FAST SEQUENCING DEMODULATION METHOD AND APPARATUS

Abstract: A fast sequencing method and apparatus for use in a radio navigation system of the GPS type. Satellite-­ coded (A-D) signals are simultaneously received from a plurality of sources. The signals are decoded in a single channel by multiple samplings of each signal per each signal modulating period (T). By taking multiple signal samplings per data period, broadband receiver operation is realised, enhancing the suitability of such apparatus for navigation of medium and high g performance vehicles and the like.

Description: The present invention relates to a demodulation method, and apparatus therefor, suitable for the demodulation of simultaneous transmissions from multiple sources such as that encountered when navigating with Global Positioning System (GPS) data. GPS is a navigation system that includes a plurality of space vehicles (satellites) that transmit radio frequency signals to earth. Each space vehicle's location, at any given point in time can be determined with precision by calculation from ephemeris or almanac data. Distances and velocities with respect to such locations can be determined for navigation purposes from the transit times and rate of change of transit times of signals originating at the various satellites. When signals are simultaneously received from at least four satellites, locations and directions on (or below or above) earth can be determined by trilateration methods. The need to process signals originating from multiple sources, and particularly the need to demodulate such signals, complicates the design of GPS-type system receivers. Some designs have featured separate receivers and/or separate dedicated channels in a single receiver to perform the multiple signal demodulation. Such approaches incur significant costs while increasing system power, weight and/or bulk requirements greatly. Attempts have been made to solve the above problems by multiplexing the independent signals within a single hardware channel. Such an approach is disclosed in "An Advanced NAVSTAR GPS Multiplex Receiver" by Phil Ward, IEEE Journal pp. 51 (1980.) and in United States patent Serial No.4,468,793 of Johnson et al. for "Global Position System (GPS) Multiplexed Receiver". In the system disclosed in that patent, signals from a plurality of satellites are sequentially sampled once during each transmitted data period. Figure 1A of the accompanying drawings is a diagrammatic representation of a fast sequencing scheme for multiplexing multiple source signals in accordance with the prior art patent of Johnson et al. supra. Such sequencing has been employed, and is useful for navigation in accordance with the multiple satellite GPS system wherein each of a plurality of satellites transmits navigation data (e.g. Doppler information) to an earthbound receiver. The transmissions are encoded for satellite identification. In the multiplexing method illustrated in Figure 1A, signals continuously transmitted by the GPS satellites are sampled a single time during each modulation data period. As the GPS data rate is 50 bits per second, each data bit transmitted by the satellites of the system has a duration, or period, of 20 milliseconds. Four satellite transmissions illustrate the prior art method, consistent with the requirement that the receiver "see" at least four satellites for navigation purposes. Figure 1B illustrates tracking with a single correlator system. Thus, a code tracking function is interspersed with the demodulation of GPS data. A multiple correlator system of the prior art in which code tracking takes place independently of data demodulation, is described in the Johnson et al patent (i.e. Figure 1A). As can be seen from Figure 1B, each 20 millisecond long data period is divided into five equal segments. The first four segments are utilised for sequentially sampling data from the four satellites while the fifth segment is utilised for sampling the code of one of the satellites. The 20 millisecond long sampling interval is repeated over and over, with a different satellite code sampled each time so that, over four such intervals, the code of each satellite is sequentially sampled. Thus, eighty milliseconds are required to code track all four satellite signals. The prior art multiplexing scheme described above permits one receiver channel to perform the functions of five dedicated hardware channels. Thus, significant savings are realised. While the above-described method does result in cost, bulk and power savings, such method is not sufficiently broadband for many applications. Further, the narrowband operation that is achieved by such a system is particularly sensitive to the loss of signal lock that can occur when an earth vehicle (or an airplane or ship) performs relatively "high g" manoeuvres. Navigation systems are generally rendered useless when they cannot attain and maintain "lock" onto known monuments. In the case of GPS navigation, the satellite transmitters act as the system's monuments. The exact locations of the satellites are available to the user who enters appropriate ephemeris data into the system model. The ephemeris data and status information are collected from the satellites during an initialization process known as "slow sequencing". This process requires the receiver to dwell between eighteen and thirty seconds upon each satellite, collecting data in one millisecond samples, to ensure that all required information is collected. Each time lock is lost, this rather time-consuming process must be repeated and, of course, a significant amount of navigation data will be lost to the system during each-reinitialization "break". It is an object of the present invention to obviate, or at least to mitigate the preceding and other shortcomings of the prior