# Pulsars and neutron stars/Pulsar based time standards
## Introduction

The pulsar timing method relies on the measurement of pulsar arrival times. Such arrival times are measured with an observatory clock. Of course, no clock is perfect and so a series of corrections are applied to the measured times to "transfer" the time to a realisation of terrestrial time (TT). The aim of pulsar-based time standards is to search for irregularities in the realisations of terrestrial time. If such an irregularity exists then it will affect all pulsars equally; the timing residuals for all pulsars will exhibit the irregularity in the terrestrial time realisation.
Numerous groups have attempted to detect such irregularities. To date, no significant errors have been found in the BIPM realisation of Terrestrial Time.

## Terrestrial time standards and terminology

* International Atomic Time, TT(TAI). The stability of TAI
* Terrestrial time as realised by the BIPM, TT(BIPM)
* EAL
Describe how to access the time standards and how they are formed.
Show plot of the difference between TAI and BIPM with and without a quadratic removal

## Developing a pulsar-based time standard
### Using the pulsar pulse

Pulsar-based time standards are created by inspecting the timing residuals of multiple pulsars. If the data spans for all pulsars were identical and the same fitting procedure is applied to all pulsars then the pulsar-based clock signal will simply be the weighted sum of the residuals. However, for real data sets, the pulsars are not simultaneously observed, they have different data spans, the timing model includes arbitrary phase offsets and different pulsars require different timing model fits.
Numerous authors have attempted to form a pulsar-based time scale (Guinot & Petit 1991, Petit & Tavella 1996, Rodin 2008, Rodin & Chen 2011). More recently, Hobbs et al. (2012) made use of the global, constrained fitting routines within the TEMPO2 software package to fit for a common signal present in all pulsars.

### Using the orbital motion as a clock
## Contaminants of the pulsar time standard

Any signal that is correlated between different pulsar timing residuals will be "detected" as an error in the terrestrial time standard.
* Converting from the observatory to terrestrial time. It is necessary to convert pulse arrival times that were measured at an observatory to Terrestrial Time. Any error in that correction will induce identical timing residuals in other pulsars that were observed using the same observatory clock. Hobbs et al. (2012) suggest that this correction is usually known to a few nanoseconds. However, the most direct test is to observe the same pulsar at multiple observatories and search for signals that are only seen at a single observatory.
* 'Converting from Terrestrial Time to Barycentric Time. The pulsar timing method relies on the conversion from Terrestrial Time to Barycentric time. This is currently carried out using a time ephemeris presented by Irwin & Fukushima (1999). They report that their time ephemeris is accurate at the ~5ns level.
* Astrophysical phenomena. Correlated timing residuals can occur from the interstellar medium, errors in the planetary ephemeris or even from gravitational waves. Current research efforts are attempting to determine the possible contaminations between different physical affects (see e.g., Tiburzi et al. 2015).

## Using a pulsar-based time standard

Currently no significant errors have been detected in terrestrial time standards. However, with new timing observations of more pulsars with high timing precision pulsar time standards will soon significantly improve in sensitivity. Hobbs et al. (2012) also noted that a pulsar-based timescale provides: