US20110001668A1 - Rebroadcasting method and system for navigation signals - Google Patents
Rebroadcasting method and system for navigation signals Download PDFInfo
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- US20110001668A1 US20110001668A1 US11/937,247 US93724707A US2011001668A1 US 20110001668 A1 US20110001668 A1 US 20110001668A1 US 93724707 A US93724707 A US 93724707A US 2011001668 A1 US2011001668 A1 US 2011001668A1
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- synchrolite
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/03—Cooperating elements; Interaction or communication between different cooperating elements or between cooperating elements and receivers
- G01S19/10—Cooperating elements; Interaction or communication between different cooperating elements or between cooperating elements and receivers providing dedicated supplementary positioning signals
- G01S19/11—Cooperating elements; Interaction or communication between different cooperating elements or between cooperating elements and receivers providing dedicated supplementary positioning signals wherein the cooperating elements are pseudolites or satellite radio beacon positioning system signal repeaters
Definitions
- the modulated signals are shifted in frequency to a transmit frequency. After any amplification and/or filtering, the modulated spread signals are transmitted. The transmission is either continuous or intermittent.
- a navigation receiver receives the rebroadcast ranging signals and determines a range from the transmitted, modulated spread signals. The information may be used to determine a distance or position as a function of the rebroadcast signals.
- each channel 34 , 36 and 38 apply different spreading codes to uniquely identify the ranging signals despite transmission with different antennas in a same or overlapping frequency band. Different frequencies may be used in other embodiments. While the number of channels 34 , 36 and 38 shown is three, any useful number of transmit antennas 28 , 44 and 46 using the same or different number of channels 34 , 36 and 38 may be used. Multiple transmit antennas in a single synchrolite are valuable for the same reasons that multiple antennas connected to a single navigation receiver are valuable, as discussed above in the description of FIG. 3 .
- the frequency translator 58 is a mixer or multiplier. In one embodiment, the frequency translator 58 is a diode-ring mixer, but other now known or later developed frequency translators may be used.
- the frequency translator 58 is operable to shift in frequency the rebroadcast signals from the receive antenna 54 .
- the rebroadcast signals are shifted in frequency to any convenient intermediate frequency band, such as a band 200 MHz wide centered on 1400 MHz, using a local oscillator signal.
- the frequency translator may be omitted, making the intermediate frequency band within the receive channel 52 the received frequency band.
- the spreading code tracker 62 is a combination of one or more linear feedback shift registers (LFSR) and each code “chip” is a set of one or more digital bits. Other configurations of components or a single register may be used.
- the “C/A-code” and “P-code” trackers used in the American GPS system are examples of one type of spreading code tracker 62 in which each code chip is a single digital bit and the spreading code demodulator 60 is a BPSK demodulator.
- the spreading code tracker 62 is a field-programmable gate array.
- the spreading code tracker 62 performs a function similar to the spreading code generator 20 in FIG. 1 , computing a new state or “chip” of the spreading code at each transition of the spreading code clock signal. Rather than resetting periodically, the spreading code tracker 62 is shifted forward or held back a fraction of a chip at a time by the spreading code adjust signal generated by the navigation computer 72 .
- the pilot tone amplifier and/or filter 66 and detector 70 generate the error signals to adjust the timing of the reconstructed spreading code so that the code matches the timing of the transmitted spreading code.
- the pilot tone is detected once the two codes are approximately aligned. The alignment may be improved by adjusting the timing to maximize the amplitude of the detected pilot tone, or by delay-locked-loop techniques.
- the navigation computer 72 searches in one dimension (i.e., the timing of the reconstructed spreading code) to find the synchrolite signal.
Abstract
A “synchrolite” or rebroadcasting device allows GPS or GNSS navigation signals received at one or several locations to be processed at a separate location. The signals received by the synchrolite are added to a pilot tone and then encoded with a superimposed spread-spectrum code before being rebroadcast. The superimposed code allows signals from different synchrolites to be distinguished during the navigation process.
Description
- The present patent document is a divisional of U.S. application Ser. No. 10/837,515, filed Apr. 29, 2004, pending, the entire disclosure of which is incorporated herein by reference.
- This present invention relates generally to global navigation satellite systems (GNSS), such as the American global positioning system (GPS), the Russian Glonass system, and the European Galileo system. It relates specifically to synchrolites (i.e., frequency translators or rebroadcasters) for use with navigation signals from such GNSS systems.
- GNSS systems can be used in a “stand-alone” mode to compute the position of a single antenna, or in a “differential” mode to compute the relative positions of two or more antennas. In conventional systems, each antenna is directly connected to a navigation receiver. It is occasionally desired to break this direct connection and separate the antenna from the navigation receiver. In such cases, the signals received by the antenna are transmitted over a separate communications link to the navigation receiver. This separate communications link has traditionally been a separate radio frequency, but it could alternatively be an acoustic or optical link (see U.S. Pat. No. 5,345,244). When a separate radio frequency was used, the original receiver or antenna side of the link is often called a “frequency translator” or “rebroadcaster.”
