Measurement Vector

Overview

It is possible to choose among various measurement models for a given EKF implementation. The particular model is selected based on many factors, one being the limitations of the available measurements. This formulation was selected due to the incomplete knowledge of the magnetic environment of the system and uses the available sensor information as follows:

  1. Accelerometers “level” the system (used to compute \({^{\perp}}{\phi}{_{meas}^{B}}\) and \({^{\perp}}{\theta}{_{meas}^{B}}\)) FN
  2. Magnetometers and/or GPS heading information align the perp-frame with true or magnetic north (\({^{N}}{\psi}{^{\perp}}\))
  3. GPS position and velocity measurements update the position and velocity estimates (\(\vec{r}^{N}\) and \(\vec{v}^{N}\))

Based upon these steps, the measurement vector, \(\vec{z}_{k}\), is formed:

\[\vec{z}_{k} = { \begin{Bmatrix} { \begin{array}{c} {\vec{r}_{GPS}^{N}} \cr {\vec{v}_{GPS}^{N}} \cr {^{N}}{\vec{\Theta}}{_{meas}^{B}} \end{array} } \end{Bmatrix} }\]

with the corresponding measurement model, \(\vec{h}_{k}\):

\[\vec{h}_{k} = { \begin{Bmatrix} { \begin{array}{c} {\vec{r}_{pred}^{N}} \cr {\vec{v}_{pred}^{N}} \cr {^{N}}{\vec{\Theta}}{_{pred}^{B}} \end{array} } \end{Bmatrix} }\]

Both \({^{N}}{\vec{\Theta}}{_{meas}^{B}}\) and \({^{N}}{\vec{\Theta}}{_{pred}^{B}}\) are 3x1 column vectors containing the roll, pitch, and heading values. FN

Measurement Vector (\(\vec{z}_{k}\))

The measurement vector, \(\vec{z}_{k}\) is comprised of position, velocity, and attitude information as defined above. It is formed from sensor measurements:. However, only the GPS velocity is available directly from measurements; other information must be derived from sensor readings using the relationship described below.

Roll and Pitch Measurements

Roll and pitch values are computed from the accelerometer signal. Under static conditions, measurements made by the accelerometer consists solely of gravity and sensor noise. Along the axis pointed in the direction of gravity, the sensor measures -1 [g]. This is due to the proof-mass being pulled in the direction of gravity, which is equivalent to a deceleration of 1 [g] in the absence of gravity.

\[\vec{a}_{meas} = \vec{a}_{grav} = -\vec{g}\]

Static roll and pitch values are determined by noting that gravity is constant in the N-Frame (perp-Frame):

\[\begin{split}\vec{g}^{N} = \vec{g}^{\perp} = \begin{Bmatrix} { 0 \\ 0 \\ 1 } \end{Bmatrix}\end{split}\]

and can be transformed into the body frame through \({^{B}{R}^{\perp}}\):

\[\begin{split}\vec{g}^{B} = {^{B}{R}^{\perp}} \cdot \vec{g}^{\perp} = { \begin{pmatrix} { {^{\perp}{R}^{B}} } \end{pmatrix} }^{T} \cdot \vec{g}^{\perp} = { \begin{pmatrix} { {^{\perp}{R}^{B}} } \end{pmatrix} }^{T} \cdot \begin{Bmatrix} { 0 \\ 0 \\ 1 } \end{Bmatrix}\end{split}\]

Using the definition of \({^{\perp}{R}^{B}}\) (discussed in Attitude Parameters) and expanding the equation, the accelerometer measurements can be related to roll and pitch angles:

\[\vec{g}^{B} = -\vec{a}_{meas}^{B}\]
\[\begin{split}\begin{Bmatrix} { \begin{array}{c} {-sin \begin{pmatrix} { {^{\perp}{\theta}^{B}} } \end{pmatrix}} \cr {cos \begin{pmatrix} { {^{\perp}{\theta}^{B}} } \end{pmatrix} \cdot sin \begin{pmatrix} { {^{\perp}{\phi}^{B}} } \end{pmatrix}} \cr {cos \begin{pmatrix} { {^{\perp}{\theta}^{B}} } \end{pmatrix} \cdot cos \begin{pmatrix} { {^{\perp}{\phi}^{B}} } \end{pmatrix}} \end{array} } \end{Bmatrix} = { \begin{Bmatrix} { \begin{array}{c} {-a}_{mx}^{B} \\ {-a}_{my}^{B} \\ {-a}_{mz}^B \end{array} } \end{Bmatrix} }\end{split}\]

From this result, the angles corresponding to the accelerometer signal are found:

\[{^{\perp}}{\phi}{_{meas}^{B}} =atan2(-a_{my}^{B},-a_{mz}^{B} )\]
\[{^{\perp}}{\theta}{_{meas}^{B}} =-asin(-\hat{a}_{mx}^{B} )\]

where, \(\hat{a}_{mx}^{B}\) is the x-axis acceleration value normalized by the total acceleration magnitude:

\[\hat{a}_{mx}^{B} = { {a_{mx}^B} \over \| {\vec{a}_{meas}^{B}} \|}\]

Normalization of the y and z-axis accelerometer values can be performed. However this is not required as the \(atan\) function uses the ratio of the two (the normalization factor cancels out).

Heading Measurements

Heading measurements are determined from the following:

  1. Magnetometers
  2. GPS Velocity

Magnetometer-Based Heading

Magnetometers measure the local magnetic field at a high DRs but the readings can be affected by hard and soft-iron disturbances in the system or by changes in the external magnetic field. Hard and soft-iron effects are local to the system and can be accounted for; external field disturbances cannot be corrected.

