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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 supportssemi-coherent and  incoherent (and possibly semi-coherent)  dedispersion out to considerably higher DMs.Semi-coherent dedispersion here refers to the process of partially removing dispersive smear coherently such that there is a reduced residual dispersive smear per channel after incoherent dedispersion. The amount of dedispersion to be removed coherently would be defined by the the two requirements outlined above. Remaining dispersion would then be dealt with using incoherent dedispersion.  We do not present here a scientific or technical justification for incoherent dedispersion limits however rudimentary calculations based on the residual dispersive smearing in a single frequency after fine channelisation by to  between 1k and 64k channels shows the following to be reasonable: \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 of 6,000 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} 
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