art. According to one aspect of the invention, there is provided a fast sequencing method for sampling N simultaneously received signals in a multiple correlator system wherein each of said signals is modulated with data at a rate of 1/T, each data bit being of duration T, and wherein said signals are received and sequentially sampled by a single channel, characterized by: a) selecting an integer M > 1; b) determining a sampling interval s = T/ (N x M); c) sequentially sampling, for each sampling interval s, the data from each one of said N signals M times during each data modulation period T such that each one of said N signals is sampled once during each interval of duration N x s, the samples from each signal being uniformly spaced with interval N x s between samples from the same signals; and d) sequentially repeating step c) over multiple periods of data modulation. According to a second aspect of the invention, there is provided a fast sequencing method for sampling N simultaneously received, individually coded signals in a single correlator system wherein each of said signals is modulated with data at a rate of 1/T, each data bit being of duration T and wherein said signals are received and sequentially sampled by a single channel, characterized by: a) selecting an integer M > 1; b) determining a sampling interval s = T/ ((N + 1) x M); c) sequentially sampling data from each one of said N signals for sampling intervals equal to s; d) sampling the code of one of said N signals for a sampling interval; e) sequentially repeating steps c) and d) M times for each period of data modulation T, so that the code of said one of said N signal is sampled M times; f) sequentially repeating step e) over at least N periods of data modulation so that the code of each of the N signals is tracked for at least one period T. According to a third aspect of the invention, there is provided apparatus for demodulating N coded signals, each of said signals being modulated with data at a rate of 1/T, comprising, in combination: a) means for sequentially selecting a sample of each of said N signals; b) means for sequentially gating each of said selected N signals at MxN times the data rate where M is an integer that is greater than one; c) N summers each of said summers being arranged to sum said samples of one of said N signals via a multiplexing switch arranged to sequentially direct said selected gated signal samples to said summers at M times the data rate; and d) means for storing N signal sums generated during each data period T. Preferred embodiments of the present invention address and overcome inherent limitations of the prior art that prevent realisation of the significant hardware savings afforded by fast sequencing. Such limitations relate to the limited "g capability" suffered by the prior art discussed above. Although a receiver has a two-sided 3dB bandwidth of 500 Hz during the slow sequencing initialisation process as a result of the one millisecond dwell time employed, this decreases to 25 Hz during the subsequent navigation mode when the prior art method is utilised. This is a reflection of the fact that the prior art sampling period is 20 milliseconds. In accordance with the Nyquist criterion, the bandwidth is one-half of the sampling frequency (50 Hz). For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:- FIGURES 1A and 1B are diagrammatic representations of fast sequencing multiplexing schemes in accordance with the prior art; FIGURE 2 is a like diagrammatic representation of a single correlator fast sequencing scheme according to one embodiment of the invention; FIGURE 3 is a diagrammatic representation of a multiple correlator fast sequencing scheme in accordance with an alternative preferred embodiment of the invention; and FIGURE 4 is a schematic diagram of apparatus for providing fast sequencing multiplexing of multiple source signals. In the figures like numerals refer to like features throughout. Figure 2 is a diagrammatic illustration of the fast sequencing method for multiplexing a plurality of signals in accordance with one embodiment of the invention As in the prior art example of Figure 1B, the illustrated method is adapted to a receiver system that utilises a single correlator. Thus, code tracking is combined with carrier signal demodulation, permitting a single receiver channel to accomplish the functions of five dedicated hardware channels. As such the cost advantages of the illustrated prior art method are similarly realised with the present invention. According to the method illustrated in Figure 2, signals from the satellites are multiplexed onto the single receiver channel. Unlike the prior art, however, the data modulation period is divided into four equal time intervals 10, 12, 14 and 16. Each of such intervals is further divided into a sufficient number of equal time subintervals, such as the subintervals 18, 20, 22, 24 and 26 that make up the interval 14, to permit all required carrier and code sampling to occur during each interval. The method bears some superficial resemblance to the prior art method insofar as it sequentially samples the data from the satellites. This, of course, is not unexpected insofar as both methods address the problem of multiplexing a plurality of signals onto a single sampling channel. However, the prior art does not address significant problems that occur in operational navigation systems, particularly the problem relating to signal "lock" and associated bandwidth problems. By subdividing the period of the data modulation into one millisecond sampling sub-intervals, the sampling period for the method in accordance with the embodiment of Figure 2 becomes 5 milliseconds. In accordance with the Nyquist criterion, the bandwidth of the system is one half of the 200 Hz sampling