Abstract:
The excess space and weight capacity of a conventional geosynchronous-transfer launch vehicle is used to deploy satellites to a low-earth orbit (LEO). In a preferred embodiment, an orbit-transfer vehicle provides the navigation, propulsion, and control systems required to transport a payload satellite from a geosynchronous-transfer orbit (GTO) to a predetermined low-earth orbit. Upon entering low-earth orbit, the payload satellite is released from the orbit-transfer vehicle. To reduce the fuel requirements for this deployment via the orbit-transfer vehicle, a preferred embodiment includes aerobraking to bring the satellite into a low-earth orbit. In a preferred embodiment of this method of deployment, the provider of the orbit-transfer vehicle identifies and secures available excess capacity on geosynchronous-transfer launch vehicles, and allocates the excess capacity to the satellites requiring low-earth orbit deployment, thereby providing a deployment means that is virtually transparent to the purchaser of this deployment service.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of aerospace, and in particular to the deployment of small satellites in low-earth orbit (LEO). 
     2. Description of Related Art 
     Satellites, because of their unobstructed fields of view of large areas of the earth, are often the preferred technical solutions to a variety of communications and monitoring problems. The high cost of satellite deployment, however, often precludes their use from a cost-efficiency viewpoint. In order to distribute the high costs of deployment among a large number of applications or users, the majority of deployed satellites are those that handle a multitude of tasks, or a multitude of customers for the same task. To minimize the loss of available access time to or from the satellite from or to the location on earth being serviced by a satellite, many satellites are placed in geosynchronous orbit. A geosynchronous orbit tracks the revolution of the earth, so that the satellite appears to be fixed over the same area of the earth, thus providing continual access to that area. Due to the physics involved, a geosynchronous orbit is approximately 36,000 kilometers above the earth. To provide reliable communications over this long distance, a geosynchronous satellite requires highly sensitive receivers and/or highly powerful transmitters. Because of the aforementioned economic and technical requirements, geosynchronous satellites generally weigh a ton or more, and cost (in 1999) hundreds of millions of dollars to deploy to the selected geosynchronous orbit. 
     As contrast to large geosynchronous satellites, the use of small satellites at low-earth orbit (LEO) are becoming increasingly common. Copending U.S. patent applications “SATELLITE COMMUNICATION SYSTEM WITH A SWEEPING HIGH-GAIN ANTENNA”, U.S. Ser. No. 09/045,971, filed Mar. 21, 1998 for Fleeter et al, Attorney docket AA980217; “LOW-COST SATELLITE COMMUNICATION SYSTEM”, U.S. Ser. No. 09/045,970, filed Mar. 21, 1998 for Fleeter et al, Attorney docket AA980218; “IN SITU REMOTE SENSING”, U.S. Ser. No. 09/130,854, filed Aug. 7, 1998 for Richard Fleeter, Attorney docket AA980723; “RF INSPECTION SATELLITE”, U.S. Ser. No. 09/267,942, filed Mar. 11, 1999 for Hanson et al, Attorney docket AA990202 illustrate the use of low cost satellites for a variety of applications, and are incorporated by reference herein. Low-earth orbits are typically hundreds of miles above the earth, rather than thousands of miles. Because of their order of magnitude closer proximity to earth, satellites in low-earth orbit require significantly less communicating and monitoring power and sensitivity than the satellites in geosynchronous orbit. Because they are not stationary above any location on the earth, multiple satellites in low-earth orbit are required to provide continuous coverage of a particular area on earth. Because multiple satellites are required in low-earth orbit to provide continuous coverage, a low-earth orbit satellite system is particularly well suited to applications that employ low cost satellites. As advances continue to be made in electronic circuit density and efficiency, the number of communication and monitoring applications that can be embodied in small, low cost satellites continues to increase. 
     Deployment of a small, less than five hundred pound, satellite into low-earth orbit typically costs, in 1999 dollars, between seven and ten million dollars. Because a plurality of satellites is required to provide continuous coverage of an area, the overall cost of deploying constellations of low-earth orbit satellites can often amount to hundreds of millions of dollars. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a lower cost means for deploying a satellite into low-earth orbit. It is a further object of this invention to provide a method for economically brokering the deployment of a satellite into low-earth orbit. It is a further object of this invention to provide an orbit-transfer vehicle to effectively deploy small satellite systems to low-earth orbit. 
