Apparatus and method
APPARATUS AND METHOD The present invention relates to an apparatus and method and in particular but not exclusively for the acquisition of signals. In an example of a global navigation satellite system satellites orbiting the earth in known orbit paths with accurately known positions are used. These satellites transmit signals which can be received by a receiver on earth. Using signals received from four or more satellites, the receiver is able to determine its position using trigonometry. The signals transmitted by the satellite comprise pseudo-random codes. The accuracy of the determination of position is dependent on factors such as the repetition rate of the code, the components of the receiver and atmospheric factors. GALILEO is a European initiative for a global navigation satellite system which provides a global positioning service. It has been proposed that GALILEO be interoperable with the global positioning system GPS and GLONASS, the two other global satellite navigation systems. It should be appreciated that the term GNSS is used in this document to refer to any of these global positioning systems. GALILEO currently has a system of thirty satellites, twenty-seven operational satellites with three operational in-orbit spares. The proposed frequency spectrum for GALILEO has two L-bands. The lower L-band, referred to as E5a and E5b, operate in the region of 1164 MHz to 1214 MHz. There is also an upper L-band operating from 1559 MHzto 1591 MHz. In GPS and GALILEO, signals are broadcast from satellites which include the pseudo random codes which are processed at a receiver to determine position data. The processing involves first determining the relative offset of the received codes with locally generated versions of the codes (acquisition) and then determining the position once the relative offset is determined (tracking). Both acquisition and tracking involve correlating received signals with a locally generated version of the pseudo random codes over an integration period. In spread spectrum systems, acquisition may be difficult because it is two dimensional (frequency and time). A further difficulty is that because the signals are much weaker inside as compared to outside, it is much more difficult to acquire signals indoors. In particular, the indoor operation of GNSS requires the reception of signals attenuated by at least 20dB from the outdoor equivalents. Acquisition is carried out by a trial and error searching of cells corresponding a frequency and phase range. The number of cells in the time domain is for example 4092. The number of cells in the frequency domain increases with a drop in signal strength. This however may be reduced with use of a temperature controlled crystal oscillator TCXO. The time required to search a cell may increase one hundred fold from outdoors to indoors. For example for indoors, each cell may take milliseconds because of the weaker signal strength. This results in a greatly increased search time for indoor receivers. This problem may be addressed by using parallelism in the frequency domain, for example sixteen fast Fourier transform channels or by parallelism in the time domain, using parallel correlators. To achieve parallelism may require faster clocks and/or more hardware which may be disadvantageous. Additionally, more hardware and/or faster clocks may require increased power. In any event, one limit is the stability of the reference clock which may prevent bandwidth reduction to the degree required for indoor sensitivity. As already mentioned the indoor signals can be attenuated by at least 20 dB from their outdoor equivalents. To increase the sensitivity by 20 dB for the indoor signals means integrating for a hundred times longer. However, this may be difficult to achieve because as the coherent integration period is extended, the bandwidth of the channel narrowed. This in turn requires many more searches to be carried out and eventually the stability of the reference oscillator becomes a limiting factor as a signal appears wander from one frequency to another, even before acquisition is completed. This results in a spreading of the energy, preventing further gain. In addition, the modulation method used may provide a limit on the integration time. Thus there may be problems in performing integration with such signals. The integration time may be limited by the accuracy of a local clock and the frequency shifts caused by relative motion of the satellite and receiver. Aspects of some embodiments of the invention may be seen from the appended Apparatus comprises: a first correlator configured to correlate a first signal component with a first code to provide a first output, said first signal component having a carrier frequency and data; a second correlator configured to correlate a second signal component with a second code to provide a second output, said second signal component having the same carrier frequency as the first signal component and the same data as the first signal component, said data on the second signal component being delayed with respect to the data on the first signal component; and a processor configured to process the first and second outputs, said data on said first output being aligned with the second output to provide frequency information about said carrier. 1. Apparatus comprising: a first correlator configured to correlate a first channel with a first code to provide a first output, said first channel having a carrier frequency and data; a second correlator configured to correlate a second channel with a second code to provide a second output, said second channel having the same carrier frequency as the first channel and the same data as the first channel, said data on the second channel being delayed with respect to the data on the first channel; and a processor configured to process the first and second outputs, said data on said first output being aligned with the second output to provide frequency information about said carrier. 2. Apparatus as claimed in claimed in claim 1, comprising a delay, said delay configured to delay said first output and provide a delayed first output to said processor. 3. Apparatus as claimed in claim 2, wherein said delay is configured to delay said first output such that said data in said first output is aligned with the data in said second channel. 