Pseudo random code modulated signal combiner

A circuit and associated method for determining the heading of a vehicle or device with a satellite ranging system receiver is provided. The circuit requires only a single front end RF stage. A single RF stage can be used because the incoming signal received at the first antenna is delayed with respect to the signal received at the second antenna and the two signals are merged together, to form a combined signal. The combined signal can be down converted and sampled in a single stage. The samples are then separately correlated to detect the data associated with the portion of the signal attributed to each antenna. The differences between the measured data are then used to calculate the relative orientation (heading and pitch) of the device upon which the two antennas are disposed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to digital receivers for use with pseudo random noise (PRN) encoded signals such as those used in satellite ranging systems.

2. Background Information

There are a number of satellite ranging systems that are currently deployed and additional systems are to be deployed in the near future. Each of these configurations is based upon transmission of ranging signals in particular frequency bands. More specifically, the present United States Global Positioning System (GPS) is based on transmission of ranging signals in two frequency bands known as L1, which is at a center frequency of 1575.42 MHz and L2, centered at 1227.6 MHz. To enhance the reliability and availability of this system, additional GPS signal structures are planned (e.g. L5, L2C). In addition, other satellite ranging systems are being deployed or have been deployed such as that of the Russian Federation, i.e., GLONASS (with two signal structures: G1 and G2), and the European GALLILEO system with multiple signal structures (referred to herein as: E1, E2 . . . E5, etc.)

The system satellites transmit precisely timed signals that contain a number of components, namely, a plurality of pseudo-random noise (PRN) codes and data. The signals allow for precise determination of latitude, longitude, elevation and time. A digital receiver operating in a known manner, receives the PRN-encoded signals and essentially synchronizes local versions of the transmitted codes to the received codes, that is, the receiver tracks the received codes, to determine time differences of arrival and Doppler measurements. The digital data, which consists of information such as the satellite ephemeris, (i.e. position, current time of day, and system status information), is also transmitted by each satellite on at least one carrier frequency as a low frequency (typically 50 Hertz) signal. After synchronization of the local PRN codes, and their carriers, the receiver obtains the data. The receiver then uses the information provided by the data and the times of arrival and so forth to produce pseudoranges for the respective satellites in view and to determine its global position using the pseudoranges.

As noted, a satellite ranging signal receiver receives a composite signal consisting of one or more of the signals transmitted by the satellite within view (within a direct line of sight) as well as noise and interfering signals. By determining the transmission time from at least four satellites and knowing each satellite's ephemeris, the receiver can calculate the pseudoranges and thus its three-dimensional position, its velocity and the precise time of day.

When calculating heading information of a mobile device, such as a boat, aircraft or other vehicle, several receivers and corresponding antennas are located on the vehicle, spaced apart from one another. The antennas receive signals from the same set of satellites and determine their global positions as described above. Once the position of each antenna is known, the position information can be used to calculate a precise directional heading of the boat or other vehicle upon which the antennas are mounted. Alternately and preferably interferometric differences in the measured phase data can be used to determine attitude and relative orientation of antennas, as is well know in the art. However, a separate receiver has typically been required for each antenna being used to make such a heading calculation, thus leading to a costly system for receiving and analyzing inputs from each of the antennas used to provide the requisite information.

In U.S. Pat. No. 6,844,847 entitled BOAT POSITIONING AND ANCHORING IN A SYSTEM, of Gounon, which issued on Jan. 18, 2005, a receiver is described that includes a multiplexer which multiplexes the signals from two separate antennas together such that the receiver can separately utilize the signals from each antenna to track the respective PRN codes and determine the global positions of the antennas. However, this receiver tracks the codes in the signals provided by a given antenna only half of the time and thus, the tracking operations are susceptible to loss of phase lock. If phase lock is lost, the receiver must re-align the local codes in order to determine the antenna positions reliably, and the results of the heading calculations are therefore delayed or may be interrupted entirely.

There remains a need, therefore, for a receiver which, inter alia, receives and simultaneously processes signals from more than one antenna and uses information from each respective antenna to calculate the heading of an associated vehicle. There remains a further need for a receiver architecture which is of a reduced size and cost, and which accommodates multiple antenna signals at a nominal increase in receiver complexity.

SUMMARY OF THE INVENTION

The disadvantages of prior techniques are overcome by the present invention, which provides a pseudo random code modulated signal combiner and receiver assembly for use with a satellite ranging system receiver that receives signals using at least two antennas. The receiver, which simultaneously processes the signals from all of the antennas, utilizes a single front end RF downconversion stage and multiple channels. Accordingly, the architecture is less complex than utilizing two receivers. The assembly is used for determining the directional heading of a vehicle, boat, aircraft, or other device.