     A launch of geosynchronous satellites typically includes one or two large, multi-ton, satellites that are deployed at the geosynchronous altitude of 36,000 kilometers via a large multi-stage rocket system, such as an Ariane system. Typically, after allocating the available space and weight capabilities of the rocket system to the primary payload of the one or two large satellites, some excess space and weight allocation remains. For example, if an Ariane launch vehicle can accommodate four tons, and the primary payload satellites are 1½ and 2 tons each, the launch vehicle has an excess capacity of a half ton. Because the marginal cost of adding one or two small satellites is minimal, this excess space or weight capacity can be brokered for the deployment of small satellites at substantially less cost than the primary payload, often less than a quarter of the cost per pound charged to the primary payload satellites. 
     The expressed objects of this invention, and others, are achieved by providing a means of utilizing the excess space and weight capacity that is typical of a launch of large geosynchronous satellites to deploy small satellites at a low-earth orbit. Specifically, this invention provides a method of deployment of small satellite systems to low-earth orbit from a geosynchronous-transfer launch vehicle. In a preferred embodiment, an orbit-transfer vehicle provides the navigation, propulsion, and control systems required to transport a payload satellite from a geosynchronous-transfer orbit (GTO) to a predetermined low-earth orbit (LEO). Upon entering low-earth orbit, the payload satellite is deployed from the orbit-transfer vehicle. To reduce the fuel requirements for this deployment via the orbit-transfer vehicle, a preferred embodiment includes aerobraking to bring the satellite into a low-earth orbit. In a preferred embodiment of this method of deployment, the provider of the orbit-transfer vehicle identifies and secures available excess capacity on geosynchronous-transfer launch vehicles, and allocates the excess capacity to the satellites requiring low-earth orbit deployment, thereby providing a deployment means that is virtually transparent to the purchaser of this deployment service. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
     FIG. 1 illustrates a conventional deployment of a geosynchronous satellite via a geosynchronous-transfer launch vehicle. 
     FIG. 2 illustrates an example deployment of a low-earth orbit (LEO) satellite using a geosynchronous-transfer launch vehicle in accordance with this invention. 
     FIG. 3 illustrates an example method of facilitating the use of excess capacity of a geosynchronous-transfer launch vehicle for deploying a low-earth orbit satellite in accordance with this invention. 
     FIG. 4 illustrates an example orbit-transfer vehicle in accordance with this invention. 
     FIG. 5 illustrates an alternative example orbit-transfer vehicle in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a conventional deployment of a geosynchronous satellite via a geosynchronous-transfer launch vehicle. Note that the figures are presented herein for illustration purposes. Although the relative size of the illustrated orbits are approximately to scale relative to the illustrated size of the earth  130 , the specific details of the orbits should not be interpreted as being representative. A geosynchronous-transfer launch vehicle (not shown) containing one or more satellites for deployment in geosynchronous orbit  150  is launched  101  from the earth  130 . The geosynchronous-transfer launch vehicle is configured to achieve a geosynchronous-transfer orbit  120  whose apogee  121  is tangent to the geosynchronous orbit  150 . Some time after achieving the geosynchronous-transfer orbit  120 , the one or more geosynchronous satellites (not shown) are released from the geosynchronous-transfer launch vehicle. All items that are released by the geosynchronous-transfer launch vehicle have the same velocity as the geosynchronous-transfer launch vehicle when they are released, and therefore continue to travel in the geosynchronous-transfer orbit  120 . Each geosynchronous satellite contains a means for applying thrust  102 , preferably at or near apogee  121 , to enter an intended tangential geosynchronous orbit  150 . 
