GB2475675A - Navigation receiver operable with multiple navigation systems - Google Patents

Abstract

Navigation receiver using signals from one or more navigation transmitters in each of two navigation systems using different frequency bands 1, 3. The receiver comprises a receive chain (16, 18, 22, 24, 28, Fig.4) which down converts signals received in a tuneable receive frequency band 5a-e determined by tuning a local oscillator (20, Fig.4) such that the tuneable receive frequency band corresponds to the first frequency band and at least a sub-band of the second in separate slots in a time slot sequence (Fig.5). A similar receivers may switch between single-system and multi-system mode (Fig.9); be tuned to avoid a sub-band of the second system frequency band related to satellites inside an estimated `obscured zone' (Figs.6-7); or calculate its position based on positions of one satellite in each system and ranges to these satellites at a single time, the range to the satellite of the second system being estimated from a range to the satellite at a different time and the motion of that satellite (Fig. 8).

Description

Improvements to Navigation Receivers

Field of the Invention

The present invention relates generally to navigation receivers, and more specifically to methods, apparatus and computer software relating to navigation receivers having the capability to receive signals from one or more navigation transmitters operating according to a first navigation system and from one or more navigation transmitters operating according to a second navigation system.

Background of the Invention

A navigation receiver receives signals transmitted by navigation transmitters in one navigation system, using a navigation system technology appropriate to that navigation system. A navigation receiver may be a dedicated apparatus or may be equipment including functionality other than navigation, such a mobile handset for a cellular wireless system.

The Global Positioning System (GPS) and the GLObal NAvigation Satellite System (Glonass or GLONASS) are examples of satellite navigation systems which, at many locations, enable a user to obtain reliable and accurate position information, as well as typically velocity and timing information. Other satellite navigation systems are planned, including the European Galileo system, the Indian Regional Navigation Satellite System (IRNSS), and the Compass & Beidou satellite navigation systems.

Other, non-satellite, navigation systems also are also known. Pseudolite navigation systems, are ground-based alternatives to satellite navigation systems. Other terrestrial navigation systems, such as LOng RAnge Navigation (LORAN), are also known, along with systems in which cellular radio network access nodes are used as navigation transmitters and others in which Wi-Fi network access points are used as navigation transmitters.

To provide a reliable three dimensional position estimate, using a navigation system, the receiver needs to solve a set of equations of four unknowns; the three dimensional position coordinates and the receiver clock error. Therefore the receiver ideally needs to acquire and track at least four navigation transmitter signals. Furthermore, since satellites are not stationary, accurate information about their position and velocity at the time of transmission is required in the case of satellite navigation systems. This information can be either decoded from satellite signals themselves with a typical rate of S0bits/second, which could be a time consuming process and vulnerable to low signal to noise ratio (SNR) conditions, or obtained from a terrestrial server in form of assisted data information at the beginning of the navigation process. The information may be communicated to the receiver using a terrestrial wireless communication system such as a cellular wireless network. In some situations the receiver may not be able to receive four acceptable satellite signals from satellites of one system. For example when a user is located in a typical urban area, the line of sight view may be partially blocked by the surrounding buildings, so that reception from some satellites, typically at low elevation angles, may be blocked. In such cases the accuracy of a satellite navigation system may be degraded.

It may be possible to achieve a two dimensional position estimate to useful degree of accuracy on the basis of three navigation transmitter signals, by making assumptions about the elevation of the position, for example on the basis of past position estimates or according to elevation estimates from sources such as other positioning or navigation systems.

However, if less than a predetermined number of navigation transmitters are available for reception by a receiver, the performance of the position estimation may not be acceptable, or the location estimation may not be possible, using the navigation system. In a satellite navigation system, and other triangulation systems, the predetermined number of navigation transmitters required is typically three.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided a navigation receiver capable of determining a position on the basis of signals received from one or more navigation transmitters operating according to a first navigation system using a first frequency band and from one or more navigation transmitters operating according to a second navigation system using a second frequency band, different to the first frequency band, the receiver comprising: a receive chain arranged to down convert signals received in a tuneable receive frequency band using a tuneable local oscillator, the tuneable receive frequency band being determined by the tuning of the local oscillator; a sampler arranged to sample the down converted signals; and a processor, wherein the tuneable local oscillator is arranged to be tuned to a sequence of frequencies such that the tuneable receive frequency band corresponds to the first frequency band in first time slots and the tuneable receive frequency band corresponds to at least part of the second frequency band in second time slots, the first and second time slots forming different elements of a time slot sequence, wherein the sampler samples a plurality of samples of the signal in each first and each second time slot, and wherein the processor is arranged to determine at least a position associated with the receiver on the basis of at least the sampled signals in the first and second time slots.

An advantage of tuning the tuneable local oscillator to a sequence of frequencies such that the tuneable receive frequency band corresponds to the first frequency band in first time slots and the tuneable receive frequency band corresponds to at least part of the second frequency band in second time slots, the first and second time slots forming different elements of a time slot sequence, is that the down converter hardware may be time shared between operation to receive signals from navigation transmitters operating according to the first navigation system and operation to receive signals from navigation transmitters operating according to the first navigation system, thereby offering an economical implementation by avoiding hardware duplication.

Preferably, the sampler uses the same sampling clock to sample the signal in each first and each second time slot, with an advantage that any error in the sampling clock frequency will be common to the two navigation systems, so that the effect of such an error on a position estimate made on the basis of measurements from both systems will substantially be removed.

In embodiments of the invention, the first navigation system uses code division multiple access, and the tuneable receive frequency band has a fixed bandwidth substantially the same as the signals of the first navigation system, with an advantage that a signal from any navigation transmitter operating according to the first navigation system will occupy a band that will be sampled and may be used to determine a position, whereas other signals, that may constitute interference, are not sampled.

Preferably, the second navigation system uses frequency division multiplexing wherein a tuneable receive frequency band used in the second time slot is narrower in bandwidth than the second frequency band, with an advantage that the same filter defining the tuneable receive frequency band may be used for use in the first and second time slots, allowing an economical implementation. It should be noted that, in a frequency division multiplexed navigation system, a part of the frequency band used by the navigation system may contain the whole bandwidth of signals relating to a sub-set of the navigation transmitters operating according to the navigation system, whereas a part of the frequency band used by a code division multiplexed system would not typically contain the whole bandwidth of any signals relating to specific navigation transmitters, since the signals from each navigation transmitter typically occupy the same band as each other.

