GB2597674A

GB2597674A - Assessing distances between transceivers

Info

Publication number: GB2597674A
Application number: GB2011759.4A
Authority: GB
Inventors: James Maxim Andrew; Bryers Alastair; Ian Belcher David; Paul George Randall Andrew
Original assignee: Entotem Ltd
Current assignee: Entotem Ltd
Priority date: 2020-07-29
Filing date: 2020-07-29
Publication date: 2022-02-09
Anticipated expiration: 2040-07-29
Also published as: GB2597674B; GB202011759D0

Abstract

The assessment of distance between a first transceiver and a second transceiver by measuring a round-trip time-of-flight of a transmitted radio signal is shown. A first transceiver has a first transmitter 501, a first receiver 502 and an evaluator 503 for evaluating the strength of a received signal. A similar second transceiver has a second transmitter and a second receiver. The first transmitter is configured to increase the transmission power of the first transmitter by modifying the operation of a power amplifier 509, if the signal strength is evaluated to be below a first threshold. Similarly, a decrease in transmission power is made if the signal strength is evaluated to be above a second threshold. These thresholds are selected to reduce the introduction of errors due to automatic changes being made to the level of amplification provided by a low noise amplifier 513 in the receiver 502.

Description

Assessing Distances Between Transceivers

CROSS REFERENCE TO RELATED APPLICATIONS

This is the first application for a patent directed towards the invention and the subject matter.

BACKGROUND OF THE INVENTION

The present invention relates to assessing a distance between a first transceiver and a second transceiver by measuring a round-trip time-of-flight of a transmitted radio signal.

It is known to calculate a distance between a first transceiver and a second transceiver by measuring a round-trip time-of-flight. Each transceiver includes a radio transmitter and a radio receiver. Systems are synchronised, such that the receiver of one is operational when the transmitter of the other is operational. It is also known for receivers to include variable gain amplifiers for extending the maximum range of transceiver systems, by variably amplifying low-level incoming signals. This tends not to create problems for conventional data transmission. However, the present inventors have found that gain changes of this type can introduce errors when using these systems for timeof-flight measurements.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method as set out in claim 1.

According to a second aspect of the present invention, there is provided an apparatus as set out in claim 13.

Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings. The detailed embodiments show the best mode known to the inventors and provide support for the invention as claimed. However, they are only exemplary and should not be used to interpret or limit the scope of the claims. Their purpose is to provide a teaching to those skilled in the art. Components and processes distinguished by ordinal phrases such as "first" and "second" do not necessarily define an order or ranking of any sort.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figure 1 illustrates tags moving within an environment; Figure 2 illustrates a fixed anchor performing a ranging operation with respect to a mobile tag; Figure 3 illustrates discontinuities in distance correction values; Figure 4 shows an example of a mobile tag; Figure 5 shows a schematic representation of an example of a transceiver; Figure 6 shows procedures for implementing a method of assessing a distance between a first transceiver and a second transceiver; Figure 7 illustrates procedures for assessing distance and procedures for evaluating intensity identified in Figure 6; and Figure 8 details procedures for adjusting output power identified in Figure 6.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Figure 1 As illustrated in Figure 1, in this embodiment, distances are measured by a round-trip time-of-flight of a transmitted radio signal. In an embodiment, many signals of this type are transmitted such that, based on many range values, it is possible to put forward a best assessment as to what is considered to be the actual distance of a second transceiver from a first transceiver. The environment of Figure 1 may be considered as a room within a building, within which a first fixed transceiver 101 and a second fixed transceiver 102 have been deployed. These fixed transceivers may also be referred to as "anchors", from which the distance of one or more mobile transceivers may be assessed.

In the embodiment of Figure 1, a first mobile transceiver 111 is provided along with a second mobile transceiver 112. The mobile transceivers may also be identified as "tags" such that, as illustrated in Figure 1, a first tag 111 may move in the direction of dashed arrow 113 to a new location 114; while, in this example, the second tag 112 remains stationary.

