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\end{itemize}  In the case of the SKA1\_Low DM limits, we assume that users will have the capability to select a specific frequency range for observations, e.g. if the observation band is selected to be 150-350 MHz, then the limiting DM will be defined by a frequency of 150 MHz.   Above the DM limits set by the above requirements we suggest that the pulsar timing instrumentation of SKA1 supports incoherent (and possibly semi-coherent) dedispersion out to considerably higher DMs. We do not present here a scientific or technical justification for incoherent dedispersion limits however rudimentary calculations based on To determine acceptable DMs to support, we assume  the residual dispersive smearing in a single frequency after fine channelisation minimum channelwidth that should be handled internally  to between 1k the pulsar timing instruments on SKA1. Assuming a minimum channel with of $\sim$1 Hz on SKA1\_Low  and 64k channels shows $\sim$20 Hz on SKA1\_Low, find  the following requirements  to be reasonable: suitable.  \begin{itemize}  \item \textbf{SKA1-SYS\_REQ-XXXX:} \textit{\bf SKA1\_Mid Pulsar Timing incoherent dedispersion DM limits.} The SKA1\_Mid when in pulsar timing mode shall time pulsars using incoherent dedispersion up to a dispersion measureof 10,000 pc cm$^{-3}$.  \item \textbf{SKA1-SYS\_REQ-XXXX:} \textit{\bf SKA1\_Low Pulsar Timing incoherent dedispersion DM limits.} The SKA1\_Low when in pulsar timing mode shall time pulsars using incoherent dedispersion up to a dispersion measure of 1,000 pc cm$^{-3}$.  \end{itemize}  \begin{itemize}  \item \textbf{SKA1-SYS\_REQ-XXXX:} \textit{\bf SKA1\_Mid Pulsar Timing incoherent dedispersion DM limits.} The SKA1\_Mid when in pulsar timing mode shall time pulsars using incoherent dedispersion up to a dispersion measure  defined by ${\rm DM} = t_{\rm res}(4.15\times10^3 (\nu^{-2} - (\nu+20\ {\rm Hz})^{-2}) (\nu+\delta\nu)^{-2})  )^{-1}$ pc cm$^{-3}$. Where $\nu$ is the bottom frequency of the current observing band band, $\delta\nu=20\pm50\%$ Hz  and $t_{\rm res}$ is a user-defined effective time resolution. \item \textbf{SKA1-SYS\_REQ-XXXX:} \textit{\bf SKA1\_Low Pulsar Timing incoherent dedispersion DM limits.} The SKA1\_Low when in pulsar timing mode shall time pulsars using incoherent dedispersion up to a dispersion measure defined by ${\rm DM} = t_{\rm res}(4.15\times10^3 (\nu^{-2} - (\nu+1\ {\rm Hz})^{-2}) (\nu+\delta\nu)^{-2})  )^{-1}$ pc cm$^{-3}$. Where $\nu$ is the bottom frequency of the current observing band band, $\delta\nu=1\pm50\%$ Hz  and $t_{\rm res}$ is a user-defined effective time resolution. \end{itemize}   
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