Preferably, the second time slots comprise a first portion in which the tuneable receive frequency band corresponds to a first part of the second frequency band and a second portion in which the tuneable receive frequency band corresponds to a second part of the second frequency band, with an advantage that two parts of the second frequency band may be received sequentially. So, for example, when the second navigation system is a frequency division multiplexed system, a first sub-band used by a first navigation transmitter or navigation transmitters may be received alternately with a second sub-band used by another navigation transmitter or navigation transmitters.

In preferred embodiments, the first and/or second navigation system is a satellite navigation system. Preferably, the first navigation system is a Global Positioning System (GPS) system and the second satellite navigation system may be a Global Navigation Satellite System (Glonass) system. The GPS and Glonass systems may be particularly advantageously used together using a receiver with a constant band width tuneable receive band, since although the frequency bands used by the GPS system and the Glonass system have different band widths, the frequency division multiplexing of the Glonass system is arranged so that frequencies used by a plurality of satellites fall within a band width corresponding with that of the GPS system. As a result, if the bandwidth of the tuneable receive band corresponds to that used by the GPS system, then signals from a plurality of satellites may be received in the second time slots.

In accordance with a second aspect of the present invention, there is provided a method of selecting a sub-band for the reception of signals, at a satellite navigation receiver capable of receiving signals from one or more satellites operating according to a first satellite navigation system using a first frequency band and from one or more satellites operating according to a frequency division multiplexed satellite navigation system using a second frequency band, the method comprising: receiving signals from satellites operating according to the first satellite navigation system, and on the basis of information received by the receiver relating to the first satellite navigation system, determining a set of satellites of the first satellite navigation system from which acceptable signals are not received and from which signals would be expected to be received in an unobstructed environment; estimating a zone of directions of arrival from which satellite reception is not expected on the basis of the directions of arrival associated with the set; determining a set of satellites operating according to the frequency division multiplexed satellite navigation system from which signals are expected, on the basis of the estimated zone of directions of arrival from which satellite reception is not expected and on the basis of information received by the receiver relating to the frequency division multiplexed satellite navigation system; and selecting a sub-band for the reception of signals, the selected sub-band being a sub-band of the second frequency band for the reception of signals from a satellite in said determined set.

An advantage of selecting a sub-band for the reception of signals by the abovementioned method is that the sub-band will correspond to frequencies used by satellites from which signals are expected to be received.

Preferably, the information received by the receiver relating to the first and second satellite navigation systems is received via a connection to a terrestrial wireless network.

It is particularly advantageous to receive information relating to the second satellite navigation system via a connection to a terrestrial wireless network, rather than via a satellite operating according to the second satellite navigation system, as this enables the information to be received in advance of selecting a sub-band.

Preferably, the information received by the receiver relating to the first and second satellite navigation systems is almanac and/or ephemeris information, with the advantage that this allows the calculation of the positions of satellites, so that an appropriate zone of directions of arrival from which satellite reception is not expected can be determined, on the basis of which a set of satellites operating according to the frequency division multiplexed satellite navigation system can be determined from which signals are expected.

In accordance with a third aspect of the present invention, there is provided a method of signal processing for calculating a position of a satellite navigation receiver on the basis of signals received from one or more satellites operating according to a first satellite navigation system and from one or more satellites operating according to a second satellite navigation system, the method comprising: determining an estimate of a first range between a first satellite, operating according to the first satellite navigation system, and the receiver, using the first satellite navigation system, the first range corresponding to a first time; determining an estimate of a second range between a second satellite, operating according to the second satellite navigation system, and the receiver, using the second satellite navigation system, the second range conesponding to a second time, different to the first time; determining an estimate of a third range between the second satellite and the receiver, the third range corresponding to the first time, on the basis of a determined motion of the second satellite and the estimate of the second range; and calculating the position of the receiver on the basis of at least the first and third ranges.

An advantage of the abovementioned method is that reception of signals from one or more satellites operating according to a first satellite navigation system may be time multiplexed with reception from one or more satellites operating according to a second satellite navigation system, thereby allowing time sharing of parts of the radio receiver, and range estimates from each system may be used together to estimate a position.

Preferably, the determined motion of the satellite is determined on the basis of at least a Doppler frequency calculation applied to a signal received from the second satellite, with the advantage that a Doppler frequency calculation may have been carried out for a different purpose in the receiver so that the information may be obtained with little additional cost.

Preferably, the determined motion of the satellite is determined on the basis of at least ephemeris information, with the advantage that the Doppler frequency calculation may confirm, by comparison with ephemeris information, whether or not the receiver is substantially static.

Preferably, the method further comprises determining an estimate of a fourth range between a third satellite, operating according to the second satellite navigation system, and the receiver, using the second satellite navigation system, the fourth range corresponding to a third time, different to the first time and the second time; determining an estimate of a fifth range between the third satellite and the receiver, the fifth range corresponding to the first time, on the basis of a determined motion of the second satellite and the fourth range; and calculating the position of the receiver on the basis of at least the first, third and fifth range, with the advantage that calculations can be made on the basis of signals received from satellites the signals from which fall into different sub-bands, the reception of which is time multiplexed.

In accordance with a fourth aspect of the present invention, there is provided a method of selecting a mode of operation of a navigation receiver, the receiver being capable of determining a position on the basis of signals received from one or more navigation transmitters operating according to a first navigation system and from one or more satellites operating according to a second navigation system, the method comprising: determining a number of navigation transmitters operating according to the first navigation system from which acceptable signals are received; and selecting, on the basis of the number, between: a single system mode of operating, in which the position of the receiver is calculated on the basis of signals received by the receiver from one or more navigation transmitters operating according to the first navigation system; and a multi-system mode of operation in which the position of the receiver is calculated both on the basis of signals received by the receiver from one or more navigation transmitters operating according to the first navigation system and on the basis of signals received from one or more navigation transmitters operating according to the second navigation system.

An advantage of the abovementioned method is that when there are sufficient navigation transmitters operating according to a first navigation system from which acceptable signals can be received, determination of a position may take place using a first navigation system without timesharing with another system, allowing rapid determination of a position, or power may be saved be operating the receiver intermittently.

Preferably, the selection of the multi-system mode is on the basis of the number being less than 3 and more than zero. The advantage of selecting multi-mode operation when the number is less than three is that when signals are received from less than three navigation transmitters, reliable determination of a position is unlikely to be achieved, even in two dimensions, so that multi-mode operation would be preferable in order to increase the probability of receiving signals from three or more navigation transmitters.

