Method and apparatus for precise relative positioning in multiple vehicles

A system and method for determining the position and velocity of a plurality of vehicles relative to a host vehicle using GPS data. The method includes building a graph of the vehicles that define weighted baselines between each of the vehicles and the host vehicle and each of the vehicles where the weighted baselines define a geometric dilution of precision between the vehicles. The method then determines the optimal baseline between the host vehicle and each of the other vehicles using the weighted baselines based on the lowest geometric dilution of precision. The method then computes the relative position and velocity between all of the vehicles and the host vehicle using the optimal baselines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a system and method for determining the position and velocity of a plurality of vehicles relative to a host vehicle and, more particularly, to a system and method for determining the position and velocity of a plurality of vehicles relative to a host vehicle using GPS signals, where the system calculates optimal baselines between the vehicles.

2. Discussion of the Related Art

Short-baseline precise relative positioning of multiple vehicles has numerous civilian applications. By using relative GPS signals in real-time, a vehicle can establish a sub-decimeter level accuracy of relative positions and velocities of surrounding vehicles (vehicle-to-vehicle object map) that are equipped with a GPS receiver and a data communication channel, such as a dedicated short range communications (DSRC) channel. This cooperative safety system can provide position and velocity information in the same way as a radar system.

For precise relative positioning, a vehicle needs to broadcast its raw GPS data, such as code range, carrier phase and Doppler measurements. The bandwidth required to do this will be an issue in a crowded traffic scenario where a large number of vehicles are involved.

Data format defined in The Radio Technical Commission for Maritime Service Special Committee 104 (RTCM SC104) contains unwanted redundancy. For example, message type #1 (L1C/A code phase correction) uniformly quantizes corrections with a 0.02 meter resolution. The pseudo-range measurements are thus represented in a range of ±0.2×215meters. However, the pseudo-range measurements are generally limited to about ±15 meters. It is thus noted that excess bandwidth wastage occurs if the RTCM protocol is directly used in a cooperative safety system.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a system and method are disclosed for determining the position and velocity of a plurality of vehicles relative to a host vehicle using GPS information. The method includes building a graph of the vehicles that define weighted baselines between each of the vehicles and the host vehicle and each of the vehicles where the weighted baselines define a geometric dilution of precision between the vehicles. The method then determines the optimal baseline having the lowest geometric dilution of precision between the host vehicle and each of the other vehicles using the weighted baselines based on the geometric dilution of precision. The method then computes the relative position and velocity between all of the vehicles and the host vehicle using the optimal baselines.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed to a system and method for determining the position and velocity of a plurality of vehicles relative to a host vehicle using GPS signals and optimal-spanning tree analysis is merely exemplary in nature, and is in no way intended to limit the invention or it's applications or uses.

FIG. 1illustrates a communications architecture10for a host vehicle12and a remote vehicle14. The host vehicle12and the remote vehicle14are each equipped with a wireless radio16that includes a transmitter and a receiver (or transceiver) for broadcasting and receiving wireless packets through an antenna18. Each vehicle includes a GPS receiver20that receives satellite ephemeris, code range, carrier phase and Doppler frequency shift observations. Each vehicle also includes a data compression and decompression unit22for reducing the communication bandwidth requirement. Each vehicle also includes a data processing unit24for constructing a vehicle-to-vehicle (V2V) object map. The constructed V2V object map is used by vehicle safety applications26. The architecture10may further include a vehicle interface device28for collecting information including, but not limited to, vehicle speed and yaw rate.

