5.6 Methods of Large-Scale Navigation
353
Fig. 5.10 Schematic diagram of pulse phase ambiguity between pulsar and spacecraft
pulse period signal makes the measurement of the pulse arrival-time produce the
problem of phase ambiguity. Which pulse cycle radiated from a pulsar is arriving at
the spacecraft? The ambiguity of pulse phase from the pulsar to spacecraft is shown
in Fig. 5.10, from which for an epoch t, the total pulse phase can be expressed as
(t) = N (t 0 ) + N (t − t 0 ) + φ(t),
(5.108)
where t 0 is the initial observation epoch; N(t 0 ) is the integer ambiguity of pulse phase
at the initial observation epoch; N(t-t 0 ) is the integer of pulse phase cycle counts from
t 0 to t; φ(t) is the fractional part of the phase within a pulse cycle at epoch t.
If the pulse signal emitted from the pulsar is detected at the initial observation
epoch t 0 , and always tracked and measured continuously, then the N(t-t 0 ) can automatically be counted by the detector system. Moreover, the phase φ(t) within a pulse
cycle is also observable. Consequently, only N(t 0 ) is an unknown number related to
the initial observation epoch, pulsar’s position and spacecraft’s position, with integer
property. In the entire observation process, as long as the pulsar is continuously
tracked and measured, the N(t 0 ) is always a constant.
In fact, the number of the integer cycles from the pulsar to spacecraft need not
to be investigated in the XPNAV, but the difference value of integer cycles of the
spacecraft relative to the SSB or certain known reference point. Therefore, the integer