Preferably, in the case of the first navigation system being a satellite navigation system, when the number is zero, a terrestrial mode of operation is selected, in which the position of the receiver is calculated on the basis of a terrestrial navigation system. If no signals are received with acceptable quality from satellites operating according to the first navigation system, then it is likely that signals are heavily obstructed, so it is likely that signals would be received with acceptable quality from less than three satellites operating according to a second satellite navigation system. In this circumstance, it would be more reliable to calculate the position of the receiver on the basis of a terrestrial navigation system.

Further aspects of the invention are set out in the appended claims and further features and advantages of the invention will be apparent form the following description of preferred embodiments of the invention, which are given by way of example only.

Brief Description of the Drawings

Figure 1 is a schematic diagram showing frequency bands used by GPS and Glonass satellite navigation systems; Figure 2 is a schematic diagram showing a receiver in a location in which signals from satellites at low elevations is obstructed; Figure 3 is a schematic diagram showing a distribution of satellites in a hemisphere in which signals from some are obstructed; Figure 4 is a schematic diagram showing a receiver according to an embodiment of the invention; Figure 5 is a schematic diagram showing shows a time slot sequence according to an embodiment of the invention; Figure 6 is a flow diagram showing a method of selecting a sub-band for the reception of signals according to an embodiment of the invention; and Figure 7 is a schematic diagram showing estimated zones of directions of arrival from which satellite reception is not expected according to an embodiment of the invention; Figure 8 is a schematic diagram showing time references for range calculations corresponding using GPS and Glonass satellite navigation systems according to an embodiment of the invention; and Figure 9 is a flow diagram showing a method of selecting a mode of operation of a satellite navigation receiver according to an embodiment of the invention.

Detailed Description of the Invention

By way of example an embodiment of the invention will now be described in the context of a satellite navigation receiver capable of determining a position on the basis of signals received from one or more satellites operating according to a GPS satellite navigation system and from one or more satellites operating according to a Glonass navigation system. However, it will be understood that this is by way of example only and that other embodiments may involve the use of navigation transmitters operating according to other navigation systems.

Figure 1 shows frequency bands used by GPS and Glonass satellite navigation systems; it can be seen that the frequency band used by GPS 1 is different to the frequency band used by Glonass 3.

The GPS satellite navigation system uses code division multiple access (CDMA) to multiplex signals transmitted from different satellites, whereas Glonass uses Frequency Division Multiple Access (FDMA) to multiplex signals transmitted from different satellites. So, in the Glonass system, it is possible to make use of a sub-band of the frequency band 3 used by the system in order to receive signals from a sub-set of satellites. In figure 1, five sub-bands are shown, designated A 5a, B 5b, C Sc, D 5d and E Se, each sub-band in this example encompassing the frequencies used by three respective satellites.

Alternatively, a system in which each sub-band encompasses the frequencies used by four satellites may be advantageously employed. Parts of the sub-bands may overlap. For example, four sub-bands may be employed, centred on 1598.90625 MHz (covering Glonass frequencies designated as -7, -6, -5 and -4), 1601.15625 MHz (covering Glonass frequencies designated -3, -2, - 1, and 0), 1603.6875 MHz (covering Glonass frequencies designated 1, 2, 3, and 4) and 1604.53125 MHz (covering Glonass frequencies designated 3, 4, 5, and 6). It can be that the last two bands listed overlap; this may be advantageous in arranging the downconversion frequency plan to avoid problems with image frequencies, particularly in the case where a direct conversion architecture is employed, in which the second local oscillator 26 and mixer 24 of the arrangement shown in figure 4 are omitted.

It will be seen from figure 1 that the Glonass system uses a wider frequency band 3 than the frequency band 1 used by the GPS system. Typically, the GPS system uses a bandwidth of approximately 2 -2.5 MHz whereas the Glonass system uses a bandwidth of approximately 12 MHz. It should be noted that the GPS system transmits signals in bands other than that shown in figure 1, for example for use by high precision and military receivers. However, typically a receiver intended for civilian consumer equipment is designed to receive the frequency band 1 shown in Figure 1.

Figure 2 illustrates a scenario in which signals from one or more satellites may be blocked. A receiver 4 is shown in a typical urban street canyon, that is to say a street with buildings 2a, 2b on either side that may obstruct satellites. In the case of figure 1, the building height is shown as approximately 1 5m, and it can be seen that satellites at low elevation angles would be obstructed, since a line of sight view is typically required for good signal reception at frequencies in the 1-2 GHz range used by satellite navigation systems. It should also be noted that satellite systems are typically arranged so that the system operates close to the noise floor, in order to limit transmission power at a satellite, and so a relatively small amount of attenuation may render a signal from a satellite unacceptable. Of course, there are situations in which the height of buildings is considerably greater than 1 Sm, in which case the blockage may affect signals received from satellites at greater elevation angles than those shown in the example of figure 2.

Figure 3 is a hemispherical view showing an example of positions of satellites using GPS and Glonass satellite navigation systems. The position around the circumference of the circles of figure 3 represents an azimuth angle and the position along a radius represents elevation, the centre representing a point directly overhead. This type of diagram may be referred to as a top of head view. The arrangement of satellites may be referred to as a constellation.

Figure 3 shows, as an illustrative example, as squares the position of satellites, using either GPS or Glonass, from which acceptable signals are not received 6a... 6h, due to blocking in a scenario similar to that illustrated by figure 2. Positions of satellites using GPS from which acceptable signals are received 8a, 8b, 8c, 8d are shown as circles and positions of satellites using Glonass from which acceptable signals are received lOa, lOb, lOc, lOd are shown as triangles. It can be seen that there is a zone of directions of arrival from which satellite reception is expected, represented by the area within the broken line 12. Outside this zone, satellite reception is not expected, as it may be blocked.

In the particular arrangement shown in figure 3, it can be seen that there are four GPS satellites and four Glonass satellites from which acceptable signals may be received. As has been explained, it should therefore be possible to determine a position with good accuracy using either satellite navigation system on its own.

However, the satellites are not in geostationary orbits, and so their positions change with time, and so a situation may arise in which acceptable signals may be received from fewer satellites. Also, in street canyons with higher buildings than those shown in figure 2, acceptable signals may be received from fewer satellites, that is to say fewer satellites may be "visible".

As has been explained, if less than three satellites are available for reception by a receiver, the performance of the position estimation may not be acceptable, or the location estimation may not be possible, using a single satellite navigation system.

For use in particular in locations where insufficient satellites from a single system are visible, a receiver is provided according to an embodiment of the invention that is capable of determining a position on the basis of signals received from one or more satellites operating according to a GPS satellite navigation system and from one or more satellites operating according to a Glonass navigation system. Such a receiver is illustrated in figure 4.