Adjustment of the magnetic field measurement for hard/soft-iron disturbances can be performed according to the following equation:

\[\vec{m}_{corr}^{B} = R_{SI} \cdot S_{SI} \cdot {R_{SI}}^{T} \cdot (\vec{m}_{meas}^{B} - \vec{m}_{bias}^{B} - \vec{m}_{HI}^{B} )\]

where \(\vec{m}_{meas}^{B}\) is the measured magnetic field vector in the body-frame, \(\vec{m}_{HI}^{B}\) is the hard-iron disturbance, and \(R_{SI}\) and \(S_{SI}\) are the soft-iron disturbances. Note: for this analysis the magnetometer bias is neglected; assumed to be negligible or lumped in with the hard-iron.

Hard and soft-iron parameters are estimated by performing a magnetic-alignment maneuver. Note that the application of these corrections do not adjust individual magnetometer channels to match the actual field strength. Only the relative magnetic field is corrected, resulting in a unit-circle for the xy magnetic-field. However, as shown later, this enables the heading to be calculated from the corrected signal.

Heading calculation

The heading is computed using the fact that, in the magnetic NED-frame, the y-axis component of the magnetic field is zero. In the true-north NED-frame this is not the case; a magnetic declination angle corrects for this. The magnetic field at a given point can be found using the World Magnetic Model (WMM) or from NOAA’s website (https://www.ngdc.noaa.gov/geomag-web/#igrfwmm). In San Jose, CA, the magnetic field estimates are provided in Table 4:

MagFieldStrength

Table 4: Magnetic Field Components based on WMM

Figure 4 illustrates the relationship between the Lines of constant Lat/Lon, the NED-frame, and the perp-frame. Declination is specified with \(\delta\) and heading is specified with \(\psi\).

MagFieldNandBFrames

Figure 4: Relationship of Magnetic-Field to N and B-Frames

The magnetic field vector, \(\vec{b}\), can be broken down into two components:

  1. the xy-plane component and
  2. the vertical component

The relationship between heading and magnetic field is based on the components of \(\vec{b}^{N}\) as measured in the NED-frame:

\[\begin{split}\vec{b}^{\perp} = {^{\perp}{R}^{N}} \cdot \vec{b}^{N} = {^{\perp}{R}^{N}} \cdot \begin{pmatrix} { b_{xy} \\ 0 \\ b_{z} } \end{pmatrix}\end{split}\]

Expanding the expression results in the following:

\[\begin{split}\begin{Bmatrix} { b_{x}^{\perp} \\ b_{y}^{\perp} \\ b_{z}^{\perp} } \end{Bmatrix} = \begin{Bmatrix} { b_{xy} \cdot cos{ \begin{pmatrix} { {^{N}{\psi}^{\perp}} } \end{pmatrix} } \\ -b_{xy} \cdot sin{ \begin{pmatrix} { {^{N}{\psi}^{\perp}} } \end{pmatrix} } \\ b_{z}^{\perp} } \end{Bmatrix}\end{split}\]

From this, the heading is computed:

\[tan{ \begin{pmatrix} { {^{N}{\psi}^{\perp}} } \end{pmatrix} } = { {b_{xy} \cdot \sin{ \begin{pmatrix} { {^{N}{\psi}^{\perp}} } \end{pmatrix} }} \over {b_{xy} \cdot \cos{ \begin{pmatrix} { {^{N}{\psi}^{\perp}} } \end{pmatrix} }} } = { {-b_{y}^{\perp}} \over {b_{x}^{\perp}} } = { {-m_{corr,y}^{\perp}} \over {m_{corr,x}^{\perp}} }\]

Note: the values for \(b_{x}^{\perp}\) and \(b_{y}^{\perp}\) are the corrected and ‘leveled’ values of the measured magnetic-field in the body-frame; roll and pitch estimates are used to level the signal via \({^{\perp}{R}_{pred}^{B}}\).

\[{\vec{m}_{corr}^{\perp}} = {^{\perp}{R}_{pred}^{B}} \cdot {\vec{m}_{corr}^{B}}\]

Note: as this calculation only corrects the magnetic-field in the xy body-frame, the heading solution is best when the system is nearly level. he solution begins to degrade as the roll and pitch increase. This can be accounted for by adjusting the measurement covariance matrix, \(R\), accordingly. Additionally, the solution also begins to degrade as the iron in the system increases.

GPS Heading

Heading is also provided directly from the GPS messages. The four messages currently decoded by the IMU381/OpenIMU firmware provide true heading via messages listed in Table 6.

Table 6: GPS Messaging and Heading Measurement
System Message Description Units
NovAtel BESTVEL
Actual direction of motion over
ground (track over ground) with
respect to True North
[deg]
NMEA VTG True track made good [deg]
SiRF
Geodetic Navigation
Data – Message ID 41
Course Over Ground
(COG, True)
[deg x 100]
ublox NAV-VELNED Heading of motion 2-D [deg]

of the PS readings and angles derived from accelerometer readings (equations provided in Measurement Covariance section):

GPS Position and Velocity

GPS-based position is derived from the GPS lat/lon/alt message (BestPos, GGA, etc) and converted to NED-position using the WGS84 model.

GPS-based velocity is obtained from the BestVel, etc message. However, the NMEA message does not provide vertical velocity, derived from or accounted for in other ways. In all cases the N and E-velocity is calculated from heading and ground speed. The relationship is:

\[ \begin{align}\begin{aligned}v_{N} = v_{XY} * \cos{ \begin{pmatrix} { {^{N}{\psi}^{\perp}} } \end{pmatrix} }\\v_{E} = v_{XY} * \sin{ \begin{pmatrix} { {^{N}{\psi}^{\perp}} } \end{pmatrix} }\end{aligned}\end{align} \]