FIG. 2is a flowchart diagram38showing the operation of the processing unit24in the architecture10. The processing unit24is triggered once new data is received at decision diamond40. A first step collects the ephemeris of the satellites, i.e., orbital parameters of the satellites at a specific time, code range (pseudo-range), carrier phase observations, and vehicle data of the host vehicle12at box42. A second step determines the position and velocity of the host vehicle12that serve as the moving reference for the later precise relative positioning method at box44. A third step compresses the GPS and vehicle data at box46. A fourth step broadcasts the GPS and vehicle data at box48. A fifth step collects wireless data packets from remote vehicles at box50. A sixth step decompresses the received data packets and derives the GPS and vehicle data of each remote vehicle at box52. A seventh step constructs a V2V object map using the precise relative positioning method at box54. A eighth step outputs the V2V object map to the high-level safety applications for their threat assessment algorithms at box56.

The data processing unit24can be further described as follows. Let X1,X2, . . . , XKbe K vehicles. Let Xibe the state of the i-th vehicle, including the position and velocity in Earth-centered and Earth-fixed coordinates (ECEF). Let XHbe the state of the host vehicle12, where 1≦H≦K. Let X be the states of the satellite, including the position and velocity in ECEF coordinates, which can be determined by the ephemeris messages broadcast by the j-th satellite.

FIG. 3illustrates a flow chart diagram60for precisely solving the relative position and velocity vector between the vehicles. The diagram60includes an on-the-fly (OTF) joint positioning and ambiguity determination module62that receives information from various sources, including vehicle data at box64, stand-alone position of a vehicle at box66, satellite ephemeris at box68and double differences of GPS observations at box70, discussed below. The module62outputs the other vehicle's position velocity and ambiguities at box72. It is noted that the absolute coordinates of one vehicle are required. In a system that only contains moving vehicles, the coordinates of the moving reference are simply estimated using a stand-alone positioning module to supply the approximate coordinates of the reference base coordinates.

Double differential carrier phase measurements for short baselines are used to achieve a high positioning accuracy. Carrier phase measurements are preferred to code measurements because they can be measured to better than 0.01λ, where λ is the wavelength of the carrier signal, which corresponds to millimeter precision and are less affected by multi-path than their code counterparts. However, carrier phase is ambiguous by an integer of the number of cycles that has to be determined during the vehicle operation.

Let the host vehicle Xhbe the moving reference station. Let bihbe a baseline between the host vehicle Xhand the remote vehicle Xi. The following double-difference measurements of carrier phase, code and Doppler measurements can be written as:
d=H(XH,bih)bih+λN+νih(1)
Where H(XH,bih) is the measurement matrix depending on the moving host vehicle XHand the baseline bih, λ is the wavelength of the carrier, N is the vector of the double-difference of ambiguities and νihis the unmodelled measurement noise. Without loss of generality, it is assumed that equation (1) is normalized, i.e., the covariance matrix of νihis an identity matrix.

The heart of the flow chart diagram60is the on-the-fly (OTF) joint positioning and ambiguity determination module62. In the module62, a (6+J−1)-dimension state tracking filter is employed to estimate the three position and the three velocity components, as well as J−1 float double-difference of ambiguities as:
d={tilde over (H)}(XH,bih)s+νih(2)
Where {tilde over (H)}(XH,bih) is the extension of H(XH,bih) and the joint state

Note that the matrix {tilde over (H)}(XH,bih) is not very sensitive to changes in the host vehicle Xhand the baseline bih. With the process equation of the baseline available, it is usually sufficient to use the host vehicle XHand the predicted estimate {tilde over (b)}ihof the previous time instant. Therefore, when the value d is available, a better estimate of the baseline bihcan be obtained by the filtering described below.

Let the process equation of the baseline bihbe:
bih(t+1)=ƒ(bih(t))+w(3)
Where w denotes un-modeled noise.

In equation (3), ƒ is the function that expresses the dynamical model of the baseline. Some candidates of the dynamical model are a constant velocity model (CV) or a constant turning model (CT). Linearizing equation (3) in the neighborhood of the prediction of the baseline {tilde over (b)}ihof the previous cycle and including the double-difference of ambiguities N gives:

s⁡(t+1)=[I00F]⁢s⁡(t)+[0u]+[0I]⁢w(4)
Where I is an identity matrix

Now the OTF joint filtering procedure can be written in Algorithm 1, described below.