Signals are received from satellites at antenna 14, and are then passed to a receive chain 16, 18, 22, 24, 28. The receive chain is arranged to down convert signals received in a tuneable receive frequency band using a tuneable local oscillator 20, the tuneable receive frequency band being determined by the tuning of the local oscillator 20. The receive chain typically comprises a front end filter, typically a surface acoustic wave (SAW) filter, and a low noise amplifier (LNA) 16, followed by a mixer 18 to mix the received, filtered and amplified signals with signals from local oscillator 20, so that a tuneable receive band is defined by an intermediate frequency (IF) band pass filter 22. A second local oscillator signal is then provided by a frequency divider 26 to a second mixer 24. Signals from the IF band pass filter 22 are mixed to a second IF by the second mixer 24, and the signals are sampled by a sampler, that is to say analogue to digital converter 28. The local oscillator 20 and divider 26 may derive their frequency references from a common frequency source 30.

The tuneable local oscillator is arranged to be tuned to a sequence of frequencies such that the tuneable receive frequency band corresponds to the GPS frequency band 1 in first time slots and the tuneable receive frequency band corresponds to at least part 5a, 5b, Sc, Sd, 5e of the Glonass frequency band 3 in second time slots, the first and second time slots forming different elements of a time slot sequence. The designations "first" and "second" are arbitrary, and it is not meant to be implied that time slots for receiving GPS signals precede those for receiving Glonass signals at start up.

The sampler samples a plurality of samples of the signal in each first and each second time slot, and the processor is arranged to determine at least a position associated with the receiver on the basis of at least the sampled signals in the first and second time slots. The processor is thus provided with signals from satellites operating according to GPS and satellites operating according to Glonass in a time shared manner.

Digitised signals from the ADC are passed to a processor, comprising a measurement engine 36, typically timeshared between operation on signals from each satellite, a positioning engine 38, a controller 34, and an extended assisted GPS (AGPS) aiding functional block 32.

The measurement engine 36 performs frequency shift measurements for signals from each received satellite operating according to the GPS or Glonass system as applicable. The frequency shift measurements are a calculation of so called pseudo Doppler shift, and also time of arrival is calculated, and from this an estimate of range between the satellite and the receiver calculated, known as a pseudo range. The calculated quantities are known as "pseudo" range and Doppler, since they are derived using a clock that may have inaccuracies. The effect of the inaccuracies may be removed by subsequent calculation.

Pseudo range and pseudo Doppler measurements for each satellite, including satellites operating according to each of the GPS and Glonass systems, are then passed to the positioning engine 38 to determine a position, that is to say to determine a position of the receiver and therefore a user position.

Pre-checking and calculation 40 is performed followed by a calculation of position, and typically also a calculation of velocity and time (a PVT calculation) 42. The calculated position, velocity and time values are passed to a Kalman filter, which determines an estimate of the position, velocity and time based on a history of estimated data. A determined position is then output from the positioning engine 38.

A position associated with the receiver is therefore determined on the basis of at least the sampled signals in the first and second time slots, i.e the determination of position is determined at least on the basis of signals received from satellites operating according to both the GPS and Glonass satellite navigation systems.

In this embodiment, the sampler uses the same sampling clock to sample the signal in each first and each second time slot that is used to sample the signals from satellites operating according to the GPS and Glonass satellite systems. It is beneficial that any sampling clock error is the same for both systems, in order for a position calculation based on signal from satellites operating according to both systems to be accurate.

Figure 5 shows a typical time slot sequence according to an embodiment of the invention, in particular a sequence of first time slots 15, 21a, 21b used for reception of signals from satellites operating according to the GPS system and second time slots 17, 19, 23a, 23b for reception of signals from satellites operating according to the Glonass system.

Initially, when a position is required to be determined, acquisition process is carried out during a timeslot 15, 17 that may be longer than timeslots used for subsequent tracking once signals have been acquired. An acquisition process is a well known process in satellite navigation systems which is required in order for range estimates to be carried out; satellites have to be identified from which acceptable signals can be received, the location of the satellites needs to be established (typically using information such as almanac and ephemeris information that may be received using a terrestrial communication system, typically with a wireless link such as a cellular wireless network connection), and timing parameters need to be established.

Figure 5 shows a shorter period allocated for Glonass acquisition 17 than for Glonass acquisition 17. This may be the case if fewer Glonass satellites than GPS satellites need to be acquired.

Following acquisition, the processor carries out a tracking process, in which position estimates are updated using signals received from acquired satellites. Timeslots for tracking 19, 21a, 23a, 21b, 23b are typically shorter than those used for acquisition 15, 17. In the example illustrated in figure 5, GPS acquisition is allocated 30 seconds, Glonass acquisition is allocated 10 seconds, and each tracking period is set at 200ms. The time allocated to the acquisition phase may be variable, according to the time needed for the acquisition process; this will depend on factors including how recently a position determination, also called a position fix, was last performed, which will in turn affect the relevance of stored system parameters. Generally, acquisition times, when using aiding by information received using a terrestrial communication system, are in the region of 5s -60s and a tracking time in the region 0.1-0.5 s has been found to offer good performance. In particular, a first and second timeslot period of 0.2s when the processor is in the tracking phase has been found to provide good performance, so that re-acquisition at the beginning of the timeslot is quick and a typical drift in sampling clock frequency between one timeslot and the next is acceptable.

There may typically be a period at the beginning of a timeslot when samples of received signals are discarded, due to the need to leave time for the local oscillator frequency to settle after tuning.

It may also be advantageous to disable processing for parts of timeslots in order to reduce power consumption.

The time slots in which signals are received from the Glonass system may comprise a first portion in which the tuneable receive frequency band corresponds to a first part, that may be referred to as a sub-band, 5a of the Glonass frequency band 3 and a second portion in which the tuneable receive frequency band corresponds to a second part or sub-band 5b of the Glonass frequency band 3. In this way, reception of signals in one sub-band may be time multiplexed with signals in another sub-band, allowing signals to be processed received from Glonass satellites whose signals fall into different sub-bands.

Since in the present embodiment the Glonass frequency band 3 is divided into sub-bands, not all of which may be required to be received, a method of selecting a sub-band for the reception of signals is provided.

Signals are received from satellites operating according to the GPS satellite navigation system (GPS satellites), and on the basis of information received by the receiver relating to the GPS navigation system, a set of GPS satellites is determined from which acceptable signals are not received, but from which signals would be expected to be received in an unobstructed environment.

That is to say, these are "invisible" satellites.

