GB2353648A

GB2353648A - Repeater unit for satellite positioning system

Info

Publication number: GB2353648A
Application number: GB9928646A
Authority: GB
Inventors: Anthony Peter Hulbert
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1999-08-27
Filing date: 1999-12-06
Publication date: 2001-02-28
Also published as: GB9928646D0; GB9920286D0

Abstract

In order to determine location of objects, 15, in areas of poor or no reception from satellites, 9, 10, 11, 12, local repeaters, 5, 6, 7, 8 are provided. Each repeater determines its position and preferably transmits a GPS-type signal at the same frequency with position information to a given area. The transmitted codes may be different to those used by the satellite and other repeater units. At least four satellite or repeater signals should be received by the mobile unit to determine its position. In one embodiment the repeater stations are battery powered and attached to the tops of buildings, 1, 2, 3, 4, transmitting signals downwardly to mobile stations at street level. Code assignment and height determination may be carried out on installation or automatically.

Description

IMPROVEMENTS IN OR RELATING TO SATELLITE NAVIGATION The present invention

relates to improvements in or relating to 5 satellite navigation.

Satellite navigation systems such as GPS, are well known and include a receiver terminal which must be capable of receiving signals from a minimum of four satellites in order to determine both the location of the terminal and time. In operation of a system, four pseudo range measurements are made, to deterTnine the distance of each satellite from the terminal, which facilitates computation of the location of the terminal in three dimensions, plus a determination of time itself. However, in dense urban areas having many high rise buildings, there are so-called 'urban canyons' in which it is not possible to 'see' four satellites and so accordingly four satellite signals can not always be received. Indeed, there may be some places in which no satellite signals at all can be received.

The next generation of satellite navigation systems, following on from GPS, will be much more accurate, and will be designed to determine location to accuracies of the order of a centimetre or less. This degree of accuracy will open up new applications, many of which would be ideally suited to operation in densely populated cities. Thus, navigation coverage of cities will be more important for second generation systems than for present systems. It would be possible to deploy an entirely separate additional, nonsatellite system to provide coverage of city areas. However, this would be unattractive since it would require separate or dual mode receivers and a costly new support system. Moreover, a separate additional allocation of frequencies would probably also be required.

It is therefore an object of the present invention to provide a low cost repeater for satellite navigation signals which overcomes the problems mentioned above.

It is also an important object of this invention to provide an improved satellite navigation/position determination system which will operate in areas such as city areas, where satellite signal reception is problematic.

In accordance with one aspect of the present invention, there is provided a repeater unit comprising:- a receiving antenna for receiving signals from at least one satellite; a transmitting antenna for transmitting a signal; and processing means for processing each received signal and for generating the signal for transmission in accordance with a received signal.

In accordance with another aspect of the present invention, there is provided a satellite navigation system comprising a plurality of satellites and a plurality of repeater units as described above.

In accordance with a further aspect of the present invention, there is provided a satellite navigation/position determination system includes a plurality of repeater terminals, each comprising means operative to provide signals indicative of repeater terminal position in three dimensions, a code generator arranged to generate a coded signal indicative of the said position as determined, a transmitter operative to transmit the coded signal as modulation on the same carrier frequency as satellite signals, and a navigation terminal positioned remotely of the repeater terminals but within signal reception range of at least one of said repeater terminals and zero or more satellites to a total of at least four repeater terminals or satellites, which navigation terminal comprises an RF receiver tuned to the frequency of the satellite signals, a first demodulator/position determination unit operative in dependence upon reception by the RF receiver of the satellite signals (when present), for providing first signals indicative of pseudo ranges of the navigation terminal with respect to the satellites and second demodulator/position determination unit operative in dependence upon reception by the RF receiver of the coded signals (when present), for providing second signals indicative of pseudo ranges of the navigation terminal with respect to the repeater terminals, computer means for computing the terminal's position in dependence upon at least- four pseudo ranges obtained either from satellites or from repeater terminals or ftorn any combination thereof and display means which serve to provide a display indicative of the position of the navigation terminal as computed.

