Implemented PVT Algorithms¶
In this section we provide the theoretical aspects behind the GNSS Compare’s PVT algorithms. The information here can be associated with the following Java classes:
PedestrianStaticExtendedKalmanFilter- this one sounds a bit strange, however bear with us as explanations will be given when the filter tunning is explained
Extended Kalman Filter¶
One of the estimation techniques implemented in the GNSS Compare framework is the Kalman Filter. Taking into account that the measurement model is linearized about the time predicted position, in fact the implementation is an Extended Kalman Filter (EKF).
In this section we describe the theoretical aspects of the EKF implementation such that the curious minds can understand easily what is behind GNSS Compare’s awesome algorithms. We are interested to implement the EKF for two types of users: a static user and a dynamic user.
Therefore we will describe how the state vector is defined, or in other words, the vector containing the parameters that we wish to estimate (hint: the parameters are related to the GNSS Compare’s user position!), and also what dynamic and measurements models we have considered. And as bonus we will also write about the tunning of the EKFs.
First things first! Let’s remember the Kalman Filter equations, the implemented ones, in order to make the rest of this section more enjoyable.
We have the time prediction of the state vector (x) and it’s variance-covariance matrix (P):
In the next step we can compute the innovation vector (gamma) and it’s variance-covariance matrix (S) with the help of the obsevation vector (z), the observation matrix (H) and the measurement noise matrix (R):
We are almost there, we just need to compute the famous Kalman gain (K)!
Finally the measurement update step is:
However, before explaining how the EKF for the static user and the dynamic user was implemented, we still need to talk about the measurement model based on the GNSS pseudoranges retrieved from the smartphone’s GNSS receiver. If you are familiar with this concepts, you can skip the following section.
Pseudorange measurement model¶
For a code-based pseudorange (PRc) we have the following (non-linear) equation taking into account the satellite clock bias (dtS), the delay caused by the ionosphere (dion), the delay caused by the troposphere (dtrop) and the receiver noise (epsilon).
We know, there are more effects that are perturbing the GNSS measurements, however we wish to keep things as simple as possible and the interested persons can always consider more error sources in the GNSS Compare’s code.
The above equation is non-linear because of the geometric distance (rho) between the receiver and the GNSS satellite. Luckly we can linearize it if we have knowledge about an approximated position of the receiver (X0, Y0, Z0), which we do! We do have from the time prediction step of the EKF. Taking this into account and applying a first order Taylor series expansion we obtain:
On the left side of the equation we have moved every term that can be computed. The subscript 0 means that those parameters are estimated by using the approximate receiver position information. On the right hand side we have the unknowns (dX, dY, dZ, dtR) and their coefficients. Based on the linearized pseudorange equation one can form the observation matrix (H).
Practical advise: Take care that the unknowns from the linearized pseudorange equations are not the same as the position related unknowns that we are estimating directly in the EKF state vector. Check the GNSS Compare code ( e.g.,
StaticExtendedKalmanFilter class ) to understand how this is handled.
Good, now we can see how the EKF was implemented for the static user and the dynamic user!
In the case of a static user we have the following state vector at the epoch k:
In the above expression X, Y and Z are the coordinates in Earth Centered Earth Fixed (ECEF) frame and the last two parameters are the receiver clock bias and the receiver clock drift. All the parameters are expressed in units of meters.
Now that the state vector is defined, we can move on by choosing the dynamic model. Before moving further let’s think a bit about this aspect. A static user doesn’t change his/hers position, therefore this means that over time the X, Y, Z coordinates remain the same! We only have to take care of how we model the dynamic behavior of the receiver’s clock, which is approximated to be:
Having in view all of this information we can define the transition matrix (F) of the filter as:
We are almost done with the dynamic model elements. The only thing that we need now is the process noise matrix (Q). Because the process noise matrix contains the uncertainty we have in the dynamic model that we consider, we have to define it accordingly. In the static case we can assume that the user is not moving and that the receiver clock has some frequency and phase errors. In order to fully understand this reasoning, the interested reader is advised to check the following book: Introduction to Random Signals and Applied Kalman Filtering by Robert Grover Brown and Patrick Y. C. Hwang.Therefore, the process noise matrix is approximated to be:
In the above expression the receiver clock related parameters are expressed as:
The parameter h-2 and h0 are the Power Spectral Densities (PSD) of the random walk frequency noise and of the white noise, as defined in the suggested book above. Some typical values for a low quality Temperature Compensated Crystal Oscillator (TCXO) are 2e-20 and 2e-19 (in seconds). A practical advise before using this values is to take care that we are dealing with the parameters of a variance-covariance matrix and also that they have to be converted in units of meters (remember that we have expressed the receiver clock states in units of meters).