In the example shown in Figure 1, only two anchors are provided and only two tags are provided, for illustrative purposes. To locate the actual position of tags within the environment, an embodiment includes more anchors, such that positions of tags may be identified with reference to their distance from a significant number of fixed anchors. Furthermore, many environments will include substantially more tags, possibly hundreds or thousands, such that a scheduling operation is required, allowing a sufficient number of ranging operations to be performed over sufficiently small intervals, thereby allowing the tags within the environment to be located and tracked substantially in real time.

Thus, multiple distancing operations are scheduled, to allow the position of many tags to be identified. To locate the position of a particular tag, many distancing operations are required, each specifying the distance of a particular tag from a particular anchor. Furthermore, given the inherent difficulties of deploying return time-of-flight techniques, an embodiment performs many ranging calculations, each derived from a specific radio transmission and then retransmission, from which distance values are assessed.

To summarise, an embodiment is required to make accurate distance measurements, possibly performing many ranging transmissions, to identify the actual distance of a specific tag with respect to a specific anchor. Having determined distances of this type, for a specific tag with respect to several anchors, a computational technique is required to determine the actual location of the tags within the environment.

In some environments, all transceivers may occupy a common plane, thereby creating a two-dimensional problem. However, many real-life applications may position tags at different heights and buildings may include many floors. Thus, in such environments, a three-dimensional solution is required. Furthermore, with many tags present within the environment, many distance assessments are required, therefore the scheduling of these operations should be optimised to make good use of the available frequencies and timeslots.

Distance assessment is performed by instructing an anchor, such as the first anchor 101, to initiate an exchange with a tag by transmitting a set-up packet. This packet identifies a start channel, the number of ranging exchanges to take place and which of two antennas to use. The antennas are mutually orthogonal, to ensure that communication is not lost due to polarisation mismatch and, in an embodiment, both antennas are selected. This approach facilitates the mitigation of multi-path issues. If two distant measurements are obtained and one is longer than the other, the shorter of these two is selected, on the basis that this range will have incurred fewer reflections and therefore provides a more accurate assessment of the actual distance.

In this description, a single specific radio transmission and retransmission allows a single range to be calculated based on the return timeof-flight. In an embodiment, many ranging operations of this type are performed within a ranging interval using mutually different transmission characteristics. After considering all of these range values identified during a ranging interval, an individual distance is assessed, thereby producing a single distance output per iteration.

The first fixed anchor 101 receives power from an external power supply 121, with a similar second external power supply 122 supplying power to the second fixed anchor 102. In an embodiment, power is derived from a powerover-ethernet system, on the basis that such a supply is likely to include an uninterruptable power supply and will therefore not experience a nonoperational state should the general power supply fail.

An ethernet connection may also be used to establish communications with a network processor. However, in an embodiment, system data communications will occur within the radio network itself. Furthermore, the radio network established for ranging purposes may also provide a platform for other data-transmission applications.

The radio network itself may follow established LoRa protocols, to provide a long-range, low-power wide area network, based on spread spectrum modulation.

Figure 2 Fixed anchor 101 is illustrated in Figure 2 performing a ranging operation with respect to the first mobile tag 111. In an embodiment, the fixed transceivers and the mobile transceivers are implemented as Semtech SX1280 devices, produced by Semtech Corporation of Camarillo, California USA.

Distance measurements are made between the first anchor 101 and the first mobile tag 111 by measuring the time taken for a radio signal 201 to be transmitted from the anchor 101 to the tag 111 and then the time taken for a second radio signal to be returned back, by being transmitted by the first tag 111 and returned to the first anchor 101. Measured durations are then converted to ranges with reference to the speed of propagation.

Time-of-flight calculations of this type provide accurate measurements when transmissions occur in the direction of line-of-sight, as indicated by a first propagation path 201 and a second propagation path 202. However, problems with this approach can occur due to the presence of reflections. Thus, as an alternative to adopting the second transmission path 202, for example, the return communication could adopt a third transmission path 203, resulting in a longer transmission time and a resulting erroneous evaluation of range.

To mitigate these issues, each distance assessment does not rely upon a single evaluation of range. In particular, in an embodiment, many ranging transmissions are made using different transmission frequencies; or more specifically, different "chirps" within the chirp spread spectrum technology of LoRa WAN. Furthermore, mutually orthogonally displaced antennas are deployed from which, as previously described, the group of ranges providing the shorter distance is selected in preference to the longer distance.

Within an established radio protocol, each anchor, with a permanent power supply, may be established as a ranging master which then performs a ranging operation with a mobile tag, identified as the ranged slave. To determine the range of the slave from the master, the ranging master 101 sends a ranging request to the ranged slave 111, which in turn returns a synchronised response back to the master. The master measures and interpolates the time elapsed between the ranging request and the response, such that the measured time reported by the master is the round-trip time between the master and the slave; with the actual propagation of radio waves occurring at the speed of light. The resulting measured time is therefore indicative of the measured round-trip distance with additional timing errors. Protocols within the devices themselves attempt to compensate for these errors, resulting in the generation of output data representing an assessment of the distance of the slave 111 from the master device 101.

Within the devices, static sources of measurement error are corrected by calibration. However, further errors may be introduced due to reference oscillator drift and analogue group delay. The master's timing measurement and the slave's synchronisation are performed using local crystal reference oscillators, therefore any offset in timing between the master crystal oscillator and the slave crystal oscillator will result in an erroneous distance measurement. However, the same reference oscillator is used to derive both the timing for ranging operations and the radio frequency carrier at 2.4 gigahertz. Consequently, it is possible to measure the frequency error between the transmitter and the receiver to reliably indicate timing offsets.

The SX1280 devices, deployed in an embodiment, are configured to use automatic gain control to adapt the gain of receivers to the received signal strength. This known approach facilitates the reception of both low power signals, at the limits of sensitivity, and high-power signals at short range.

However, this approach also impacts upon the operation of the return time-of-flight measurements, in that the delays through low noise variable gain amplifiers at the receivers change as a function of the amplifier gain.

To facilitate adjustments of this type, the signal strength of the received signal (received signal strength indication, RSSI) is measured. It is assumed that the channel remains static for the duration of a single ranging exercise and that the signal power seen by the master gives an indication of the receiver gain used by both master and slave. In this implementation, the ranging RSSI differs slightly from conventional measurements of this type in that, instead of indicating a value in absolute power, the ranging RSSI indicates the received signal power relative to a signal power threshold. From empirical measurements, it is possible to construct a lookup table of ranging RSSI values with reference to range measurement correction. A plot of correction values of this type is illustrated in Figure 3.

Figure 3 Experiments have shown that distance correction values do not vary in a continuous way but exhibit discontinuities, resulting in the establishment of a first plateau 301, a second plateau 302 and a third plateau 303.

These discontinuities exist due to amplifier gain stages being selected and the present inventors have found that significant errors may occur when gain changes of this type take place during a ranging operation.

Figure 4 An example of a mobile tag 111 is illustrated in Figure 4. This represents the second transceiver and is mobile within the environment. As such, the second transceiver receives energy from a local battery 401.

The fixed transceivers, such as anchor 101, are each configured as a master and the mobile transceivers, such as tag 111, are each configured as a slave. Furthermore, the slave transceiver is configured to minimise radio transmissions to thereby conserve energy and maximise the operational life of the battery 401.

In the embodiment of Figure 4, an attachment device 402 is provided for attaching the second mobile transceiver to a person. Alternative attachment devices may be used for attachment to material assets within the environment.

Figure 5 A schematic representation of an example of a transceiver is shown in Figure 5. The operation is substantially similar for an anchor 101 and for a tag 111. The transceiver shown in Figure 5 may therefore be considered as a first transceiver for anchor 101, working with a substantially similar second transceiver such that, in combination, they provide an apparatus for assessing a distance by measuring a round-trip time-of-flight of a transmitted radio signal. The first transceiver 101 has a first transmitter 501 along with a first receiver 502. In addition, a processing circuit 503, including circuitry for modulation and demodulation, includes an evaluator for evaluating the strength of a signal received by the first receiver 502.

