Abstract: A system and method are provided for detecting local interference in GPS signals. A GPS receiver is capable of determining its GPS coordinates. A memory is capable of storing an initial location of the GPS receiver and a user-defined range of error. A processor is programmed to determine whether GPS coordinates from the GPS receiver differ from the initial location by more than a range of error, and for issuing a warning in response thereto.

Abstract: A method for monitoring of the substitution at least one high quality position measurement with a set of upgraded low quality position measurements comprising the steps of: (A) selecting a high quality source of position measurements of the object from a group of high quality sources; (B) obtaining at least one high quality position measurement of the object by using the high quality source; (C) saving at least one high quality position measurement of the object; (D) obtaining at least one low quality position measurement of the object by using a low quality source; (E) substantially continuously monitoring and checking if the currently available high quality position measurement of the object is of substantially high quality; (F) if at least one currently available high quality position measurement of the object is of substantially high quality, using at least one high quality position measurement of the object for navigation purposes of the object; (G) if the currently available high quality position measur

Abstract: A satellite positioning system and method of maintaining the positioning and clock data of a satellite using as few as one or two monitoring stations is disclosed. Each satellite of the positioning system determines its own positioning and clock data relative to other satellites in a constellation. One or more satellite signal monitoring stations receive relative clock and positioning data from a plurality of satellites in the constellation. A constellation solution which is applicable to all of the satellites in the constellation is calculated by treating the constellation as a rigid body at a fixed point in time. The constellation solution is transmitted to a satellite of the constellation which disseminates the solution through a crosslink network within the constellation for use by each satellite to update its individual positioning and clock data as needed.

Abstract: GPS signals are typically weak and thus easily interfered with by other radio transmissions in the same or adjacent frequency bands. Interference can be especially problematic when the GPS receiver is co-located with a communications device that includes a radio transmitter, such as a cellular telephone. The transmitted signal from the co-located communication device can overload (or saturate) the GPS receiver front-end designed to receive weak GPS signals. In such a situation no useful information can be extracted from the received GPS signals originating from the GPS satellites. Described herein is a novel apparatus and method that can be used to minimize the effect of co-located interference on a GPS receiver.

Abstract: The Fault Detection and Exclusion (FDE) system for use in navigational systems that rely upon multiple ranging signals, such as GPS satellites, uses an FDE algorithm to detect, as soon as possible, whether a fault exists in a signal associated with one or more of the GPS satellites. The system makes this determination by comparing a computed residual error with a first fault detection threshold. After determining that the computed residual error has exceeded the first, relatively low, fault detection threshold, the system transmits a signal to one or more external systems indicating that one or more signals associated with the various GPS satellites may be faulty. The system then monitors horizontal estimated position error (HUL) until this value has met or exceeded a relatively high fault isolation threshold value. The system then attempts to isolate and exclude the faulty satellite from the current navigational solution.

Abstract: Method in a Global Positioning System (GPS) receivers, including determining pseudorange (PNR) measurements for at least four satellites (210), determining a coarse time (220) corresponding to the pseudorange measurement, determining an offset time (240) between a periodic GPS event of one of the four satellites and the coarse time, determining a time correction delta (250) based upon the period of the Periodic GPS event, the offset time and the coarse time if an error of the coarse time is less than ½ the period of the periodic GPS event, and determining corrected time (260) based upon the coarse time and the time correction delta if the error of the coarse time is less than ½ the period of the periodic GPS event.

Abstract: A portable terminal carried by a user and in wireless communication with a local area network for displaying data based on the physical location of the user. A plurality of network nodes is distributed throughout a venue, such as a shopping mall or a store, in which products are offered for sale. The location of the user is determined by the location of the access node in communication with the terminal. The data may include a recommendation to purchase the product.