Based the directions of arrival associated with the set of invisible satellites, a zone of directions of arrival from which satellite reception is not expected is estimated, that may be referred to as a blind zone. This may encompass angles within a certain angular range of the directions of arrival of invisible satellites. It is assumed that Glonass satellites with directions of arrival in the blind zone will also be invisible. A set of Glonass satellites from which signals are expected is then determined, on the basis of information received by the receiver relating to the Glonass satellite navigation system, in particular information, such as almanac and ephemeris information, in comparison with information relating to the blind zone; the information may be received by the receiver via a connection to a terrestrial wireless network A sub-band is selected for the reception of signals from a satellite in the determined set of Glonass satellites from which signals are expected.

It may be that signals from several Glonass satellites fall into the same selected sub-band. It may also be the case that more than one sub-band is selected in order to receive a desired number of Glonass satellites. Each sub-band will then be received in a time shared manner as has already been mentioned.

Figure 6 is a flow diagram showing a method of selecting a sub-band for the reception of signals.

In steps 52 and 54, a set of satellites of the GPS system are determined from which acceptable signals are not received but from which signals would be expected to be received, on the basis of information relating to the GPS system received by the receiver from the assisted GPS server. The information would typically include almanac information. The acceptability of satellite signals may be judged, for example, on the basis of quality of capture, that is to say degree of success of the acquisition process.

In steps 56 and 58, a zone of directions of arrival from which satellite reception is not expected is estimated on the basis of directions of arrival in elevation and azimuth associated with the set determined in steps 52 and 54.

In steps 60 and 62, a set of Glonass (GLS) satellites, also known as space vehicles (SV), from which signals are expected, are determined on the basis of the estimated zone of directions of arrival from which satellite reception is not expected and on the basis of information received by the receiver from the AGPS server relating to Glonass.

In steps 64, 66 and 68 a sub-band for the reception of signals is selected, the selected sub-band being a sub-band of the frequency band used by Glonass, and the receiver is configured according to the selected sub-band. The sub-band may be selected on the basis of e.g. a priority list of candidate Glonass satellites.

The priority in the priority list may be determined by a number of factors such as the elevation of the satellites.

Figure 7 shows an example of estimated zones of directions of arrival from which satellite reception is not expected (blind zones). It has been determined that acceptable signals cannot be received from GPS satellites at positions 6a, 6b and 6c (it is not assumed that the positions shown in figure 7 correspond with those in figure 3). A blind zone 74a is estimated for angles of arrival in the vicinity of 6a, and similarly a blind zone 74b is estimated for angles of arrival in the vicinity of 6b and a blind zone 74c is estimated for angles of arrival in the vicinity of 6c.

Since the position, velocity and time calculation 42 carried out by the positioning engine 38 is based on pseudo range information corresponding to different times, a method of signal processing is provided for calculating a position of a satellite navigation receiver on the basis of signals received from one or more satellites operating according to a first satellite navigation system (e.g. GPS) and from one or more satellites operating according to a second satellite navigation system (e.g. Glonass). The two systems may be transposed in the description; it does not matter which is the first or the second system.

An estimate is determined of a first range between a first satellite, operating according to the first satellite navigation system, and the receiver, using the first satellite navigation system, the first range corresponding to a first time.

An estimate is then determined of a second range between a second satellite, operating according to the second satellite navigation system, and the receiver, using the second satellite navigation system, the second range corresponding to a second time, different to the first time. The second time is different to the first time, since the GPS and Glonass systems are time multiplexed, and so operate at different times.

An estimate is then determined of a third range between the second satellite and the receiver, the third range corresponding to the first time, on the basis of a determined motion of the second satellite and the estimate of the second range. That is to say that the range estimate corresponding to the second time slot is adjusted for the motion of the satellite between the first time and the second time, to give a third range.

The position of the receiver may then be calculated on the basis of at least the first and third ranges. That is to say that pseudo range information detennined by the measurement engine 36 from signals received from GPS satellites can be used together with corrected pseudo range information determined by the measurement engine 36 from signals received from Glonass satellites in the positioning engine 38 to determine a position.

Estimates of the position and motion of the second satellite, derived from the Glonass system, are mapped to a GPS time reference and to a GPS position reference. The mapping between the Glonass position reference and the GPS position reference is known a priori. The mapping between the Glonass time reference and the GPS time reference is achieved by taking into account the time difference between the GPS and Glonass time reference systems that is known a priori and also taking into account a fine correction based on a time difference measured at the receiver between a time corresponding to the start of a GPS sub frame and a time corresponding to the start of a Glonass string.

The calculation of a position of the receiver is determined based on at least the first and third ranges and the positions of the first and second satellites referred to the GPS time and frequency reference system (and preferably on other basis of other ranges and satellite positions in addition).

In order to adjust for the motion of the second satellite between the first time and the second time, estimates of the position and motion of the second satellite derived from the Glonass system are mapped to a GPS time reference and to a GPS position reference.

The motion of the satellite may be determined on the basis of a Doppler frequency calculation applied to a signal received from the second satellite, that is to say a pseudo Doppler measurement relating to the Glonass satellites and/or on the basis of ephemeris information.

The method can be extended to be applicable to the case where the timeslots allocated to reception of the Glonass system are sub-divided into portions for receiving frequency sub-bands, by correcting the pseudo ranges as above, but separately for the frequency sub-bands.

Figure 8 shows time references for range calculations corresponding using GPS and Glonass satellite navigation systems, in the case where the timeslots allocated to reception of the Glonass system are sub-divided into portions for receiving frequency sub-bands. Range calculations using GPS satellites correspond to time 57a, range calculations using Glonass satellites correspond to time 53a for a first frequency sub-band and to 55a for a second frequency sub-band. The range calculations corresponding to times 53a and 55a are mapped to be appropriate to time 57a before being used to calculate the position. Similarly, in the following time slots, range calculations corresponding to times 53b and 55b are mapped to be appropriate to time 57b.

In the embodiment as described thus far, it is assumed that the receiver is operating in multi-mode mode of operation, specifically dual mode (GPS!Glonass). However, if sufficient satellites are visible to operate in a single mode, it is not necessary to operate in the multi-mode mode, and there may be an advantage to operating in a single mode, in terms of reduced power consumption or the allocation of more processor resources to the single mode operation. So, there is provided a method of selecting the mode of operation of the receiver. The technique is described in terms of the single mode of operation being to use the GPS system, but it should be understood that the two systems may be transposed, so that the single mode of operation may be to use the Glonass system.