The apparatus also includes a second transceiver having a second transmitter (substantially similar to the first transmitter 501) along with a second receiver (substantially similar to the first receiver 502).

For each transceiver, a switch 504 selects a first antenna 505 or a second antenna 506. These antennas are mutually offset, such that their transmission characteristics define different phase polarisations. In an embodiment, both antennas are used, resulting in ranging transmissions using the first antenna 505, considered to be in a first group, along with ranging transmissions taking place using the second antenna 506, considered to be placed in a second group. Furthermore, the switch 504 also controls connection of the selected antenna to the transmitter 501 or to the receiver 502. Thus, when a ranging exchange takes place, the transmitter at the first transceiver is selected while the receiver at the second transceiver is selected.

The signal is then returned and these functionalities are reversed, such that the transmitter is connected to the selected antenna at the second transceiver and the receiver is connected to the selected antenna at the first transceiver. In this embodiment, the transmitter 501 includes a crystal oscillator 507 supplying a phase-locked loop 508 which in turn receives control signals from the processing circuit 503. A modulated output from the phase-locked loop 508 is supplied to an output power amplifier 509 prior to being supplied to the switch 504.

The processing circuit 503 supplies transmitter control signals on a transmitter control line 510. Transmitter status is returned to the processing circuit 503 from the transmitter via a transmitter status line 511. Having performed a ranging exchange, the first transceiver will have performed an evaluation of the signal strength of the signal received by the first receiver 502. In response to this evaluation, it is possible for the processing circuit 503, using the transmitter control line 510, to increase the transmission power of the first transmitter 501 by increasing the transmission power of the output power amplifier 509. Thus, if the signal strength is evaluated to be below a first threshold, the transmission power is increased. If the signal strength is evaluated to be above a second threshold, the transmission power is decreased.

In the receiver 502, a mixer 512 receives an output from the phase-locked loop 508. When receiving, the switch 504 supplies an input signal to a low noise amplifier 513 and the output from this low noise amplifier is supplied to the mixer 502 before being conveyed to an analogue-to-digital converter 514 for application to the processing circuit 503. The processing circuit 503 receives details of receiver status on a receiver status line 515. Furthermore, the processing circuit 503 supplies receiver control signals to the receiver on a receiver control line 516.

In response to receiving status information from the receiver, the processing circuit 503 may determine that the received signal is too low and may therefore instruct the low noise amplifier 513 to increase receiver gain.

Thus, the first transceiver has a first input amplifier configured to amplify input signals received by the first receiver. In this embodiment, the first threshold and the second threshold are selected to reduce the introduction of errors due to automatic changes being made to the level of amplification provided by the first amplifier.

Thus, the embodiment seeks to maintain communication between the first transceiver and the second transceiver at an appropriate transmission power, thus ensuring that data is received at the receiver and the transmitted signal is not too high or too low. However, to avoid the introduction of errors, the embodiment seeks to make changes to the power amplifier 509 in preference to allowing changes to be made to the low noise amplifier 513. In particular, the first transceiver is configured to reduce the introduction of errors by preventing the first input amplifier from switching between amplification stages.

In an embodiment, the first transceiver and the second transceiver are configured to assess a distance during a ranging interval. Many ranging transmissions occur during the ranging interval, with these transmissions having mutually different characteristics. An assessment of distance is then made in response to processing many ranging evaluations.

In an embodiment, the evaluator is configured to evaluate a signal strength value for a ranging interval by considering many signal strength values identified form individual ranging transmissions. In an embodiment, the signal strength value is evaluated by averaging the ranging signal strength values. In an embodiment, the different transmission characteristics include different transmission frequencies. More specifically, the LoRa protocol may be adopted using a chirp spread spectrum modulation by representing each bit of payload information by multiple chirps of information. This in turn results in the presentation of many channels, such that all channels may be used to perform a ranging exercise and similar ranging operations may be performed in parallel, given that different channels are multiplexed at different times.