frequency. Thus, a four-fold increase in bandwidth is realised over the prior art arrangement. This represents a significant improvement in a critical area of navigation performance as the ability of the GPS receiver to reacquire satellite signals or to acquire new signals during fast sequencing can place severe constraints upon the practical application of such a system. Figure 3 illustrates an alternative embodiment of the invention wherein the receiver is assumed to employ a multiple correlator configuration. As mentioned above, such a system permits simultaneous code and carrier tracking to take place. Once again, the period of the modulation is divided into intervals of equal duration. Since a time slot need not be utilised for code sampling, only five intervals 28, 30, 32, 34 and 36 are employed. Each interval is further partitioned into sub-intervals of one millisecond duration, such as the sub-intervals 38, 40, 42 and 44 that make up the interval 32. As a result of the additional interval attained when codes are sampled apart from the carriers, the embodiment of Figure 3 achieves an even greater improvement in receiver bandwidth than that of the prior method of the invention. The carrier sampling period is four milliseconds. Thus, by the Nyquist criterion, the two-sided 3dB bandwidth for carrier tracking is, in this instance, 125 Hz, a five-fold improvement over the prior art in this critical area of performance. Figure 4 is a block diagram of receiver apparatus for performing fast sequencing in accordance with Figure 3. GPS satellite transmissions energize an antenna 46 to communicate corresponding signals to conventional front end receiver electronics 48 for performing necessary filtering, amplification, down conversion, and Doppler removal functions. The antenna 46 is simultaneously energised by a plurality of satellite transmissions and a corresponding plurality of intermediate frequency (IF) signals is generated. As all GPS satellite transmissions are encoded, the IF signals include coding. A code discriminator 50 accepts the conditioned IF outputs of the front end electronics 48 and, responsive to the output of a code generator 52, passes a selected signal therethrough, blocking all others. The code discriminator 50 effectively strips the coding from the outputs. Thus, the code discriminator serves as a type of parallel to serial convertor, passing data serially to the multiplexing switch 56 in a preselected sequence. It can also be considered to act as a filter. The multiplexed outputs of the discriminator 50 are applied serially to a data demodulator 54. The demodulator extracts the data bits therefrom. A multiplexing switch 56 sequentially directs the data bits to a plurality of summers 58. In the instance of a GPS-type navigation system four summers 58 would be adequate, although more could be employed. The outputs of the summers 58 are applied to a plurality of registers 60, each of which is arranged to accept the output of one of the summers 58. The multiplexing switch 56 is controlled by the output of a divide-by-N counter 62 wherein N is the number of transmitting satellites. A signal whose frequency is calculated as follows: (number of satellites) (number of sub-intervals)/(data bit period) is applied to the counter. Thus the switch 56 multiplexes the stream of data at a rate that is equal to the number of sub-intervals divided by the period of the modulating data bits. This corresponds to the division of data into sub-intervals of the modulating data period as discussed above with reference to the methods of Figures 2 and 3. Data is clocked into the registers 60 under control of a divide-by-M counter 64 where M is the number of sub-intervals. Thus the frequency of the output of the counter 64 is equal to the modulating data bit rate. By clocking each of the registers 60 one time per bit period, an output from the summers 58 is entered into the corresponding one of the registers 60 each data period. In addition to the other advantages offered by the present invention, the demodulation of data through a process in which a plurality of samples of each data bit is taken adds reliability that is not found in the single sample systems of the prior art. Present-day computers are easily capable of performing the sampling rates required by the present invention. As the number of sub-intervals becomes larger and larger and the number of times each data bit is sampled increases the system gets closer and closer to, and therefore approaches the inherent advantages of, the continuous tracking offered by the costly multiple-channel systems of the prior art. The code generator 52 may be one of a number of devices for providing the predetermined GPS satellite codes. Such apparatus may comprise an array of registers or a device that relies upon a look-up table stored within a read only memory (ROM) such as that disclosed in pending United States patent application Serial No.527,124 (Patent No. 4,642,647) of Sturza et al. for "Signal Generator for Radio Navigation System". The codes stored within the generator 52 are clocked out (and input to the code discriminator 50) by the output of the divide-by-N counter 62. The output of the counter 62 comprises a repeating sequence of the numbers 1 through N, each designating a particular GPS satellite and thereby calling up its code. As mentioned earlier, the frequency of this output is equal to the number of sub-intervals divided by the period of the modulating data. The phases of the codes output from the generator 52 are adjusted by means of a code tracking loop 66. The loop 66, preferably a delay lock loop, accepts the output of the code discriminator 50, which includes the coding of the signal that is passed therethrough, and, accordingly, outputs an updated phase estimate to the generator 