CN104301868A - High-precision indoor positioning method based on frame round-trip-time and time-of-arrival ranging technology - Google Patents

Abstract

The invention provides a high-precision indoor positioning method based on the frame round-trip-time and time-of-arrival ranging technology. By adopting the TOA differential measurement method, a system scheme meeting the requirement for high-precision positioning of mobile terminal devices is provided. Based on frame round-trip measurement information, the differential measurement method is adopted for time information to reduce the frame processing time delay error by embedding the time information of frame transmission. Because the high-precision indoor positioning method based on the frame round-trip-time and time-of-arrival ranging technology is adopted, the time synchronization problem is effectively solved through the frame round-trip-time and time-of-arrival ranging technology besides achieving the traditional indoor positioning function, and the phase error and the frequency error of the node clock and the standard clock are eliminated by using the differential measurement method for ranging.

Description

The high accuracy indoor orientation method with the ranging technology time of advent is come and gone based on frame

Technical field

The present invention relates to a kind of indoor positioning field, especially a kind of high-precision indoor orientation method.

Background technology

Along with advancing by leaps and bounds of information technology, people emerge in an endless stream for the demand of obtaining information, especially day by day increase the perception demand of spatial positional information.Aspect out of doors, the maturation of satellite technology impels the development in an all-round way of global positioning system (Global Positioning System, GPS) technology.There is due to GPS the features such as round-the-clock, high accuracy, automatically measurement, be all used widely in each side such as military affairs, traffic, mapping and people's daily lifes.The most important condition of GPS technology application is namely good to empty sighting condition, just seems beyond one's ability to help for indoor positioning navigation.So other technologies approach must be sought for this particular surroundings indoor.

At present, although the indoor positioning such as ultrasonic technology, REID, infrared technology, super-broadband tech, Bluetooth technology can reach higher positioning precision, but need a large amount of transducers and extra hardware device support, there is in practical application larger limitation.Along with the maturation of IEEE802.11 wireless technology, universal all over the world of WIFI, its coverage rate is more and more wider, become the standard configuration of the communication apparatus such as smart mobile phone, panel computer especially, and a lot of region such as enterprises, airport, school, warehouse, hospital etc. all provide WIFI to support, the indoor positioning technologies based on WIFI arises at the historic moment.Than existing location technology, WIFI locates responsible existing network, and do not need extra hardware device, use cost is low.But, due to the existence of the disturbing factor such as multipath signal and multipath fading, often cause the positioning precision based on indoor transmissions model to become and be difficult to reach requirement.

The position feature information fingerprint chart database existed in traditional indoor orientation method is too huge, and in tuning on-line matching process, computational complexity is high, poor real; And along with the change in the interior space bad border, based on the position feature information of received signal strength (RSS) technology also in continuous change, coupling accuracy can be affected to a certain extent; The existing localization method based on the time of advent (TOA, Time Of Arrival) technology better cannot process the propagation delay time on clock crystal oscillator error and node, cannot reach the indoor hi-Fix satisfied the demands.

Summary of the invention

In order to overcome the deficiencies in the prior art, the present invention proposes a kind of based on the round (RTT of frame, Round Trip Time) and the time of advent (TOA, Time Of Arrival) the high accuracy indoor orientation method of ranging technology, by adopting TOA differential measuring method, provide a kind of system schema meeting the indoor hi-Fix requirement of mobile terminal device.

The metrical information that this case comes and goes based on frame, by embedding the temporal information of frame transmission, adopts differential measuring method to reduce frame processing delay error to temporal information.The technical solution adopted for the present invention to solve the technical problems is:

(1) RTT and TOA methods combining completes the time measurement of frame transmitting procedure

By mobile terminal, namely MT (Mobile Terminal) sends wireless signal to the AP (Access Point) in its wireless signal effective range, carry out at least 3 nearest AP of selected distance MT by the handshake mechanism in 802.11 wireless protocols and participate in location, after AP has selected, MT sends inquiry frame to each selected AP, selected AP returns acknowledgement frame to MT after receiving the inquiry frame of MT, obtains frame round-trip transmission time T _rTT, in MT and selected AP any one AP frame transmitting procedure in, MT is at t ₁moment sends inquiry frame, and selected AP is at t ₂moment receives inquiry frame, and selected AP is at t ₃moment sends acknowledgement frame, and MT is at t ₄moment receives acknowledgement frame, t ₁it is the time that frame heading that MT sends appears at MT antenna port; t ₃it is the time that frame heading that selected AP sends appears at selected AP antenna port; t ₂it is the time that transmission frame heading starts to arrive selected AP reception antenna port; t ₄that transmission frame heading starts to arrive the MT reception antenna port time, wherein t ₁come by MT clock with the record of t4, t ₂and t ₃record come by the clock selecting AP;

Otherwise if first initiated to measure by selected AP, send inquiry frame, the role of whole transmitting procedure MT and selected AP can exchange;

(2) adopt TOA differential measuring method to eliminate or reduce measure error

As described in step (1), after round-trip transmission terminates, the time point t that selected AP records it ₂and t ₃send MT to, by MT, frame round-trip transmission time T is calculated to time data _rTTas follows:

T _RTT＝(t ₄-t ₁)-(t ₃-t ₂) (1)

T _rTTfor the difference of the time difference on MT and selected AP, namely difference measurement, the distance D between MT and selected AP is that transmission speed is multiplied by the one-way transmission time, i.e. the half of frame round-trip transmission time, obtains:

$D=C\·\frac{T_{\mathrm{RTT}}}{2}---\left(2\right)$

Wherein C represents the light velocity;

(3) step (1) and the method described in (2) is repeated, the distance between MT and each selected AP can be measured, the distance between the coordinate of each selected AP and MT is utilized to be radius, set up the equation of a circle that at least 3 is the center of circle with selected AP, their common intersection is exactly the coordinate of MT, MT coordinate can be solved, complete location;

(4) in step (1), MT is at t ₁=t ' ₁+ ε _mTmoment sends inquiry frame, and selected AP is at t ₂=t ' ₂+ ε _aPmoment receives inquiry frame, then at t ₃=t ' ₃+ ε _aP' returning acknowledgement frame, MT is at t ₄=t ' ₄+ ε _mT' receive acknowledgement frame, t here ₁, t ₂, t ₃, t ₄for time measured value, t ' ₁, t ' ₂, t ' ₃, t ' ₄for the standard time of correspondence, ε _mT, ε _aP, ε _aP', ε _mT' be the measured value of each correspondence and the error between the standard time, error is divided into again the clock of node M T and AP and the phase error of standard time clock and frequency error, wherein phase error is that the clock of node M T and AP is according to the time delay of time synchronizing signal alignment time, frequency error is the accumulated error that between twice time synchronized, the drift of nodal clock crystal oscillator frequency produces, in the process of step (1), node M T and synchronous standard time AP deadline are t ₀if, T _ifrom t ₀to the time interval of measuring the moment, elapsed time T _ithe standard time of posterior nodal point MT and AP is t ' _i=t ₀+ T _i, (i=1,2 ... 4), t ' is obtained ₁=t ₀+ T ₁by standard time clock is clocked quarter, quarter that node M T clocks is:

t ₁＝t' ₁+u+η(t ₁-u-t ₀) (3)

Wherein u is the phase error of MT clock and standard time clock, and η is clock self crystal oscillator frequency coefficient of deviation, then η (t ₁-u-t ₀) for node is from the Measuring Time (t of oneself ₀+ u) moment is to t ₁the cumulative errors of the crystal oscillator frequency drift in moment, after abbreviation:

$t_1=\frac{t_1^{\′}+u-\mathrm{\η}(t_0+u)}{1-\mathrm{\η}}=\frac{t_0+T_1+u-\mathrm{\η}{t}_0-\mathrm{\ηu}}{1-\mathrm{\η}}=t_0+u+\frac{T_1}{1-\mathrm{\η}}---\left(4\right)$

Then MT clock receives acknowledgement frame t ₄moment can be expressed as:

$t_4=t_0+u+\frac{T_4}{1-\mathrm{\η}}---\left(5\right)$

Formula (5) deducts formula (4) and can obtain:

$t_4-t_1=\frac{T_4-T_1}{1-\mathrm{\η}}=\frac{t_4^{\′}-t_1^{\′}}{1-\mathrm{\η}}\≈(t_4^{\′}-t_1^{\′})(1+\mathrm{\η})=(t_4^{\′}-t_1^{\′})+\mathrm{\η}(t_4^{\′}-t_1^{\′})---\left(6\right)$