- Frequency translators were first used in the early days of the American GPS program. In the late 1970's, various agencies desired to flight-test missiles and track them with GPS accuracy, but the existing GPS receivers were too large and heavy to fit inside a missile. The solution was a device (called a “GPS translator” at the time) which received signals in the GPS frequency band and retransmitted them in a telemetry frequency band from the missile to a navigation receiver on the ground, such as disclosed in U.S. Pat. No. 5,729,235.
- These frequency translators had at least one local oscillator which is independent of the GPS satellite system clocks. The frequency translation is accomplished by mixing in a radio frequency mixer the local oscillator frequency with the signals received through the antenna, producing translated signals at a new frequency. The apparent frequencies of the GPS signals from the perspective of the navigation receiver on the ground are the sum or difference of the actual GPS frequencies with the frequency of the local oscillator on board the missile. To improve the navigation accuracy, a signal known as a “pilot tone” was added to the output of the translator in the same band as the translated GPS signals. The pilot tone was derived from the local oscillator in such a way that the respective frequencies had a fixed mathematical relationship. Spread spectrum pilot tones have been suggested. The navigation receiver on the ground then measured the frequency of the pilot tone, computed the frequency of the translator's local oscillator using the known mathematical relationship, and adjusted the received frequencies of the translated GPS signals to remove the effect of the translator's local oscillator.
- The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below includes a method and systems for rebroadcasting radio signals and/or receiving rebroadcast radio signals, such as ranging signals. Ranging signals are defined herein as radio signals containing features which can be measured by a properly designed receiver to determine a quantity related to the distance or “range” between the transmitter and such a receiver, or between two such receivers. Other applications include rebroadcasting a band of radio frequency signals, such as cellular telephone signals, from a remote location to a more developed location.
- In one aspect, ranging signals are received, modulated with a spreading code and rebroadcast. For example, a frequency translator or rebroadcaster for GNSS signals modulates with a spreading code so that several different sets of GNSS signals may be rebroadcast in the same frequency band at the same time using code division multiple access (CDMA) techniques. Each channel of a synchrolite receives signals in a given GNSS band through a given antenna, shifts or translates those signals to another frequency band chosen for rebroadcast, optionally adds a pilot tone signal, modulates the set of signals with a CDMA spreading code, and then transmits this set of modulated signals through a transmitting antenna. The term “synchrolite” is used to include frequency translators and rebroadcasters as well as similar units with additional features. A single set of GNSS signals may also benefit from modulation with a spreading code and rebroadcast.
- In other aspects, a synchrolite may translate GNSS signals originally broadcast in several different frequency bands (such as the L1, L2, and L5 signals of the American GPS system) and rebroadcast them in a single frequency band with the individual sets of signals distinguished by different spreading codes. A synchrolite may receive GNSS signals through several different antennas and rebroadcast them through a same transmit antenna with the signals from individual antennas distinguished by different spreading codes. A synchrolite may receive GNSS signals through a single antenna and rebroadcast them through several different antennas with the signals from individual antennas distinguished by different spreading codes.
- In yet another aspect, a receiver for these rebroadcast signals may receive in only one frequency band, such as the rebroadcast frequency band. The ranging, position and/or navigation computations assume that each set of signals received in that band is affected in the same way by imperfections in the receiver.
- Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments and may be later claimed independently or in combination.
- The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.
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FIG. 1 is a block diagram of one embodiment of a single-channel system for rebroadcasting signals received in a single GNSS frequency band through a single receiving antenna and a single transmitting antenna; -
FIG. 2 is a block diagram of another embodiment of a multiple-channel system for rebroadcasting signals received in multiple GNSS frequency bands through a single receiving antenna and a single transmitting antenna; -
FIG. 3 is a block diagram of yet another embodiment of a multiple-channel system for rebroadcasting signals received in a single GNSS frequency band through multiple receiving antennas and a single transmitting antenna; -
FIG. 4 is a block diagram of yet another embodiment of a multiple-channel system for rebroadcasting signals received in a single GNSS frequency band through a single receiving antenna and multiple transmitting antennas; -
FIG. 5 is a block diagram of one embodiment of a single channel of a ranging receiver for receiving rebroadcast ranging signals; and -
FIG. 6 is a block diagram of another embodiment of a ranging receiver for receiving rebroadcast signals from one or more synchrolites. -
