GB2506090A

GB2506090A - Navigation system

Info

Publication number: GB2506090A
Application number: GB9309415A
Authority: GB
Inventors: Francis Giles Overbury; Simon John Gale
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1993-05-06
Filing date: 1993-05-06
Publication date: 2014-03-26
Anticipated expiration: 2013-05-06
Also published as: GB2506090B; GB9309415D0

Abstract

A navigation system having a plurality of transmitting stations each of which transmits a sequence of short repeating codes at a fast digital rate and superimposed thereon a long single code at a second slower digital rate, and of a duration equal to the duration of the sequence of short repeating codes, such that correlation of the long code in a receiver will yield an unambiguous timing signal relevant to the sequence of short repeating codes. One of said stations may be a master transmitting codes to which transmissions from other stations are locked, and its operation monitored and master status transferred to another station in the event of a fault. Transmitting stations may adjust their frequencies if a receiver detects interference.

Description

NAVIGATION SYSTEM

This invention relates to a navigation system utilising fixed terrestrial transmission stations.

A common type of navigation system uses a plurality of transmission stations each transmitting signals which are phase locked to the signals from one of the stations (called a master' station). The signals are digitally encoded signals conveying information relating to the stations. A receiver in a vehicle, vessel or aircraft can, by utilising the information from a master station and two or more other stations concurrently, determine its position in relation to the positions of the stations and hence its absolute geographical position. Currently there are in operation marine navigation systems such as DECCA, OMEGA and LORAN-C. For aircraft, marine and land vehicles there is also the recently introduced Global Positioning System (OPS) in which the transmitting stations are earth orbiting satellites.

For systems such as GPS where the transmitted signals consist primarily of long pseudo-random digital codes there can be problems of equipment complexity and power consumption, as well as problems arising from interference, either accidental or wilful.

The principles involved in using long pseudo-random codes is that the code sequence from a station is correlated with a like code sequence held in the receiver to determine timing information relevant to the link from the station to the receiver. The timing information relates to the path length of the link so that by determining the path lengths from two stations range information can be extracted, while information from three stations can give range and azimuthal position and from four stations range, azimuth and elevation information.

In general, long spread spectrum codes are correlated by serial correlators, this process requiring that the incoming code be multiplied bit by bit one after the other by the model code held in the receiver, results being continuously integrated. The degree of correlation between model and incoming code is decided at the end of the long series of multiplications; if correlation is not achieved the code is slid along and another try made. Early and late gates may be used. Long acquisition time, dependent on the uncertainty of initial positioning, are inevitable.

The alternative which yields an instant correlation at the moment that the whole received code is enclosed by the correlator, thus requiring no acquisition time, is the parallel correlator. This mechanism requires that each bit of received code is multiplied by the model, which has the total code length, all multiplications are added, the result being the correlation at each point of the code.

When final correlation is signalled by a high integration peak N2 multiplications and additions will have taken place (where N is the total number of chips in the code). In the case of a long code this may involve an unacceptable degree of complexity and processing power.

According to the present invention there is provided a navigation system having a plurality of transmitting stations each of which transmits a digital code wherein each digital code comprises a sequence of short repeating codes at a fast digital rate and superimposed thereon a long single code at a second slower digital rate, the second code being of a duration equal to the duration ot the sequence of short repeating codes, such that correlationof the long code in a receiver will yield an unambiguous timing signal relevant to the sequence of short repeating codes.

An embodiment of the invention will now be described with reference to the accompanying diagram which illustrates the * relationship between a sequence of short repetitive digital codes and a superimposed long single code.

In a receiver the long sequence of short codes PS is fed into a parallel correlator which matches the short code PS, thus a number of peaks are generated without the need for acquisition every time one of the short codes PS passes through the short code correlator.

Each one of the short code sequence peaks are time positioned to an accuracy defined by the chip rate but, at this point are at low signal-to-noise ratio because the total short code sequence length has not passed. Each output from the short code correlation is passed into an integrator register of length equal to the short code PS, each cell of which corresponds to a define deposition of the short code sequence. As the short code appears again and again.

equally spaced in time, the summed content of the cell which corresponds to the correlation time of the short code in the total code sequence, will increment. At the end of the total code sequence period, inspection of the row of integrator will define a correlation peak consistent with a signal-to-noise appropriate to coherent correlation over the total period and defined to a time accuracy appropriate to the chipping rate. However, the timing, although being consistent with the accuracy of the chip rate, is multiply ambiguous by the number of times that the short code is repeated in the sequence. All information as to the coarse position of the short sequence pulse was lost in the second stage of integration, nor was it available at the time that the multiple correlations were generated; at that stage the signal-to-noise was not adequate for positioning.