52. This phase is utilised by the generators 52 to line up its code outputs with the code inputs to the discriminator 50. The code of a single satellite is examined throughout each sampling interval. This, of course, includes a number of sub-intervals. Thus, the phase of a single code is adjusted M times during a data period. The output of the code discriminator 50 is also applied to a carrier tracking loop 68. The carrier tracking loop 68 accepts the output of a controller 70 that is programmed to supply initial doppler frequency estimates per interval for satellites as designated by the divide-by-N counter 62. Such estimates may be derived from the satellite ephemeris models stored within the controller 70. The carrier tracking loop 68 supplies the actual doppler information to the controller 70, enabling the controller to update the initial doppler frequency estimates for the following interval. The loop 68, which may comprise a conventional Costas-loop configuration, determines an updated doppler frequency estimate from the initial estimate provided by the controller 68 and the output of the front end electronics 48. This estimate is updated M times per interval (per satellite) as the output of the code discriminator 50 changes at this rate under the control of the divide-by-N counter 62-clocked code generator 52. The front end electronics 48 utilises the doppler information it receives from the carrier tracking loop 68 to adjust the satellite frequency passbands of its various filters. As the electronics 48 receives a plurality of estimates per satellite per period of modulating data, the bandwidth of the system shown in Figure 4 is increased in accordance with the present invention over systems of the prior art in which a single sample is taken per satellite per modulating data interval. Thus the harmful results of loss of lock (through maladjustment of the receiver front end electronics) are minimized by the incorporation of apparatus in accordance with the described embodiment into GPS-type receivers. By incorporating the teachings of the invention into a receiver, one is thus able to achieve the cost and simplicity of manufacture, maintenance and operation offered by the fast sequencing of a plurality of transmissions onto a single data channel without unnecessarily complicating navigation in high g environments. Practically, the invention offers the operational advantages of continuous tracking without sharing inherent lack of economy.

Claims: A fast sequencing method for sampling N simultaneously received signals in a multiple correlator system wherein each of said signals is modulated with data at a rate of 1/T, each data bit being of duration T, and wherein said signals are received and sequentially sampled by a single channel, characterized by: a) selecting an integer M > 1; b) determining a sampling interval s = T/ (N x M); c) sequentially sampling, for each sampling interval s, the data from each one of said N signals M times during each data modulation period T such that each one of said N signals is sampled once during each interval of duration N x s, the samples from each signal being uniformly spaced with interval N x s between samples from the same signals; and d) sequentially repeating step c) over multiple periods of data modulation. A fast sequencing method for sampling N simultaneously received, individually coded signals in a single correlator system wherein each of said signals is modulated with data at a rate of 1/T, each data bit being of duration T and wherein said signals are received and sequentially sampled by a single channel, characterized by: a) selecting an integer M > 1; b) determining a sampling interval s = T/ ((N + 1) x M); c) sequentially sampling data from each one of said N signals for sampling intervals equal to s; d) sampling the code of one of said N signals for a sampling interval; e) sequentially repeating steps c) and d) M times for each period of data modulation T, so that the code of said one of said N signal is sampled M times; f) sequentially repeating step e) over at least N periods of data modulation so that the code of each of the N signals is tracked for at least one period T. A method as defined in claim 1 or 2, wherein said data is transmitted from at least four signal sources. A method as defined in any one of claims 1 to 3 further characterised in that said data rate is 50 bits per second, four signals are received and each sampling interval is one millisecond in duration. A method as defined in any preceding claims wherein said signals are transmitted from GPS system satellites. A fast sequencing apparatus for sampling N simultaneously received signals, each of said signals being modulated with data at a rate of 1/T, each data bit being of duration T, comprising, in combination: a) means (50) for sequentially selecting a sample of each of said N signals and for sequentially gating each of said selected N samples at MxN times the data rate where M is an integer that is greater than one; b) N summers (58) each of said summers being arranged to sum said selected gated samples of one of said N signals via a multiplexing switch (56) arranged to sequentially direct said selected gated samples to said summers (58) at M times the data rate; and c) means (60) for storing N signal sums generated by respective summers during each data period T. Apparatus as defined in claim 6 and including: a) means (66) for adjusting the phases of the codes of said received signals in response to said selected gated signals and b) means (52) coupled to the adjusting means for generating in-phase codes which control said means (50) for sequentially gating said samples.

Inventors: STURZA, MARK ALAN
Applicants: LITTON SYSTEMS, INC.
Application year: 1987
Main classification: G01S5
IPC codes: H04B7, G01S19