FIG. 1 shows asystem 10 for rebroadcasting ranging signals. Thesystem 10 represents one embodiment of a synchrolite for use in any application in which the relative positions of two points is to be measured. For example, the synchrolite may be fixed with respect to the surface of the earth. A navigation receiver, moving nearby, determines its position relative to the fixed synchrolite. The navigation receiver might be used to survey the locations of various points in the area around the synchrolite, or to control the motion of a vehicle or aircraft relative to a synchrolite placed on a runway, in a garage, or at an arbitrary point of interest. Alternatively, the synchrolite may be moving and the navigation receiver may be fixed. This could be useful if the synchrolite is smaller, lighter, or less expensive than the navigation receiver, or if the navigation data is desired at a fixed point. Applications include tracking people or animals across a large and possibly partly obscured area. Alternatively, both the synchrolite and the navigation receiver may be moving. Such applications include formation flying or driving of several aircraft or vehicles, automatic control of a farm implement or bulldozer blade relative to the tractor or bulldozer to which it is attached, or automatic guidance of an object dropped from an aircraft. Other applications may be used for any of the fixed or moving embodiments discussed above. - The
system 10 includes areceive antenna 12, a receive amplifier and/orfilter 14, afrequency translator 16, a signal combiner 18, amodulator 20, a spreadingcode generator 22, anotherfrequency translator 24, a transmit amplifier and/orfilter 26, atransmit antenna 28, afrequency synthesizer 30 and areference oscillator 32. Additional, different or fewer components may be used, such as having only one or nofrequency translators amplifiers 14 and/or 26 and combinations thereof. Thesystem 10 is a single-channel synchrolite in one embodiment. The dotted lines at 34 represent the single synchrolite channel. For use with multiple channels, thereference oscillator 32 andfrequency synthesizer 30 are common to all or a sub-set of channels within a given synchrolite. - The receive
antenna 12 is a GPS antenna, such as theAndrew 40 series, a microwave antenna, or other now known or later developed antenna for ranging signals. Ranging signals broadcast by a GNSS system or a land-based transmitter, such as spread spectrum or code division multiplexed signals, are received by the receiveantenna 12. In one embodiment, the receiveantenna 12 and transmitantenna 28 are part of the same physical antenna structure, constructed so that the phase centers of the receive and transmitantennas - The receive amplifier and/or
filter 14 is a low-noise amplifier and a separate filter appropriate for the frequencies of interest, such as the L1, L2 or L5 frequencies of the American GPS system. Amplification without filtering or filtering without amplification may be used. Other amplifiers and/or filters may be used. - The
frequency translator 16 is a mixer or multiplier connected with themodulator 20. “Connected with” includes direct or indirect connection. In one embodiment, thefrequency translator 16 is a diode-ring mixer, but other now known or later developed frequency translators may be used. Thefrequency translator 16 is operable to shift in frequency the ranging signals from the receiveantenna 12. The ranging signals are shifted in frequency to any convenient intermediate frequency band, such as aband 20 MHz wide centered on 175 MHz, using a local oscillator signal. In one embodiment, bothfrequency translators frequency translator synchrolite channel 34 the same as either the received or transmitted frequency band. - The
signal combiner 18 connects between the receiveantenna 12 and themodulator 20. Thesignal combiner 18 is a microstrip quadrature hybrid, summer, common electrical node or other now known or later developed combiner for summing two signals together. Thesignal combiner 18 is operable to combine the ranging signals with a pilot tone. The pilot tone is provided by thefrequency synthesizer 30 at a frequency within or near the intermediate frequency band. The pilot tone is added to the intermediate frequency signals by thesignal combiner 18. In a simplified embodiment, the pilot tone andsignal combiner 18 are skipped or omitted. - The
modulator 20 connects with the receiveantenna 12 to receive ranging signals. Themodulator 20 is a mixer, multiplier, a spreading modulator, a diode-ring mixer or other now known or later developed modulator. In one embodiment, themodulator 20 is a digital modulator, such as a binary phase shift keying (BPSK) modulator, quadrature phase shift keying (QPSK) modulator, m-phase shift keying (mPSK), or quadrature amplitude modulator (QAM). Themodulator 20 is operable to modulate ranging signals from the receiveantenna 12 with a spreading code. For example, themodulator 20 modulates the combined ranging and pilot tone signals at the intermediate frequency with the spreading code. - The spreading
code generator 22 generates a repeatable stream or sequence of digital or analog voltages or values known as “chips”. In one embodiment, the spreadingcode generator 22 is a combination of one or more linear feedback shift registers (LFSR) and each code “chip” is a set of one or more digital bits. Other configurations of components or a single register may be used. The “C/A-code” and “P-code” generators used in the American GPS system are examples of one type of spreadingcode generator 22 in which each code chip is a single digital bit and the spreadingcode modulator 20 is a BPSK modulator. In one embodiment, the spreadingcode generator 22 is a field-programmable gate array. The spreading code generator creates a stream of code “chips,” usually in the form of digital bits or groups of bits. The stream is of a fixed pattern of code chips. At each event of the spreading code clock signal, the next code chip in the pattern is selected and output by the spreading code generator. When the end of the pattern is reached, the spreading code generator starts again at the beginning of the pattern. An event on the spreading code reset signal may also cause the spreading code generator to restart at the beginning of the pattern. The spreading code repeats at a rate determined by the spreading code clock rate divided by the length of the fixed code pattern. - The