In equation (1), the measurement matrix H(XH,bih) plays an important role for the above single baseline positioning method to converge to the correct solution. A geometric dilution of precision (GDOP), i.e., [H(XH,bih)]−1, affects the quality of the estimate of the baseline bih. It can be verified that the GDOP depends on the number of shared satellites and the constellation of the common satellites for the baseline bih. For example, when visible shared common satellites between the remote vehicle and the host vehicle are close together in the sky, the geometry is weak and the GDOP value is high. When the visible shared common satellites between the remote vehicle and the host vehicle are far apart, the geometry is strong and the GDOP value is low. Thus, a low GDOP value represents a better baseline accuracy due to the wider angular separation between the satellites. An extreme case is that the GDOP is infinitely large when the number of shared satellites is less than four.

FIG. 4is a diagram of vehicles82,84and86, where the vehicle82is a host vehicle, used to illustrate the discussion above. A baseline88(bAB) is defined between the vehicles82and84, a baseline90(bBC) is defined between the vehicles84and86and a baseline92(bAC) is defined between the vehicles82and86. A building94is positioned between the vehicles82and86, and operates to block signals of certain satellites so that the vehicles82and86only receive signals from a few of the same satellites. Particularly, the vehicle82receives signals from satellites1,9,10,12,17and21, the vehicle84receives signals from satellites1,2,4,5,7,9,10,12,17and21and the vehicle86receives signals from the satellites1,2,4,5,7, and9. Thus, the vehicles84and86receive signals from common satellites1,2,4,5,6, and9and the vehicles82and84only receive signals from common satellites1and9. Therefore, the vehicles82and86do not receive signals from enough common satellites to obtain the relative position and velocity because it takes a minimum of four satellites.

It is noted that there is more than one solution for positioning among multiple vehicles. Consider the scenario shown inFIG. 4where the host vehicle82needs to estimate the relative positions and velocities of the vehicles84and86, i.e., the baselines bABand bAC, respectively. The baseline bACcan be estimated either directly using the single baseline positioning method or can be derived by combining two other baseline estimates as:
bAC=bAB+bBC(5)

Similarly the baseline bABhas two solutions. It can be verified that the qualities of the two solutions are different. The goal is to find the best solution. As shown inFIG. 4, due to the blockage caused by the building94, the quality of the estimate of the baseline bACis degraded because less than four shared satellites (PRN1,9) are observed. On the other hand, the baseline bACinferred from the baselines bABand bBCis better than the direct estimate of the baseline bAC.

The concept fromFIG. 4can be generalized by introducing a graph G with the vertices denoting the vehicles and edges denoting the baselines between two vertices. Let the weights of the edges be the GDOP of the baseline between two vehicles. The goal is to find a spanning tree, i.e., a selection of edges of G that form a tree spanning for every vertex, with the host vehicle assigned as the root so that the paths from the root to all other vertices have the minimum GDOP.

FIG. 5(a) is a diagram of such a weighted graph100showing the host vehicle at node102and the other vehicles at nodes104, where an edge or baseline106between the host node102and the nodes104and between the other nodes104is giving a weight determined by a suitable GDOP algorithm.FIG. 5(b) shows an optimal-spanning tree108with the non-optimal edges or baselines removed.

FIG. 6is a flow chart diagram110showing a process for defining the weighted graph100shown inFIG. 5(a) and the optimal-spanning tree108shown inFIG. 5(b). The flow chart diagram110includes steps of building the weighted graph100of nodes at box112and then finding the optimal span of the graph100at box114. The algorithm then computes the baseline of an edge in the graph100at box114, and determines whether all of the edges in the span of the graph100are processed at decision diamond118, and if not, returns to the box116to compute the next baseline. The algorithm then computes the relative positions and velocities of all of the vehicles relative to the host vehicle at box120.