In accordance with an illustrative embodiment of the invention in which the signal combiner and receiver assembly are deployed on a boat, for example, a first antenna is mounted on the bow of the boat and a second antenna is mounted on the stern of the boat. A GPS receiver located on the boat receives signals from the same set of satellites at the first antenna and the second antenna. In accordance with the invention, the signal received at the first antenna (referred to hereinafter as the “first signal”) is subjected to a fixed delay. More specifically, the pseudo random code modulated first signal is delayed with respect to the pseudo random code modulated signal at the second antenna (the “second signal.”) This delay allows, in accordance with the invention, the second signal to be merged with the delayed first signal to form a single, combined signal. The first signal is sufficiently delayed to allow two separate correlation peaks to occur in signal processing. In an illustrative embodiment of the invention, this delay may be 1 to 2 code chips, for example. The combined signal then is down converted to an intermediate frequency signal thus requiring only a single RF down conversion stage. In other words, there is no need for separate RF down conversion stages for the first and second signals, respectively.

In accordance with one aspect of the invention, the combined signal is then digitized and is used as an input to two correlation channels per satellite code. The first correlation channel includes a PRN code generator, which produces a locally generated code which is correlated with the combined signal to track the PRN code in the first signal. The first channel thus produces correlation measurements from which the position of the first antenna can be determined. The second correlation channel includes a second PRN code generator and a second set of correlators. The second channel is programmed to use the information from the portion of the combined signal representing the second signal, and thus, to ignore the code from the first antenna. When the correlation peak with respect to the second signal is located, the associated correlation measurements are used to determine the position of the second antenna.

The distance between the two antennas is known, and once the position of each antenna is determined, the directional heading of the boat can be determined with reference to magnetic north or another directional reference point.

Interferometric processing of the measured pseudorange and/or carrier phase data can also be used to determine the relative orientation of the two (or more) antennas.

In an alternative embodiment of the invention, the satellite ranging system receiver signal processing electronics can be designed such that only one PRN code generator per satellite code is required. The code produced by the single PRN code generator is supplied to a first set of correlators after passing through a delay element that applies an adjusted delay to the local code, to account for the fixed delay which was introduced into the first signal and also to account for the differences in the times of arrival of the code to the first and second antennas. The correlation measurements produced by the first set of correlators is then used to calculate the position of the first antenna. The PRN code generator also passes the local PRN code to a second set of correlators, which compares the local code with the code from the second antenna and produces correlation measurements that are used to determine the position of the second antenna, and also to control the code phase of the PRN code generator. The position information corresponding to each of the antennas is then used to calculate the heading as discussed. The signal processing portion of the receiver can also be programmed to make other calculations with respect to the vehicle, such as ground track, and the like.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

FIG. 1is a block diagram of a conventional arrangement100of GPS receivers that are used to determine heading. The arrangement100includes a first antenna102and a second antenna104which receive ranging signals from multiple satellites that are then in view (line of sight) of the receivers. With respect to the first antenna102, a receiver (illustrated in dashed block106) includes an RF band pass filter110, which is a low insertion loss filter having a selected pass band centered at a desired carrier frequency. The band pass filter110should have a sufficiently wide range to allow several harmonics of the PRN codes to pass. The filtered signals then pass through a low noise amplifier114.

Thereafter, downconversion is performed using a mixer120which downconverts the received signal from the carrier frequency to a desired intermediate frequency by mixing the received signal with a locally generated signal of an appropriate frequency that is produced by local oscillator122. Appropriate amplification is then performed by the amplifier124. An intermediate frequency band pass filter128is provided having a sufficiently narrow bandwidth to remove any undesired frequencies, but sufficiently wide to keep the desired frequency and one or more harmonics. The automatic gain control device130is used as a pre-amplification stage to adjust signal strength so that it is appropriate for sampling by the A/D converter140. Digital samples obtained by the A/D converter140are passed to the signal processing circuitry152, which acquires and tracks the satellite codes.

As shown in the prior art drawing, conventional systems require a separate receiver160for the signals from the second antenna104. This separate receiver160requires its own set of the components as in receiver106, namely a band pass filter161, a low noise amplifier162, a mixer163and an associated local oscillator164, an amplifier165and an intermediate frequency filter166, as well as an automatic gain control device167. The second receiver160then feeds an analog signal to the A/D converter170to provide digital samples of the signal received from the second antenna104to the signal processing circuitry172for the second receiver. As is apparent fromFIG. 1, there is thus duplication in the components in the receiver100.