In an embodiment, forty channels may be available in a form compatible with the Bluetooth (RTM) low energy channel plan. A frequency hopping approach is adopted, such that a single ranging exchange is performed on a single frequency which is then followed by a hop to the next frequency for a similar exchange to take place. In an embodiment, this sequence is performed for the first antenna and then repeated for the second antenna. This represents a complete ranging interval, allowing a distance to be assessed from a total of eighty ranging measurements. Furthermore, many exchanges of this type may be multiplexed by performing a similar frequency hopping sequence but relatively out of phase. Sophisticated scheduling is therefore required to make optimum use of the available transmission environment.

Figure 6 Procedures for implementing a method of assessing a distance between a first transceiver and a second transceiver by measuring a round trip time-of-flight of a transmitted radio signal are shown in Figure 6. At step 601 an initial power is established at the first transceiver which then starts a ranging interval at this initial power level.

At step 602, a ranging transmission occurs on a selected channel, with transmission taking place from the first transceiver for reception by the second transceiver.

At the second transceiver, at step 603, the signal is re-transmitted on the selected channel back to the first transceiver.

These interactions are repeated within the ranging interval, thereby producing a first group of ranging values relating to ranges calculated using the first antenna pair, along with a second group of ranging values calculated while using the second antenna pair. Thus, in an example, a ranging interval involves a total of eighty radio exchanges that may be divided into two groups, with each group making use of all forty available channels.

At the first transceiver, at step 604, a distance is assessed to produce an actual distance value. This may also be identified as a range but as used herein, "range" refers to a range calculated from an individual radio action and "distance" refers to an actual assessment of distance made from many individual ranges.

At step 605, the first transceiver makes an evaluation of signal intensity whereafter, at step 606, it is possible for an adjustment of output power to be made. Thus, the intensity of a received radio signal is evaluated, whereafter transmitter power is increased if the intensity is below the first threshold, with the transmitter power being decreased if this intensity is above a second threshold. In an embodiment, this operation is performed to reduce the introduction of ranging errors due to automatic changes being made to input amplification, as described with reference to Figure 5.

If an adjustment to output power has been made at step 606, a transmission is instigated at step 607 instructing the second transceiver, acting as a slave, to make a similar local adjustment to output transmission power.

Thus, at step 608, the second transceiver adjusts output power in anticipation of the next ranging transmissions.

Thus, on the next cycle, ranging transmissions are initiated by the first transceiver at step 609, with similar signals being returned by the second transceiver at step 610 In this example, a question is asked at step 611 as to whether the process is to continue and when answered in the affirmative, control is returned to step 604 for a further assessment of distance to be implemented. Alternatively, the procedures may be terminated by the question asked at step 611 being answered in the negative.

In an embodiment, it would be possible for specific radio transmissions to take place exclusively for the evaluation of signal strengths. However, in an embodiment, radio transmissions made to perform the actual distance measuring operations are themselves considered, with reference to their own signal strength measurements, to facilitate this dynamic adjustment.

In an alternative embodiment, each slave is configured to return its own ranging RSSI or some equivalent information in the ranging response. This information provides an indication of the analogue delay introduced by the slave's electronics when returning the ranging response symbols.

In an embodiment deploying SX1280 devices, an internal calibration delay register is present, that allows for design specific delays, such as those due to PCB trace and antenna delays, to be automatically compensated in the ranging procedures. In this deployment, the ranging master's analogue delay, taken from the master calibration value register, is added to the ranging slave analogue delay value (the slave calibration value) which is returned by the slave in the ranging response message. The total calibration delay is then subtracted from the actual measured ranging delay. This arrangement allows interoperability between different physical hardware designs of master and slave devices.

In a specific deployment, the processor could choose to either return the ranging RSSI or equivalent information as a separate value in the ranging response or it could use the ranging RSSI information to dynamically modify the calibration value that is already returned by the slave device. These approaches require an assumption to be made to the effect that the low noise amplifier 502/602 gains and hence delays, are the same at both the master and the slave devices.