Wherein t ' ₄-t ' ₁=t ₀+ T ₄-(t ₀+ T ₁)=T ₄-T ₁;

Accordingly, t in formula (6) ₄-t ₁for the time difference of Measuring Time, t' ₄-t' ₁for the time difference of standard time, η (t' ₄-t' ₁) be error, the time difference that error equals the standard time is multiplied by clock self crystal oscillator frequency coefficient of deviation η, and η is 10 ^-5the order of magnitude, η (t' ₄-t' ₁) as error component, negligible, obtain the time difference t of the Measuring Time of same clock ₄-t ₁equal the time difference t' of the standard time of its correspondence ₄-t' ₁, make frame T two-way time in formula (1) _rTTcan calculate with the time difference of standard time, thus eliminate error, the precision of measurement result improves, thus positioning precision is improved.

The invention has the beneficial effects as follows and come and go the high accuracy indoor orientation method with the ranging technology time of advent owing to have employed based on frame, except reaching the function of conventional chamber inner position, frame comes and goes (RTT) and the time of advent (TOA) ranging technology efficiently solves time synchronization problem, and differential measuring method is used in range finding, eliminate phase error and the frequency error of nodal clock and standard time clock.

Accompanying drawing explanation

Fig. 1 is the frame measurement of round trip time block diagram of indoor positioning of the present invention, and wherein MT is at t ₁moment sends inquiry frame, and AP is at t ₂moment receives inquiry frame, then at t ₃return acknowledgement frame, MT is at t ₄receive acknowledgement frame, MLME is MAC layer management entity.

Fig. 2 is the fundamental system model framework chart of indoor positioning, in figure, and AP ₁, AP ₂, AP ₃, AP ₄be respectively access point, MT _i(i=1 ... n) be mobile terminal.

Embodiment

Below in conjunction with drawings and Examples, the present invention is further described.

(1) RTT and TOA methods combining completes the time measurement of frame transmitting procedure

By mobile terminal, namely MT (Mobile Terminal) sends wireless signal to the AP (Access Point) in its wireless signal effective range, carry out at least 3 nearest AP of selected distance MT by the handshake mechanism in 802.11 wireless protocols and participate in location, after AP has selected, MT sends inquiry frame to each selected AP, selected AP returns acknowledgement frame to MT after receiving the inquiry frame of MT, obtains frame round-trip transmission time T _rTT, in MT and selected AP any one AP frame transmitting procedure in, MT is at t ₁moment sends inquiry frame, and selected AP is at t ₂moment receives inquiry frame, and selected AP is at t ₃moment sends acknowledgement frame, and MT is at t ₄moment receives acknowledgement frame, t ₁it is the time that frame heading that MT sends appears at MT antenna port; t ₃it is the time that frame heading that selected AP sends appears at selected AP antenna port; t ₂it is the time that transmission frame heading starts to arrive selected AP reception antenna port; t ₄that transmission frame heading starts to arrive the MT reception antenna port time, wherein t ₁come by MT clock with the record of t4, t ₂and t ₃record come by the clock selecting AP;

Otherwise if first initiated to measure by selected AP, send inquiry frame, the role of whole transmitting procedure MT and selected AP can exchange;

(2) adopt TOA differential measuring method to eliminate or reduce measure error

As described in step (1), after round-trip transmission terminates, the time point t that selected AP records it ₂and t ₃send MT to, by MT, frame round-trip transmission time T is calculated to time data _rTTas follows:

T _RTT＝(t ₄-t ₁)-(t ₃-t ₂) (1)

T _rTTfor the difference of the time difference on MT and selected AP, namely difference measurement, the distance D between MT and selected AP is that transmission speed is multiplied by the one-way transmission time, i.e. the half of frame round-trip transmission time, obtains:

$D=C\·\frac{T_{\mathrm{RTT}}}{2}---\left(2\right)$

Wherein C represents the light velocity;

(3) step (1) and the method described in (2) is repeated, the distance between MT and each selected AP can be measured, the distance between the coordinate of each selected AP and MT is utilized to be radius, set up the equation of a circle that at least 3 is the center of circle with selected AP, their common intersection is exactly the coordinate of MT, MT coordinate can be solved, complete location;

