\textcolor{red}{Title: Analysis and sampling strategy}
Given the sensor fusion algorithm and the sensor model we can now describe which dynamical limitations it imposes and therefore how the sensor can be sampled. In this section we will discuss the general features that characterize an effective sampling strategy. In the next section complementary a simulation evaluation of the available trade-offs will be performed.
Given the sensor fusion algorithm and the sensor model we can now describe which dynamical limitations the sensor imposes and therefore how it can be sampled. In this section we will discuss the general features that characterize an effective sampling strategy. Complementary in the next section a simulation evaluation of the available trade-offs will be performed.
\subsection{The dynamics}
The model discussed in section~\ref{sec:gps} points out two dynamics that characterize the sensor: the availability of the ephemeris data and the availability of ranging data. The two are characterized by very different time scales, both in temrs of acquisition time and validity.
The model discussed in section~\ref{sec:gps} points out two dynamics that characterize the sensor: the availability of the ephemeris data and the availability of ranging data. The two are caracterized by very different time scales, both in terms of acquisition time and validity.
The ephemeris data live in the time scale of minutes requiring betwen 30 to 59 seconds to be aquired and being valid for 30 minutes. This poses two main constraints. First at the startup a delay equivalent to the acquisition time of the ephemeris data will be present -- this is the so called \emph{Time To First Fix} TTFF. Second during regular working conditions every 30 minutes the ephemeris data must be updated, requiring the antenna to be turned on for enough time and affecting the battery drain of the sensor.
The ranging data are instead characterized by a time scale of the order of milliseconds. They require from 2 to 10 milliseconds to be acquired and they have istantaneous validitiy since they are the ones used to compute the present position. This means that there is a lower bound the sampling period of the sensor under which the sampling will be equivalent to keep the sensor always turned on. Also for some applications with real time constraints the varying delay for the acquisition could be critical.
The ranging data are instead caracterized by a time scale of the order of milliseconds. They require from 2 to 10 milliseconds to be acquired and they have istantaneous validitiy since they are the ones used to compute the present position. This means that there is a lower bound the sampling period of the sensor under which the sampling will be equivalent to keep the sensor always turned on. Also for some applications with real time constraints the varying delay for the acquisition could be critical.
Another important consequence of the sampling policy is the observability of the event \texttt{lost\_visibility}. This is in fact observable only when the antenna is on and is listening to the visible satellites. When a satellite disappears, if the antenna is turned off the device wont observe this and at the next sampling it will have to acquire new ephemeris data before being available of providing new positioning (assuming that enough satellites are visible).
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@@ -24,7 +24,7 @@ Given these considerations we designed a simple sampling stategy that tries to k
\textbf{The controller}: The logical controller sends a \texttt{turn\_on} signal at the start up in order to collect the ephemeris data (state 2). Then once those are available it starts cycling between states 3 and 4 through alternatively triggering the \texttt{turn\_off}, \texttt{turn\_on} signals. For readability, in the same way as we did in the sensor model, the states in which the antenna is turned on are filled with green.
When the ephemeris data are about to expire or the sensor loses visibility of the satellites it was tracking, then the controller goes back to state 2 and keeps the antenna on in order to update the ephemeris data. In the case that the ephemeris data are only about to expire the sensor can actually still be sampled, represented by taking the transition \texttt{sensor in position\_avaialable}. The previous ephemeris data are in fact available and the ranging data come along with the fetching of the satellites signals.
When the ephemeris data are about to expire or the sensor loses visibility of the satellites it was tracking, the controller goes back to state 2 and keeps the antenna on in order to update the ephemeris data. In the case that the ephemeris data are only about to expire the sensor can actually still be sampled, represented by taking the transition \texttt{sensor in position\_avaialable}. The previous ephemeris data are in fact still valid and the ranging data come along with the fetching of the satellites signals.