frequency translator 24 is a mixer or multiplier connected with themodulator 20. In one embodiment, thefrequency translator 24 is a diode-ring mixer, but other now known or later developed frequency translators may be used. Thefrequency translator 24 is operable to shift in frequency the ranging signals from themodulator 20. The signals (e.g., the originally received GNSS signals and the pilot tone, together modulated by the spreading code) are converted from the intermediate frequency band to the desired transmit frequency band by thefrequency translator 24 using a local oscillator signal. - The transmit amplifier and/or
filter 26 is a medium power amplifier, a band pass filter or combinations thereof. In one embodiment, separate amplifiers and filters are used, but a combined device may be provided. High pass or low pass filters may be used. Low or high power amplifiers may also be used. - The transmit
antenna 28 is a patch antenna and connects with an output of themodulator 20. Other now known or later developed transmit antennas may be used. The transmitantenna 28 transmits or rebroadcasts the ranging signals and pilot tone modulated by the spreading code to any interested receivers. - The bandwidth of the entire signal path or
channel 34, including the received ranging signals, the various intermediate frequency signals, and the transmitted signal, is wide enough to convey all or substantially all of the signals transmitted by the GNSS systems, but a less bandwidth may be used. The bandwidth of the signal path from the spreadingmodulator 20 through the transmitantenna 28 is further wide enough to convey a substantial portion of the spread combined intermediate frequency signal where the portion is sufficient to reliably despread and recover the various signals in the navigation receiver. - The
reference oscillator 32 is a crystal oscillator, such as an ovenized crystal oscillator. Other now known or later developed oscillators may be used. Thereference oscillator 32 generates a signal at any convenient reference frequency, such as 10.000 MHz. The oscillating output of theoscillator 32 is provided to thefrequency synthesizer 30 for use by thechannel 34. Separate oscillators are used in other embodiments. - The
frequency synthesizer 30 is a group of phase-locked-loop oscillator modules, but other frequency dividers or multipliers may be used. Thefrequency synthesizer 30 generates the local oscillator signals at different frequencies for thefrequency translators - A microcontroller controls operation of the synchrolite or
channel 34. For example, the microcontroller programs the phase-locked-loop modules of thefrequency synthesizer 30 and initializes other components, such as the field programmable gate array of the spreading code generator. -
FIG. 1 also represents the method for rebroadcasting ranging signals. The blocks forFIG. 1 represent a process that uses the same or different components than discussed above. Ranging signals are received, such as receiving ranging signals from a GNSS satellite, a satellite-based GNSS augmentation service such as WAAS or EGNOS, or a land-based transmitter. The ranging signals are within at least one frequency band. After any filtering and amplification, the frequency of the ranging signals is optionally shifted prior to modulation with a spreading code. The intermediate frequency ranging signals are combined with a pilot tone prior modulation with the spreading code. The pilot tone may be modulated to include information, such as status or operational information of thechannel 34. The combined ranging signals and pilot tone are modulating with a spreading code. The modulated signals are shifted in frequency to a transmit frequency. After any amplification and/or filtering, the modulated spread signals are transmitted. The transmission is either continuous or intermittent. A navigation receiver receives the rebroadcast ranging signals and determines a range from the transmitted, modulated spread signals. The information may be used to determine a distance or position as a function of the rebroadcast signals. - In one embodiment, a feedback loop is used to calibrate the shifting as a function of the received ranging signals. The
reference oscillator 32 may contain a GNSS receiver to calibrate the reference frequency as accurately as possible. Techniques for calibrating an oscillator using GNSS signals are known; see for example U.S. Pat. No. 5,274,545, the disclosure of which is incorporated herein by reference. The GNSS receiver processes ranging signals through a signal splitter connected to the receiveantenna 12 or to any other receive antennas. -
FIG. 2 shows a block diagram of one embodiment of a multiple-channel synchrolite to rebroadcast signals received in multiple GNSS frequency bands through asingle receiving antenna 12 and asingle transmitting antenna 28. Multiple transmit or receive antennas may be used. Each of thesynchrolite channels channel 34 ofFIG. 1 . For example, eachchannel synchrolite channel filter 14 operates for the frequency band chosen for that channel, and the first and possibly second local oscillator signals for the twofrequency translators channel code generator 22 in each channel generates a unique spreading code, so that the signals rebroadcast by the varioussynchrolite channels oscillator 32 andfrequency synthesizer 30 are common to all or a sub-set of thechannels frequency synthesizers 30 may be used for each or sub-sets ofchannels - Each
channel FIG. 1 , and rebroadcasts the processed ranging signals. The frequency bands received may include signals from other GNSS systems such as the Russian Glonass or the European Galileo systems, from augmentation systems such as the American wide area augmentation system (WAAS) and local area augmentation system (LAAS) or the European EGNOS, from pseudolites, or even from systems not originally intended for navigation such as commercial communication or broadcast signals. - The rebroadcast signals from the
various channels channel - The rebroadcasting of multiple GNSS frequency bands is useful for the same reasons that the reception of multiple GNSS frequency bands is useful in conventional GNSS positioning. For example, the use of measurements in different frequency bands can improve the process of determining carrier-phase integer ambiguities in carrier-phase differential GPS (CDGPS) navigation, also called real-time kinematic (RTK) navigation.
