spacing D, This would enable the arrays on the filament to transmit between a transmitter and a receiver as many channels of information at as many carrier frequencies as there were separate arrays on the one filament, i.e., arrays each having a ditferent D. By this means, one filament could meet any bandwidth requirement.

An important exception to this limitation of length of an array occurs when the requirements of the communication system are that a single wide band is more useful than the equivalent total bandwidth made up of several smaller bandwidth arrays. For this case at the cost of a loss in gain of the array of 25% from that of the limited length maximum gain array, the array can be made as toward the center of the earth by virtue of the gradient 1 of the earths gravitational field. The maximum tension in the antenna filament 10, due to the gravity gradient, is reduced from the magnitude of the force of gravity on the filament at the orbit altitude by a factor proportional to the ratio of the length of the filament to the distance from the center of the filament to the center of the earth.

The deployment of the filaments is accomplished by dispensing the filament coil into space :with the filament being held in place on a lightweight spool during this launching phase by being covered with a subliming material such as napthalene. After the coil is dispensed from the launch vehicle the napthalene sublirnes, allowing the filament to unwind from the spool, one turn at a time.

The thickness of the filament may be reduced to make the payload weight a negligible factor, as a glass fiber .002 cm. in diameter will yield an adequate bandwidth capability to the array. A 1,000 foot filament of this thickness Weighs less than M of a pound. On the other hand, added thickness to the filament may be added if a longer life expectancy to the filament is required, But, at a less weight penalty, the life expectancy can also be increased using redundant spaced filaments connected together at convenient intervals along the filament.

An embodiment of the invention is illustrated at FIG- UR-E 4, showing a flatted filament or ribbon 20 of insulating material, providing additional protection against micro-meteorite damage when exceptionally long life is required. This ribbon is also provided .with conducting elements 12 regularly spaced therealong at a distance D apart. The reflecting bands 12 of FIG. 4 form horizontal dipoles at right angles to the axis of the flattened filament. These horizontal dipoles may be used to give a greater reflected signal level for small angles for a or B.

A technique to reduce any bending resulting from solar radiation is to maintain a constant rotation of the antenna array or 20 about its axis. A constant rotation may be maintained using solar radiation pressure on solar paddles. These paddles, denoted by the numeral 30 of FIGURE 5, are attached to the antenna array and cause rotation by using the familiar principle of coating opposite surfaces thereof with reflecting and absorbing materials, respectively. The rotation rate may be governed by employing paddles having coiled, flexible ends 34, as shown at FIGURE 6. The unrolling of the coils due to the centrifugal force as the rotation ing the ends 34 to unroll. As the coils unroll, surfaces coated similar to the surfaces on the opposite side are exposed so as to slow down the rotation rate.

The relative size of the paddles, as shown, will ordinarily be much smaller. This is because spinning would take place if only a very small turning torque existed. To get an estimate of the magnitude of torque required, a continuous torque of 3X 10* dyne centimeters would result in an increase of 1 radian per day in the rotation rate of a 1 mil diameter filament 3000 feet long. This torque would result from an uncompensated solar pressure acted on a reflecting surface A that had a movement arm L with the following value for the product of L and A:

This value of the product would exist from a 10 sq. cm. area with a moment arm equal to the diameter of the 1 mil filament. Then, the required unbalanced solar presrate increases caus- 5 one side and blackened on the other side and the mirror image of this area was placed on the other side of the filament with the surfaces of all areas being at temperatures that maintained thermal equilibrium for the environment of the solar rflux on a spinning filament.

In addition to mechanical schemes used to provide damping to the spinning torque, of which the technique illustrated in FIGURE 6 is an example, electromagnetic damping torques may also be used. A simple technique is to make electric generators out of a very small fraction of the dipoles 12 by removing the coating in small vertical strips from the middle of both sides of a dipole to thereby define a conducting loop, which generates an electromotive force in the magnetic field of the earth proportional to the spin rate. To limit the spin rate to one hundred revolutions per second with an applied solar torque of 3 10- dyne centimeters, approximately one hundred dipole conducting loops would be adequate. This amount of spin would be much too small to have any measurable elfect in disturbing the equilibrium of the filament. The extremely small eifect of this spin upon the filament motion results from the extremely small moment of inertia about the spin axis.

