In situ remote sensing

The in situ remote sensing system of this invention includes a plurality of sensors that are distributed about an area of interest, and a satellite communications system that receives communications signals from these sensors. The satellite communications system determines the location of each sensor at the time that the sensor transmits its communications signal, thereby facilitating the mapping of the value of the sensed parameter about the area of interest. In a preferred embodiment, the satellite communications system transmits a trigger signal to the sensors that are within the satellite antenna's field of view, and the sensors transmit only in response to the trigger signal. The sensors may be conventional active transmitters or passive transponders that receive their operating energy from a received trigger signal. The sensing devices within the sensor may also provide the operating energy for the sensor using, for example, photocells, piezoelectrics, and thermocouples.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to the field of satellite systems, and in particular
 to the field of remote sensing of events and parameters via satellite.
 2. Description of Related Art
 Satellites are often used to sense events that occur at remote locations.
 Weather satellites use optical and radar devices to sense the presence of
 clouds, lightning, and other atmospheric phenomena. Reconnaissance
 satellites use optical, infrared, and other sensing devices to detect
 occurrences of events on the earth's surface. Scientific satellites use a
 variety of sensing equipment to detect occurrences of events throughout
 the universe.
 The cost of placing a satellite into orbit and subsequently controlling the
 satellite significantly increases the relative cost of each sensor in the
 satellite. Additionally, sensors designed for space must be robust, and
 require robust ancillary equipment to support their continued operation in
 a relatively harsh environment. As a result, the cost of placing a sensor
 in space is significantly higher than the cost of placing a similar sensor
 on the earth. The additional cost of placing the sensor into orbit,
 however, allows the sensor to cover a substantially greater area of the
 earth's surface. It also allows the detection of events in areas that are
 inaccessible or dangerous for direct placement of the detector.
 Satellites can also be used to communicate information from individual
 sensing devices that are placed at remote locations. For example, weather
 buoys containing sensing equipment are placed at specific locations across
 the oceans, and the information from the sensing equipment is communicated
 via satellites to a centralized weather station. The "FireSat" system of
 reference [1] discloses the placement of transmitters in a forest. Each
 transmitter contains a thermocouple and has an associated unique
 identifier. When a forest fire occurs, the thermocouple turns the
 transmitter on and the transmitter transmits the unique identifier. A
 satellite receives the transmission and relays the transmission to a
 control station that coordinates the firefighting efforts based upon the
 location of each of the uniquely identified transmitters. These land-based
 remote sensing systems can be less costly to implement than satellite
 remote sensing systems, because the satellite need only contain
 communications equipment, rather than remote sensors designed for each
 type of phenomena being detected.
 The conventional land-based remote sensing system presumes an a priori
 knowledge of the remotely placed sensors, and/or a unique identification
 of each sensor. Each uniquely identified thermocouple transmitter in the
 "FireSat" system, for example, has an associated assigned location, as
 does each ocean weather buoy. If the thermocouple transmitter in the
 forest is moved by a creature of the forest, or by a vandal, the
 transmission of its unique identifier will convey erroneous information,
 because the control station will direct the firefighting efforts to the
 original location of the transmitter. The satellite remote sensing system,
 on the other hand, only requires a knowledge of where the satellite is
 located and the orientation, or viewing angle, of the sensing device that
 is detecting the phenomena. The satellite remote sensing system, however,
 requires a particular sensing device (optical, radar, infrared, etc.) in
 the satellite for each of the phenomena being detected. To distinguish
 details of the event or phenomenon being sensed, the resolution of the
 satellite remote sensing system for each sensing device must also be high,
 which further adds to the high cost of such a system. The satellite remote
 sensing system is also subject to premature obsolescence. As time
 progresses, the infeasibility of replacing or augmenting the equipment on
 a satellite with newer technologies makes satellite remote sensing systems
 increasingly less competitive with land based remote sensing systems.
 Therefore, a need exists for a remote sensing system that eliminates the
 need for a satellite that contains a sensing device for each sensed
 phenomena, and also eliminates the need to place sensing devices at
 predetermined locations. A need also exists for a remote sensing system
 that does not require a unique identification of each remote sensor. A
 further need exists for a high resolution sensing system that does not
 require high resolution sensing devices.