The step of computing the baselines to obtain the minimum GDOP in the flow diagram110can be performed any algorithm suitable for the purposes described herein. A first algorithm, referred to as Algorithm 1, is based on a single baseline precision positioning. Given the previous estimate of the joint state ŝ(t−1) with its covariance matrix {circumflex over (P)}(t−1); double difference d; GPS time stamp of the receiver tR; satellite ephemeris E; the system dynamical of equation (1); the measurement equation (2); the covariance matrix Q of the noise term w in equation (3); and the covariance matrix R of the noise term ν in equation (2).

The updated estimate of the joint state ŝ(t) and the covariance matrix {circumflex over (P)}(t) at time t can be solved as follows:

2. Compute the innovation error as:
e=d−{tilde over (H)}({tilde over (s)})

With {tilde over (H)}={tilde over (H)}(Xh,bih)

4. Compute Kalman gain as K={tilde over (P)}{tilde over (H)}TS−1.

5. Output the updated estimate ŝ={tilde over (s)}+Ke and the covariance matrix {circumflex over (P)}=(I−K{tilde over (H)}){tilde over (P)}.

The precise relative positioning for multiple vehicles can also be determined by the following algorithm, referred to as Algorithm 2.

1. Build a weighted graph G of vehicles where each vehicle is a vertex and adding an edge between two vertices if the number of shared observed satellites is larger or equal to four. Let the root denote the host vehicle.

2. The weight of an edge is equal to the geometric dilution of precision (GDOP) of the common satellites observed by the two vehicles, i.e., det [H(Xi,bik)]−1in equation (1) for the weight of the edge between the vertices i and j.

3. Use dynamic programming (either revised Bellman-Ford algorithm of Algorithm 3 or Dijkstra algorithm of Algorithm 4) to find a spanning tree such that the path from any other nodes has the best satellite geometry (minimum GDOP) for positioning.

4. For all E in the graph G do

5. Determine the baseline as represented by the edge E by the algorithms described in Algorithm 1.

6. end for

7. Compute relative positions and velocities from the vehicles to the host vehicle based on the graph G.

Algorithm 3 is a Reversed Bellman-Ford Algorithm

Given the graph G with the vertices V={νi|1≦i≦|V|}, the edges E={ek|1≦k≦|E|} and the weights of edges {wk|1≦k≦|E|}; and the source of vertex H.

Ensure: The span tree T and C:

1.for all vertex v in the set of vertices do2.if v is the source then3.Let the cost (v) be 0.4.else5.let the cost(v) be ∞.6.end if7.Let predecessor(v) be null.8.end for9.for i from 1 to |V| − 1 do10.for each edge ekin E do11.Let u be the source vertex of e. Let v be the destinationvertex of ek.12.if cost(v) is less than max(wk, cost(u)), then13.Let cost(v)=max(wk, cost(u)).14.Let predecessor(v) = u.15.end if16.end for17.end for18.Construct the span tree T using the predecessor(v) for allvertices.

Algorithm 4 is a Revised Dijkstra Algorithm:

Given the graph G with the vertices V={νi|1≦i≦|V|}, the edges E={ek|1≦k≦|E|} and the weights of edges {wk|1≦k≦|E|}; and the source of vertex H.

Ensure: The span tree T and G:

1.for all vertex v in the set of vertices do2.if v is the source H then3.Let the cost (v) be 0.4.else5.let the cost(v) be ∞.6.end if7.Let predecessor(v) be null.8.end for9.Let the set Q contain all vertices in V.10.for Q is not empty do11.Let u be vertex in Q with smallest cost. Remove u fromQ.12.if for each neighbor v of u do13.Let e be the edge between u and v. Let alt =max(cost(u), weight(e)).14.if alt<cost(v) then15.cost(v)=alt16.Let predecessor(v) be u.17.end if18.end for19.end for20.Construct the span tree T using the predecessor(v) for allvertices.

