Positioning system

[0001] The invention concerns a positioning system where a receiver uses signals emitted by satellites, pseudolites and similar in order to determine its position. Well-known systems of this type are the GPS system and the GLONASS system.

[0002] Often the position of a receiver must be determined in places where the sources of primary positioning signals as emitted by, in particular, satellites are obscured from view and, as a consequence, the signals are extremely weak or virtually non-existent, e.g., in woods, buildings, mines etc. Several methods have been developed to allow positioning under such circumstances. In known systems of the generic type (see G. -I. Jee et al., ‘Indoor Positioning using TDOA Measurements from Switching GPS Repeater’, ION GNSS 2004, pp. 1970–1976 or J. Caratori et al., ‘UPGRADE RnS Indoor Positioning System in an Office Building’, ION GNSS 2004, pp. 1959–1969) several secondary positioning signals are produced and each of them transmitted by a respective transmit antenna. In each case the secondary positioning signal corresponds to the input signal, restricted to a certain time slot specifically assigned to the signal in question.

[0003] In this system time slots must be assigned centrally and the receiver or receivers must be synchronized to the transitions between subsequent secondary positioning signals. The system is relatively complicated and requires expensive hardware. As the input signal is repeated by the transmit antennas essentially unchanged, disturbance of nearby outdoor receivers or indoor receivers capable of processing weak primary positioning signals cannot be excluded.

[0004] It is an object of the invention to provide a system of the generic type which is simple and, in particular, does not require central administration of the transmit antennas or synchronisation of the receiver.

[0005] It is a further object of the invention to provide a system which does not produce signals which might interfere with the primary positioning signals emitted by satellites, pseudolites or similar.

[0006] The system according to the invention can easily be realised in such a way that no license for its use is required in that only signals whose spectrum is contained in an ISM band, in particular the 2.4 GHz ISM band, are used.

[0010] On the roof of a building 1 a receive antenna 2 for receiving primary positioning signals which usually stem from GPS satellites but may in part come from pseudolites and similar devices as well, is mounted. It is connected by an RF cable to a repeater 3 which comprises a bandpass filter 4 and a low noise amplifier 5 whose output is connected to four parallel conversion circuits 6a–d. Each of the conversion circuits 6a–d comprises an oscillator 7 for producing a sine-shaped conversion signal with a specific fixed frequency and a mixer 8 for mixing the same with the output signal of low noise amplifier 5.

[0011] The outputs of conversion circuits 6a–d are each connected by an RF cable to one of transmit antennas 9a;b;c;d which are mounted at fixed locations inside building 1, via one of transmit circuits 10a;b;c;d. Each of the latter contains an interface connecting it to a LAN cable 11 and is capable of converting data received via the same into 802.11 WLAN format and vice versa for data transmitted in the opposite direction so each of the transmit antennas 9a–d serves at the same time as a WLAN access point.

[0012] Combined signals 12a–d which contain secondary positioning signal as well as WLAN components are transmitted by transmit antennas 9a–d and received by a receiver 13. The latter comprises (s. FIG. 2) a receiver antenna 14 which is connected to an RF front end for preprocessing received RF signals, the RF front end comprising a first part 16a containing a low noise amplifer and a second part 16b. A back-conversion circuit 15 comprises a synthesizer 17 for producing a sine-shaped back-conversion signal and a mixer 18 between the first part 16a and the second part 16b of the RF front end for mixing the same with the signal from receiver antenna 14. The preprocessed and back-converted signal from receiver antenna 14 is fed to a GPS decoder 19 and the merely preprocessed but not back-converted signal to a WLAN decoder 20.

[0013] The outputs of both the GPS decoder 19 and the WLAN decoder 20 are delivered to a controller 21 which extracts data defining the position of receiver 13 from them and controls a display 22 in order to make them accessible to a user. Controller 21 also controls synthesizer 17 in such a manner that the back-conversion signal assumes the frequencies of the conversion signals produced by the oscillators 7 in conversion circuits 6a–d in a cyclical sequence, each of them during a fixed time slot of, e.g., between 0.3 sec and 0.5 sec. For normal, in particular, outdoor use of receiver 13, controller 21 can inactivate the synthesizer 17.

[0014] Receive antenna 2 receives primary positioning signals from a number of satellites and possibly pseudolites or similar. This input signal whose carrier frequency is close to the 1.572 GHz GPS frequency is filtered in bandpass filter 4 and amplified by low noise amplifier 5. Then the carrier frequency is up-converted by conversion circuits 6a–d and shifted to four different frequencies in the 2.4 GHz ISM band while the signals are not modified otherwise such that they—apart from their being filtered and amplified—essentially conform to the input signal as received by receive antenna 2. Each of those secondary positioning signals is then furnished to one of transmit circuits 10a;b;c;d where a WLAN signal is superposed and the combined signal 12a;b;c;d output via the respective transmit antenna 9a;b;c;d. The WLAN signal is in 802.11 format with a carrier frequency also in the 2.4 GHz ISM band and contains data like the positions of the transmit antennas 9a–d and the signal time delays associated with each of them. The spectra of the secondary positioning signals as well as of the WLAN signals are contained in the 2.4 GHz ISM band and therefore far removed from the spectrum of the primary positioning signals. There is practically no risk of interference.

[0015] In the receiver 13, the superposition of combined signals 12a–d is received by receiver antenna 14, amplified in the first part 16a of the RF front end, then down-converted and its carrier frequencies shifted back to the original values close to 1.572 GHz and the resulting signal, after having been further preprocessed in the second part 16b of the RF front end, fed to GPS decoder 19 where it is filtered and analysed. At the same time, the unconverted signal goes, also via RF front end 16a,b, to WLAN decoder 20 where the relevant data, in particular the positions of transmit antennas 9a–d and the signal time delays associated with them are extracted and delivered to controller 21.

[0016] The back-conversion of the combined signals 12a–d is carried out by mixing the signal received by receiver antenna 14 with the back-conversion signal from synthesizer 17 which corresponds to each of the conversion signals produced by the oscillators of the conversion circuits 6a;b;c;d during its assigned time slot. As a consequence, during its respective time slot, the carrier frequency of the GPS component of, e.g., combined signal 12a as transmitted via transmit antenna 9a which conforms to the input signal of receive antenna 2 is converted back and assumes its original carrier frequency.

[0017] The back-converted GPS component of combined signal 12a is then isolated and analysed in GPS decoder 19 while the GPS components of the combined signals 12b;c;d from transmit antennas 9b;c;d are suppressed. The analysis yields position data—those of receive antenna 2—as well as a clock bias which is fed to controller 21. Switching from combined signal 12a to combined signal 12b by appropriately changing the frequency of the output signal of synthesizer 17, controller 21 will cause GPS decoder 19 to then analyse the GPS component of combined signal 12b which will yield the same position data but usually a different clock bias. From the differences in clock bias—corrected for the signal time delays—which are gained from cyclically switching through the combined signals 12a–d as well as the known positions of transmit antennas 9a–d the position of the receiver 13 can then be determined by an algorithm of the type used in conventional TDOA calculations. If the synthesizer 17 is not active, the receiver 13 operates like an ordinary GPS receiver.

[0018] The usefulness of the invention is not limited to the above-described application. In particular, it may as well be used in larger mobile objects like ships, trains, aeroplanes etc. where the position of the receive antenna is not fixed.