 BRIEF DESCRIPTION OF THE INVENTION
 The invention provides for the use of remote sensors that are distributed
 about the site of the phenomena being sensed, and a satellite system that
 determines the location of each sensor based upon the receipt of a
 transmission from the sensor. The remote sensor can be an autonomous
 transmitter or a transponder that emits a signal in response to the
 receipt of a triggering signal. The remote sensor may transmit its
 location explicitly, or its location may be determined by the satellite
 system based upon the characteristics of the received signals. By
 providing dynamic location determination means, the remote sensors may be
 arbitrarily located while assuring the proper association between the
 sensed event and the location of the sensed event. In a preferred
 embodiment, the remote sensors are designed to be of minimal complexity
 and cost, thereby allowing a multitude of sensors to be deployed to effect
 field measurements of phenomena about the area of interest. In this
 preferred embodiment, the resolution of the system is determined by the
 distribution of the low cost sensors, rather than by the resolution of a
 sensor that is deployed in a satellite.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 illustrates an example remote sensing system that includes a
 plurality of sensors 100, a satellite 150, and ground station 160. The
 satellite 150 and ground station 160 form a satellite communications
 system for relaying the information received at the satellite 150 from the
 sensors 100 to the ground station 160. Example sensors 101-105 of the
 plurality of sensors 100 are shown to be located at locations L1-L5
 respectively. The sensors 100 are designed to measure a value of a
 parameter P. As illustrated in FIG. 1, example sensor 101 detects a value
 P1 of the parameter P; sensor 102 detects a value P2; sensor 103 detects a
 value P3; sensor 104 detects a value P4; and sensor 105 detects a value
 P5.
 The parameter P may be, for example, temperature. Each sensor 101-105 in
 this example contains a means for detecting or measuring temperature. In
 the simplest example, each sensor 101-105 includes a thermocouple that is
 in an off state when the detected temperature is less than a particular
 value and is in an on state when the detected temperature is above the
 particular value. The value of each parameter P1-P5 in this example will
 be either "on" or "off", depending upon whether the temperature at each of
 the sensors 101-105 is above or below the particular value, respectively.
 Each of the example sensors 101-105 communicates their respective parameter
 value P1-P5 to the satellite 150. In the two state example above, the
 parameter value may be communicated by the presence or absence of a
 transmission: receiving a transmission from a sensor signifies an `on`
 state, otherwise, the sensor is assumed to be in the `off` state. That is,
 a null transmission is used to convey a parameter value. As is common in
 the field of telemetry, the value of the parameter P may be a continuous
 value, and the sensor 100 communicates this value at a particular level of
 precision. In the `on` off example, the level of precision is one bit of
 information. Conversely, the parameter P may be used to adjust an analog
 characteristic, such as the frequency, amplitude, or phase of the
 transmitted signal; in such an example, the level of precision is
 virtually unlimited.
 The parameter P is not limited to a temperature measurement. Transducers
 are commonly available to convert a myriad assortment of parameters into
 an electrical characteristic. Thermodynamic transducers are available that
 measure thermodynamic and transport properties. Similarly, pressure,
 motion, and acceleration transducers are commonly available, as well as
 chemical transducers that can detect or measure the presence of particular
 chemicals or groups of chemicals, such as salinity testers, smoke
 detectors, carbon monoxide detectors, pollutant detectors, and the like.
 Radiation transducers of various types are also commonly available. Flux
 field measuring devices are also available that measure sunlight flux,
 charged particle flux, and neutral particle flux, and various transducers
 are commonly available for measuring properties of electromagnetic fields,
 such as the orientation and strength of the earth's magnetic field.
 Note that by deploying the sensors in situ, parameters that are difficult
 or impossible to measure from space can be sensed. Additionally, each
 sensor 101-105 can be designed to the desired degree of accuracy and
 precision, independent of the satellite 150. In accordance with a
 preferred embodiment of this invention, the communication method and
 protocol from the sensors 100 to the satellite 150 and on to the ground
 station 160 is independent of the particular sensing device used, and
 independent of the parameter being measured. In this manner, the same
 satellite communications system can be used to remotely sense a variety of
 parameters. The same satellite communications system can be used when new
 transducer technologies become available, or when new situations arise
 that require in situ remote sensing. For example, in the field of
 microelectronic manufacturing (MEM), expectations of future technologies
 include the ability to provide mass-spectrometer capabilities on a single
 MEM chip. As new technologies are developed, the remote sensing systems in
 accordance with this invention can be configured to employ these new
 sensing technologies by deploying new sensors 100, rather than by
 launching new satellites 150 containing the new sensing technologies.