This spinning technique is a preferred technique to minimize thermal bending, first since the spinnnig will be difficult to prevent, second because spinning about the vertical axis in a vertical field of force would tend to straighten out any residual bending forces, as these would be minimized by the averaging effect of the spinning motion, third an initial spin could exist for many cases. The many cases for which an initial spin would exist would occur when the filament had been previously twisted prior to being placed on the spool before the dispensing process. This twisting could be done to insure that any set or creep that occurred while the filament was in place on the spool was reduced to a minimum.

Another technique that may be used to reduce thermal bending of the antenna array when heavier filaments or ribbons are used, is to have flexible coupling at regular spacings along the vertical axis of the array or much thinner filaments connecting segments of the antenna array.

For the user with both high signal level and'wide information bandwidth requirements, these requirements can be met through using additional filaments with each filament having a difierent spacing between reflectors. These additional arrays are placed in similar orbits or can be placed on the same filament so that the transmitter beam illuminates the required number of filaments so that the required signal level and bandwidth is obtained.

For extremely high signal level requirements, but with lower bandwidth requirements, a satellite system is designed so that many antennas with the same reflector spacing D may be in the beam width at the same time. In this case, the signals may be the total of the randomly phased reflected signals of all the illuminated antenna arrays when appropriate signals of a and t! are used. This increases still further the increase in the signal level than is available from the Project West Ford technique.

In order to show some of the advantages and capabilities of the arrays of this invention, it is necessary to compute the signal power and bandwidth obtainable from these arrays as well as the improvement in the signal power obtainable from these arrays compared to the signal power from active satellites. First, then the power from a passive satellite into a receiver is given by:

P PtGQGfAf4L (41rR (41R?) where P is the transmitted power G, is the transmitter antenna gain G is the gain of the filament array A is the eflective area of the antenna for the receiver sure would result if this 10- cm. area was silvered on A is the effective area of the filament array 7 R is the distance to the filament from the transmitter R is the distance to the receiver from the filament.

For the case where the passive satellite is the filament consisting of the array of vertical dipoles, we have 2L sin {3 where B, as in FIGURE 2, is the angle between the filament and receiver and L is the length of the filament.

Also

where D and D are the diameters of the parabolic antennas:

For this case then:

.056P.D D L sin a sin 8 h R Rg Since the curvature of the wavefront can cause an unwanted difference in phase of regularly spaced scatterers, then this unwanted difference in phase can result in a limit to the length of the array. Using as a limit that for the worst geometry case at a given altitude, the ends of the array can be excited at no more than 90 difference in phase from the center of the array, the following limit in length as a function of altitude is imposed:

where h is the altitude of the orbit, R is the radius of the earth.

The worst case for which this maximum length is computed is for a horizon transmission or reception from the array. Then using this length and considering a horizon to horizon transmission, the following equation results for the power at the receiver:

Then computing the following ratio of P to KT, where KT=.4 10 when T is 290 K., for the following assumed values:

D =D :30 feet Pt=10 kw. 7\=3.75 cm.

Gives and For example, for an orbit with an altitude of 2,000 miles it can be seen that 1.4 megacycles of information can be sent over the assumed link with a 20 db power ratio between the signal and the noise power from a 290 K. source over the same bandwith. The curve can be scaled to give the capabilities of other communication links.

For transmissions between 6 foot antennas, the above ratios would be decreased by 625 so that for this link a two-way voice channel could be maintained with the above 20 db ratio between signal and noise and between locations 6,700 miles apart. If the power of the transmitter was reduced to 1 kW., then the ratio between the signal power and the noise power at 290 would be reduced to 10 db. For 100 watt transmitter a signal of 240 cycles per second would give the same signal to noise power ratio over the 6 foot antennas.

Using the West Ford 20 kw. transmitters and 60 foot antennas would provide a 35 db signal to noise ratio for the 1.4 mc. bandwidth of signal and an even higher signal to noise ratio if the West Ford low noise receivers were used.

