Patent Description:
The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as motorcars.

Contemporary vehicle positioning solutions are using a global navigation satellite system (GNSS) in order to obtain heading information about said vehicle. However, the heading information is usually only provided when said vehicle is moving.

Moreover, global navigation satellite system information is usually combined with information of at least one inertial measurement unit (IMU) or other vehicle data to increase the availability of said heading information. This, however, is only possible when said vehicle has first been moving. Furthermore, this also suffers from sensor calibration issues where inaccurately calibrated sensors will cause said information to drift, i.e. become inaccurate.

Moreover, there is also an issue with the global navigation satellite system providing said heading information of the antenna (which is not the same as the attitude, e.g. the direction, of said vehicle).

One option to solve the above mentioned problems is to combine the global navigation satellite system heading information, inertial measurement unit data, vehicle speed and wheel base (and thereby the slip angle) to calculate the attitude of said vehicle instead of the antenna heading. However, this is challenging since it is hard to estimate the effective wheel base of a vehicle, in particular a truck, with dynamic geometries and properties, such as liftable rear axles, different load conditions etc..

<NPL>) discloses a system for baseline-constrained GNSS attitude measurement.

<NPL>) discloses a further approach to baseline-constrained attitude estimation using GNSS sensors with a baseline of the order of <NUM> meter.

<CIT> discloses a vehicle attitude determination method, in which candidate attitude values from GNSS are evaluated as to their compatibility with a heading read from an inertial instrument (INS) and a known baseline distance.

<CIT> discloses an attitude determination method, in which the precise attitude determination is guided by a known baseline distance. Optionally the attitude is determined in view of a heading computed from a code measurement of a GPS signal. Because no phase calibration is performed in a code measurement, it may include an error of up to one symbol duration; this is less accurate than the regular use of the GPS signal.

An object of the invention is to provide a method for estimating the attitude of a vehicle, which provides an accurate vehicle attitude.

The object is achieved by a method according to claim <NUM>.

Thus, a method is proposed, wherein a vehicle having two antennas is used to estimate the attitude of said vehicle, in particular by using a relative position vector and said separation of said antennas.

By placing two global navigation satellite system antennas, and in particular two global navigation satellite system receivers, on said vehicle providing raw satellite observables, it is possible to apply real time kinematic (RTK) algorithms to find the relative position of the antennas enabling to calculate the attitude of said vehicle. Moreover, as long as the real time kinematic integer ambiguity fix for said antennas is known, it is possible to calculate the fix, even when the truck is stationary.

An advantage of the provided method is to calculate the attitude of a vehicle in situations where it would not be possible with known global navigation satellite system receivers, in particular by utilizing two antennas, e.g. two global navigation satellite system antennas on said vehicle.

When said vehicle is moving and not turning, e.g. the yaw rate of said vehicle equals zero, the attitude of said vehicle is equal to the direction of movement of said antennas (GNSS heading). Knowing this and the separation of said antennas, the integer ambiguity fix may be calculated. Once a possible candidate is evaluated, said candidate may validated by analysing the residuals of said relative position calculation. For example, by the following steps: <NUM>) Is said vehicle moving at high enough speed to provide accurate GNSS heading? <NUM>) Is said vehicle currently not turning (yaw rate = <NUM>)? <NUM>) Calculate said relative position of said antennas using current GNSS heading and the known separation of the antennas. Additionally, use roll estimate if available or assume roll is zero. <NUM>) Calculate said integer ambiguities for RTK fix based on relative position estimation. <NUM>) Calculate said relative position using the ambiguity candidates and validate the solution using the residuals. <NUM>) Validate the solution using know properties (residuals, separation, roll, GNSS heading, etc.).

In a preferred embodiment, the step of detecting that said vehicle is moving and not turning comprises the step of: estimating the movement speed of said vehicle.

Preferably, the movement speed is estimated via vehicle sensors, e.g. by at least one wheel speed sensor.

In a preferred embodiment, the step of detecting that said vehicle is moving and not turning also comprises the step of: estimating the yaw rate of said vehicle.

Thus, the yaw rate is used to determine whether said vehicle is turning or not.

In a preferred embodiment, the method further comprises the step of: storing the integer ambiguity fix, in particular for the antennas.

Preferably, the data is stored in a memory or a memory device. Preferably, the stored data comprises at least the real time kinematic integer ambiguity fix.

In particular, said integer ambiguity fix is stored to be used while the fix is valid, preferably until the phase-lock is lost. Thus, said integer ambiguity is preferably not stored to a persistent memory.

Thus, the proposed method may also be used while said vehicle is stationary, in particular by using said stored data.