 In accordance with this invention, the satellite communications system
 150-160 includes he ability to determine the location of each sensor at
 the time that the sensor transmits. In the example of FIG. 1, sensors
 101-105 are located at locations L1-L5 within an area of interest. The
 satellite communications system 150-160 determines each of the locations
 L1-L5 of the sensors 101-105 when they each transmit their detected
 parameter value P1-P5, respectively. In the case of a null transmission
 from one of the sensors 101-105, the satellite communications system
 150-160 determines the location of the null-transmitting sensor as the
 last determined location of that sensor, or an estimated location of the
 null-transmitting sensor based upon prior movements of that sensor.
 Alternatively, the satellite communications system 150-160 can ignore
 nulltransmissions. In a preferred embodiment that uses a null transmission
 to convey a particular parameter value, the most benign value of the
 parameter is associated with a null transmission.
 That is, for example, if the parameter being detected is whether the carbon
 monoxide level is above or below a particular value, a null transmission
 is associated with the level being below the particular value. In this
 manner, a non-benign detection effects a non-null transmission, from which
 the satellite communication system determines the most recent location of
 the sensor.
 In a preferred embodiment, to minimize the cost and complexity of the
 satellite 150, the ground controller 160 includes a processor 165 that
 determines the location of each sensor 101-105 based on the signals 151
 that are relayed from the satellite 150. By determining the location L1-L5
 of each sensor 101-105 at the time that the each sensor 101-105 transmits
 its measured parameter value P1-P5, each measured parameter value can be
 associated with a corresponding location, as shown at the output of the
 processor 165 in FIG. 1. From these corresponding parameter values and
 locations, a map 170 of the occurrence of each parameter value about the
 area of interest can be produced. For example, each location L1-L5 may be
 represented by a coordinate or set of coordinates, such as a latitude and
 a longitude on the earth's surface. The map 170 is a conventional overlay
 of the parameter values P1-P5 at each coordinate upon a geographic
 representation of the area of interest. Other representations would be
 common to one of ordinary skill in the art. For example, isobars can be
 drawn representing locations having substantially the same parameter
 value, or differing colors or intensities of colors can be used that
 correspond to differing parameter values. Similarly, the map 170 may be
 configured to display changes of parameter values, rather than absolute
 parameter values.
 Note that the location determination is dynamic, in that the determined
 location represents the actual location of each sensor 100 when each
 sensor 100 is transmitting. If the sensors 100 are mobile, the
 corresponding mapping 165 of parameter values P to locations L will change
 if either the parameter value P or the location L changes. In this manner,
 the map 170 will be continually updated to reflect these changing values.
 For example, floating sensors 100 that contain oil sensing devices can be
 deposited about the area of an oil leak at sea. These floating sensors 100
 may be deposited about the area by dropping them from an aircraft as it
 traverses the area around and upon the oil slick. Each of the sensors 100
 will report the presence or absence of oil at their location. The location
 of each sensor 100, and the location of the oil slick, will be affected by
 the wind and by the current of the water, but not identically. By
 maintaining a dynamic determination of the location and parameter value of
 each sensor 100, the map 170 will contain an accurate representation of
 the location and extent of the oil slick, as different sensors enter and
 leave the oil slick due to the influences of wind and current.
 Note also that because the satellite communication system determines a
 location associated with each transmitted parameter directly, there is no
 need to identify each individual sensor 100 that is transmitting to the
 satellite. In the FireSat system, and the ocean buoy example, each sensor
 and buoy must be uniquely identified, because in each of these systems,
 each sensed parameter value P must be associated with a specific sensor
 before an association with a location is determined. That is, in a
 conventional system, a transmission signature uniquely identifies each
 transmitter. This transmission signature may be a characteristic of the
 signal itself, such as the frequency of the signal, or it may be an
 encoding contained within the signal, such as a transmission preamble that
 contains a unique pattern of bits that identify each transmitter. Based
 upon the transmission signature, the conventional system determines the
 location of the particular transmitter. The generation of a unique
 identifier per transmitter increases the cost of each transmitter. In a
 preferred embodiment of this invention, each transmitter need not have a
 unique transmission signature. Each transmitter may be identical, thereby
 achieving the economic advantages of mass production.