     FIG. 2 illustrates an example method of deploying a low-earth orbit (LEO) satellite using a geosynchronous-transfer launch vehicle in accordance with this invention. As noted above, a geosynchronous orbit  150  is located 36,000 kilometers above the earth  130 . A low-earth orbit  110 , on the other hand, is located well within 1500 kilometers of the earth  130 . The Space Shuttle, for example, typically orbits the earth at an altitude between 160 and 600 kilometers. For ease of reference, the term geosynchronous-transfer launch vehicle is used to define a launch vehicle that deploys its payload at an elevation above the earth that is substantially higher than a low-earth orbit, because the most common use of a high-altitude launch vehicle is for geosynchronous satellite deployment. 
     In accordance with this invention, an orbit-transfer vehicle (not shown) containing the low-earth orbit satellite (not shown) is launched  101  from the earth  130  via a conventional geosynchronous-transfer launch vehicle (not shown), such as an Ariane. The geosynchronous-transfer launch vehicle releases the orbit-transfer vehicle into geosynchronous-transfer orbit  120 . Some time after release, preferably at the apogee  121  of the geosynchronous-transfer orbit  120 , the orbit-transfer vehicle fires  103  its integral propulsion device (not shown) to force itself, and the attached low-earth orbit satellite, out of the geosynchronous-transfer orbit  120 , and begins a forced descent toward the earth  130 . Another firing  104  of the propulsion device can thereafter be used to place the orbit-transfer vehicle directly into a low-earth orbit  110 , but such a direct geosynchronous-transfer orbit to low-earth orbit transfer will require a substantial amount of fuel to reduce the kinetic energy of the orbit-transfer vehicle sufficiently to remain in the low-earth orbit  110 . 
     In accordance with an aspect of this invention, aerobraking is used to facilitate the geosynchronous-transfer orbit to low-earth orbit transfer. Aerobraking uses the friction of the earth&#39;s atmosphere to reduce the kinetic energy of a spacecraft. Illustrated in FIG. 2, the firing  103  of the propulsion device provides a thrust to force the orbit-transfer vehicle close to the earth  130 , at  113 . In a preferred embodiment, the firing  103  of the propulsion device is controlled to provide a perigee  113  to within a hundred kilometers above the earth  130 . At this nominal hundred kilometer altitude, the atmosphere of the earth is sufficiently dense so as to impart a frictional force that reduces the kinetic energy of the orbit-transfer vehicle, and its attached low-earth orbit satellite. This reduction in kinetic energy results in an apogee  114  of the orbit-transfer vehicle that is less than its original geosynchronous-transfer orbit apogee  121 . Upon achieving apogee  114 , the orbit-transfer vehicle is drawn toward the earth  130  again, and reaches perigee  115 , also within 100 kilometers of the earth  130 . The friction of the earth&#39;s atmosphere at this low altitude again reduces the kinetic energy of the orbit-transfer vehicle, and the resultant apogee  116  is less than the prior apogee  114 . Subsequent traversals  117  of the earth&#39;s atmosphere will continue to reduce the kinetic energy of the orbit-transfer vehicle, further lowering each subsequent apogee  118 . 
     When sufficient kinetic energy is removed from the orbit-transfer vehicle, the decreasing apogee  111  of the orbit-transfer vehicle substantially approaches the altitude of the intended low-earth orbit  110 . At this apogee  111 , the orbit-transfer vehicle effects the firing  105  of the integral propulsion device to force an ascent of the orbit-transfer vehicle so as to raise the perigee of the orbit-transfer vehicle, and attached low-earth orbit satellite, beyond the earth&#39;s atmosphere, thereby preventing further decreases of apogee. The preferred firing  105  places the orbit transfer vehicle, and attached low-earth orbit satellite, into a symmetric low-earth orbit  110 , with a perigee that is substantially equal to the apogee  111 . In a preferred embodiment, over one hundred aerobraking orbits are made before firing the integral propulsion device at  105 . Typically, the orbit-transfer vehicle releases the low-earth orbit satellite at this low-earth orbit  110 , and thereafter the low-earth orbit satellite operates independently, as it would have, had it been launched directly from the earth  130  to the low-earth orbit  110 . That is, although the low-earth orbit satellite may travel hundreds of thousands of miles to reach an orbit  110  that is only a few hundred miles above the earth&#39;s surface, its operation is substantially independent of this rather circuitous deployment scheme. 
     Because the deployment methods in accordance with this invention can be effected without affecting the satellite payload, this invention provides a means for brokering lower cost services for the deployment of low-earth orbit satellites. Generally, a satellite is used as a component of a ground-based system, such as a communications network, a research facility, and the like. The user, or owner, of the satellite is not necessarily fluent in the intricacies of rocketry and orbital mechanics. A satellite deployment broker provides the interface services and support between the owner of the satellite and the provider of a launch vehicle. In accordance with the principles of this invention, a satellite deployment broker can extend the range of potential low-earth orbit satellite launch vehicle providers to include geosynchronous-transfer launch vehicle providers having excess capacity. FIG. 3 illustrates, for example, a flow diagram for allocating launch services for low-earth orbit satellite deployments. At  210 , the satellite(s) requirements are determined, including the required orbit parameters, the size and weight of the satellite, and so on. At  220 , the cost of a conventional low-earth orbit launch that satisfies the requirements are determined or estimated. At  230 , the availability of excess capacity on scheduled geosynchronous orbit launch vehicles is determined, and a cost is negotiated for using this excess capacity. The overall cost of deploying the low-earth orbit satellite via a geosynchronous-transfer launch is the cost of using the excess launch capacity of the geosynchronous-transfer launch vehicle plus the cost of the orbit-transfer vehicle for transporting the satellite from the geosynchronous orbit to the low-earth orbit, as determined at  240 . If, at  250 , the overall cost of the geosynchronous-transfer launch and orbit-transfer is less than the conventional low-earth orbit launch, the deployment is effected by attaching  260  the satellite to the orbit-transfer vehicle and launching  270  the orbit-transfer vehicle with satellite via the geosynchronous-transfer launch vehicle. Thereafter, the orbit-transfer vehicle effects the deployment  280  of the satellite to a low-earth orbit as discussed above. If, at  250 , the cost of the conventional low-earth orbit launch is less expensive than the geosynchronous-transfer launch, the satellite is deployed  290  via the conventional low-earth orbit launch. Note that a deployment of a constellation of satellites to low-earth orbits in accordance with this invention can involve a combination of low-earth orbit and geosynchronous-transfer launches, depending primarily on the availability and cost of excess capacity on scheduled geosynchronous-transfer launch vehicles. 
     FIG. 4 illustrates an example orbit-transfer vehicle  300  in accordance with this invention. The example orbit-transfer vehicle  300  includes an adapter element  310  that provides a conventional means  315  for securing the vehicle  300  to the geosynchronous-transfer launch vehicle (not shown), a body element  320  that provides a cavity for holding a fuel cell  340 , and an attitude determination and control system  330  that provides the navigation, propulsion, and control systems required to transport the satellite from a geosynchronous-transfer orbit to a predetermined low-earth orbit. The adapter element  310  and body  320  in a preferred embodiment include shielding to enable the orbit-transfer vehicle  300  to withstand the heat that is induced by the atmospheric friction during aerobraking. The elevation at perigee  113 ,  115 ,  117  determines the required degree of shielding. Correspondingly, the elevation at perigee  113 ,  115 ,  117  determines the number of aerobraking orbits required to provide a sufficient reduction in the kinetic energy of the orbit-transfer vehicle  300  to achieve a low-earth orbit, given the capacity of the fuel cell  340 . Jets  325  on the body element  320  effect the thrust required to effect the orbit-transfer, under the control of the attitude determination and control system  330 . A satellite  380  is designed to be mounted within the body  320 , and is released from the orbit-transfer vehicle  300  when the appropriate low-earth orbit is achieved. Alternative arrangements will be evident to one of ordinary skill in the art in view of this disclosure. For example, FIG. 5 illustrates an orbit-transfer vehicle  400  for use with a satellite  480  having an integral body  420  and conventional means  315  for securing the satellite  300  to a geosychronous transfer launch vehicle (not shown). 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. For example, the orbit-transfer components  430  can be built into the satellite  480 , thereby eliminating the need to detach the components  430  upon achievement of the low-earth orbit.