In a more preferred embodiment, the attitude of said vehicle is estimated while said vehicle is stationary, in particular by using said stored integer ambiguity fix.

In a further embodiment, the method also comprises the step of: defining a search space for possible candidates values of the attitude and evaluating all possible candidate within in the search space with respect to the relative position vector.

Preferably, the search space for possible candidates is defined based on the uncertainty of the GNSS heading, which in turn is based on the number of satellites, vehicle speed and yaw rate.

The GNSS heading is then used together with the known antenna separation to estimate a relative position uncertainty.

The relative position uncertainty can then be used to, for each, satellite define the potential ambiguity factors to be included in the search space.

Thus, the proposed method evaluates not one but all possible candidates within said search space, in particular defined by an estimated uncertainty.

In particular, there are factors which could influence the possibility of finding the correct fix, including not knowing the roll of said vehicle or inaccuracies in the yaw rate information, e.g. by sensor errors. Defining a search space, however, may overcome this.

According to a second aspect of the invention, the object is achieved by a computer program comprising program according to claim <NUM>.

According to a third aspect of the invention, the object is achieved by a computer readable medium according to claim <NUM>.

According to a fourth aspect of the invention, the object is achieved by an estimation unit according to claim <NUM>.

According to a fifth aspect of the invention, the object is achieved by a movement estimation device according to claim <NUM>.

According to a sixth aspect of the invention, the object is achieved by a vehicle comprising an above or below described movement estimation device.

In one embodiment, said vehicle further comprises at least a first antenna and a second antenna for communicating with a global navigation satellite system, in particular to provide raw satellite observables.

In a preferred embodiment, said vehicle also comprises at least a first receiver and a second corresponding receiver for communicating with the global navigation satellite system.

In a more preferred embodiment, said vehicle also comprises at least one movement speed estimation unit having at least one movement speed sensor. Preferably, said vehicle also comprises at least one yaw rate estimation unit.

Hence, said vehicle comprises at least one sensor obtaining that said vehicle is moving and not turning.

Still other objects and features of embodiments herein will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits hereof, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

<FIG> shows a vehicle <NUM>, in particular a truck, having an attitude A and interacting with a global navigation satellite system <NUM> via a first and a second antenna <NUM>, <NUM> in order to obtain said attitude A of said vehicle <NUM>.

Hence, said vehicle comprises at least a first antenna <NUM> and a second antenna <NUM> having a separation d to each other for communicating with the global navigation satellite system <NUM>. In a preferred embodiment, said vehicle <NUM> also comprises two receivers for communicating with the global navigation satellite system <NUM>.

Moreover, said vehicle <NUM> also comprises a movement estimation device <NUM> and a movement speed estimation unit <NUM>.

Said movement estimation device <NUM> comprises a computer program <NUM>, a computer readable medium <NUM> and an estimation unit <NUM>.

Said movement estimation device <NUM> is further adapted for performing said above or below described method <NUM> for estimating the attitude A of said vehicle <NUM>.

In particular, said movement estimation device <NUM> is connected to said antennas <NUM>, <NUM> and said movement speed estimation unit <NUM>.

Preferably, said antennas <NUM>, <NUM> are installed at the roof of said vehicle <NUM> and the movement speed estimation unit <NUM> comprises a movement speed sensor <NUM>, which is arranged for estimating the angular velocity of at least one wheel of said vehicle <NUM>.

Said global navigation satellite system <NUM> comprises at least a plurality of satellites <NUM>, <NUM>, <NUM>, <NUM> interacting with said antennas <NUM>, <NUM> of said vehicle <NUM>, in particular via the signals S<NUM>, S<NUM>', S<NUM>; S<NUM>', S<NUM>, S<NUM>', S<NUM>, S<NUM>'.

Thus, said vehicle <NUM> is adapted for double difference via said antennas <NUM>, <NUM>.

One way of estimating said attitude A of said vehicle <NUM> is proposed in <FIG> and/or <FIG>.

<FIG> shows a vehicle <NUM> having two antennas <NUM>, <NUM> in a topview, in particular the topview of said vehicle <NUM> in <FIG>.

Said vehicle <NUM> has an attitude A and said antennas <NUM>, <NUM> are mounted at the roof at said vehicle <NUM>, having a separation d to each other. Preferably, said antennas <NUM>, <NUM> are installed at said roof such that said separation d is square to said attitude A.

<FIG> shows an embodiment of a method <NUM> for estimating the attitude of a vehicle, preferably a truck as shown in <FIG> and/or <FIG>.

The method <NUM> comprises the steps of: detecting that said vehicle is moving and not turning <NUM>; calculating an integer ambiguity fix <NUM>; and validating the integer ambiguity fix <NUM>.