Firstly, the number of satellites is determined that are operating according to the first satellite navigation system (e.g. GPS) from which acceptable signals are received; Depending on the number the number, a single system mode of operating may be selected, in which the position of the receiver is calculated on the basis of signals received by the receiver from one or more satellites operating according to the first satellite navigation system (e.g. GPS), or a multi-system mode of operation may be selected, in which the position of the receiver is calculated both on the basis of signals received by the receiver from one or more satellites operating according to the first satellite navigation system (e.g. GPS) and on also on the basis of signals received from one or more satellites operating according to the second satellite navigation system (e.g. Glonass).

Typically, the multi-system mode would be selected if the number is less than 3 and more than zero. That is to say, it may be acceptable to operate in a single mode (e.g. GPS mode) when only 3 satellites are providing acceptable signals, if for example only a two dimensional position estimate, or fix, is required.

Alternatively, the multi-system mode may be selected if the number is less than 4. That is to say, it may be acceptable to operate in a single mode (e.g. GPS mode) only when at least 4 satellites are providing acceptable signals, if for example a three dimensional position fix is required.

A terrestrial mode of operation may be selected when the number is zero, in which the position of the receiver is calculated on the basis of a terrestrial navigation system. A suitable tenestrial navigation system may be base on positioning in a cellular radio system, such as using a cell ID. If no satellites are visible operating according to one satellite system then it is likely that reception is highly obstructed, so that no or at least few satellites operating according to another satellite system will be visible.

Figure 9 is a flow diagram showing a method of selecting a mode of operation of a satellite navigation receiver. At steps 80, 82 and 84, GPS signals are acquired, typically using assisted GPS.

At step 86, the number of GPS satellites from which acceptable signals are received is determined.

If the determined number is zero, a cell identifier from a cellular radio system is used to determine a position, i.e. to make a fix at step 88.

If the determined number is three, a two dimension fix is made and a three dimensional fix is attempted at step 90.

If the determined number greater than three, three dimensional GPS tracking is enabled at step 92 If the determined number greater than zero and less than three, it is detennined which Glonass (GLS) bands and satellites (space vehicles, SVs) to acquire at step 94.

At step 96, the determined Glonass satellites are acquired.

At Step 96 a position is determined.

The operation of the controller 34, the measurement engine 36, and the positioning engine 38 as illustrated in figure 4 will now be explained in more detail. Typically, the controller 34, the measurement engine 36, and the positioning engine 38 are implemented in software.

The controller 38 controls GPS signal processing channels, selects the satellite to be searched, calculates frequency and code phase search windows and decides on the appropriate start mode (AGPS, cold, warm or hot start).

Assisted GPS mode is used if a last fix is sufficiently out of date as to require that information regarding the positions of satellites is required. Cold, warm and hot start refer to re-acquisition modes, to be selected according to the time elapsed since the last update of tracking parameters. The satellite is selected based on its priority calculation which is made relying on previous search results and the amount of a priori data such as almanac, ephemeris and previous position and fix time. The control software can be involved in predicting almanac data if the latter is not available. Also it controls Measurement Engine and Position Engine process sequences, communicate between the two engines and external Location Manager.

The Measurement Engine 36 reads digitized RF samples from the ADC 28, and acquires visible satellites, and then starts tracking the active channels simultaneously. This unit performs digital processing of GPS signals and estimates navigation parameters required to solve the navigation task. The main procedures of this unit are: * Acquisition & Re-acquisition: search for the signal both in frequency and C/A code phase then identify the existence of the signal and confirm detection.

* Signal Tracking: after a successful acquisition of a GPS satellite signal, approximate estimations of both code alignment and Doppler shift are obtained. In order to attain accurate measurements and ensure continuous adjustments of these two entities a tracking process of both code phase, using first order delay locked loop (DLL), and Doppler frequency, using combination of frequency locked ioop (FLL) and phase locked loop (PLL) has to be carried out immediately after detection.

* Bit Synchronization: uses 20 coherent correlators with 20 ms accumulation time. Deferent time positions of bit boundary are used in CA code generation process, Estimation of bit boundary corresponds to the correlator with a maximum output value.

* Frame Synchronization: preamble is searched to determine the start of each sub-frame. To avoid false preambles we need to check parity bits of the navigation words. The frame synchronization is achieved when the best hypothesis, which has the maximum number of successfully parity checks, has at least 10 successfully checked.

The outputs of ME are the raw data of frames, pseudo-range and pseudo-Doppler measurement for each active channel, and channel status such as SNR.

The Positioning Engine (PE) 38 uses pseudo-range and pseudo-Doppler measurements, as well as satellite ephemeris to calculate receiver position, velocity, and time (PVT). Also PE determines satellite positions relative to the receiver (azimuth and elevation angles), and Dilution of Precision (DOP). The latter gives an estimate of the positional accuracy based on the angular separation between the satellites used to calculate the receiver's position.

The positioning engine includes the following functions: * NAV Message Decoder: decodes the data from the 50 bps Li NAV message. The message consists of a 1500 bit long frame made up of five sub-frames, each sub-frame being 300 bits long. The decoder includes parity based error checking, so that only valid data is returned to the positioning engine. It is based on the description given in the document [1].

* Satellite Position & Velocity Calculation: calculates the position of each satellite in ECEF (x, y, z) coordinates based on the ephemeris and clock correction parameters given in sub-frames 1, 2, and 3, and the time of signal transmission as determined from the pseudo-range measurements.

* Corrected Pseudo-range Calculation: clock correction parameters are broadcast in sub-frame 1. In order to reduce the positioning errors, the measured pseudo-ranges must be corrected to account for small time differences between the clocks onboard each satellite, and relativistic effects.

* Calculate Receiver Position 3D: the receiver knows the exact satellite ECEF positions at a given time of transmission. It knows the pseudo-range, or the difference in time of reception, for each satellite. In order to calculate the receiver ECEF position it is necessary to calculate the exact range, or transmission delay, between each satellite and the receiver.

There are4 unknowns, therefore a minimum of 4 satellites are required to do the calculation. When there are more than 4 satellites, the problem may be generalized as an over-determined linear algebraic equation, which has no exact solution. The least squared solution is calculated iteratively using a pseudo-inverse.

* Calculate Receiver Position 2D: To calculate a 2D position, we require a previous good 3D position fix. By assuming that the altitude is fixed to the last known value, it is possible to calculate a 2D position using only 3 satellites.