In this particular implementation, signal strength information is available from a register identified as RSSI and the embodiment aims to maintain this value at around seventy-five dB, representing substantially the middle of a largest flattest region, such that it is therefore most likely to result in operation with the same low noise amplifier gain at both ends.

Within the protocol, transmission power is specified, such that anchors and tags operate at similar transmission powers when performing a ranging operation. The present embodiment makes use of dynamically controlled transmission power to inhibit adjustments to variable-gain input amplifier stages.

Figure 7 Procedures 604 for assessing distance are illustrated in Figure 7. At step 701, the first group of transmissions is selected, relating to the first aerial pair, from which a median average is calculated. Use of the median (as distinct from the mean) is preferred, as ranging values at the extremes will clearly be erroneous. Thus, at step 702, a similar median range value is calculated for the second group.

At step 703, the median value calculated at step 701 is compared with the median value calculated at step 702 and from these, the shortest median value is selected as the output distance value. The shortest is selected because a longer range will tend to be less accurate than a shorter range, given that the extended distance will be caused by reflections, as described with reference to Figure 2.

Procedures 605 for evaluating intensity are also shown in Figure 7. For each of the eighty ranging interactions, a signal strength measurement will be made. On this occasion, it is preferred to calculate a mean value of the signal strength derived from all interactions. Thus, a signal strength is accumulated at step 711 whereafter, at step 712, a question is asked as to whether a further signal strength value is available. When answered in the affirmative, the next value is accumulated at step 711 and the process repeated until all values have been accumulated. This is then divided by the total number of interactions that have taken place to provide a mean value of signal strength.

Figure 8 Procedures 606 for adjusting transmitter power are shown in Figure 8. At step 801 an internal flag is reset and in this reset condition, no adjustment will be made to transmitter power. However, if, subsequently, the flag is set, an adjustment will be made on this iteration.

Having received the second signal 704, a question is asked at step 802 as to whether the strength of the signal is above eighty dB. In this embodiment, a signal strength of seventy-five dB is considered optimal, with eighty dB representing an upper bound and seventy dB representing a lower bound.

Thus, if the signal strength is above eighty dB, the question asked at step 802 is answered in the affirmative and output power is reduced at step 803. Thus, at step 803, a local adjustment to output power is made. However, it is now necessary to convey this information back to the remote tag to enable power adjustment step 608. To achieve this, a register is updated at step 804 and the flag is set at step 805.

If the question asked at step 802 is answered in the negative, to the effect that the received signal strength is not above eighty dB, a question is asked at step 806 as to whether the received signal strength is below seventy dB. If answered in the affirmative, output power is increased at step 807, which, again, results in a local adjustment. A register is updated at step 808 and the flag is set at step 809. Control then continues to step 810. However, if the question asked at step 806 is answered in the negative, control is directed to step 810.

At step 810 a question is asked as to whether the flag has been set. If answered in the negative, no further action is required. However, if the question asked at step 810 is answered in the affirmative, the register, updated at step 804 or at step 808, is read, to facilitate the transmission of a power adjustment command at step 607, which is then implemented at step 608.