- In a method of operation,
FIG. 2 represents receiving ranging signals in at least two frequency bands, shifting the ranging signals of the at least two frequency bands to a single rebroadcast frequency band, modulating the ranging signals of each frequency band with a different spreading code, and transmitting at a rebroadcast frequency band. The transmitted signals correspond to the ranging signals from each of the at least two frequency bands. -
FIG. 3 shows one embodiment of a multiple-channel synchrolite to rebroadcast signals received in a single GNSS frequency band through multiple receivingantennas single transmitting antenna 28. Each of thesynchrolite channels channel 34 inFIG. 1 . Each channel receives ranging signals through a different receiveantenna FIG. 1 , and rebroadcasts them. The rebroadcast signals from thevarious channels modulator 20 of eachchannel antennas channel - Multiple receive
antennas - Although
FIG. 3 shows a synchrolite which rebroadcasts the L1 frequency band of the American GPS system, other, any or all of the frequency bands discussed herein may be used. Any of the receiveantennas corresponding synchrolite channel FIG. 2 . - In a method of operation,
FIG. 3 represents receiving ranging signals with a plurality of receive antennas, modulating the ranging signals of each of the plurality of receive antennas with a different spreading code, shifting the ranging signals to a rebroadcast frequency band, and transmitting signals responsive to each of the different spreading codes from a single antenna. The received ranging signals are at a same frequency band for each of the plurality of the receive antennas. The transmission at the rebroadcast frequency band of the modulated ranging signals for each receive antenna is performed simultaneously or at a same time, but may be performed sequentially. -
FIG. 4 shows one embodiment of a multiple-channel synchrolite to rebroadcast ranging signals received in a single frequency band through asingle receiving antenna 12 and multiple transmittingantennas synchrolite channels synchrolite channel 34 discussed above forFIG. 1 . Eachchannel antenna 12, processes them in the manner described in the discussion ofFIG. 1 , and rebroadcasts the ranging signals through a separate transmitantennas various channels modulators 20 of eachchannel channels antennas channels FIG. 3 . - Although
FIG. 4 shows a synchrolite which rebroadcasts only the L1 frequency band of the American GPS system, any or all of the frequency bands discussed herein may be used. In this case, the receiveantenna 12 could receive multiple frequency bands, and thecorresponding synchrolite channel FIG. 2 . Similarly, a single synchrolite may use multiple receive and multiple transmit antennas, such as for rebroadcasting ranging signals received in multiple bands through any or all of the receive antennas. Any combination of the systems ofFIGS. 1-4 may be used. Furthermore, it may prove convenient for a given synchrolite to transmit the rebroadcast signals in more than one frequency band. - In a method of operation,
FIG. 4 represents receiving the ranging signals in a first frequency band, modulating the ranging signals with at least two different spreading codes, and separately transmitting the modulated ranging signals for each of the at least two different spreading codes from separate antennas. The separate transmission is simultaneous, but may be sequential. The transmissions from each of the antennas are at a same frequency band, but different or overlapping frequency bands may be used. - In an alternate embodiment, any combination of the synchrolite channels within a given synchrolite may transmit their rebroadcast signals in an interrupted or pulsed manner rather than continuously. This may be useful in reducing interference between synchrolites or between channels within a given synchrolite. Different channels may have the same or different components.
- In yet another alternate embodiment, any synchrolite channel may rebroadcast signals from any useful band, including but not limited to navigation signals, such as the signals transmitted by GNSS systems (GPS, Glonass, Galileo, etc), by GNSS augmentation systems (WAAS, LAAS, EGNOS, etc), by other synchrolites, by pseudolites, or by other radio navigation systems (Loran, VOR, TACAN, ILS, MLS, ADF beacons, marine radio beacons, etc). A synchrolite channel may alternatively rebroadcast signals from other systems not originally intended for navigation, such as satellite broadcasting, satellite communications, terrestrial broadcasting, and terrestrial communications. In other alternate embodiments, the pilot tones transmitted by various synchrolites or synchrolite channels within a given system may be adjusted to be on substantially the same frequency, to be on differing frequencies within the common rebroadcasting band, or even to be slightly outside the rebroadcasting band. Similarly, the rebroadcast signal bands themselves may be adjusted to fall on substantially the same set of frequencies, on slightly different frequencies, or on adjacent or non-overlapping frequencies within the same overall transmission band.
- In another alternate embodiment, the pilot tones of one or more synchrolite channels may be modulated with arbitrary data to be communicated from the synchrolite to the navigation receiver, provided that the data modulation is applied in a way which presents a low or acceptable level of interference to the signal recovery process within the navigation receiver. For example, the data is modulated on the pilot tone using BPSK modulation and timed so that a new data bit is presented to the data modulator at precisely the time that the spreading code generator restarts at the beginning of the spreading code pattern.