Abstract: A method and system for controlling a movable appendage on an aerospace vehicle independent from the position of the aerospace vehicle. First and second beacons containing actual position information for the movable appendage are tracked and acquired by first and second beacon tracking sites. The beacon tracking sites report the azimuth and elevation data relating to the position of the movable appendage to a processor where a correction command is determined based on an error calculated from the position data and a desired position. The correction command is communicated to the movable appendage so that appropriate correction to the desired position can be made.

Abstract: A synthetic aperture radar (SAR) compensates for ionospheric distortions based upon measurement of the group delay, particularly when operating in the VHF/UHF band. The SAR is based upon a multi-input multi-output (MIMO) technique for estimating the effective ionospheric conditions, which is referred to as the group delay approach. The group delay approach is divided into a 1-dimensional (range) approach and a 2-dimensional (range and cross-range) approach. The group delay measures the effective or observed TEC, which is used to reduce the ionospheric distortion.

Abstract: A synthetic aperture radar (SAR) for a moveable platform includes an antenna, a radar transmitter and radar receiver cooperating with the antenna. A radar processor is connected to the radar transmitter and radar receiver to account for the Faraday rotation introduced by propagation through the ionosphere by estimating an individual ionospheric distortion for each received echo pulse based upon a measured Faraday rotation, and reducing the ionospheric distortion for each received echo pulse based upon the estimated individual ionospheric distortion associated therewith for providing a compensated echo pulse.

Abstract: Systems and methods for estimating the bias and scale factor in a speed sensor and pitch sensor using a modified adaptive filter. A filter learns adaptively in a random manner to estimate the errors in the vehicle speed and pitch angle. The aiding measurements used in this approach are exemplarily DGPS travel distances. The filter adaptively estimates error source values during the availability of DGPS, and can then use these estimates to aid an INS during DGPS outages.

Abstract: An off-gimbal pointing system is improved using filtering of resolver and gyro responses respectively applied at the output of the resolvers and gyros for attenuating high frequencies resolver responses and gyro responses that effectively degrades the high frequency responses that are matched and above the control system bandwidth for improving the overall dynamic control of the off-gimbal pointing system for rejecting the affects of base motion disturbances and vibrations.

Abstract: A method for calculating instantaneous characteristics of a satellite in orbit equipped with a GNSS receiver, capable of performing measurements of pseudo-distance, when fewer than four transmitters are simultaneously in visibility. This method pre-processes quantities measured by the receiver to deliver measurements of pseudo-distance, pseudo-speed and instantaneous pseudo-accelerations, mathematically processes the measured quantities, enhanced by knowledge of complementary data concerning the orbit of the satellite and the receiver, delivering plural solutions, and physically filters the solutions to deliver position and speed of the satellite and clock bias and drift of the receiver.

Abstract: A GPS (Global Positioning System) receiver and a navigation device and system utilizing the same includes a receiver which sequentially generates GPS data including current position information, based on signals received from GPS satellites. The GPS receiver further includes at least one filter which performs filtering with respect to the GPS data sequentially generated by the receiver and generates positioning data including the current position information, and a positioning data output device which selects a filter according to a selection signal and outputs the positioning data in accordance therewith.

Abstract: The Fault Detection and Exclusion (FDE) system for use in navigational systems that rely upon multiple ranging signals, such as GPS satellites, uses an FDE algorithm to detect, as soon as possible, whether a fault exists in a signal associated with one or more of the GPS satellites. The system makes this determination by comparing a computed residual error with a first fault detection threshold. After determining that the computed residual error has exceeded the first, relatively low, fault detection threshold, the system transmits a signal to one or more external systems indicating that one or more signals associated with the various GPS satellites may be faulty. The system then monitors horizontal estimated position error (HUL) until this value has met or exceeded a relatively high fault isolation threshold value. The system then attempts to isolate and exclude the faulty satellite from the current navigational solution.