GPS measurements are correlated through the latent vector of the position and velocity of a GPS receiver, which can be expressed as follows.

Let X be a six-dimension latent state vector consisting of the positions and velocities in ECEF coordinates. Let C be a satellite constellation including the positions and velocities of the satellites in ECEF coordinates, which can be determined by the ephemeris messages broadcasted by the satellites. Let a GPS measured quantity O include the code range, carrier phase and Doppler shift for the receiver from the satellites. As a result, the measurement equation can be written as;
O=h(X,β,{dot over (β)},C)+ν  (6)
Where β is the host receiver clock error, {dot over (β)} is the change rate of β and ν is the un-modeled noise for GPS measurements including biases caused by ionospheric and tropospheric refractions, satellite orbital errors, satellite clock drift, multipath, etc.

FIG. 7is a block diagram of a system130including a stand-alone absolute positioning module132receiving satellite observations at box134and satellite ephemeris at box136. Prediction observations from the positioning module132and the satellite observations are provided to an adder138where the difference between the signals is encoded by an encoder140. The encoded signals from the encoder140and the absolute positioning velocity signals from the positioning unit132are provided as vehicle absolute positioning and velocity signals and compressed innovation errors at box142.

The stand-alone absolute positioning module132monitors the input of measurements including code range, carrier phase and Doppler shift, input of satellite constellation C, and vehicle data (e.g., wheel speed and yaw rate). The module132generates the absolute position and velocity of the GPS receiver {circumflex over (X)}. The module132also generates the predicted GPS measurements Õ as expressed by a function h as:
Õ=h({circumflex over (X)},C)  (7)

Therefore, the innovation error e can be defined as:
e=O−Õ(8)

It can be verified that the innovation error vector has two properties. The components are mutually uncorrelated and for each component, the variance is much less than the counterpart of the GPS measurements O. Thus, standard data compression methods, such as, but not limited to, vector quantization or Huffman coding, can be applied to the innovation error e and achieves good compression performance.

FIG. 8is a block diagram150illustrating the inversion operation of the compression module and shows the steps of how to recover the GPS measurements from the received compressed data from a wireless radio module. Particularly, compressed innovation errors at box152are provided to a decoder154and vehicle absolute position and velocity signals at box156and satellite ephemeris signals at box158are provided to a compute predicted observations module160. The signals from the decoder154and the observations module160are added by an adder162to provide satellite observations, such as code range, carrier phase and Doppler frequencies at box164.

Let the estimate of the absolute position and velocity of the GPS receiver be {circumflex over (X)}. The compressed innovation errors is decoded to obtain the corresponding innovation error e. One can verify that the predicted measurements Õ can be computed from the estimate of the absolute position and velocity of the GPS receiver {circumflex over (X)} and the satellite constellation C as:
Õ=h({circumflex over (X)},C)  (9)

Thus, the recovered GPS measurements can be computed as:
Õ=Õ+e(10)

The GPS measurements are highly correlated with time. This makes them well suited to compression using a prediction model of the latent state vector X. Let the process equation of the latent state at time instant t be:
X(t+1)=ƒ(X(t))+w(11)
Where ƒ is the system process function of the host vehicle (e.g., constant velocity model, or constant turning model) where the GPS receiver is mounted on the roof of the vehicle and w is the un-modeled noise in the process equation.

The residuals w in equation (11) are well suited to compression by encoding the difference between the current state vector and the predicted state vector from the previous time instant.

FIG. 9is a system170for a proposed compression scheme. A stand-alone position and velocity estimator172monitors the input of GPS measurements O(t) at time instant t and the prediction of the latent state vector {tilde over (X)}(t) from the previous time t−1, and generates the new estimate of the latent state vector {circumflex over (X)}(t). An observation prediction model module174calculates the observation prediction Õ(t) using equation (6). A Huffman encoder I module176encodes the difference between the input O(t) and the model prediction Õ(t) from an adder184using variable-length coding based on a derived Huffman tree. A unit delay module178stores the previous latent state vector {circumflex over (X)}(t−1). A state prediction model180calculates the prediction of the latent state Õ(t). A Huffman encoder II module182encodes the difference between the latent state vector {circumflex over (X)}(t) and the model prediction {tilde over (X)}(t) from an adder186using variable-length coding based on a Huffman tree.