 A variety of techniques can be employed to determine the location of the
 sensor when the sensor transmits. FIG. 2 illustrates a direct
 determination method, wherein the sensor 100 includes a location
 determining device 210, as well as the parameter sensing device 220. A
 transmitter 230 receives the location L 211 from the location determining
 device 210, and the parameter value P 221 from the parameter sensing
 device 220, and communicates a communications signal 231 that contains an
 encoding of both the location L 211 and the parameter value P 221. As
 noted above, a particular parameter value 221 may effect a null
 transmission, wherein neither the location 211 nor the parameter value 221
 is transmitted. The location determining device 210 can be any commonly
 available location determining device, such as a Global Positioning System
 (GPS) device, an inertial navigation device, a LORAN device, and the like.
 The satellite communications system 150-160 determines the coordinates of
 the location of the transmitter at the time of transmission by decoding
 the location 211 from the communications signal 231.
 FIG. 3 illustrates the use of multiple satellites 150a, 150b, 150c to
 determine the location of a sensor 100. The sensor 100 transmits a
 communications signal 231 that contains an encoding of the parameter value
 PI and that has signal characteristics C. The signal characteristics C may
 include the frequency, phase, amplitude, and time of the transmitted
 communications signal 231.
 As the communications signal 231 propagates to the satellites 150a, 150b,
 150c, the signal characteristics C change, relative to each satellite. For
 example, the time of arrival of the communications signal 231 at each
 satellite will in most cases be different at each satellite, and different
 from the time of transmission. Similarly, the frequency of the
 communications signal 231 that is received at each satellite will differ,
 depending upon the relative motion of each satellite relative to the
 sensor 100 (the Doppler effect). Each satellite 150a, 150b, 150c
 communicates a relayed signal to the ground station 160, and this relayed
 signal contains the modified characteristics C1, C2, and C3 of the
 communications signal 231a, 231b, and 231c as it was received at each
 satellite 150a, 150b, and 150c, respectively. These characteristics may be
 explicitly or implicitly communicated to the ground station 160. For
 example, the differing received frequencies may be communicated by merely
 relaying the communications signal 231a, 231b, 231c to the ground station
 160 and having the ground station 160 determine the relative frequency
 shifts based on these received communications signals 231a, 231b, 231c
 from each satellite 150a, 150b, 150c. As would be evident to one of
 ordinary skill in the art, the frequency of the received communications
 signals from each satellite will also be affected by the relative motion
 of each satellite relative to the ground station 160, and must be
 compensated for in the determination of the frequency shifts at each
 receiving satellite. Alternatively, each satellite 150a, 150b, and 150c
 may determine the pertinent characteristics of the received communications
 signal 231a, 231b, 231c and explicitly communicate the determined
 characteristics C1, C2, C3 to the ground station 160. For example, each
 satellite may append the time of receipt of the communications signal 231
 at the satellite to the signal that is relayed to the ground station 160.
 The ground station 160 determines the coordinates of the location of the
 transmitting sensor 100 based upon these received signal characteristics
 C1, C2, C3 using techniques that are common in the art. For example, if
 the time of receipt at each satellite is identical, the sensor 100 must
 lie along a loci of points that are equidistant from each satellite. Under
 typical conditions, the loci of points that are equidistant from each of
 four or more satellites is a single point or a loci of points about a
 small area. Note that the individual satellites 150a, 150b, 150c may, in
 fact, be a single satellite 150 at different locations over time. The
 frequency shift of the communications signal 231a, 231b, 231c at each of
 these different locations, for example, can be used to determine the
 location of the transmitting sensor 100, particularly if the transmitting
 frequency at the sensor 100 is known.
 In a preferred embodiment, the satellite 150 includes a high-gain
 directional antenna. By using a high gain antenna at the satellite 150,
 the transmit power of the sensor 100 can be substantially reduced, thereby
 substantially reducing the cost of the sensor 100. By definition, a high
 gain antenna is a directional antenna. The high gain is achieved by
 focusing the field of view of the antenna along an axis of orientation of
 the antenna. Signals, including noise, outside the field of view of the
 antenna are substantially attenuated, thereby providing for a higher
 signal to noise ratio for those signals within the field of view. A
 directional antenna also facilitates the determination of the location of
 a sensor 100, because if the satellite 150 receives the communications
 signal 231 from the sensor 100, the sensor 100 must lie within the
 directional antenna's field of view. FIG. 4 illustrates an example of a
 directional antenna 155 on the satellite 150 to determine of the location
 of a sensor 100. The directional antenna 155 is oriented along an
 orientation axis 158, and is designed to have a field of view 159 that
 about the orientation axis 158. The satellite 150 contains an orientation
 controller 152 that controls the orientation A of the antenna 155. As is
 common in the art, an array of high gain antenna elements may be
 distributed about the satellite 150, and the orientation controller 152
 controls the orientation A via the selection of one or more of the high
 gain elements. Upon receipt of a communications signal 231 from the sensor
 100, the satellite 150 appends the orientation A of the orientation axis
 158 at the time of receipt of the signal 231 to the signal 151 that is
 relayed to the ground station 160. The processor 165 of the ground station
 determines the location of the sensor 100, based on the location of the
 satellite 150 and the angle A of the orientation axis 158 of the antenna
 155. For example, if the sensor 100 is known to be at a particular
 altitude, then the intersection of the orientation axis 158 with a
 spherical surface defining this altitude defines the location of the
 sensor 100. Alternatively, the aforementioned frequency or time
 differential between the transmission and reception of the communications
 signal 231 can be used to determine the distance between the satellite and
 the sensor 100, and the point along the orientation axis at this distance
 from the satellite defines the location of the sensor 100. Also, the
 aforementioned use of multiple satellites, each having a directional
 antenna, can be used to determine the location of each, sensor 100, using
 conventional radio direction finding techniques.