Abstract: A method and apparatus for locating mobile device over a broad coverage area using a wireless communications link that may have large and unknown latency. The apparatus comprises at least one mobile device, a reference network, a position server, a wireless carrier, and a location requester. The mobile device is in communication with the wireless carrier and receives global positioning system (GPS) signals from a plurality of satellites in the GPS satellite constellation. The reference network is coupled to the position server and provides GPS data. The mobile receiver receives GPS signals, performs rudimentary signal processing and transmits the processed signals to the wireless carrier. The wireless carrier passes the signals on to the position server. The position server processes the mobile receiver's GPS data and the reference network ephemeris data to identify the location of the mobile receiver. The location is sent to the location requester.

Abstract: Systems, devices and methods are provided for determining the appropriate or desired geographical correction source for SBAS corrections. One aspect provided herein is a method. According to one method embodiment, a Space Based Augmentation System (SBAS) correction message is received from a selected SBAS satellite. It is determined whether at least one criterion is satisfied for using the selected SBAS satellite as a correction source and then processing the correction message received therefrom. A second SBAS satellite is selected from which to receive SBAS correction messages upon determining that at least one criterion is not satisfied for using the selected SBAS satellite as a correction source. One example of SBAS is the Wide Area Augmentation System (WAAS) used in North America Other aspects and embodiments are provided herein.

Abstract: Methods and structures are provided for reducing pointing errors &zgr; of satellite antennas and for generating broad field-of-view satellite attitude acquisition patterns. In one method embodiment, satellite transmit beams have estimated pointing attitudes &bgr; and are transmitted to overlap on a ground-based receiving terminal which has a known terminal location &lgr; and which measures received signal strengths &agr;. Pointing errors &zgr; of the transmit beams are then determined from the estimated pointing attitudes &bgr;, the terminal location &lgr; and the signal strengths &agr; and the pointing errors &zgr; are subsequently reduced by revising the pointing attitudes &bgr;. Other method embodiments utilize known signal-strength functions and antenna signals with known signal parameters such as frequencies and/or modulations.

Abstract: Upon reception of four GPS signals from GPS satellites and determining four pseudo ranges, along with ephemeris data previously stored in the GPS receiver, the location of the GPS receiver and real time clock time error is determined. The GPS receiver is in possession of four pseudo ranges and determines four unknown coordinate values (x, y, x, and time) identifying the location of the GPS receiver and real time clock error. The process of solving for four pseudo range formulas simultaneously with each pseudo range formula having an unknown “x”, “y” “z”, and time coordinates of the GPS receiver, results in identification of the coordinates and time of the GPS receiver. In a similar process, the GPS receiver receiving four GPS signals from four GPS satellites is able to determine four pseudo ranges. Using the four pseudo ranges, four pseudo range equations unknown values for “x”, “y”, “z” and time can be solved for simultaneously.

Abstract: A portable tester verifies the operational status of a satellite-based navigation ground facility. The portable tester includes a receiver and a computer. In a preferred embodiment, the receiver obtains Global Position System (GPS) signals from a plurality of satellites and Very High Frequency (VHF) broadcasts from the ground facility. The computer compares the data obtained from the receiver with expected results and determines if the ground facility is operational if not operational, the computer determines a subsection of the ground facility causing the failure. If the ground facility is not operating properly, it may not transmit the proper differential corrections and glidepath information to an aircraft. As a result, a pilot may not receive accurate information regarding the aircraft's position. This may result in landings that do not closely follow a prescribed glidepath, which could be disastrous.

Abstract: A signal deformation monitor that corrects for bias caused by receiver front end signal deformation is disclosed. The signal deformation monitor includes a correlator that determines a plurality of correlation measurements along a correlation curve. The correlation measurements are transformed by a correlation transformation. The correlation transformation subtracts the mean of the correlation measurements over all tracked satellites from each of the correlation measurements calculated by the correlator. The transformed correlation measurements are independent of the front end signal deformation, resulting in a deformation monitor that will more accurately detect satellite signal deformation.