In a first step <NUM>, the movement of said vehicle is detected and also whether said vehicle is turning or not. If said vehicle is not turning and the movement speed of said vehicle is high enough to provide accurate GNSS heading, a second step is started.

In the second step <NUM>, an integer ambiguity fix is calculated, in particular by using a relative position calculation and the separation of said first and second antenna, in particular the separation of said antennas of said vehicle as shown in <FIG> and/or <FIG>.

In a third step <NUM>, the integer ambiguity fix is validated, in particular by analysing the residuals of the relative position calculation, as mentioned in said second step <NUM>.

With continued reference to <FIG>, there will be described an alternative embodiment of a method <NUM> for estimating the attitude of a vehicle, preferably a truck as shown in <FIG> and/or <FIG>.

The method <NUM> comprises the steps of: detecting that said vehicle is moving and not turning and obtaining a heading of at least one of the antennas <NUM>; calculating an integer ambiguity fix corresponding to the relative position vector of the first and second antenna <NUM>; and determining the attitude of the vehicle using the integer ambiguity fix <NUM>.

In a first step <NUM>, the movement of said vehicle is detected and also whether said vehicle is turning or not. If said vehicle is not turning and the movement speed of said vehicle is high enough to provide an accurate GNSS heading, a heading v is obtained for at least one of the antennas. (Clearly, the GNSS heading may be an approximation of the attitude A of the vehicle. To estimate the attitude A with improved accuracy, however, this method <NUM> uses the GNSS heading as one of several inputs. ) Then a second step is initiated.

In the second step <NUM>, the relative position vector D is estimated which satisfies r<NUM> = r<NUM> + D, where r<NUM>, r<NUM> are position vectors of the first and second antennas <NUM>, <NUM>. An estimate D* of the relative position vector D will be used to derive an integer ambiguity fix.

In the third step <NUM>, the attitude A of the vehicle is determined, wherein candidate values of the attitude obtained from the GNSS are validated by analysing residuals in respect of the estimated relative position vector estimate D*, which is applied to find the integer ambiguity fix. The estimate D* corresponds to the number of wavelengths, per GNSS satellite <NUM>, <NUM>, <NUM>, <NUM>, by which a carrier phase measurement is to be corrected in a double-differenced comparison of data received at the first and second antennas <NUM>, <NUM>. The correction may be termed integer ambiguity fix.

The integer ambiguity estimation may include solving a constrained optimization problem, as in <NPL>.

The validation may alternatively apply a LAMBDA method, as in above-cited XP056006728 or XP056010162. The ratio test disclosed therein may include applying a threshold, with which the calculated ratio of integer residuals is compared. The threshold may be a number which depends on the expected noisiness of the carrier phase measurements. Alternatively or additionally, it may be required that the threshold be satisfied for several consecutive samples, such as <NUM>-<NUM> samples.

In particular, the third step <NUM> may comprise minimizing an objective function over <MAT> the space of integer triplets, wherein the objective function includes a term expressing the difference between D*, the estimate of the relative position vector on the basis of the GNSS heading, and D̂, a float estimate of the relative position vector computed from GNSS position vector candidates representing the first and second antennas <NUM>, <NUM>. Such an objective function will penalize any GNSS position vector candidates that are incompatible with the estimate of the relative position vector based on the GNSS heading. Conversely, the objective function will tend to validate position vector candidates that are in good agreement with D*. The validated position vector candidates are generally a useful basis for determining the attitude A of the vehicle.

Alternatively, the third step <NUM> can be implemented by modifying the method disclosed in <NPL>, in which a Kalman filter is used to estimate low-precision floating-point estimates of the carrier ambiguities. In the Kalman filter, according to this reference, the floating-point estimates are improved by including a measurement with low uncertainty of the distance between two antennas; a residual of this distance (baseline residual) is appended to the measurement vector of the Kalman filter, as one new component. The present invention makes available an estimate D* of the relative position vector between the two antennas <NUM>, <NUM>, so that three new components can be added to the measurement vector of the Kalman filter; this may improve the accuracy of Martin's method significantly.

The present approach is efficient at least when the GNSS heading has better accuracy than the GNSS position vector candidates. This condition is satisfied in a broad range of situations, since the GNSS heading is computed from a plurality of GNSS measurements. Therefore, the GNSS heading can be obtained using a state-of-the-art method, e.g., by reading an output from a commercially available GNSS receiver during uniform movement of the vehicle <NUM>.