A solution is provided by the device of the present invention, which is illustrated inFIG. 2.FIG. 2depicts a signal combiner and GPS receiver assembly200, which receives signals from two separate antennas202and204. As noted, by way of illustrative example, antenna202is located, for example, on the bow of a ship and antenna204is located on the stern of the ship. Alternatively, the receiver200is readily adaptable such that the antennas may be located in two separate locations of an aircraft, or on another vehicle, such as a remotely controlled military vehicle or other device. The antennas202and204receive ranging signals from one or more satellites. For calculation of the heading of the vehicle upon which antennas202and204are deployed, the signal of interest is a signal from the same satellite such that the signals received at antennas202and204include the same PRN codes.

In accordance with an illustrative embodiment of the invention, the signal from the antenna202is amplified with amplifier208, to accommodate the insertion loss of the delay element, and subjected to a fixed delay using the delay circuit210. The amount of the delay is selected such that the signal from the first antenna202is delayed with respect to the signal from the second antenna204in such a manner that when the two signals are combined, two separate PRN code correlation peaks can be determined—one for each of the signals. As will be understood by those skilled in the art, the C/A PRN code in a GPS satellite ranging signal is 1023 bits long, and these bits are also known to those skilled in the art as “code chips,” with each code chip representing a 1 or a 0. The rate at which the GPS signal is transmitted is such that a different code chip is transmitted every microsecond. In accordance with the present invention, the fixed delay is equivalent to at least one code chip such that cross correlation between the two codes is avoided. The signal combiner and receiver assembly of the present invention has been tested and deemed to be fully operational using a 1.75 code chip delay between the two signals.

The delayed first signal associated with the antenna202and the second signal received from the second antenna204are merged by a suitable signal combiner220, to form a combined signal. The combined signal is introduced to an appropriate front end filter223to remove any noise which may have been introduced in the combining stage. The signal is then amplified appropriately using the low noise amplifier224. A mixer230, then downconverts the combined signal to a desired intermediate frequency (IF) by mixing the signal with a signal produced by a local oscillator232. The IF combined signal is then amplified further using amplifier236and passed through a low noise filter240.

The IF combined signal is next sampled and converted to digital values by the single A/D converter250. The A/D converter250is regulated by an appropriate sampling clock (not shown). The digital counterparts of the combined analog signal which include portions from the delayed first signal associated with antenna202and the second signal from the antenna204, are passed to the signal processing circuitry260as described further herein with reference toFIG. 3.

FIG. 3illustrates one illustrative embodiment of the signal processing circuitry260(FIG. 2) in which there are two channels for processing the incoming signal from samples of the combined signal. More specifically, the first channel302includes a first PRN code generator304which generates a local PRN reference signal corresponding to the PRN code associated with the satellite from which the first and second signals have been received. A PRN code signal comparison circuit306receives as inputs the locally generated code from the PRN code generator304and the samples of the combined signal305. The compare circuit306includes at least two correlators (not shown). In a first mode, the correlators can be used for acquiring the PRN code. In that mode, the first correlator is configured as an early correlator and the second correlator is configured as a late correlator. A second mode is used for PRN code tracking and in the second mode the first correlator is configured as an early minus late correlator and the other correlator is a punctual correlator. Alternatively, three correlators may be used. In either mode, the correlators operate in a conventional manner to produce correlation measurements which are used by position circuitry320in a conventional manner to acquire and track the received code and ultimately to determine the distance of the receiver from the satellite being tracked.

In addition, the position and heading calculation signal processing circuit320also generates a first feedback signal322to be used for phase synchronization of the first PRN code generator304. Further details regarding the correlation process are provided in commonly owned U.S. Pat. No. 5,101,416, issued on Mar. 31, 1992, to Fenton, et al., which is incorporated by reference herein in its entirety.

In accordance with the present invention, a second channel, which includes a second PRN code generator310is programmed to search for a second PRN code in the incoming samples. The second channel thus operates the second PRN code generator to produce a local code that precedes or trails the code produced by the first PRN code generator by the length of the fixed delay. More specifically, when a correlation peak is determined in the first compare circuit306, the associated code timing information is communicated to the second PRN code generator310. The second compare circuit312then searches the code at one or more code chips away from the peak found with respect to the first antenna to locate a different correlation peak which is attributable to the signals from the second antenna. In this way, the portion of the combined signal, which is associated with the second antenna,204(FIG. 2) is identified. In a manner similar to compare circuit306, the compare circuit312produces correlation measurements to the signal processing circuit320, which produces a second feedback signal324for phase synchronization of the second PRN code generator310, and also the position information.