Abstract: A method for substituting at least one high quality position measurement with a set of upgraded low quality position measurements comprising the steps of: selecting a high quality source of position measurements of an object; obtaining at least one high quality position measurement of the object by using the high quality source; wherein each high quality position measurement of the object satisfies the threshold of acceptability requirement for the object; saving at least one high quality position measurement of the object taken at at least one high quality epoch timing coordinate; selecting a low quality source of position measurements of the object; obtaining at least one low quality position measurement of the object using the low quality source; if at least one high quality position measurement of the object is available and substantially recent, using at least one high quality position measurement of the object for navigation purposes of the object; if each high quality position measurement of the object

Abstract: Methods and apparatus are described and illustrated for producing GPS corrections, comprising: collecting measurements from a plurality of network reference stations; determining network corrections from the measurements; determining residual errors at one or more vernier-cell reference stations; and preparing vernier-cell corrections to compensate the residual errors within a vernier-cell region. Network correction streams are described and illustrated which contain network corrections derived from a plurality of network reference stations and residual error corrections derived from one or more vernier-cell reference stations. Methods and apparatus are described for employing such network correction streams in a virtual reference station to produce corrections and/or virtual measurements for use in a GPS receiver.

Abstract: A signal deformation monitor that corrects for bias caused by receiver front end signal deformation is disclosed. The signal deformation monitor includes a correlator that determines a plurality of correlation measurements along a correlation curve. The correlation measurements are transformed by a correlation transformation. The correlation transformation subtracts the mean over all tracked satellites from each of the correlation measurements calculated by the correlator. The transformed correlation measurements are independent of the front end signal deformation, resulting in a signal deformation monitor that will more accurately detect satellite signal deformation.

Abstract: A method and apparatus for correlating a satellite signal with a reference code is described. In an example, samples of a satellite signal having a repeating code are received. At least one fast convolution between the samples and the pseudorandom reference code is performed. A correlation peak is detected within a result of the at least one fast convolution. A code phase of the repeating code relative to the pseudorandom reference code is estimated at the correlation peak. The samples are then correlated with the pseudorandom reference code using the code phase as an initial value.

Abstract: A data detection circuit within a global positioning system (GPS) satellite receiver operates to detect and decode data sent in a spread spectrum signal. The data detection circuit receives input from a radio receiver, the information containing data from a plurality of satellites. The data is supplied to a circular memory device, which determines which data corresponds to which satellite. The memory device sends the received signal to a matched filter, which decodes the signal received from each satellite. This signal is analyzed to determine whether a phase inversion due to data modulation on the received signal is present. The phase inversion can occur at boundaries, known as data epochs, in the received signal, and corresponds to data in the received signal. This data contains information relating to the position of each satellite and is collected by the data detection circuit for use by the GPS receiver.

Abstract: A method and system for using two-way ranging navigation to accurately determine the location of a transponder platform above-Earth or a fixed or mobile target on the ground or on water. The two-way ranging navigation measurements are used as calibration references, thereby improving the positioning accuracy of GPS. The system includes GPS and a two-way ranging navigation system for taking position measurements of a target. A correction factor is determined as a function of the measurements and the GPS position is adjusted by the correction factor. The method for calibrating GPS using two-way ranging navigation involves taking a two-way ranging navigation measurement and a GPS measurement of a target, determining a correction factor as a function of the measurements, and correcting the GPS position by taking a second GPS measurement and adjusting it by the correction factor.

Abstract: An apparatus to permit real time movements and events experienced by one or more individuals to be recorded for subsequent replay in a virtual reality domain. The apparatus comprises a GPS receiver for receiving a plurality of GPS signals, a processor operatively connected to the GPS receiver and programmed to calculate the position of the receiver relative to the surface of the earth, a memory module operatively connected to the processor, and means to permit data corresponding to the movement of the GPS receiver and data corresponding to events experienced by the operator to be downloaded into the memory of a secondary processor. The processor stores in the memory module data corresponding to the calculated position of the GPS receiver and the movement of the GPS receiver for a given time interval. The memory module also stores data corresponding to specific events experienced by an operator of the apparatus during the time interval.

Abstract: A global positioning system comprising: (a) a GPS receiver configured to (1) receive a plurality of signals from a plurality of visible GPS satellites, and (2) produce a plurality of pseudorange measurements from the received signals, the pseudorange measurements being indicative of the GPS receiver's position and having an amount of ionospheric delay error contained therein, (b) a processor configured to estimate the amount of ionospheric delay in the pseudorange measurements by (1) estimating an amount of global ionospheric delay attributable collectively to the plurality of visible GPS satellites, and (2) estimating a plurality of amounts of local ionospheric delays, each local ionospheric delay being attributable to a different visible GPS satellite. Preferably the processor implements the ionospheric delay estimations using a modified Kalman filter.

Abstract: Apparatuses, methods, and systems of transferring correction information are described. In certain implementations, correction factors relate to stored or calculated values, and a correspondence between a correction factor and a value is indicated by a predetermined order of the correction factors. In one application, a method according to an embodiment of the invention is used to transmit correction factors relating to the positions of physical objects. For example, such a method may be used to transmit correction factors relating to the positions of space vehicles within a Global Positioning Satellite (‘GPS’) system.

Abstract: A global positioning system comprising: (a) a GPS receiver configured to (1) receive a plurality of signals from a plurality of visible GPS satellites, and (2) produce a plurality of pseudorange measurements from the received signals, the pseudorange measurements being indicative of the GPS receiver's position and having an amount of ionospheric delay error contained therein; (b) a processor configured to estimate the amount of ionospheric delay in the pseudorange measurements by (1) estimating an amount of global ionospheric delay attributable collectively to the plurality of visible GPS satellites, and (2) estimating a plurality of amounts of local ionospheric delays, each local ionospheric delay being attributable to a different visible GPS satellite. Preferably the processor implements the ionospheric delay estimations using a modified Kalman filter.

Abstract: A method, apparatus for reducing errors in a plurality of beacon beams is disclosed. The method comprises the steps of computing quantized channel weights {tilde over (W)}c from channel weights Wc for at least some of the channels; estimating the quantization error &Dgr;Ba for each of the beacon beams from a difference between the channel weights Wc and the computed quantized channel weights {tilde over (W)}c; and adding the estimated quantization error &Dgr;Ba to the beacon beams. Similarly, a beacon biases equivalent to the beacon asymmetry error can be computed by the ground beacon beam forming software and uploaded to the on-board software for error compensation. The apparatus comprises one or more means, such as a processor communicatively coupled to a memory storing instructions for performing these operations.

Abstract: The invention relates to a method for determining the correlation phase between a signal received at a receiver and a replica sequence. A matched filter multiplies samples (21) of the received signal with samples (22) of the replica and sums the resulting products to obtain a correlation value for a specific correlation phase. The samples of the received signal and the replica are shifted relative to each other for each correlation phase that is to be checked. In order to reduce the computational load, it is proposed that results obtained in the correlation calculations for one correlation phase are used by the matched filter also for calculations for a subsequent correlation phase. The invention relates equally to a corresponding receiver, to an electronic device comprising such a receiver, to a device cooperating with such a receiver and to a corresponding system.

Abstract: A method, apparatus for reducing errors in a plurality of beacon beams is disclosed. The method comprises the steps of computing quantized channel weights {tilde over (W)}c from channel weights Wc for at least some of the channels; estimating the quantization error &Dgr;Ba for each of the beacon beams from a difference between the channel weights Wc and the computed quantized channel weights {tilde over (W)}c; and adding the estimated quantization error &Dgr;Ba to the beacon beams. Similarly, a beacon biases equivalent to the beacon asymmetry error can be computed by the ground beacon beam forming software and uploaded to the on-board software for error compensation. The apparatus comprises one or more means, such as a processor communicatively coupled to a memory storing instructions for performing these operations.

