  This paper reports on single-pulse radio observations of PSR B1702-19 and
their implications for pulsar emission theories. These observations were made
with the Westerbork Synthesis Radio Telescope at 1380 and 328 MHz. The PA-swing
is used to constrain possible geometries of the pulsar and the single-pulse
data is analysed for subpulse modulation correlations between the main pulse
and interpulse. We confirm earlier conclusions that the dipole axis of this
pulsar is almost perpendicular to its rotation axis, and report that both its
main pulse and interpulse are modulated with a periodicity around 10.4 times
the pulsar's rotation. Allowing for the half-period delay between main pulse
and interpulse the modulation is found to be precisely in phase. Despite small
secular variations in the periodicity, the phase-locking continues over all
timescales ranging up to several years. The precision of the phase locking is
difficult for current emission theories to explain if the main pulse and
interpulse originate from opposing magnetic poles. We therefore also explore
the possibility of a bidirectional model, in which all the modulated emission
comes from one pole, but is seen from two sides and slightly displaced by
aberration and time-delay. In this model the unmodulated emission is directed
to us from the opposite pole, requiring the emission of the main pulse to
originate from two different poles. This is difficult to reconcile with the
observed smooth PA-swing. Whichever model turns out to be correct, the answer
will have important implications for emission theories.
