344
5 X-ray Pulsar-Based Navigation: Theories and Experiments
speed of spacecraft and the field of view of the detector. The slower does the spacecraft
rotate, and the larger is the field of view of the detector, the shorter the imaging time
required.
5.6.2 Transformation of Photon Arriving Time
5.6.2.1 Transformation of Timescales
In the XPNAV, the time of the X-ray photons radiated from pulsars arriving at the
spacecraft is measured by the onboard atomic clock in the spacecraft-body coordinate
system, which is the proper time. The process of transforming the proper time of the
X-ray photons arriving at the spacecraft into the TCB or TDB is known as timescale
transformation. The flow chart of the transformation relations among the proper
time and several commonly used timescales is shown in Fig. 5.9.
Without loss of generality, taking the spacecrafts orbiting the Earth as an example,
the scale transformation of arrival-time of the photons is discussed here. Supposed
that the time of the photons arriving at the spacecrafts is measured with the GPST,
the specific transformation steps and formulas are as follows.
Fig. 5.9 Transformations between proper time and commonly used timescales