Although the signal to noise power in the above case is sufficient, the bandwidth of information that a single antenna array can transmit must also be considered. This bandwidth of information is a function of a length of the array and for the case where the maximum length is used the following relation is obtained:

The maximum bandwidth for the maximum length array for a 2,000 mile altitude orbit is 360 kc. The 1.4 m. bandwidth of information could still be transmitted over one filament array using the assumed link with the 30 foot antennas or using a West Ford link if proper coding was used.

However, without using this coding, it would be necessary to utilize more than one filament of this array length to transmit this signal bandwidth. In this case the required number of four filaments for the transmission of the 1.4 mc. signal bandwidth could be within the beamwidth of the transmitter for the link with the 30 or 60 foot transmitting antenna,'if sufficient number of arrays had been placed into one ring with random spacings or if the required number of four arrays had been placed on one filament. By placing several thousand of these filaments into a ring, then with a very high probability, the transmitter beamwidth from a 30 or 60 foot antenna can be used to locate along the satellite ring a region where 4 or more arrays can be used to transmit the required information.

After the glass or plastic filaments are drawn, the conductive reflective bands may be placed thereon following conventional chemical, electrical and vapor plating procedures.

If chemical plating is employed, satisfactory results have been obtained by using a solution of palladium chloride and/or palladium chloride and tin chloride containing about 3 to 4 grams of reagent per gallon. The PdCl or the mixture of PdCl and SnCl provides a uniform coating of the reducing agent on desired portions of the fiber. Following the coating of the elements with the solution of the reducing agent an aqueous solution of the salt of the metal to be plated onto the coated surfaces is applied to the fibers. A very satisfactory metallic coating has been provided by using a plating solution of grams of CuSo -5H O per liter of solution. Such a solution will have a pH of from about 5.7 to 6.3 with a specific gravity of 1.10 at 70 F. It has also been found that uniformity of the coating is enhanced by adding to the solution a 1% solution of sodium hydroxide. In general, 5 to 15 minutes in such a plating bath will provide a uniform metallized coating on the glass surfaces.

It has been found that coatings of this nature generally have a very uniform thickness and that a metal layer is generally deposited to a thickness of about /3 micron and further immersion of the element in the plating solution will generally not increase the thickness of the metal layer.

From the above, it is seen that the ability to steer or to direct the transmitted signals stems from the regular spacing of the reflecting bands or dipole elements, and expressed by the relation n)\=D (cos a-i-cos 19). It is to further noted that the capture and continuing orientation of a filament by the gravity gradient so that it is always pointing towards the center of the earth, thus utilizing this relation, requires a minimal length of approximately ten feet.

What is claimed is:

1. A passive communication satellite system comprising, in orbit around the earth, an elongated filament, said elongated filament being of an insulating material and provided with a plurality of spaced reflecting elements along the longitudinal axis thereof at regular intervals, said filament being oriented so that its longitudinal axis is normal to the surface of the earth.

2. The system of claim 1 wherein said filament is in the form of a flat ribbon.

3. The system of claim 1 wherein said reflecting elements are in the form of bands of a conductive material.

4. The system of claim 1 wherein said filament carries means to rotate it about its axis in response to solar radiation forces.

5. The system of claim 1 wherein the filament is at least ten feet in length.

6. The passive communication system of claim 1, wherein said elongated filament is flexible.

7. The method of establishing a passive communication system comprising the step of placing in orbit around the earth a coiled and flexible filament of insulating material having a plurality of regularly spaced reflecting elements along the length thereof and allowing the gravitational gradient of the earth to uncoil and thereby elongate the said filament to thereby place it in orbit around the earth in an extended configuration with its longitudinal axis directed towards the center of the earth.

References Cited UNITED STATES PATENTS 2,624,003 12/ 1952 Iams 343-785 3,057,579 10/ 1962 Cutler et a1 343705 3,128,467 4/1964 Lanctot 343785 3,165,751 l/ 1965 Clark 343705 3,168,263 2/1965 Kamm 343-705 3,202,998 8/1965 Hoffman 343785 3,277,479 10/ 1966 Struble 244-455 2,936,453 5/ 1960 Coleman.

3,144,606 8/1964 Adams et al 343-909 20 ELI DIEBBRMAN, Primary Examiner.

US. Cl. X.R.

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