* Dilution of Precision Calculation: DOP is used to describe the effect of the geometric strength of the satellite constellation on the accuracy of the position calculation. Good (low) DOP occurs when there is wide angular separation between the satellites used in the position solution. The DOP is derived from the diagonal elements of the covariance matrix of the least squares solution. HDOP (horizontal dilution of precision) is part of the NMEA GPGSA sentence.

* Kalman Filter: adjust the final PVT output between prediction and measurement. Its decision is based on both adopted dynamic model and calculated standard deviation of measurements.

The above embodiments are to be understood as illustrative examples of the invention.

It should be noted that, whilst the above specific embodiments have been described in relation to satellite navigation systems, other, non-satellite navigation systems, such as pseudolite navigation systems, LORAN navigation systems, cellular radio network navigation systems and Wi-Fi network navigation systems, may be incorporated as one or more of the systems used by a multi-system navigation receiver in accordance with the invention.

It should also be noted that a combined GPS/Galileo system may constitute the first or second navigation system or satellite navigation system.

Furthermore, third or further navigation systems or satellite navigation systems may be employed in a similar way to the second navigation and satellite navigation systems. For example, when a GPS or combined GPS/Galileo system can receive less than 3, or alternatively 4, satellites (or other transmitters) with acceptable quality, the system may try to acquire Glonass or Compass satellites or transmitters from any other satellite navigation system or navigation system in order to determine the position of the receiver.

It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments.

Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims (24)