(4) in step (1), MT is at t ₁=t ' ₁+ ε _mTmoment sends inquiry frame, and selected AP is at t ₂=t ' ₂+ ε _aPmoment receives inquiry frame, then at t ₃=t ' ₃+ ε _aP' returning acknowledgement frame, MT is at t ₄=t ' ₄+ ε _mT' receive acknowledgement frame, t here ₁, t ₂, t ₃, t ₄for time measured value, t ' ₁, t ' ₂, t ' ₃, t ' ₄for the standard time of correspondence, ε _mT, ε _aP, ε _aP', ε _mT' be the measured value of each correspondence and the error between the standard time, error is divided into again the clock of node M T and AP and the phase error of standard time clock and frequency error, wherein phase error is that the clock of node M T and AP is according to the time delay of time synchronizing signal alignment time, frequency error is the accumulated error that between twice time synchronized, the drift of nodal clock crystal oscillator frequency produces, in the process of step (1), node M T and synchronous standard time AP deadline are t ₀if, T _ifrom t ₀to the time interval of measuring the moment, the standard time of elapsed time Ti posterior nodal point MT and AP is t ' _i=t ₀+ T _i, (i=1,2 ... 4), t ' is obtained ₁=t ₀+ T ₁by standard time clock is clocked quarter, quarter that node M T clocks is:

t ₁＝t' ₁+u+η(t ₁-u-t ₀) (3)

Wherein u is the phase error of MT clock and standard time clock, and η is clock self crystal oscillator frequency coefficient of deviation, then η (t ₁-u-t ₀) for node is from the Measuring Time (t of oneself ₀+ u) moment is to t ₁the cumulative errors of the crystal oscillator frequency drift in moment, after abbreviation:

$t_1=\frac{t_1^{\′}+u-\mathrm{\η}(t_0+u)}{1-\mathrm{\η}}=\frac{t_0+T_1+u-\mathrm{\η}{t}_0-\mathrm{\ηu}}{1-\mathrm{\η}}=t_0+u+\frac{T_1}{1-\mathrm{\η}}---\left(4\right)$

Then MT clock receives acknowledgement frame t ₄moment can be expressed as:

$t_4=t_0+u+\frac{T_4}{1-\mathrm{\η}}---\left(5\right)$

Formula (5) deducts formula (4) and can obtain:

$t_4-t_1=\frac{T_4-T_1}{1-\mathrm{\η}}=\frac{t_4^{\′}-t_1^{\′}}{1-\mathrm{\η}}\≈(t_4^{\′}-t_1^{\′})(1+\mathrm{\η})=(t_4^{\′}-t_1^{\′})+\mathrm{\η}(t_4^{\′}-t_1^{\′})---\left(6\right)$

Wherein t ' ₄-t ' ₁=t ₀+ T ₄-(t ₀+ T ₁)=T ₄-T ₁;

Accordingly, t in formula (6) ₄-t ₁for the time difference of Measuring Time, t' ₄-t' ₁for the time difference of standard time, η (t' ₄-t' ₁) be error, the time difference that error equals the standard time is multiplied by clock self crystal oscillator frequency coefficient of deviation η, and η is 10 ^-5the order of magnitude, η (t' ₄-t' ₁) as error component, negligible, obtain the time difference t of the Measuring Time of same clock ₄-t ₁equal the time difference t' of the standard time of its correspondence ₄-t' ₁, make frame T two-way time in formula (1) _rTTcan calculate with the time difference of standard time, thus eliminate error, the precision of measurement result improves, thus positioning precision is improved.

In order to enable above-mentioned purpose of the present invention, feature and advantage become apparent more, are described in further detail below for WIFI indoor positioning to this case.Obviously, the embodiment that this place describes is only a part of embodiment that the present invention sets forth in conjunction with WIFI signal.Based on scheme of the present invention, those of ordinary skill in the art not making the embodiment obtained under the prerequisite of creative work, all within protection scope of the present invention.

First, mobile terminal (as mobile phone, panel computer, notebook etc.) namely MT regioselective time, should first be linked in the WIFI system of room area (as enterprises, airport, school, warehouse, hospital etc.).WIFI inquiry frame signal is sent to the AP (wireless device such as router) of its WIFI signal effective range by MT, at least 3 AP carrying out selected distance MT nearest by the handshake mechanism in 802.11 wireless protocols participate in position fixing process, and the selection principle of AP is: choose the response sequencing of MT according to AP.After AP has selected, MT sends inquiry frame to each AP, and AP returns acknowledgement frame to MT after receiving the inquiry frame of MT, obtains frame round-trip transmission time T _rTT.For the frame transmitting procedure of MT and arbitrary AP, MT is at t ₁moment sends inquiry frame, and AP is at t ₂moment receives inquiry frame, and AP is at t ₃moment sends acknowledgement frame, and MT receives acknowledgement frame (all moment are the time that frame heading starts the antenna port occurring or start to arrive MT and AP) in the t4 moment.Wherein special concern, the record of each time point is all completed by respective nodes self clock, and frame measurement of round trip time block diagram as shown in Figure 1.

