# Measurement generation¶

Nyx allows custom implementations of a MeasurementDevice, which this MathSpec cannot cover. Therefore, this page focuses on the implementation for GroundStation: this is the structure used for the default orbit determination setups which rely on Earth based ground stations.

## Range and range-rate calculation¶

The embeded ground stations are initialized with a latitude $$\phi$$, a longitude $$\lambda$$, a height $$h$$, an elevation mask, a range noise level and a range-rate noise level.

The position of this ground station is converted into the frame of the spacecraft to generate a measurement between a spacecraft ($$r$$, for receiver) and a ground station ($$t$$, for transmitter). In the following, the subscript $$r$$ corresponds to the radius of that orbital state and $$v$$ the velocity vector of that state. Further, if a symbol has a frame associated to it, e.g. $$^{\text{SEZ}}\mathbf{\rho}$$, then it is a vector (the bold font may not that visible).

The range in the IAU Earth fixed frame is then computed:

$^{\text{IAU Earth}}\mathbf{\rho} = \mathbf{r}_r - \mathbf{t}_r$

That range vector is then converted in the SEZ frame using the algorithm from Vallado, where $$R_i$$ corresponds to a rotation by the $$i$$-th axis:

$^{\text{SEZ}}\mathbf{\rho} = R_2\left(\frac \pi 2 - \phi\right)~R_3(\lambda)~\cdot~^{\text{IAU Earth}}\mathbf{\rho}$

The elevation is then computed as follows:

$el= \sin^{-1}\left(\frac {^{\text{SEZ}}\mathbf{\rho_z}}{|^{\text{SEZ}}\mathbf{\rho}|}\right)$

A Gaussian/normal PDF is sampled with the range and range-rate noises to noise-up the true range and range rate computations, respectively noted $$\mathcal{N}(\rho)$$ and $$\mathcal{N}(\dot\rho)$$. The range is computed trivially computed:

$\rho = |^{\text{SEZ}}\mathbf{\rho}| + \mathcal{N}(\rho)$

And the range-rate is computed as:

$\dot\rho = ^{\text{SEZ}}\mathbf{\rho} \cdot \frac{(\mathbf{r}_v - \mathbf{t}_v)}{\rho} + \mathcal{N}(\dot\rho)$

## Measurement sensitivity matrix¶

In a Kalman filter, the sensitivity matrix, noted $$\tilde{H}$$, relates the filter covariance, the filter gain, the measurements and the noise of the measurement. Like the state transition matrix, the sensitivity matrix is a partials matrix of size $$N\times M$$, where $$N$$ is the size of the measurement and $$M$$ is the size of the state to be estimated. For example, if the measurement is the range $$\rho$$ and the range-rate $$\dot{\rho}$$, and the estimated state is the position $$\{x,y,z\}$$ and velocity $$\{\dot x, \dot y, \dot z\}$$, then the sensitivity matrix is written as follows.

$\begin{equation} \label{sensitivity} \tilde H = \begin{bmatrix} \frac{\partial \rho}{\partial x} & \frac{\partial \rho}{\partial y} & \frac{\partial \rho}{\partial z} & \frac{\partial \rho}{\partial \dot x} & \frac{\partial \rho}{\partial \dot y} & \frac{\partial \rho}{\partial \dot z} \\ \frac{\partial \dot \rho}{\partial x} & \frac{\partial \dot \rho}{\partial y} & \frac{\partial \dot \rho}{\partial z} & \frac{\partial \dot \rho}{\partial \dot x} & \frac{\partial \dot \rho}{\partial \dot y} & \frac{\partial \dot \rho}{\partial \dot z} \\ \end{bmatrix} \end{equation}$

Using the same methodology as previously, it is evident that the sensitivity matrix may be computed by simply defining a hyper-dual space whose size is equal to that of the state to be estimated. The equations which return the range $$\rho$$ and the range-rate $$\dot\rho$$ from an input state will then automatically also return the components of the sensitivity matrix.

Note

The ground stations currently do not support light-time corrections or tropospheric attenuations. This will be part of the 1.1.0 release.

Last update: 2021-06-10