1. Claims 1. A navigation receiver capable of determining a position on the basis of signals received from one or more navigation transmitters operating according to a first navigation system using a first frequency band and from one or more navigation transmitters operating according to a second navigation system using a second frequency band, different to the first frequency band, the receiver comprising: a receive chain arranged to down convert signals received in a tuneable receive frequency band using a tuneable local oscillator, the tuneable receive frequency band being determined by the tuning of the local oscillator; a sampler arranged to sample the down converted signals; and a processor, wherein the tuneable local oscillator is arranged to be tuned to a sequence of frequencies such that the tuneable receive frequency band corresponds to the first frequency band in first time slots and the tuneable receive frequency band corresponds to at least part of the second frequency band in second time slots, the first and second time slots forming different elements of a time slot sequence, wherein the sampler samples a plurality of samples of the signal in each first and each second time slot, and wherein the processor is arranged to determine at least a position associated with the receiver on the basis of at least the sampled signals in the first and second time slots.
2. 2. A navigation receiver according to claim 1, wherein the sampler uses the same sampling clock to sample the signal in each first and each second time slot.
3. 3. A navigation receiver according to claim 1 or claim 2, wherein the first navigation system uses code division multiple access, the tuneable receive frequency band has a fixed bandwidth substantially the same as the signals of the first satellite navigation system.
4. 4. A navigation receiver according to any preceding claim, wherein the second navigation system uses frequency division multiplexing and wherein a tuneable receive frequency band used in the second time slot is narrower in bandwidth than the second frequency band.
5. 5. A navigation receiver according to any preceding claim, in which the second time slots comprise a first portion in which the tuneable receive frequency band corresponds to a first part of the second frequency band and a second portion in which the tuneable receive frequency band corresponds to a second part of the second frequency band.
6. 6. A navigation receiver according to any preceding claim, wherein the first navigation system is a Global Positioning System (GPS) system and the second navigation system is a Global Navigation Satellite System (GLONASS) system.
7. 7. A method of selecting a sub-band for the reception of signals, at a satellite navigation receiver capable of receiving signals from one or more satellites operating according to a first satellite navigation system using a first frequency band and from one or more satellites operating according to a frequency division multiplexed satellite navigation system using a second frequency band, the method comprising: receiving signals from satellites operating according to the first satellite navigation system, and on the basis of information received by the receiver relating to the first satellite navigation system, determining a set of satellites of the first satellite navigation system from which acceptable signals are not received and from which signals would be expected to be received in an unobstructed environment; estimating a zone of directions of arrival from which satellite reception is not expected on the basis of the directions of arrival associated with the set; determining a set of satellites operating according to the frequency division multiplexed satellite navigation system from which signals are expected, on the basis of the estimated zone of directions of arrival from which satellite reception is not expected and on the basis of information received by the receiver relating to the frequency division multiplexed satellite navigation system; and selecting a sub-band for the reception of signals, the selected sub-band being a sub-band of the second frequency band for the reception of signals from a satellite in said determined set.
8. 8. A method according to claim 7, wherein the information received by the receiver relating to the first and second satellite navigation systems is received via a connection to a terrestrial wireless network.
9. 9. A method according to claim 7 or claim 8, wherein said information received by the receiver relating to the first and second satellite navigation systems is almanac and/or ephemeris information.
10. 10. A method of signal processing for calculating a position of a satellite navigation receiver on the basis of signals received from one or more satellites operating according to a first satellite navigation system and from one or more satellites operating according to a second satellite navigation system, the method comprising: determining an estimate of a first range between a first satellite, operating according to the first satellite navigation system, and the receiver, using the first satellite navigation system, the first range corresponding to a first time; determining an estimate of a second range between a second satellite, operating according to the second satellite navigation system, and the receiver, using the second satellite navigation system, the second range conesponding to a second time, different to the first time; determining an estimate of a third range between the second satellite and the receiver, the third range corresponding to the first time, on the basis of a determined motion of the second satellite and the estimate of the second range; and calculating the position of the receiver on the basis of at least the first range, the third range, a position of the first satellite and a position of the second satellite.
11. 11. A method according to claim 10, wherein said determined motion of the second satellite is determined on the basis of at least a Doppler frequency calculation applied to a signal received from the second satellite.
12. 12. A method according to claim 10 or claim 11, wherein said determined motion of the second satellite is determined on the basis of at least ephemeris information.
13. 13. A method according to any one of claim 10 to claim 12, wherein said position of the second satellite and/or said determined motion of the second satellite are determined on the basis of a mapping between a GPS time reference and a Glonass time reference, the method comprising: correcting an a priori estimate of the time difference between the GPS and the Glonass time reference based on a time difference measured at the receiver between a time corresponding to the start of a GPS sub frame and a time corresponding to the start of a Glonass string.
14. 14. A method according to any one of claim 10 to claim 13, further comprising: determining an estimate of a fourth range between a third satellite, operating according to the second satellite navigation system, and the receiver, using the second satellite navigation system, the fourth range corresponding to a third time, different to the first time and the second time; determining an estimate of a fifth range between the third satellite and the receiver, the fifth range corresponding to the first time, on the basis of a detennined motion of the second satellite and the fourth range; and calculating the position of the receiver on the basis of at least the first, third and fifth range.
15. 15. A method of selecting a mode of operation of a navigation receiver, the receiver being capable of determining a position on the basis of signals received from one or more navigation transmitters operating according to a first navigation system and from one or more navigation transmitters operating according to a second navigation system, the method comprising: determining a number of navigation transmitters operating according to the first navigation system from which acceptable signals are received; and selecting, on the basis of the number, between: a single system mode of operating, in which the position of the receiver is calculated on the basis of signals received by the receiver from one or more navigation transmitters operating according to the first navigation system; and a multi-system mode of operation in which the position of the receiver is calculated both on the basis of signals received by the receiver from one or more navigation transmitters operating according to the first navigation system and on the basis of signals received from one or more navigation transmitters operating according to the second navigation system.
16. 16. A method according to claim 15, wherein the selection of the multi-system mode is on the basis of the number being less than 3 and more than zero.
17. 17. A method according to claim 15 or claim 16, wherein the first navigation system is a satellite navigation system, further comprising selecting a terrestrial mode of operation, in which the position of the receiver is calculated on the basis of a terrestrial navigation system, when the number is zero.
18. 18. A method according to any one of claim 7 to claim 17, wherein the first navigation system is a Global Positioning System (GPS) system and the second navigation system is a Global Navigation Satellite System (GLONASS) system.
19. 19. A satellite navigation receiver arranged to select a sub-band for the reception of signals, the satellite navigation receiver being capable of receiving signals from one or more satellites operating according to a first satellite navigation system using a first frequency band and from one or more satellites operating according to a frequency division multiplexed satellite navigation system using a second frequency band, wherein the satellite navigation receiver: is arranged to receive signals from satellites operating according to the first satellite navigation system, and on the basis of information received by the receiver relating to the first satellite navigation system, to determine a set of satellites of the first satellite navigation system from which acceptable signals are not received and from which signals would be expected to be received in an unobstructed environment; is arranged to estimate a zone of directions of arrival from which satellite reception is not expected on the basis of the directions of arrival associated with the set; is arranged to determine a set of satellites operating according to the frequency division multiplexed satellite navigation system from which signals are expected, on the basis of the estimated zone of directions of arrival from which satellite reception is not expected and on the basis of information received by the receiver relating to the frequency division multiplexed satellite navigation system; and is arranged to select a sub-band for the reception of signals, the selected sub-band being a sub-band of the second frequency band for the reception of signals from a satellite in said determined set.
20. 20. A computer readable medium encoded with computer executable instructions for causing a satellite navigation receiver to select a sub-band for the reception of signals, the satellite navigation receiver being capable of receiving signals from one or more satellites operating according to a first satellite navigation system using a first frequency band and from one or more satellites operating according to a frequency division multiplexed satellite navigation system using a second frequency band, by: receiving signals from satellites operating according to the first satellite navigation system, and on the basis of information received by the receiver relating to the first satellite navigation system, determining a set of satellites of the first satellite navigation system from which acceptable signals are not received and from which signals would be expected to be received in an unobstructed environment; estimating a zone of directions of arrival from which satellite reception is not expected on the basis of the directions of arrival associated with the set; determining a set of satellites operating according to the frequency division multiplexed satellite navigation system from which signals are expected, on the basis of the estimated zone of directions of arrival from which satellite reception is not expected and on the basis of information received by the receiver relating to the frequency division multiplexed satellite navigation system; and selecting a sub-band for the reception of signals, the selected sub-band being a sub-band of the second frequency band for the reception of signals from a satellite in said determined set.
21. 21. A signal processor for calculating a position of a satellite navigation receiver on the basis of signals received from one or more satellites operating according to a first satellite navigation system and from one or more satellites operating according to a second satellite navigation system, wherein the signal processor is: arranged to determine an estimate of a first range between a first satellite, operating according to the first satellite navigation system, and the receiver, using the first satellite navigation system, the first range corresponding to a first time; arranged to determine an estimate of a second range between a second satellite, operating according to the second satellite navigation system, and the receiver, using the second satellite navigation system, the second range corresponding to a second time, different to the first time; arranged to determine an estimate of a third range between the second satellite and the receiver, the third range corresponding to the first time, on the basis of a determined motion of the second satellite and the estimate of the second range; and arranged to calculate the position of the receiver on the basis of at least the first range, the third range, a position of the first satellite and a position of the second satellite.
22. 22. A computer readable medium encoded with computer executable instructions for causing a signal processor to calculate a position of a satellite navigation receiver on the basis of signals received from one or more satellites operating according to a first satellite navigation system and from one or more satellites operating according to a second satellite navigation system, by: determining an estimate of a first range between a first satellite, operating according to the first satellite navigation system, and the receiver, using the first satellite navigation system, the first range corresponding to a first time; determining an estimate of a second range between a second satellite, operating according to the second satellite navigation system, and the receiver, using the second satellite navigation system, the second range conesponding to a second time, different to the first time; determining an estimate of a third range between the second satellite and the receiver, the third range corresponding to the first time, on the basis of a determined motion of the second satellite and the estimate of the second range; and calculating the position of the receiver on the basis of at least the first range, the third range, a position of the first satellite and a position of the second satellite.
23. 23. A navigation receiver capable of determining a position on the basis of signals received from one or more navigation transmitters operating according to a first navigation system and from one or more navigation transmitters operating according to a second navigation system, wherein the navigation receiver is: arranged to determine a number of navigation transmitters operating according to the first navigation system from which acceptable signals are received, and arranged to select, on the basis of the number, between: a single system mode of operating, in which the position of the receiver is calculated on the basis of signals received by the receiver from one or more navigation transmitters operating according to the first navigation system; and a multi-system mode of operation in which the position of the receiver is calculated both on the basis of signals received by the receiver from one or more navigation transmitters operating according to the first navigation system and on the basis of signals received from one or more navigation transmitters operating according to the second navigation system.
24. 24. A computer readable medium encoded with computer executable instructions for causing a navigation receiver to select a mode of operation, the receiver being capable of determining a position on the basis of signals received from one or more navigation transmitters operating according to a first navigation system and from one or more navigation transmitters operating according to a second navigation system, by: determining a number of navigation transmitters operating according to the first navigation system from which acceptable signals are received; and selecting, on the basis of the number, between: a single system mode of operating, in which the position of the receiver is calculated on the basis of signals received by the receiver from one or more navigation transmitters operating according to the first navigation system; and a multi-system mode of operation in which the position of the receiver is calculated both on the basis of signals received by the receiver from one or more navigation transmitters operating according to the first navigation system and on the basis of signals received from one or more navigation transmitters operating according to the second navigation system.