-
FIG. 5 shows asystem 50 for receiving rebroadcast ranging signals, such as a navigation receiver. Thesystem 50 includes achannel 52 of a navigation receiver to receive and process signals from a single channel of a rebroadcast synchrolite, such as the one shown inFIG. 1 . Thesystem 50 includes a receiveantenna 54, a receive amplifier and/orfilter 56, afrequency translator 58, amodulator 60, a spreadingcode tracker 62, a pair of amplifiers and/orfilters GNSS receiver 68, apilot tone detector 70, anavigation computer 72, afrequency synthesizer 74 and anoscillator 76. Additional, different or fewer components may be provided. - The receive
antenna 54 is a patch antenna, microwave antenna, or other now known or later developed antenna for ranging signals. Ranging signals rebroadcast by one or more synchrolite channels are received by the receiveantenna 54. - The amplifier and/or filter 56 are a low-noise amplifier and a separate filter appropriate for the frequencies of interest, such as the rebroadcast frequency. Amplification without filtering or filtering without amplification may be used. Other amplifiers and/or filters may be used. The received rebroadcast signals are amplified and/or filtered by the receive amplifier and/or
filter 54. - The
frequency translator 58 is a mixer or multiplier. In one embodiment, thefrequency translator 58 is a diode-ring mixer, but other now known or later developed frequency translators may be used. Thefrequency translator 58 is operable to shift in frequency the rebroadcast signals from the receiveantenna 54. The rebroadcast signals are shifted in frequency to any convenient intermediate frequency band, such as a band 200 MHz wide centered on 1400 MHz, using a local oscillator signal. In a simplified embodiment, the frequency translator may be omitted, making the intermediate frequency band within the receivechannel 52 the received frequency band. - The
despreading demodulator 60 is a mixer, multiplier, a diode-ring mixer or other now known or later developed demodulator. In one embodiment, thedemodulator 60 is a digital demodulator, such as a binary phase shift keying (BPSK) demodulator, quadrature phase shift keying (QPSK) demodulator, m-phase shift keying (mPSK), or quadrature amplitude demodulator (QAM). Thedemodulator 60 connects with the receiveantenna 54 to demodulate the rebroadcast signals as a function of the spreading code used to modulate the signals. The spreading code which was applied to the synchrolite signal by the spreadingmodulator 20 inFIG. 1 is removed from the intermediate frequency band rebroadcast signals by thedespreading modulator 60 using the reconstructed spreading code. - The spreading
code tracker 62 is a combination of one or more linear feedback shift registers (LFSR) and each code “chip” is a set of one or more digital bits. Other configurations of components or a single register may be used. The “C/A-code” and “P-code” trackers used in the American GPS system are examples of one type of spreadingcode tracker 62 in which each code chip is a single digital bit and the spreadingcode demodulator 60 is a BPSK demodulator. In one embodiment, the spreadingcode tracker 62 is a field-programmable gate array. The spreadingcode tracker 62 performs a function similar to the spreadingcode generator 20 inFIG. 1 , computing a new state or “chip” of the spreading code at each transition of the spreading code clock signal. Rather than resetting periodically, the spreadingcode tracker 62 is shifted forward or held back a fraction of a chip at a time by the spreading code adjust signal generated by thenavigation computer 72. - In one embodiment, the pilot tone amplifier and/or filter 66 and
detector 70 generate the error signals to adjust the timing of the reconstructed spreading code so that the code matches the timing of the transmitted spreading code. The pilot tone is detected once the two codes are approximately aligned. The alignment may be improved by adjusting the timing to maximize the amplitude of the detected pilot tone, or by delay-locked-loop techniques. In this embodiment, thenavigation computer 72 searches in one dimension (i.e., the timing of the reconstructed spreading code) to find the synchrolite signal. - As noted in the discussion of
FIG. 1 , an alternate embodiment of the synchrolite channel may omit the generation of the pilot tone. The corresponding embodiment ofFIG. 5 may omit the detection of the pilot tone (components 66 and 70). In this embodiment, thenavigation computer 72 uses the signal strength of one or more GNSS signals, as reported by theGNSS receiver 68, to detect and refine the alignment of the reconstructed spreading code. TheGNSS receiver 68 report no or minimal signal strength for a given rebroadcast signal unless the reconstructed spreading code for that signal is aligned with the spreading code transmitted by the synchrolite. Thenavigation computer 72 searches in two dimensions, the timing of the reconstructed synchrolite spreading code and the timing of the reconstructed spreading code of the ranging signal. Both dimensions of the search align before any significant signal strength is detected and before the timing of either reconstructed code may be improved. If thenavigation receiver 50 contains anindependent GNSS receiver 68, and the synchrolite is known to be within a certain radius (equivalent to n GNSS code chips) of thenavigation receiver 50, then the correct timing of the ranging signals rebroadcast by the synchrolite may be found to lie with the range of 0 to 2n GNSS code chips later than the timing of the GNSS code received directly by theindependent GNSS receiver 68. Thenavigation computer 72 uses this information to restrict the second dimension of the search to the known range, thus reducing the time required for the entire search. - The pair of amplifiers and/or