4. Apparatus as claimed in any preceding claim wherein said data in said second channel is delayed with respect to said data in the first channel by n symbols. 5. Apparatus as claimed in claim 4, wherein n is equal to 1. 6. Apparatus as claimed in any preceding claim, wherein said frequency has a value of F -/+ x where F is the target transmission frequency and x is an error. 7. Apparatus as claimed in claim 6, comprising a down convertor configured to down convert the first channel and said second channel by a frequency value of substantially F. 8. Apparatus as claimed in claim 6 or 7, wherein said processor is configured to process said first and second outputs to cancel out components of said channels to provide frequency information. 9. Apparatus as claimed in any preceding claim, wherein said frequency information comprises phase information. 10. Apparatus as claimed in claim 9, wherein said phase information comprises a phase difference between the carrier of the first channel and the carrier of the second channel. 11. Apparatus as claimed in claim 10 when appended to claim 4, wherein in said phase difference is determined over n symbols. 12. Apparatus as claimed in any preceding claim wherein the first and second codes are different. 13. Apparatus according to any preceding claim further comprising a mixer arranged to correlate said first and second outputs to provide the third output. 14. Apparatus as claimed in any preceding claim wherein at least one of said correlators comprises a mixer. 15. Apparatus as claimed in any preceding claim further comprising data recovery circuitry operable to receive said first and second output. 16. Apparatus as claimed in claim 15 wherein said data recovery circuitry is operable to combine said first and said second outputs and output a data signal based on said difference, representative of said data. 17. Apparatus according to claim 16 wherein said second channel a mixer operable to extract a pilot signal from said output of the second correlator. 18. An integrated circuit or chip set comprising an apparatus as claimed in any preceding claim. 19. A positioning device comprising an apparatus as claimed in any of claims 1 to 17. 20. A positioning device as claimed in claim 19, wherein said device comprises one of a satellite navigation device and a mobile communication device. 21. A method comprising: correlating a first channel of a received signal with a first code to provide a first output, said first channel having a carrier frequency and data; correlating a second channel of said received signal with a second code to provide a second output, said second channel having the same carrier frequency as the first channel and the same data as the first channel, said data on the second channel being delayed with respect to the data on the first channel; providing frequency information about said carrier by processing the first and second outputs, said data on said first output being aligned with the second output to. 22. The method as claimed in claimed in claim 21, comprising delaying said first output and processing said delayed first output. 23. The method as claimed in claim 22, wherein said delaying said first output further comprises delaying said first output such that said data in said first output is aligned with the data in said second channel. 24. The method as claimed in any of claims 21 to 23 wherein said data in said second channel is delayed with respect to said data in the first channel by n symbols. 25. The method as claimed in claim 24, wherein n is equal to 1. 26. The method as claimed in any of claims 21 to 25, wherein said frequency has a value of F -/+ x where F is the target transmission frequency and x is an en'or. 27. The method as claimed in claim 26, further comprising down-converting said first channel and said second channel by a frequency value of substantially F. 28. The method as claimed in claim 26 or 27, wherein said processing said first and second outputs further comprises cancelling out components of said first and second signal to provide frequency information. 29. The method as claimed in any of claims 21 to 28 wherein said frequency information comprises phase information. 30. The method as claimed in claim 29, wherein said phase information comprises a phase difference between the carrier of the first channel and the carrier of the second channel. 31. The method as claimed in claim 30 when appended to claim 24, further comprising determining said phase difference over n symbols. 32. The method as claimed in any of claims 21 to 31 wherein the first and second codes are different. 33. The method according to any of claims 21 to 32 further comprising: mixing said first and second outputs to provide the third output. 34. The method as claimed in any of claims 21 to 33 wherein at least one step of correlating further comprises mixing. 35. The method as claimed in any of claims 21 to 34 further comprising receiving said first and second output by data recovery circuitry. 36. The method as claimed in claim 35 further comprising: combining said first and said second output and outputting a data signal, representative of said data. 37. The method according to claim 36 wherein said method further comprises: extracting a pilot signal from said second output by a mixer. 38. A computer program configured to perform one or more of the steps of any one of claims to 21 to 37. 39. A signal comprising a first channel having a first code a carrier frequency and data and a second channel having a second code, the same carrier frequency as the first channel and the same data as the first channel, said data on the second channel being delayed with respect to the data on the first channel. 40. Apparatus comprising: a first correlator configured to correlate a first signal component with a first code to provide a first output, said first signal component having a carrier frequency and data; a second correlator configured to correlate a second signal component with a second code to provide a second output, said second signal component having the same carrier frequency as the first signal component and the same data as the first signal component, said data on the second signal component being delayed with respect to the data on the first signal component; and a processor configured to process the first and second outputs, said data on said first output being aligned with the second output to provide frequency information about said carrier.