Concerning the second step <NUM>, the following is a possible approach to determine D*, the estimate of the relative position vector on the basis of the GNSS heading v. The true relative position vector D satisfies: <MAT> <MAT> <MAT> where ez denotes a basis vector of a local east-north-up (ENU) reference frame. Condition (<NUM>) expresses the known separation distance. Condition (<NUM>) holds for zero roll when the antennas are at equal height, but regardless of the pitch of the vehicle. Condition (<NUM>) holds when the separation is square to the attitude A and the vehicle is not turning. An estimate satisfying conditions (<NUM>), (<NUM>) and (<NUM>) is: <MAT> where × denotes vector product. This estimate may be applied to find an integer ambiguity fix for determining the attitude A.

Here, it has been assumed that one GNSS heading is used, from either the first or second antenna <NUM>, <NUM>. In variations of the method, two GNSS headings v<NUM>, v<NUM> may be used to find the integer ambiguity fix. For example, an average <MAT> of the two GNSS headings may be used to compute D*. Alternatively, two separate estimates <MAT> are computed on the basis of a respective GNSS heading v<NUM>, v<NUM>, and an average of both, D* = <MAT>, is fed into the third step <NUM>. Further alternatively, the two separate estimates <MAT> are computed and differences with respect to each estimate are included in the objective function.

A generalized version of the method <NUM> can be performed also in situations where condition (<NUM>) does not apply. If the first and second antennas <NUM>, <NUM> are mounted such that the true relative position vector D differs by an angle α with respect to the lateral direction of the vehicle, then D* according to the above equation is to be rotated back, namely, by an angle -α with respect to ez. This is accounted for by the following expression for D* in the ENU reference frame: <MAT>.

The first, simplified expression for D* corresponds to the special case α = <NUM>.

The carrier ambiguities remain constant as long as the GNSS receiver maintains phase lock; the integer ambiguity fix is valid for this duration. Therefore, the method <NUM> may comprise an additional step of storing the integer ambiguity fix, which is available after the third step <NUM>, in a memory. The stored data may be used in order to estimate the attitude A later, even after the vehicle <NUM> has moved to a different position and/or orientation. This way, the later attitude estimation can be performed while no fresh GNSS heading is available, as may be the case during slow driving, much manoeuvring, reversing or when the vehicle <NUM> is stationary. The stored integer ambiguity fix may be retrieved and used for the purpose of estimating the attitude A in each of these situations.

<FIG> shows a preferred embodiment of a method <NUM> for estimating the attitude of a vehicle, preferably a truck as shown in <FIG> and/or <FIG>.

The method <NUM> comprises the steps of: detecting that said vehicle is moving and not turning <NUM>; calculating an integer ambiguity fix <NUM> and validating the integer ambiguity fix <NUM>.

In a first step, the movement of said vehicle is detected and also whether said vehicle is turning or not. If said vehicle is not turning and the movement speed of said vehicle is high enough to provide accurate GNSS heading, a second step is started.

The first step also comprises: estimating the movement speed of said vehicle (<NUM>) and estimating the yaw rate of said vehicle (<NUM>).

The second step also comprises a calculation wherein at least the relative position of said antennas (<NUM>, <NUM>) is calculated and/or estimated. For this, a search space may be defined (<NUM>) in order to obtain multiple possible candidates of the integer ambiguity fix. Preferably, the step also comprises: evaluating all possible candidates within the search space.

Afterwards, in a fourth step <NUM>, the validated integer ambiguity fix may be stored in a memory.

The stored data may then be used in a fifth step <NUM>, wherein the attitude of said vehicle is estimated, even when said vehicle is stationary, in particular by using said stored validated integer ambiguity fix.

Claim 1:
A method (<NUM>) for estimating the attitude (A) of a vehicle (<NUM>) by using a global navigation satellite system, GNSS, (<NUM>) having a plurality of satellites (<NUM>, <NUM>, <NUM>, <NUM>), wherein the vehicle comprises at least a first antenna (<NUM>) and a second antenna (<NUM>) having a separation (d) to each other, characterized by the steps of:
- detecting that said vehicle is moving and not turning (<NUM>) and obtaining a heading of at least one of the antennas using the GNSS, wherein the heading is obtained as a GNSS heading corresponding to the direction of movement of the antennas computed from a plurality of GNSS measurements during uniform movement of the vehicle at a speed high enough for the GNSS heading to be accurate;
- calculating an integer ambiguity fix (<NUM>) corresponding to the relative position vector between the first and second antenna using said heading and the separation (d) of said first and second antenna (<NUM>, <NUM>);
- determining the attitude of the vehicle (<NUM>), including validating candidate values of the attitude obtained from GNSS carrier phase measurements from the GNSS by analysing residuals in respect of the relative position vector, thereby validating the calculated integer fix.