filters despreading demodulator 60. Onefilter 66 is operable to isolate a pilot signal from the output of thedespreading demodulator 60. This pilot tone amplifier and/or filter 66 filters the despread intermediate frequency band to remove signals except for a narrow band around the likely frequency of the desired pilot tone. Theother filter 64 is operable to filter the despread intermediate frequency band to the bandwidth of the desired ranging signal, and amplify the ranging signal. - The
pilot tone detector 70 is an envelope detector or phase-locked loop, which may be implemented using analog or digital circuits. Thepilot tone detector 70 measures the amplitude and/or frequency of the pilot tone received from the synchrolite. - The
navigation computer 72 is a processor, microprocessor, digital signal processor, analog circuit, digital circuit, application specific integrated circuit or other now known or later developed processor with software for controlling or determining a range or a position from a plurality of ranges. Thenavigation computer 72 uses the pilot tone information to adjust the timing of the reconstructed spreading code generated by the spreadingcode tracker 62 so that the reconstructed spreading code is synchronized with the spreading code transmitted by the synchrolite channel. The method for synchronizing the transmitted and reconstructed spreading codes is analogous to the method for tracking spread-spectrum GNSS signals spread by analogous code. - The ranging
signal receiver 68 is any now known or later developed ranging signal receiver, such as a GPS receiver for operating on CDMA ranging signals. The rangingsignal receiver 68 connects with an output of thedespreading demodulator 60 and is operable to despread an input signal to determine a range. Thereceiver 68 attempts to track the ranging signals embedded within the input frequency band. - Once the transmitted and reconstructed spreading codes have been synchronized, the input to the
receiver 68 closely resembles the ranging signals received by the synchrolite channel's receive antenna 12 (seeFIG. 1 ). Some differences may exist. The signals may be delayed in passing through the electronics of thesynchrolite channel 34, and further delayed in crossing the space between the synchrolite channel's transmitantenna 28 and the navigation receiver's receiveantenna 54, and still further delayed in passing through the electronics of the navigation receiver'schannel 52 up to theGNSS receiver 68. The signals may be weaker or noisier due to imperfect alignment between the transmitted and reconstructed spreading codes. The signals may be weaker or noisier and may contain additional multipath reflections due to its passage through the electronics of thesynchrolite channel 34 and through the space between the synchrolite and thenavigation receiver 50. TheGNSS receiver 68 operates with these differences in mind, such as providing a noise filter or algorithm, increasing amplification, adjusting a phase of the alignment, measuring the signals at several slightly different alignments, and other techniques now known or yet to be developed. TheGNSS receiver 68 may also processes an input band of frequencies which may be far from the original GNSS signal frequencies, depending on the frequencies chosen for the three local oscillator signals (i.e., the mixing frequency of thefrequency translators GNSS receiver 68 may be replaced by a receiver for the reflected signals. - The
reference oscillator 76 is a crystal oscillator, such as an ovenized crystal oscillator. Other now known or later developed oscillators may be used. Thereference oscillator 76 generates a signal at any reference frequency, such as 10.000 MHz. The oscillating output of theoscillator 76 is provided to thefrequency synthesizer 74 for use by thechannel 52. Separate oscillators are used in other embodiments. - The
frequency synthesizer 74 is a group of phase-locked-loop oscillator modules, but other frequency dividers or multipliers may be used. Thefrequency synthesizer 74 generates the local oscillator signal at different frequencies for thefrequency translator 58, the clock for the spreadingcode tracker 62 and/or any additional local oscillator or clock signals used by theGNSS receiver 68,navigation computer 72 orpilot tone detector 70. Thereference oscillator 76 and/or thefrequency synthesizer 74 are common to allchannels 52 within a given synchrolite receiver, but separate devices may be used for different channels or within asame channel 52. - The bandwidth of the
channel 52, including the received signals, the various intermediate frequency signals, and the input to theGNSS receiver 68, is wide enough to convey all or substantially all of the signals transmitted by ranging systems, such as 20 MHz or another range. The bandwidth of the signal path from the receiveantenna 54 through the spreadingdemodulator 60 is wide enough to convey a substantial portion of the spread combined intermediate frequency signal, such as 200 MHz or another range. The bandwidth is wide enough to reliably despread and recover the various signals in the subsequent components of thesynchrolite receiver channel 52. -
FIG. 5 also represents a method for receiving rebroadcast ranging signals that uses the same or different components than discussed above. A rebroadcast spread spectrum ranging signal is received. The ranging signal is spread for rebroadcast, spread as originally broadcast, or both. The spread spectrum ranging signal is despread as a function of a spreading code. A pilot tone may be isolated from the ranging signals after being despread based on the code used for rebroadcasting. In a feedback, the pilot tone is used to refine the despreading operation for subsequent ranging signals. Where the ranging signal includes two different spreads, such as associated with an original CDMA ranging signal being rebroadcast with another spreading code, the output of the act of despreading is despread further as a function of another spreading code. A range is determined from the despread ranging signals. - In one embodiment,