It will be appreciated that, if in a built up city environment the repeater terminals are positioned at mutually spaced locations on the top of buildings, so as easily to receive the satellite signals, and so as to be in a position to radiate the said coded signals downwardly towards streets below, a navigation terminal in a car at street level, although unable to receive the satellite signals, will still continue to function by receiving the coded signals from the repeaters.

Since the repeater terminals operate on the same frequencies as the satellite signals and use the same basic hardware system, full city coverage may be achieved relatively economically.

It may be arranged that coded signals from repeaters and satellite signals from line of sight paths (where available) are received at ground level at substantially the same power.

In use, the repeater terminals would typically be deployed on the side walls of skyscrapers at the top thereof, so as to provide visibility of the satellites for repeater terminal receivers, and visibility to street level for the repeater terminal coded signal transmitters.

It is envisaged that the repeater terminals will require little or no setting up, and preferably will require a mechanical connection only (no electrical connection) to a building supporting them. In a preferred embodiment the repeater terminals are powered by batteries charged from a solar panel, which is possible since power requirements are modest.

In accordance with yet a further aspect of the present invention, there is provided a method of operating a satellite navigation system as described above, the method comprising the steps of generating a plurality of satellite navigation signals from the plurality of navigation satellites; determining positional and reference information relating each repeater unit using the generated satellite navigation signals; receiving at least one navigation signal from one of the navigation satellites at each receiver unit; and transmitting a signal from each repeater unit on the same frequency as one of the received navigation signals.

Advantageously, the transmitted signal has a different code to those used by the satellites, and the code used for the repeater unit is different to the code used for other repeater units.

The power of the transmitted signal is controlled in accordance with the position of the associated repeater unit.

In accordance with the present invention, repeaters would operate on the same frequencies as the signals they repeat and would be arranged in such a way that the signals from the repeaters and from line of sight paths (where available) directly from the satellites are received on the ground at substantially the same power. The repeaters would typically be deployed on the walls of skyscrapers at the top, providing visibility of the satellites for the receiver and the ground for the transmitter. The repeaters are intended to require little or not setting up and, preferably, only a mechanical (no electrical) connection to the building supporting them.

In a preferred embodiment, a repeater unit would be powered by batteries charged from a solar panel. The power requirements should be modest.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which Figure I is a schematic plan view of a satellite navigation system in a city block; Figure 2 is a front view of the system as shown in Figure 1; Figure 3 is a block schematic diagram of a signal repeater terminal which forms a part of the system as shown in Figures I and 2; and Figure 4 is a block schematic diagram of a navigation terminal which forms a part of the system as shown in Figures I and 2.

Referring first to Figure I and Figure 2, a satellite navigation or positioning system in a city block, which includes buildings 1, 2, 3 and 4 comprises four repeater terminals 5, 6, 7 and 8, which are mounted on the buildings 1, 2, 3 and 4 respectively. The terminals 5, 6, 7 and 8 are mounted on the top of the buildings 1, 2, 3 and 4 on side walls at comers of the buildings 1, 2, 3, 4 so that they are in line of sight communication with satellite transmitters 9. 10, 11 and 12 and so that they are able to transmit to a navigation terminal 15 fitted in a car 16 coded signals on the same carrier frequency as is used by the satellite transmitters 9, 10, 11, 12.

Thus it will be appreciated, that although the navigation terminal 15, is screened by the buildings 1, 2, 3, 4 so that it is unable to receive signals from the satellites, it is nevertheless able to receive the coded signals transmitted by the repeater terminals 5, 6, 7, 8.