Abstract: Systems and methods for estimating the bias and scale factor in a speed sensor and pitch sensor using a modified adaptive filter. A filter learns adaptively in a random manner to estimate the errors in the vehicle speed and pitch angle. The aiding measurements used in this approach are exemplarily DGPS travel distances. The filter adaptively estimates error source values during the availability of DGPS, and can then use these estimates to aid an INS during DGPS outages.

Abstract: A method and system are disclosed for providing WAAS like corrections using a server and processor on the Internet. For one method, satellite measurement data is collected. Satellite position error corrections are computed using the collected satellite measurement data. The computed satellite position error corrections are stored. The stored satellite position error corrections can then be later retrieved and used by a server and processor on the Internet.

Abstract: A satellite command system and method that provides satellite commanding using remotely-controlled modulation of telemetry parameters on-board a satellite. Software is provided on each of the redundant processor units that creates separate DMA command input pathways to the redundant processor units. A command translator is used at a ground station to translate a command that is to be implemented on the satellite into a form ready for modulation. The command information is uplinked to the satellite by remotely manipulating (modulating) a quantity that can be measured by the on-board computer using its telemetry collection functions. The software implemented in processor units recognizes the modulation and processes it to reconstruct the command. The reconstructed command is passed to the processor unit, which then executes the command.

Abstract: A method and apparatus for radio navigation and location is disclosed. The method uses an autonomous, on-board micro-inertial navigation system to propagate the state of a station, and an angularly resolvable antenna to measure relative orientation and relative range and to receive estimated state information from one or more companion stations. Optimally estimated state information is used to permit operation in environments that are otherwise hostile to communications between stations.

Abstract: An arrangement and appertaining method utilizing the arrangement provides a cost effective way to implement an accurate and cost effective satellite positioning differential augmentation system. This hybrid system integrates a network of Ground Based Augmentation Systems (GBAS) with a Satellite Based Augmentation System (SBAS), permitting the high integrity features of the GBAS to be utilized with the much broader coverage area of the SBAS system without requiring significant expenditures that would be required for upgrading either of the systems independently.

Abstract: A thermal distortion estimation system for delivering thermal time varying distortion of various spacecraft antenna is provided which includes a measurement signal outage indicator and a storage device containing time varying distortion data. A signal is generated indicating an outage for the system received by the spacecraft and then a generated time varying distortion of the system from a previous measurement history to predict the error resulting from the thermal distortion is employed to estimate the error.

Abstract: A thermal distortion estimation system for delivering thermal time varying distortion of various spacecraft antenna is provided which includes a measurement signal outage indicator and a storage device containing time varying distortion data. A signal is generated indicating an outage for the system received by the spacecraft and then a generated time varying distortion of the system from a previous measurement history to predict the error resulting from the thermal distortion is employed to estimate the error.

Abstract: Payload performance is optimized by determining yaw trajectory employing a method which develops a mathematical expression to define the payload in terms of ‘n’ location(s) on the earth; determining the pointing error of the ‘n’ location(s); combining the error so as to product a single performance parameter and minimizing the value of the performance parameter by appropriately varying yaw.

Abstract: According to the present invention, it is possible to output from a GPS receiver positioning data having a positioning precision or a positioning rate appropriate to the intended use. In the GPS receiver 1, the first filter section 13a and the second filter section 13b perform filtering with respect to the GPS data sequentially generated at the receiving section 12, and generate positioning data. The positioning precision and the positioning rate of the first filter section 13a are different from those of the second filter section 13b. According to a selection signal from a navigation device 2, the positioning data output section 14 outputs the positioning data obtained in either the first filter section 13a or the second filter section 13b to the navigation device 2.

Abstract: A terminal Global Positioning System (TGPS) adopting a real-time self Differential Global Positioning System (DGPS) is developed for positioning a moving station in a local area. This TGPS enables not only to minimize the error ranges without any additional expensive transmitting-receiving equipment, but also to calculate an accurate correcting value through a TGPS server.