 FIG. 5 illustrates a remote sensor 100 that responds to a trigger signal
 that is transmitted from the satellite 150. The satellite 150 includes a
 communications receiver 510 and transmitter 520 for relaying the
 communications signal 231 from the sensor 100 to the ground station 160,
 and a trigger transmitter 530 for transmitting a trigger signal 531 to the
 sensor 100. To minimize the transmit power requirements, the trigger
 transmitter 530 transmits the trigger signal 531 to the sensor 100 via the
 directional antenna 155. The orientation controller 152 controls the
 orientation of the directional antenna 155, and controls a switch 540 that
 switches the directional antenna 155 between the trigger transmitter 530
 and the communications receiver 510. The orientation controller 152 also
 communicates information concerning the orientation of the directional
 antenna 155 to the ground station 160 via the communications transmitter
 520.
 The sensor 100 in FIG. 5 includes a trigger receiver 580 that receives the
 trigger signal 531 whenever the sensor 100 is within the field of view 159
 of the directional antenna 155. Upon receipt of the trigger signal 531,
 the transmitter 230 of the sensor 100 transmits a communications signal
 231, in dependence upon the value of the parameter that is sensed by the
 parameter sensing device 220.
 The use of a directed trigger signal 531 to trigger the transmission of a
 communications signal when the sensor 100 is within the field of view 159
 of the satellite 150 provides a number of advantages. The power
 utilization of the sensor 100 is substantially reduced by constraining the
 transmission of a communications signal 231 to only those intervals of
 time that the satellite antenna 155 is able to receive the communications
 signal 231 from the sensor 100. The probability of interference among the
 plurality of sensors 100 is also substantially reduced, because only those
 sensors 100 that are within the field of view 159 of the satellite 150
 will receive and respond to the trigger signal 531. In a preferred
 embodiment of the in situ remote sensing system, the sensors 100 are each
 designed so as to minimize the likelihood of interference among them. For
 example, each sensor 100 can be designed to conform to the common `listen
 before transmit` broadcast protocol. Each sensor 100 transmits only when
 no other sensor 100 is already transmitting; in this protocol,
 interference is reduced to the rare occasion of two sensors initiating a
 transmission at exactly the same time. Other techniques of interference
 minimization techniques are common in the art. In a preferred embodiment,
 each sensor 100 transmits using a CDMA (Code Division Multiple Access)
 transmission method. The CDMA transmission protocol also reduces the
 interference to the rare occasion of two sensors simultaneously initiating
 a transmission, but does not require that each sensor 100 contain a
 receiver for determining whether another sensor is transmitting.
 As is common in the art of transponders, the sensor 100 of FIG. 5 can use
 the power contained in the received trigger signal 531 to provide the
 power to transmit the communications signal 231, thereby eliminating the
 need for a discrete power supply, such as a battery, within the sensor
 100. In a conventional transponder, the receiver 580 is a high-Q resonant
 circuit that is tuned to the frequency of the trigger signal 531. Upon
 receipt of the trigger signal 531, the high-Q resonant circuit contains
 the energy required to power the sensor 100. In a simple transponder of
 FIG. 6, the function of the receiver 580 and the transmitter 230 of FIG. 5
 is effected by a single high-Q resonant circuit 680, and the sensor 220 is
 configured to affect the value of Q in response to the sensed value of the
 parameter. For example, at high-Q, the resonant circuit will continue to
 oscillate after the trigger signal 531 ceases. The continued oscillation
 forms the communications signal 231. The directional antenna 155 of FIG.