Referring now to Figure 3, the repeater terminals 5, 6, 7, 8 each comprise a receiver aerial 17, satellite signals received by which, from each of the satellites 9, 10, 11, 12 are fed to an RF receiver 18 and processed therein to determine both repeater position as indicated by output signals X, Y Z and time as indicated by an output signal T. The signals X, Y and Z indicative of position, are fed to an encoder 19 to produce a coded signal characteristic of the position of a repeater terminal. The time signal T is fed to an oscillator 20 which feeds a code generator 21 to produce a another coded signal indicative of time. The code generator 21 and the encoder 19 are arranged to feed a modulator 22 which produces an output signal at a carrier frequency corresponding to the satellite frequency which is appropriately code modulated to indicate both time and repeater position. This coded output signal from the modulator 22 is amplified in an amplifier 23 and fed via a switch 24 (provided to facilitate time multiplex operation as will hereinafter be described) to a repeater transmitter aerial 25.

In this embodiment, the position of a repeater terminal as indicated in accordance with its coded transmitted signal, and time is determined in known manner from four received satellite signals and this operation of position determination may either be automatically repeated periodically or alternatively it may be carried out on installation.

Appropriately code signals are thus radiated from each of the repeater terminals 5, 6, 7, 8 indicative of their respective positions and indicative of time. These signals are received at an Rx aerial 26 of the navigation terminal 15, as shown in Figures 1 and 2, and as shown in detail in Figure 4. The navigation terminal 15 comprises an RF receiver 27 to which signals from the aerial 26 are fed thereafter to be decoded in a repeater decoder 28 or a satellite decoder 29 as appropriate. The satellite decoder 29 operates conventionally to provide information appertaining to the position of the navigation terminal 15, which is displayed by a display in processor/di splay unit 30, and the repeater decoder utilises the received coded signals from each of the repeater terminals 5, 6, 7, 8 (which indicate both the respective positions of the terminals 5, 6, 7, 8 and time), so as to provide the same information to the display in processor/unit 30 as is provided by the satellite decoder 29.

It will be appreciated that this system may be implemented using known technology for code generation and code recognition purposes, well known to those skilled in these arts, thereby to provide for navigation in circumstances when satisfactory reception of satellite signals is not possible, and accordingly detailed description herein of these code generation and code recognition functions is believed to be unnecessary.

It will be appreciated that, when away from the city environment, this satellite navigation system will operate normally on satellite signals alone, and that when in a city environment, where satellite reception is problematic, repeater signals will be used instead of, or effectively to supplement the satellite signals.

In cases where a satellite transmits signals on more than one frequency, the repeaters may use one or more of these frequencies. It will be apparent that the coded signal transmitted by a repeater terminal, although on the same frequency as satellites, will use codes which are different to the codes used by satellites. Moreover, since it is necessary for a navigation terminal to receive and process at least four repeater signals in order to determine its position in three dimensions, and time, the codes used by different repeater terminals in a given area will be arranged to differ, so that they can be distinguished from each other.

As hereinbefore explained, the assignment of codes may be performed either manually on installation or automatically and it is apparent that the basis of deployment of repeater tenninals should be such that each terminal should be within range of another, in such a way that a chain of connections extend across a matrix of repeater terminals in a given area of coverage. Given that the position of each repeater terminal is known, appropriate codes may be computed at a designated terminal based on information aggregated from the terminals. This information may then be disseminated to all of the terminals using a suitable communication link.

Alternatively the codes may be selected according to the 2d grid reference of the repeaters, modulo a suitable distance (e.g. 150m).

The power to be transmitted from each repeater terminal should be such that, at ground level, the power received from all repeater terminals is the same and equal to the power typically received from one of the satellites, if and when visible. This implies that the power must vary as a function of the height of the building on which the repeater terminal is mounted.