Abstract: Absolute positions of global position system (“GPS”) enabled mobile communications devices, which form a wirelessly networked group of GPS mobiles, can be determined at each of the mobiles based on use of wireless communications capabilities of the mobiles for obtaining relative positioning information or range measurements for the mobiles of the group. Although the mobiles of a group cannot establish line of sight (“LOS”) communications signal reception paths with a sufficient number of GPS satellites to permit each mobile to determine its absolute position solely based on GPS signal transmissions, the mobiles can compute their respective absolute positions and the absolute positions of other mobiles if a predetermined number of LOS paths can be established and based on the obtained range or relative positioning information.

Abstract: In a system for determining a precise orbit of a satellite, a satellite control system includes a tracking, telemetry and command module for receiving telemetry data from the satellite, tracking the satellite, and performing link to the satellite; a satellite operations sub module for extracting L1 carrier phase data by processing the telemetry data, monitoring the satellite, generating telecommand data, and controlling the satellite; and a mission analysis and planning subsystem for determining the precise orbit of the satellite using the L1 carrier phase data collected by a satellite, L1/L2 carrier phase data of the reference ground stations of the GPS satellites collected by an IGS, and a path error caused by the ionosphere of data, and analyzing and planning a mission of the satellite.

Abstract: A method operates a mobile communication terminal or MCT (10) by the steps of: (A) determining, at the MCT 10, a location of the MCT in accordance with Global Positioning System (GPS) inputs, (B) transmitting a signal that indicates the GPS-determined location, the signal being transmitted from the MCT to a network service provider or NSP (30), and (C) downloading data to the MCT 10 from the network service provider 34 or from a server (40, 42) that is coupled to the network service provider through a data communications network (38). The downloaded data is selected in accordance with the GPS-determined location of the MCT. The step of determining may employ a GPS capability of the MCT or a GPS capability of a GPS accessory (28) that is coupled to the MCT. In one embodiment the downloaded data is comprised of map data, while in another embodiment the downloaded data is comprised of at least one of operating software or an operating protocol. The data may be downloaded as a background task.

Abstract: A self-monitoring satellite system is disclosed to more quickly and accurately convey satellite signal information and verification of its reliability to users of the signal. In a first aspect, the satellite includes one or more receivers on the satellite to monitor the reliability of signals transmitted from the satellite. In a second aspect, a warning signal is generated and transmitted substantially simultaneous with an unreliable satellite signal and at the same frequency as the satellite signal. Exemplary embodiments to implement the various aspects of the self-monitoring satellite system invention are configured as global positioning system (GPS) satellites, signals, and receivers.

Abstract: Power is conserved in a Global Positioning System (GPS) receiver by shutting down selected components during periods when the GPS receiver is not actively calculating the GPS receiver location. A low power time keeping circuit accurately preserves GPS time when the selected components are deactivated. When the selected components are turned on in response to a wake-up command, time provided from the low power time keeping circuit, corrected for actual operating temperatures, and data from the GPS clock temperature/frequency table, are used to recalibrate time from a GPS oscillator. Positions of the GPS satellites are then estimated such that the real GPS time is quickly determined from the received satellite signals. Once real GPS time is determined from the detected satellite signals, the selected components are deactivated. The process described above is repeated such that accurate GPS time is maintained by the low power time keeping circuit.

Abstract: Methods and structures are provided for reducing pointing errors &zgr; of satellite antennas and for generating broad field-of-view satellite attitude acquisition patterns. In one method embodiment, satellite transmit beams have estimated pointing attitudes &bgr; and are transmitted to overlap on a ground-based receiving terminal which has a known terminal location &lgr; and which measures received signal strengths &agr;. Pointing errors &zgr; of the transmit beams are then determined from the estimated pointing attitudes &bgr;, the terminal location &lgr; and the signal strengths &agr; and the pointing errors &zgr; are subsequently reduced by revising the pointing attitudes &bgr;. Other method embodiments utilize known signal-strength functions and antenna signals with known signal parameters such as frequencies and/or modulations.