 5, being a high gain antenna, detects this continued oscillation in
 response to the trigger signal. At low-Q, the resonant circuit is damped,
 and the oscillation ceases soon after the trigger signal 531 ceases and
 effects a null transmission in response to the trigger signal 531.
 The transponder of FIG. 6 may be further simplified if the parameter being
 sensed is the trigger signal from the satellite. The high-Q resonant
 circuit 680 will be energized when it is triggered; therefore, the high-Q
 resonant circuit 680 can also be used to sense the trigger signal. In this
 manner, the sensor 100 may consist only of a single high-Q resonant
 circuit 680 that is resonant at the frequency of the trigger signal. With
 nano-technology techniques commonly available, such a sensor 100 can be
 made extremely small in size, light in weight, and low in cost.
 In an example embodiment, such sensors 100 may be deployed in large
 quantities by an aircraft into the jet stream, and the satellite
 communications system 150-160 can be used to map the speed, volume, and
 direction of the jet stream over time. Similarly, the sensor 100 may be
 designed to attach itself to particular elements or forms of matter, and
 the satellite communications system 150-160 used to track the propagation
 of these particular elements or forms of matter.
 The sensor 100 of FIG. 6 receives the power to transmit the communications
 signal from the trigger signal. Another method of providing the energy
 required to power a sensor 100 of FIGS. 1-5 is via the sensing device 220.
 If the sensor 100 is sensing light, for example, the sensing device 220
 may be an array of photocells. When the light is above a particular level,
 the output of the photocells powers the transmitter 230; when there is
 insufficient light, a null transmission results. Similarly, thermal
 sensing devices generate an electromotive force in response to heat,
 piezoelectric devices generate an electromotive force in response to a
 mechanical force, and so on.
 It should also be noted that, consistent with this invention, the
 communications signal that is transmitted to the satellite may be a
 reflection of a signal that is received from another source. For example,
 the resonant circuit 680 may be energized by a trigger signal that is
 emitted from a source that is not located at the satellite, such as a
 trigger generating system that is deployed in the same area of interest as
 the sensors 100. Also, the communications signal that is transmitted to
 the satellite need not be constrained to a signal in the radio-frequency
 spectrum. In another preferred embodiment, the sensor 100 is designed to
 emit light energy, using for example, different colors of light to encode
 the value of the parameter being sensed. In a light-emitting embodiment
 that is analogous to FIG. 6, the high-Q resonant circuit 680 is an optical
 device that reflects light if the sensing device 220 senses a particular
 value, and does not reflect light otherwise. The antenna 155 and receiver
 510 in this embodiment are configured to receive the reflections of light
 from each sensor 100.
 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, FIG. 2 shows a location determining device
 210, such as a GPS device, wholly contained within the sensor 100. A GPS
 device comprises the equipment necessary to receive the GPS signals and
 the equipment necessary to determine a coordinate based upon these
 received GPS signals. To reduce the overall costs, each sensor 100 may
 only contain the necessary equipment to receive the GPS signals, and the
 ground station 160 would contain the coordinate determination equipment.
 The received GPS signals would be communicated directly to the satellite
 communications system 150-160 via the transmitter 230 and subsequent
 converted to a location coordinate by the commonly used, and easily
 accessible, equipment at the ground station 160. Similarly, in this and
 the previous examples, a majority of the processing is effected at the
 ground station 160. As would be evident to one of ordinary skill in the
 art, this processing could be distributed between the ground station 160
 and the satellite 150. In general, however, the ease of access to
 equipment at the ground station 160, and the cost per pound to launch a
 satellite 150 favors the distribution of as much processing as possible to
 ground station 160.
 REFERENCE
 1. Space Mission Analysis And Design, Second Edition, Wiley J. Larson and
 James R. Wertz (editors), Chapter 22, Design of Low-Cost Spacecraft, Rick
 Fleeter and Richard Warner, AeroAstro, pages 782-785. Published jointly by
 Microcosm, Inc. 2601 Airport Drive, Suite 230, Torrance, Calif. 90505 USA
 and Kluwer Academic Publishers, P.O. Box 17, 3300 AA, Dordrecht, The
 Netherlands. Copyright 1992 W. J. Larson and Microcosm, Inc.
 ISBN 1-881883-01-9 (pb.) (acid-free paper)
 ISBN 0-7923-1998-2 (hb.) (acid-free paper)