This setting can be achieved in a number of ways. One possibility is for an installer to enter the height of the building into the repeater terrninal. upon installation. The terminal can use this information to compute the power level to transmit. The installer can either use records of the height of buildings to derive this information or a differential altimeter unit set to zero at ground level. Alternatively, a radar altimeter may be used. In principle, this could be built into the repeater terminal although cost considerations are likely to make this unattractive. In a further embodiment, the terminals can determine their absolute altitude from the satellite navigation system. All that is then missing is the height level of the ground. This common value could be entered into all terminals prior to deployment or by reference to a database stored within all terminals.

For simplicity of operation, the repeater terminals will transmit on the same frequency as the satellite navigation system and thus isolation between the receiver and the transmitter will be required. This can be achieved in various ways, as will hereinafter be described.

One approach is to use an antenna for the satellite receiver with an upper hemispherical pattern so that gain in a downward direction is made as small as possible. The repeater terminal's transmit antenna should conversely have a pattern pointing downwards with very little gain in the upper hemisphere. The isolation can be increased by introducing a vertical separation between the receiver and transmit antennas. Since the terminals will be mounted on walls at the top of tall buildings, this vertical separation should be practical up to distances of perhaps 1 to 2m. It should be noted that the processing gain due to the spread spectrum format of the signals will also contribute to the isolation between transmit and receiver.

An alternative approach is to introduce an element of time multiplexing. In this approach, the transmitter is enabled for only part of the time and disabled by periodically opening the switch 24. Typically, the transmitter would be enabled for a greater part of the time. Given the location of the repeater terminals and the time and frequency information, a repeater terminal has all it needs to be able to construct its signals without continuously receiving the satellite signals. Periodically the repeater ten-ninal would therefore initiate operation of the switch 24, so as to switch off its transmitter in order to listen to the satellite and correct its timing and frequency reference.

The navigation terminal receivers are constructed as required for normal satellite navigation, and additional baseband processing is incorporated to search for codes corresponding to the repeater ten-ninals when necessary.

Given the relatively short range to the repeater ten-ninal and the availability of absolute time, the codes for the different repeater terminals may be assigned as a shift of a signal code. The baseband processing in the navigation terminal receiver would then only need to search for the one code in all positions. The navigation terminal receiver would preferably attempt to obtain its information ftom four or more satellites if possible. Should this not be possible, or should a greater accuracy be required than can be provided from the satellites alone, the search for repeater terminals would be activated. If the time division option is implemented at the repeater terminals then provided the navigation terminal receiver has obtained correct timing ftorn at least one satellite, it could be arranged to search only when repeater terminal transmissions were known to be in progress.

Once the code timing for a given repeater terminal is found, its spread spectrum signal could be demodulated. This would contain the location of the repeater terminal and related information. The exact timing of the signals relative to its references provides a 'pseudo range' which, in conjunction with 'pseudo ranges' from either other repeater terminals or satellites, will enable the navigation terminal receiver to compute its position and/or time.

Claims

CLAIMS:

1. A repeater unit comprising:a receiving antenna for receiving signals from at least one satellite; a transmitting antenna for transmitting a signal; and processing means for processing each received signal and for generating the signal for transmission in accordance with a received signal.

2. A repeater unit according to claim 1, further comprising means for providing signals indicative of a repeater terminal position in three dimensions.

3. A repeater unit according to claim 1 or 2, further comprising a code generator arranged to generate a coded signal indicative of the terminal position, the transmitting antenna being operative to transmit the coded signal as modulation on the same carrier frequency as the received satellite signals.

4. A satellite navigation/position determination system including a plurality of repeater terminals, each comprising means operative to provide signals indicative of repeater terminal position in three dimensions, a code generator arranged to generate a coded signal indicative of the said position as determined, a transmitter operative to transmit the coded signal as modulation on the same carrier frequency as satellite signals, and a navigation terminal positioned remotely of the repeater terminals but within signal reception range of at least one of said repeater terminals and zero or more satellites to a total of at least four repeater terminals or satellites, which navigation terminal comprises an RF receiver tuned to the frequency of the satellite signals, a first demodulator /position determination unit operative in dependence upon reception by the RF receiver of the satellite signals (when present), for providing first signals indicative of pseudo ranges of the navigation terminal with respect to the satellites and second demodulator/position determination unit operative in dependence upon reception by the RF receiver of the coded signals (when present), for providing second signals indicative of pseudo of the navigation terminal with respect to the repeater terminals, computer means for computing the terminal's position independence upon at least four pseudo ranges obtained either from satellites or from repeater terminals or from any combination thereof a display means which serve to provide a display indicative of the position of the navigation terminal as computed.

