This memo aims to address and update the following level 1 system requirements for pulsar timing with SKA1_Low and SKA1_Mid:

**SKA1-SYS_REQ-2961:**The SKA1_Mid Pulsar timing mode shall have a timing resolution of better than 100 ns.**SKA1_Mid Pulsar Timing resolution.****SKA1-SYS_REQ-2962:**The SKA1_Low Pulsar timing mode shall have a timing resolution of better than 100 ns.**SKA1_Low Pulsar Timing resolution.**

We will approach this in two ways: Firstly we will look at the maximum timing resolution that can be envisaged as being required for pulsar timing with SKA1 and secondly we will consider how multi-path scattering imposes further limits on timing resolution as a function of dispersion measure.

Figure \ref{fig:pulse_spectra} displays fluctuation power spectra derived from the long-term average profiles of two Parkes Pulsar Timing Array (PPTA) pulsars observed at 20 and 50 cm (Dai 2015). In these plots, the fluctuation power of each pulsar drops exponentially as a function of spin harmonic, eventually hitting a white noise floor. PSR J0437\(-\)4715 and PSR J2241\(-\)5236 are the worst case examples in which fluctuation power approaches the Nyquist limit where spin harmonics will be aliased. These plots indicate that 1024 phase bins is sufficient to resolve all of the structure in the mean pulse profiles of the currently known MSPs used for high-precision timing.

Greater telescope sensitivity increases the signal-to-noise ratio, causing more fluctuation power to rise out of the white noise floor; however, these plots indicate that the number of spin harmonics required to resolve all significant power is proportional only to the logarithm of the S/N; i.e. to first order, the maximum harmonic,

\[H_\mathrm{max} \sim (1 - \log S/N) / k,\]

where \(k\) is the slope of the line fit to the spectrum of the pulsar, \(S/N\) is the signal-to-noise ratio at the y-intercept (approximately the \(S/N\) in the first harmonic) and it is assumed that harmonics past \(S/N\sim1\) are not important. By eye, increasing the S/N by two orders of magnitude will add around 100 to 300 harmonics for 0437 and 2241, so 2048 phase bins should suffice for observing these pulsars with SKA1.