The sender also transmits a slower code, long code superimposed on the sequence of short fast codes and of exactly the same duration as the total short code sequence, by the use of orthogonal modulation (QPSK). This long code, when correlated by a parallel correlator clocked at the slower rate, will yield in the receiver one unambiguous time defining pulse at the end of the same long sequence period as produced the sequence of fine time defining correlation pulses. A separate slow clocked parallel correlator may be used at the same time as the fast correlator, in which case ambiguity resolution may be achieved with one pulse. Where equipment economy is essential the one correlator may be used for both coarse and line time resolution, clocked at the appropriate rates, but in this case it would be necessary to operate on successive impulses of the transmission for coarse and fine discrimination.

Thus the invention provides a means whereby a long spread spectrum sequence, required for fine time definition as well as high processing gain characteristics, may be located and correlated with a near zero acquisition time without the use of lengthy acquisition procedures and without excessive demands for processing power.

Such an end could normally only be achieved by parallel correlation of the whole code involving N2 muftiplications and additions where N is the total number of chips in the total code.

To enhance the reliability of a multiple station navigation system, wherein one station is designated a master' station and the transmissions of the other stations are locked to those of the master station, the invention also provides means for monitoring the operations of the master station and, in the event of a fault therein being detected, means for transferring the master status to another one of the stations.

Another problem associated with repetitive code transmissions arises from the fact that in spread spectrum code sequences which are repeated every millisecond the code spectrum has a line structure repeating every Kilohertz. Whereas spread spectrum type modulations are normally used in order to provide good resistance to interference, this particular line structure in the use of short repetitive code sequences does mean that there is a significant vulnerability to CW interference at these particular line offset frequencies.

Therefore another aspect of the invention provides a navigation system having at least one transmitting station and a receiving station wherein the transmitting station transmits a spread spectrum signal comprising a sequence of short repetitive digital codes, the receiving station having means for determining the frequency of a CW interfering signal and means for communicating with the transmitting station a request for the transmitting station to adjust its transmission frequencies such that the CW interference frequency ceases to coincide with a spectral line frequency in the spread spectrum signal. * 6

Claims

CLAIMS: 1. A navigation system having a plurality of transmitting stations each of which transmits a digital code wherein each digital code comprises a sequence of short repeating codes at a fast digital rate and superimposed thereon a long single code at a second slower digital rate, the second code being of a duration equal to the duration of the sequence of short repeating codes, such that correlation of the long code in a receiver will yield an unambiguous timing signal relevant to the sequence of short repeating codes.
2. A navigational system according to claim 1, wherein one of said plurality of transmitting stations is designated a master' station and transmissions of the other stations are locked to those of the master station, the system including means for monitoring the operation of the master station and, in the event of a fault therein being detected, means for transferring master status to another one of the stations.
3. A navigation system according to claim 1 or 2, wherein a receiving station includes means for determining the frequency of a CW interfering signal and means for communicating with the transmitting station a request for the transmitting station to adjust its transmission frequencies such that the CW interference frequency ceases to coincide with a spectral line frequency in the spread spectrum signal.
4. A navigation system substantially as described with reference to the accompany drawing.Amendments to the claims have been flied as follows 1. A navigation system having a plurality of transmitting stations each of which transmits a digital code wherein each digital code comprises a sequence of first repeating codes at a fast digital rate and sup!rimposed thereon a second repeating code at a second-slower digital rate, the second code being of a duration equal to the duration of the sequence of the first repeating codes.2. A navigational system according to claim 1, wherein one of said plurality of transmitting stations is designated a master station and transmissions of the other stations are locked to those of the master station, the system including means for monitoring the operation of the master station and, in the event of a fault therein being detected, means for transferring master status to another one of the stations.3. A navigation system according to claim 1 or 2, wherein a receiving station includes means for determining the frequency of a CW interfering signal and means for communicating with the transmitting station a request for the transmitting station to adjust its transmission frequencies such that the CW interference frequency ceases to coincide with a spectral line frequency in the spread spectrum signal.4. A navigation system substantially as described with reference to the accompanying drawing.