After frame round-trip transmission terminates, AP sends MT to its time point t2 and t3 recorded, and calculates T by MT to time data _rTT

T _RTT＝(t ₄-t ₁)-(t ₃-t ₂)

＝[(t' ₄+ε _MT')-(t' ₁+ε _MT)]-[(t' ₃+ε _AP')-(t' ₂+ε _AP)] (7)

＝[(t' ₄-t' ₁)-(t' ₁-t' ₂)]+[(ε _MT'-ε _MT)-(ε _AP'-ε _AP)]

In formula (7), [(t' ₄-t' ₁)-(t' ₃-t' ₂)] be real transmission time of frame, [(ε _mT'-ε _mT)-(ε _aP'-ε _aP)] be the transmission error of frame, ε _mTand ε _mT' be two errors that MT clock is measured, their difference is η (t' ₄-t' ₁), because (t' ₄-t' ₁) be 10 to the maximum ^-6the order of magnitude, η is 10 ^-5the order of magnitude, so this result is 10 ^-11the order of magnitude, the order of magnitude is too small, negligible; Same ε _aPand ε _aP' be two errors that AP clock is measured, their difference is η (t' ₃-t' ₂), and t' ₃-t' ₂aP clock, from receiving inquiry frame to the processing time sending acknowledgement frame, is be less than t' ₄-t' ₁, η (t' ₃-t' ₂) also ignore because the order of magnitude is too small, therefore, make second bracket part very little on the impact of result by difference measurement, can ignore.The T then obtained _rTTresult just equals signal and transmit the consumed time between two AP.

Coverage due to WIFI is 100m, and when MT and AP is at a distance of 100m, maximum standard time difference is 10 ^-6the order of magnitude, then 10 ^-11the time of the order of magnitude corresponds to millimetre-sized distance, relative to the precision of indoor positioning 1m, completely negligible.

In actual measurement, first (clock resolution of existing localization method is 1 μ s to need to make MT and at least 3 AP participating in locating carry out time synchronized according to 802.11 agreements, be 10ns according to the clock resolution of 802.11v agreement, the clock resolution that the present invention arranges MT and each AP is all 1ns), the clock time needing time u to adjust oneself after AP time of receipt (T of R) synchronizing signal is to the standard time, therefore in emulation experiment, the clock of setting MT and each AP all has phase error u with standard time clock, getting u is [-5,-1] discrete uniform on distributes and can only be integer, MT and AP self crystal oscillator frequency coefficient of deviation η is [-5 × 10 ^-5, 5 × 10 ^-5] on be uniformly distributed, wherein negative value represents that crystal oscillator frequency is slower than standard clock frequency, on the occasion of expression crystal oscillator frequency faster than standard clock frequency.Choose the position of an each measurement point of AP, MT all from x ∈ near AP [34,65], the region of y ∈ [34,65], experiments of measuring obtains Measuring Time and the absolute error D in true frames transmission time of traditional TOA method ₁, error mean is E [D ₁], mean square error MSE ₁=E [D ₁ ²]; For differential measuring method, T _rTTbe D with the absolute error in true frames transmission time ₂, error mean is E [D ₂], mean square error MSE ₂=E [D ₂ ²].Wherein E [] represents the expectation asking for variable.And define P=MSE ₁/ MSE ₂as gain reference value.

Experiment one two kinds of metering systems in main opposite are at T ₁=0 (T ₁for sending the time of inquiry frame being accomplished to MT from time synchronized last time) when, the whole MT measurement points in AP near zone are for the result of time synchronization error.Table 1 shows this whole measurement results:

Table 1 traditional measurement and difference measurement error result contrast

E[D 1](ns)	E[D 2](ns)	MSE 1＝E[D 1 2]	MSE 2＝E[D 2 2](×10 -6)	P(×10 9)
					1.5294	0.0016	3.7474	4.2169	7.1945

Experimental result describes TOA differential measuring method of the present invention and significantly reduces measure error, improves certainty of measurement.