So basically we are done with the static user case. That’s great as we can move to the dynamic one!
In the case of a dynamic user there are few aspects that one has to consider. Let’s start again by defining the new state vector:
We can already observe that we have three more parameters to estimate (U, V, W) which are the velocities on the X, Y and Z directions. If our state vector is modified (with respect to the static case) then our intuition will tell us that we need to define a new transition matrix and a new process noise matrix. Which is exactly what we are going to do next, therefore:
For the process noise matrix we use the approach presented in the book of Robert Grover Brown and Patrick Y. C. Hwang ( Introduction to Random Signals and Applied Kalman Filtering ). Indeed, is the third time we refer to this book in the implemented PVT algorithms section, however you can trust us that is a very good one!
The parameters Sx, Sy and Sz are the spectral amplitudes that reflect the position random process. Unfortunately, setting their values is not as straigth forward as for the receiver clock states. We have to rely on what we call the tunning process which is modifying the values in Q and R experimentally (i.e., trial and error). Just as a note, this can be avoided by designing and implementing adaptive estimators. Who knows, maybe you (the reader) will decide to implement some nice ideas now that this possibility is enabled with GNSS Compare’s flexible framework.
Practical advise: When the observation matrix (H) is being built do consider that it’s size is defined in the following way: the number of rows is the number of measurements and the number of columns is the number of unknowns. Therefore when switching from the static case to the dynamic case, H changes also. We mention this just to be sure that a possible conceptual hiccup is avoided.
Because at the moment we are dealing with a standard EKF and not an adaptive one this means that we have to assign values in the process noise matrix (Q) and in the measurement noise matrix (R) such that the filter is tunned to our situation.
Let’s start with the R matrix. We set R to be a diagonal matrix containing the variances of each pseudorange measurement. The measurement noise matrix being diagonal relies on the assumption that there is no cross-correlation between the measurements coming from different satellites ( an assumption that is not entirely represeting the reality, however it fits most of the applications ). Therefore, the diagonal elements of the R matrix are:
To keep things relatively simple, we can assign the value for the sigma 10 meters ( don’t forget to square it before putting it in R ). Another assumption that is made is that the measurements received at the k-th epoch have equal variances ( ok, this assumption is not true at all ). However here is an idea for you, maybe you can try investigating some interesting measurement weigthing methods and then compare (the main keyword of the whole project) the results you get with our not so realistic assumption. Let the researcher within you thrive!
Let’s move to the Q matrix now. For this we present three tunning examples: static, pedestrian and dynamic.
For the static case we have already seen that we only have to take care about the process noise of the receiver clock states. So the values that we are assigning to the PSD of the random walk of the frequency noise and of the white noise are:
In the above we use the c notation for the speed of light.
Intuitively we should have used the EKF designed for a dynamic user in this situation. It would only make sense as a pedestrian changes his/hers position over time. However, one must take into account that the raw measurements delivered by the smartphone’s GNSS receiver are quite noisy and if there are no other means to detect the motion of the user (e.g., using an Inertial Measurement Unit) then estimating the velocities can make our results not soo accurate. Having this situation in view we have found a workaround: we use the EKF designed for a static user and we let some process noise for the X and Y coordinates ( unless one of our users is not Superman we are not that interested in the Z direction ). This means that we have the following Q matrix:
The value 0.2 was chosen by trial and error and it fits a slow walking pedestrian. We hope that the name of the Java class
PedestrianStaticExtendedKalmanFilter makes a little bit more sense now.
Finally we have arrived at the final case regarding the tunning of the dynamic EKF. Again the following values were determined empirically:
Weighted Least Squares¶
GNSS Compare offers also the possibility to change the PVT estimator if the user whishes so. By not requiring knowledge about the dynamics, Weighted Least Squares (WLS) can be used to estimate the position using only the pseudorange measurements. However there are some drawbacks like: the quality of the estimations fully depends on the quality of the measurements and also the WLS requires a minimum number of measurements (typically 4 if we want to estimate the 3D position and the receiver clock bias).
Nevertheless is useful to have such an estimator as its behavior can be studied in real-time/post-processing in comparison with an EKF. And all this thanks to GNSS Compare!
Altough the pseudorange measurement model was presented in the EKF description we will do it one more time just for the sake of completion.
Pseudorange measurement model¶
The linearized code-based pseudorange measurement is:
Let’s also express the unit line of sight vector and the position related unknowns as:
For n observed satellites we have the following measurement model:
To estimate the vector of unknowns (x) in the WLS sense, we proceed in the following way:
Practical advise: In the WLS case, as the position is concerned, we are estimating the difference between the approximated position and the true position until this difference is below a certain threshold. We encourage you to check the
WeightedLeastSquares class to see how this is handled.