The minimum description length compression of the GPS protocol (MDLCOG) is designed as an application layer provided above a transport layer, as shown inFIG. 10. Particularly, the MDLCOG is an application layer194in a protocol stack190positioned between a GPS data layer192and a transport layer196. A network layer198is below the transport layer196and a data link layer200is at the bottom of the protocol stack190.

The MDLCOG consists of a collection of messages, known as frames, used for initializing and transmission of measurements and additional data, such as GPS time stamp and a bitmap of the observed satellites. These data frames are known as an initialization frame (I-frame), an additional data frame (A-frame), a differential frame (D-frame) and a measurement frame (M-frame).

At the beginning of the data transmission, the encoder sends an I-frame to initialize the state prediction module at the decoder. The I-frame is analogous to the key frames used in video MPEG standards. The I-frame contains the absolute position and velocity of the GPS receiver in ECEF coordinates estimated by the encoder. The I-frame is also sent whenever the difference between the current and previous estimates of the latent state X is larger than a threshold.

The A-frame contains the non-measurement data, such as the satellite list, data quality indicators, etc. The A-frame is transmitted only at start-up and when the content changes.

The most frequently transmitted frames are the D-frame and the M-frame. D-frames are analogous to the P-picture frames used in MPEG video coding standards in the sense that they are coded with reference to previously coded samples. The time series difference in the D-frame use the vehicle dynamical model expressed in equation (11). Each D-frame contains the Huffman coded difference between the current and previous estimates of the latent state X. The M-frame contains the GPS time stamp and the Huffman coded difference between the measurements O and the predicted values Õ. The M-frame is sent whenever a new GPS measurement is received, and separate frames are transmitted for L1 and L2 frequencies for the M-frame.

Once the decoder has been initialized, having received the appropriate I-frames and A-frames, the encoder transmits the quantized prediction residual for each epoch in the corresponding D-frame and M-frame. An example of the sequence of frames is shown inFIG. 11. M-frames are sent in each time epoch. At time epoch1, an I-frame and an A-frame are sent to initialize the prediction modules in the decoder. An I-frame is sent again in epoch6because the significant change in the latent state X estimate is detected. An A-frame is transmitted in epoch8since a satellite shows up at the horizon or a satellite sets down.

FIG. 12is a flow chart diagram210that outlines the procedure for building a dictionary for coding the residuals. Extensive data of dual frequency GPS data is collected at box212. At box214, the ensemble of measurement residuals e or state prediction residuals w is calculated. At box216, a specific resolution to quantize the residuals is chosen (e.g., 0.2 meter for pseudo-range as the RTCM protocol) and derives a list of symbols. At box218, the frequency of each symbol in the ensemble is calculated. At box220, A={a1,a2, . . . , an} is set, which is the symbol alphabet of size n. Then, P={p1,p2, . . . , pn} is set, which is the set of the (positive) symbol frequency, i.e., pi=frequency (ai), 1≦i≦n. A code C (A,P)={c1,c2, . . . , cn} is generated by building a Huffman tree, which is the set of (binary) code words, where ciis the codeword for ai, 1≦i≦n.

FIG. 13is a flowchart diagram230of an algorithm for encoding GPS data. The procedure starts once new data from the GPS device is received at decision diamond232, and ends if no data is received at box234. Then, the GPS data O is collected at box236that consists of pseudo-range Rj, Doppler shift Djand carrier phase Φjfrom the j-th satellite Xjthat belongs to the set C={Xj|1≦j≦J}, with J being the number of visible satellites. The value Xjconsists of the three-dimensional position of the j-th satellite in the ECEF coordinates.