Accordingly, the signals from the first antenna202are processed in the first channel302, to obtain correlation measurements for the first antenna. The signals from the second antenna204are processed in the second channel308and are used to determine the correlation measurements with respect to the second antenna. The comparison circuits306,312, accumulate the respective correlation measurements and provide the measurements to the position and heading calculation signal processing circuit320, which determines the respective positions of the two antennas and the precise heading of the vehicle on which the two antennas are mounted.

In an alternative embodiment of the invention, which is illustrated inFIG. 4, a single PRN code generator402is used along with an adjustable delay406that incorporates the fixed delay introduced by the front end circuit ofFIG. 2, and makes an appropriate adjustment to the delay to account for the fact that the signals arrive at the first antenna at a different time than the second antenna. A compare circuit408has as inputs, the locally generated code from the PRN code generator402and the samples from the combined signal407. The compare circuit408compares the delayed PRN code signal with the samples of the combined signal to produce correlation measurements that are associated with the first antenna. The correlation measurements are used by the processing circuitry to control the adjustable delay. The combined signal samples are also sent to comparison circuit409, with the combined signal407to produce correlation measurements that are associated with the second antenna. These correlation measurements are used by the processing circuitry to control the PRN code generator. When both the PRN code signal and the delayed PRN code signal are in synchronism with the corresponding antenna signals, the position and heading calculation signal processing circuit410determines the position of each antenna. From the position information thus obtained, the heading of the vehicle is determined. Interferometric processing can be used in making such determinations.

In another embodiment of the invention, the signal combiner and receiver assembly is used with a Local Area Augmentation Antenna (“IMLA”). The IMLA antenna assembly includes a first antenna, which is pointed directly upwardly, and a second antenna, which is oriented 90 degrees from the first antenna in an azimuth direction. In such a case, all of the satellite signals that are received can be combined together and used in a position solution. These results will include substantially high negative residuals and substantially high positive residuals. The high negative residuals can be attributed to one of the antennas, and the positive residuals are attributable to the other antenna. To the extent that all of the signals are from different satellites, the signals can be combined without introducing a delay into the signals from one of the antennas. However, in practice, there will typically be at least some overlap in signal reception between the two antennas, so a delay would illustratively be introduced into one of the received signals to allow the signals from each respective antenna to be separated and analyzed and a position calculation can then be performed.

In yet another embodiment of the invention, the signal combiner and receiver assembly is used on rotating bodies such as rockets and fighter aircraft where antennas are required on diametrically opposed sides of the structure to accommodate fuselage shading of the satellite signals.

The method of the present invention can be best understood with respect to the flow chart ofFIG. 5, which illustrates a procedure500. Procedure500begins at the start step502and continues to step504in which a signal is received at a first antenna from a satellite. In accordance with step506, a fixed delay is introduced to that signal to obtain a delayed signal. The procedure500then continues to step508in which a signal is received at a second antenna, which is disposed on a vehicle at a predetermined distance from the first antenna.

In accordance with step510, the delayed signal from the first antenna is merged with a signal from the second antenna to obtain a combined signal. In accordance with step512a combined signal is downconverted to an intermediate frequency. In accordance with step514, the combined signal at the intermediate frequency is digitized to obtain samples of the combined signal. In step516, the samples of the combined signal are fed to a comparison circuit in which the delayed portion of the combined signal is correlated with a first local PRN code to determine correlation measurements associated with the first antenna. Simultaneously, in accordance with step518, the signal samples are correlated with a second local PRN code. In accordance with step520, the position and relative orientation of the first antenna and the second antenna is determined, and using these positions, or the pseudorange and carrier phase measurements, the directional heading of the vehicle upon which the first antenna and the second antenna are disposed is calculated. The procedure ends at step522.

It should be understood that the present invention provides a simplified front end signal combiner and satellite ranging system receiver assembly that eliminates the requirement of separate RF downconversion and signal processing channels downstream from a point where two signals are combined. Thus, the receiver has reduced complexity and requires less space on the circuit board. This simplification can also lead to cost savings.

The foregoing description has been limited to specific embodiments of the invention. It should be apparent, however, that variations and modifications may be made to the invention, with the attainment of some or all of its advantages. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.