5. A system according to claim 4, installed in a built up city environment, wherein the repeater terminals are positioned at mutually spaced locations on top of buildings so as to receive satellite signals if so required, and so as to be in a position to radiate the said coded signals downwardly towards navigation terminals below at street level.

6. A system according to claim 5, wherein it is arranged that coded signals from repeaters and satellite signals from line of sight paths (where available) are received by navigation terminals at ground level at substantially the same power level.

7. A system according to any one of claims 4 to 6, wherein the repeater terminals have only a mechanical connection to a building supporting them and are powered by batteries charged from a solar panel.

8. A system according to any one of claims 4 to 7, wherein the coded signal transmitted by a repeater terminal, although on the same frequency as the uses codes which are different to codes used by the satellites and wherein the codes used by different repeater terminals in a given area are arranged to differ.

9. A system according to any one of claims 4 to 8, wherein the assignment of codes is performed manually on installation.

10. A system according to any one of claims 4 to 9, wherein the assignment of codes is perfonned automatically.

11. A system according to claim 10, wherein appropriate codes are computed at a designated repeater terminal based on information aggregated from the terminals appertaining to the position of each terminal, which is then be disseminated to all of the terminals.

12. A system according to any one of claims 4 to 11, wherein the height of a building to which a repeater terminal is secured is entered into the terminal on installation and used to compute the power level of transmissions.

13. A system according to claim 12, wherein the height is established using an altimeter.

14. A system according to claim 13, wherein the altimeter is built into the repeater terminal.

15. A system according to any one of claims 4 to 11, wherein the repeater terminals are arranged to determine their absolute altitude from the satellite navigation system, height above ground being entered into all terminals prior to deployment or by reference to a database stored within all terminals.

16. A system according to any one of claims 4 to 15, wherein isolation between a repeater terminal satellite receiver and its coded signal transmitter, operating on the same frequency, is achieved by use of an antenna for the satellite receiver with an upper hemispherical pattern and by arranging that the repeater terminal's transmit antenna has a pattern pointing downwardly with very little gain in the upper hemisphere.

17. A system according to claim 16, wherein vertical separation between the receiver and transmit antennas is provided.

18. A system according to any one of claims 4 to 15, wherein isolation between a repeater terminal satellite receiver and its coded signal transmitter, operating on the same frequency, is achieved by the use of time multiplexing wherein the transmitter is enabled for only part of the time.

19. A system according to claim IS, including switch means in each of the repeater terminals operable to effect periodic disablement of the said transmitter so that the transmitter and its associated receiver are time multiplexed so as to provide isolation therebetween.

20. A system according to any one of claims 4 to 19, wherein the codes for different repeater terminals are assigned as shifts of a single long code whereby baseband processing in a navigation terminal receiver is to search for the one code only.

21. A method of operating a satellite navigation system according to any one of claims 4 to 20, the method comprising the steps of, generating a plurality of satellite navigation signals from the plurality of navigation satellites; determining positional and reference information relating each repeater unit using the generated satellite navigation signals; receiving at least one navigation signal from one of the navigation satellites at each receiver unit; and transmitting a signal from each repeater unit on the same frequency as one of the received navigation signals.

22. A repeater unit substantially as hereinbefore described with reference to the accompanying drawings.

23. A satellite navigation system substantially as hereiribefore, described with reference to the accompanying drawings.