The name ``Gigatracker'' derives from the high rate of particles that this spectrometer must sustain. Due to the non-uniform 750~\si{\mega\hertz} beam rate, the particle flux presents a peak of 1.3 \si{\giga\hertz/\milli\meter^2} around the centre of the detector.\\

The Gigatracker provides precise measurements of the angle, momentum and time of the crossing particle. The resolutions to be achieved are:

\hspace{2.5em}
\begin{minipage}{0.95\textwidth}
\begin{itemize}
\item[\textbf{time}] 150 \si{\pico\second}\T\T
\item[\textbf{momentum}] $\Delta p \,/\, p \leq 0.2 \%$
\item[\textbf{direction}] 16 \si{\micro\radian}\B\B\\
\end{itemize}
\end{minipage}

Such accurate measurements are necessary in order to correctly associate the decaying kaon with the $\pi^+$ track detected downstream. Kinematic selection of $K^+ \rightarrow \pi^+ \nu \bar{\nu}$ events indeed relies on constraints based on the missing mass variable, defined as
\begin{align}
m_{miss}^2 &\equiv \left( P_K - P_\pi \right)^2\notag\\
&\simeq m_K^2 \left( 1 - \frac{P_\pi}{P_K} \right) + m_{\pi}^2 \left( 1 - \frac{P_K}{P_\pi} \right) - P_K P_\pi \theta^2_{K\pi}
\end{align}
In this equation, $P_K$ entirely derives from the Gigatracker data, and the determination of $\theta_{K\pi}$ makes use of both the GTK and the downstream STRAW spectrometer.\\

In order to limit hadronic interactions and to preserve the beam divergence, the Gigatracker is composed of three stations for a total thickness lower than 0.5 $X_0$ \cite{na62-tdr}. Each station contains 18000 300$\times$300 \si{\square\micro\meter} silicon micro-pixels 200 \si{\micro\meter} thick bump-bonded to 10 readout ASIC chips 100 \si{\micro\meter} thick. A sketch of a Gigatracker station is shown in \figurename~\ref{img:det:gtk-station}.\\

The three stations of the GTK are mounted inside the vacuum tank preceding the decay region, and they are interlaced with 4 achromat magnets as shown in \figurename~\ref{img:det:gtk-pos}.\\

An extensive amount of resources was committed to the design and development of this detector. Critical aspects are the high radiation hardness needed to sustain the rate of beam particles crossing the silicon sensors, and the very high time resolution required. In particular, the dissipation of the increasing leakage current due to radiation damage needs a complex cooling system. This will be based on a micro-channel cooling system.
% nitrogen-cooled vessel placed around the beam line.\\

\begin{figure}
\centering
\subfigure{\includegraphics[width=0.28\textwidth]{Images/pdf/na62/detector/gtk-full}\label{img:det:gtk-station}}
\hfill
%\subfigure[]{\includegraphics[width=0.71\textwidth]{Images/pdf/na62/detector/gtk-pos-full}\label{img:det:gtk-pos}}
%\subfigure[]{\def\svgwidth{0.71\textwidth}\input{Images/pdf/na62/detector/GigaTracker_Velghe-tex.pdf_tex}\label{img:det:gtk-pos}}
\subfigure{\includegraphics[width=0.69\textwidth, trim = 0cm -2.0cm 0cm 0cm]{Images/pdf/na62/detector/GigaTracker_Velghe-full2}\label{img:det:gtk-pos}}%\hfill
\caption{On the left, sketch of a Gigatracker station \cite{Ceccucci:1070584}. On the right, layout of the Gigatracker stations and magnets used to bend the beam \cite{bob-gtk}.}\label{img:det:gtk}
\end{figure}