In experiment two, parameter u and the η of the clock of MT and AP remain unchanged.Main contrast's two kinds of metering systems are at different T ₁in situation, the result difference of the whole measurement points in x ∈ [34,65], y ∈ [34,65] region.For traditional TOA, its measured value:

$t_2=\frac{t_2^{\′}+u_2-{\mathrm{\η}}_2(t_0+u_2)}{1-{\mathrm{\η}}_2}=\frac{t_0+T_1+\mathrm{\Δ}+u_2-{\mathrm{\η}}_2(t_0+u_2)}{1-{\mathrm{\η}}_2}=t_0+u_2+\frac{T_1+\mathrm{\Δ}}{1-{\mathrm{\η}}_2}---\left(8\right)$

$t_1=\frac{t_1^{\′}+u_1-{\mathrm{\η}}_1(t_0+u_1)}{1-{\mathrm{\η}}_1}=\frac{t_0+T_1+u_1-{\mathrm{\η}}_1(t_0+u_1)}{1-{\mathrm{\η}}_1}=t_0+u_1+\frac{T_1}{1-{\mathrm{\η}}_1}---\left(9\right)$

$t_2-t_1=u_2-u_1+T_1(\frac{1}{1-{\mathrm{\η}}_2}-\frac{1}{1-{\mathrm{\η}}_1})+\frac{\mathrm{\Δ}}{1-{\mathrm{\η}}_2}---\left(10\right)$

D ₁＝t ₂-t ₁-Δ (11)

Wherein Δ is the frame transmission time of MT to AP, can find out absolute error D according to formula (10) and (11) ₁value and T ₁the linear relation of value.

Table 2 T is for traditional measurement and the contrast of difference measurement error result

Can find according to table 2, the error of traditional TOA method and mean square error are along with T ₁increase and linear change, and the error of TOA difference measurement and mean square error and T ₁change have nothing to do.Therefore difference measurement effectively inhibits and to be drifted about the accumulated error caused by crystal oscillator frequency.

Distance between MT and AP is the time (half of frame round-trip transmission time) that transmission speed is multiplied by one-way transmission, namely wherein C represents the light velocity, and above-mentioned same method can be adopted to measure distance between MT and other several selected AP.In the model, by the coordinate of at least 3 selected AP and be radius with the distance of MT, set up the equation of a circle that at least 3 take AP as the center of circle, their common intersection is exactly the coordinate of MT, can solve MT coordinate, complete location, fundamental system model structure as shown in Figure 2.

Claims (1)

1. come and go the method for high accuracy indoor positioning with the ranging technology time of advent based on frame, it is characterized in that comprising the steps:

(1) RTT and TOA methods combining completes the time measurement of frame transmitting procedure

By mobile terminal, namely MT (Mobile Terminal) sends wireless signal to the AP (Access Point) in its wireless signal effective range, carry out at least 3 nearest AP of selected distance MT by the handshake mechanism in 802.11 wireless protocols and participate in location, after AP has selected, MT sends inquiry frame to each selected AP, selected AP returns acknowledgement frame to MT after receiving the inquiry frame of MT, obtains frame round-trip transmission time T _rTT, in MT and selected AP any one AP frame transmitting procedure in, MT is at t ₁moment sends inquiry frame, and selected AP is at t ₂moment receives inquiry frame, and selected AP is at t ₃moment sends acknowledgement frame, and MT is at t ₄moment receives acknowledgement frame, t ₁it is the time that frame heading that MT sends appears at MT antenna port; t ₃it is the time that frame heading that selected AP sends appears at selected AP antenna port; t ₂it is the time that transmission frame heading starts to arrive selected AP reception antenna port; t ₄that transmission frame heading starts to arrive the MT reception antenna port time, wherein t ₁come by MT clock with the record of t4, t ₂and t ₃record come by the clock selecting AP;

Otherwise if first initiated to measure by selected AP, send inquiry frame, the role of whole transmitting procedure MT and selected AP can exchange;

(2) adopt TOA differential measuring method to eliminate or reduce measure error

As described in step (1), after round-trip transmission terminates, the time point t that selected AP records it ₂and t ₃send MT to, by MT, frame round-trip transmission time T is calculated to time data _rTTas follows:

T _RTT＝(t ₄-t ₁)-(t ₃-t ₂) (1)

T _rTTfor the difference of the time difference on MT and selected AP, namely difference measurement, the distance D between MT and selected AP is that transmission speed is multiplied by the one-way transmission time, i.e. the half of frame round-trip transmission time, obtains:

$D=C\·\frac{T_{\mathrm{RTT}}}{2}---\left(2\right)$

Wherein C represents the light velocity;

(3) step (1) and the method described in (2) is repeated, the distance between MT and each selected AP can be measured, the distance between the coordinate of each selected AP and MT is utilized to be radius, set up the equation of a circle that at least 3 is the center of circle with selected AP, their common intersection is exactly the coordinate of MT, MT coordinate can be solved, complete location;

(4) in step (1), MT is at t ₁=t ' ₁+ ε _mTmoment sends inquiry frame, and selected AP is at t ₂=t ' ₂+ ε _aPmoment receives inquiry frame, then at t ₃=t ' ₃+ ε _aP' returning acknowledgement frame, MT is at t ₄=t ' ₄+ ε _mT' receive acknowledgement frame, t here ₁, t ₂, t ₃, t ₄for time measured value, t ' ₁, t ' ₂, t ' ₃, t ' ₄for the standard time of correspondence, ε _mT, ε _aP, ε _aP', ε _mT' be the measured value of each correspondence and the error between the standard time, error is divided into again the clock of node M T and AP and the phase error of standard time clock and frequency error, wherein phase error is that the clock of node M T and AP is according to the time delay of time synchronizing signal alignment time, frequency error is the accumulated error that between twice time synchronized, the drift of nodal clock crystal oscillator frequency produces, in the process of step (1), node M T and synchronous standard time AP deadline are t ₀if, T _ifrom t ₀to the time interval of measuring the moment, elapsed time T _ithe standard time of posterior nodal point MT and AP is t ' _i=t ₀+ T _i, (i=1,2 ... 4), t ' is obtained ₁=t ₀+ T ₁by standard time clock is clocked quarter, quarter that node M T clocks is:

t ₁＝t' ₁+u+η(t ₁-u-t ₀) (3)

Wherein u is the phase error of MT clock and standard time clock, and η is clock self crystal oscillator frequency coefficient of deviation, then η (t ₁-u-t ₀) for node is from the Measuring Time (t of oneself ₀+ u) moment is to t ₁the cumulative errors of the crystal oscillator frequency drift in moment, after abbreviation:

$t_1=\frac{t_1^{\′}+u-\mathrm{\η}(t_0+u)}{1-\mathrm{\η}}=\frac{t_0+T_1+u-{\mathrm{\ηt}}_0-\mathrm{\ηu}}{1-\mathrm{\η}}=t_0+u+\frac{T_1}{1-\mathrm{\η}}---\left(4\right)$

Then MT clock receives acknowledgement frame t ₄moment can be expressed as:

$t_4=t_0+u+\frac{T_4}{1-\mathrm{\η}}---\left(5\right)$

Formula (5) deducts formula (4) and can obtain:

$t_4-t_1=\frac{T_4-T_1}{1-\mathrm{\η}}=\frac{t_4^{\′}-t_1^{\′}}{1-\mathrm{\η}}\≈(t_4^{\′}-t_1^{\′})(1+\mathrm{\η})=(t_4^{\′}-t_1^{\′})+\mathrm{\η}(t_4^{\′}-t_1^{\′})---\left(6\right)$

Wherein t ' ₄-t ' ₁=t ₀+ T ₄-(t ₀+ T ₁)=T ₄-T ₁;

Accordingly, t in formula (6) ₄-t ₁for the time difference of Measuring Time, t' ₄-t' ₁for the time difference of standard time, η (t' ₄-t' ₁) be error, the time difference that error equals the standard time is multiplied by clock self crystal oscillator frequency coefficient of deviation η, and η is 10 ^-5the order of magnitude, η (t' ₄-t' ₁) as error component, negligible, obtain the time difference t of the Measuring Time of same clock ₄-t ₁equal the time difference t' of the standard time of its correspondence ₄-t' ₁, make frame T two-way time in formula (1) _rTTcan calculate with the time difference of standard time, thus eliminate error, the precision of measurement result improves, thus positioning precision is improved.