multiple channels 52 of the receive ornavigation system 50 are provided. For example, multiple despreading demodulators connect with the same or different receiveantennas 54 and operate in response to different despreading codes to uniquely identify different ranging signals.FIG. 6 shows one embodiment of anavigation receiver 50 intended to receive and process signals from one or more of the rebroadcasting synchrolites discussed herein. Three receivechannels channels channel 52 described above forFIG. 5 . Each receivechannel synchrolite channel 34 shown inFIGS. 1-4 . For simultaneous processing or reception from multiple rebroadcastingchannels channels navigation receiver 50 with six synchrolite receive channels may process signals from six single-channel synchrolites 10 (FIG. 1 ) or two three-channel synchrolites (FIGS. 2-4 ) simultaneously. A memory for sequential processing is alternatively provided. - Optionally, a
GNSS antenna 86 and associatedreceiver 84 connect with thenavigation computer 72. TheGNSS receiver 84 allows thenavigation computer 72 to make differential measurements between the positions of thesynchrolite channels GNSS receiver 84. Thereference oscillator 76 andfrequency synthesizer 74 may be shared between thesynchrolite receiver channels GNSS receiver 84. The GNSS receiveantenna 86 and the synchrolite receiveantenna 54 are positioned so that the phase centers are aligned as closely as possible, such as being adjacent to each other. The GNSS and synchrolite receiveantennas GNSS receiver 84 may be fed forward into theGNSS receivers 68 within each of thereceiver channels -
FIG. 6 also represents a method for receive rebroadcast ranging signals using the same or different components than discussed above. The received spread spectrum ranging signal is despread as a function of different spreading codes for each channel. The resulting ranging signals originally receive by the synchrolite channels are then further despread by codes corresponding to the ranging signals, such as GPS codes. The range is determined as a function of the despread ranging signals, such as through alignment of codes with or without differential measurements. The algorithms for relative navigation or determining range using synchrolites and/or GNSS signals are altered to account for signal delays through the synchrolites, generally by subtracting those delays from the measured ranges. The signal delays can be measured in real-time by the synchrolite itself or a separate reference receiver, calibrated at the factory or periodically in the field, or simply assumed as a constant valid for all copies of a given synchrolite configuration. - As one possible example of a system corresponding to
FIG. 1 , thereference oscillator 32 is an ovenized crystal oscillator such as the Milliren 230-0503; thefrequency synthesizer 30 is a group of phase-locked-loop oscillator modules such as the PSN1810A, PSN2710A, and PSN0210A from Z-Comm; the spreadingcode generator 22 is a field-programmable gate array such as the Xilinx XC2S50; the receiveantenna 12 is a GPS antenna such as theAndrew 40 series; the receive amplifier/filter 14 is a low-noise amplifier such as the M/A-Com MAAM12021 and a filter such as the Toko 4DFB-1575D-10; thefrequency translators modulator 20 are diode-ring mixers such as the Mini-Circuits SYM-2500; thesignal combiner 18 is a microstrip quadrature hybrid; the transmit amplifier/filter 26 is a medium-power amplifier such as the Mini-Circuits GALI-5 and a filter such as the Toko 4DFB-2450T-10; and the transmitantenna 28 is a patch antenna such as the Toko DACT2450CT1T. A microcontroller, such as the Motorola 9S12DP256, may be used to program the Z-Comm PLL modules and initialize the Xilinx FPGA. - While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.
Claims (10)
1. A method for receiving rebroadcast ranging signals, the method comprising:
(a) receiving a spread spectrum ranging signal;
(b) despreading the spread spectrum ranging signal as a function of a first spreading code;
(c) despreading a despread ranging signal output of (b) as a function of a second spreading code; and
(d) determining a range as a function of an output of (c).
2. The method of claim 1 further comprising:
(e) isolating a pilot tone in the output of (b);
(f) performing (b) as a function of the pilot tone.
3. The method of claim 1 further comprising:
(e) despreading the spread spectrum ranging signal as a function of the a third spreading code different than the first spreading code; and
(f) despreading an output of (e) as a function of a fourth spreading code;
wherein (d) comprises determining the range as a function of the output of (c) and an output of (d).
4. A system for receiving rebroadcast ranging signals, the system comprising:
a receive antenna;
a first despreading demodulator connected with the receive antenna, the first despreading demodulator responsive to a first spreading code;
a ranging signal receiver connected with an output of the first despreading demodulator for receiving a despread ranging signals despread with the first spreading code, the ranging signal receiver operable to despread the despread ranging signal from the output and determine a range.
5. The system of claim 4 further comprising:
a first filter connected with the despreading demodulator, the first filter operable to isolate a pilot signal from the output of the despreading demodulator, the first spreading code responsive to the pilot signal.
6. The system of claim 4 further comprising:
a second despreading demodulator connected with the receive antenna, the second despreading demodulator responsive to a second spreading code different than the first despreading code.
7. The system of claim 6 wherein the second spreading code comprises a code for a different synchrolite.
8. The system of claim 6 wherein the second spreading code comprises a code for different signals from a same synchrolite.
9. The method of claim 1 further comprising repeating (a), (b), and (c) for different ranging signals from a same synchrolite.
10. The method of claim 1 further comprising repeating (a), (b), and (c) for different ranging signals from a different synchrolite.
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US7310064B2 (en) | 2007-12-18 |
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