The algorithm then determines if the satellite map has changed at decision diamond238, and if so, the algorithm generates an A-frame at box240. Particularly, if the identities, i.e., PRN, of the satellite constellation C are changed from the previous time instant, an A-frame is generated to encode the list of the observed satellite's PRN. The frame consists of a 32-bit map, where each bit is either true or false depending on the presence data for a particular satellite.

The algorithm then estimates the stand-alone position and velocity of the vehicle at box242. In the estimating stand-alone position and velocity module, a Kalman filter is used to estimate the latent state vector X through a series of measurements O. Let the latent state vector X=(x,y,z,{dot over (x)},{dot over (y)},ż,β,{dot over (β)}) denote the concatenated vector of a three-dimensional position vector in the ECEF coordinates, three-dimensional velocity vector in the ECEF coordinates, receiver clock error, and change rate of the receiver clock error, respectively. The linearized system of equation (6) at the neighborhood of X* can be written as:
X(t+1)=FX(t)+u1+w(12)
Where F is the Jacobian matrix with respect to the latent state vector X and the nonlinear term u1=ƒ(X*)−FX*.

The measurements of equation (6) of the j-th satellite can be expanded into:

For j=1, . . . , J, where ρjis the geometrical range between the receiver and the j-th satellite {dot over (ρ)}jis the projection of the velocity vector of the j-th satellite projected onto the direction of from the receiver to the satellite, c denotes the speed of light, λ and f are the wavelength and frequency of the carrier signal, respectively, νR,νφand νDare the un-modeled measurement noise for pseudo-range, carrier phase and Doppler shift, respectively, and xj,yjand zjare the three-dimensional position of the j-th satellite in the ECEF coordinates.

Note that the quantities ρjand {dot over (ρ)}jdepend on the vector of the latent state vector X. In other words, equation (13) includes nonlinear equations in terms of the latent state vector X. These quantities are not very sensitive to changes in the latent state vector X. With the receiver's dynamic available, it is usually sufficient to use the predicted estimate {tilde over (X)} of previous time instant as the center of the linearized neighborhood X* and use it to replace the latent state vector X in equation (13). Therefore, when Rj, Φjand Djare available, a better estimate of the latent state vector X can be obtained by the filtering method described in Algorithm 5 detailed below.

Equation (13) can be linearized in the neighborhood of X* as:
Oj=HjX+u2j+νj(14)
Where Oj=[Rj,Φj,Dj]T,Hjis the Jacobian of equation (13) matrix with respect to the latent state vector X and the nonlinear term u2j=h(X*)−HjX*. Therefore, the key steps of the estimating stand-alone position and velocity module can be outlined in Algorithm 5.

The algorithm then determines whether the current state estimate X(t) and the previous state estimate X(t−1) is larger than a threshold T at decision diamond244. If the difference between the current state estimate X(t) and the previous state estimate X(t−1) is larger than the threshold T, an I-frame is generated at box248. The I-frame encodes the current state estimate X(t), including the ECEF position and velocity of the receiver. Otherwise a D-frame is generated to encode the difference X(t)−X(t−1) using Huffman codeword dictionary at box246.

The next step is to calculate the model residuals at box250for the measurements as:
e=O−h(X)  (15)

Then, the measurement model residuals are encoded using the Huffman codeword dictionary by generating the M-frame at box252. In the last step at box254, all generated frames are transmitted to the lower UDP layer196.

Given the previous estimate of the latent state {circumflex over (X)}(t−1) with its covariance matrix {circumflex over (P)}(t−1); measurements O(t); GPS time stamp of the receiver tR; satellite ephemerides E; the system dynamical equation (4); measurement equation (6); covariance matrix Q of the noise term w in equation (4); covariance matrix R of the noise term ν in equation (6).

The updated estimate of the absolute position and velocity of the receiver {circumflex over (X)}(t) at time t.