Claims

CLAIMSThe invention claimed is: 1. A method of assessing a distance between a first transceiver and a second transceiver by measuring a round trip time-of-flight of a transmitted radio signal, comprising the steps of: evaluating the intensity of a received radio signal; increasing transmitter power if said intensity is below a first threshold; and decreasing said transmitter power if said intensity is above a second threshold.
2. The method of claim 1, further comprising the step of selecting said first threshold and said second threshold to reduce the introduction of ranging errors due to automatic changes being made to input amplification.
3. The method of claim 2, wherein ranging error introduction is reduced by avoiding step changes to said input amplification due to amplification stage switching.
4. The method of any of claims 1 to 3, wherein a distance is assessed by performing a plurality of ranging transmissions during a ranging interval using mutually different transmission characteristics.
5. The method of claim 4, wherein: a received signal strength indication is obtained for each said ranging transmission; and said intensity is evaluated for a ranging interval with reference to a plurality of signal strength indications received during said ranging interval.
6 The method of claim 5, wherein a signal strength value is evaluated for a distance assessment by averaging signal strength values for each said ranging transmission.
7. The method of any of claims 4 to 6, wherein said different transmission characteristics include different transmission frequencies.
8. The method of any of claims 4 to 7, wherein said different transmission characteristics include different phase polarisations implemented by the provision of two mutually offset antennas at both the first transceiver and the second transceiver.
9. The method of claim 8, wherein said step of performing distance measuring calculations further comprises the steps of: calculating a first median distance value derived from ranges determined by a first antenna pair; calculating a second median distance value derived from ranges determined by said second antenna pair; and selecting the shorter of said first median distance and said second median distance to obtain a range value.
10. The method of any of claims Ito 9, wherein said first transceiver instructs said second transceiver to transmit at a substantially similar transmission power after performing said increasing step or said decreasing step.
11. The method of any of claims Ito 10, further comprising the steps configuring the first transceiver as a fixed master transceiver; and configuring said second transceiver as a mobile slave transceiver. of:
12. The method of claim 8 or claim 9, wherein the mobile slave transceiver is configured to minimise radio transmissions to conserve energy.
13. An apparatus for assessing a distance between a first transceiver and a second transceiver by measuring a round-trip time-of-flight of a transmitted radio signal, comprising: a first transceiver having a first transmitter, a first receiver and an evaluator for evaluating the strength of a signal received by said first receiver; a second transceiver having a second transmitter and a second receiver, wherein said first transceiver is configured to: increase transmission power of said first transmitter if said signal strength is evaluated to be below a first threshold; and decrease said transmission power is said signal strength is evaluated to be above a second threshold.
14. The apparatus of claim 11, wherein: said first transceiver comprises a first input amplifier configured to amplify input signals received by said first receiver; and said first threshold and said second threshold are selected to reduce the introduction of errors due to automatic changes being made to the level of amplification provided by said first input amplifier.
15. The apparatus of claim 14, wherein said first transceiver is configured to reduce the introduction of errors by preventing said first input amplifier from switching between amplification stages.
16. The apparatus of any of claims 11 to 13, wherein: said first transceiver and said second transceiver are configured to assess a distance during a ranging interval; a plurality of ranging transmissions are made during said ranging interval, said transmissions having mutually different transmission characteristics; and an assessment of said distance is made in response to processing a plurality of ranging evaluations.
17. The apparatus of claim 16, wherein said evaluator is configured to evaluate a signal strength value for a ranging interval by considering a plurality of signal strength values identified from individual ranging transmissions.
18. The apparatus of claim 17, wherein a signal strength value is evaluated by averaging said ranging signal strength values.
19. The apparatus of any of claims 16 to 18, wherein said different transmission characteristics include different transmission frequencies.
20. The apparatus of any of claims 16 to 19, further comprising a first antenna and a mutually offset second antenna at both said first transceiver and said second transceiver, wherein said different transmission characteristics include different phase polarisations.
21. The apparatus of claim 20, wherein said first transceiver is configured to: calculate a first median distance based on ranges determined from a plurality of transmissions using said first antennas; calculate a second median distance based on ranges determined from a plurality of transmissions using said second antenna; and select the shorter calculated median distance.
22. The apparatus of any of claims 13 to 21, wherein said first transceiver is configured to instruct said second transceiver to transmit at a substantially similar transmission power after locally increasing or decreasing transmission power.
23. The apparatus of any of claims 13 to 22, wherein said first transceiver is a fixed master transceiver and said second transceiver is a mobile slave transceiver.S
24. The apparatus of claim 23, wherein the second transceiver is personally carried.
25. A system in which distances for a plurality of mobile transceivers are assessed with respect to a plurality of fixed transceivers, wherein said first transceivers and said second transceivers are in accordance with any of claims 13 to 24.