  (abridged). We outline LBCS (the LOFAR Long-Baseline Calibrator Survey),
whose aim is to identify sources suitable for calibrating the
highest-resolution observations made with the International LOFAR Telescope,
which include baselines >1000 km. Suitable sources must contain significant
correlated flux density (50-100mJy) at frequencies around 110--190~MHz on
scales of a few hundred mas. At least for the 200--300-km international
baselines, we find around 1 suitable calibrator source per square degree over a
large part of the northern sky, in agreement with previous work. This should
allow a randomly selected target to be successfully phase calibrated on the
international baselines in over 50% of cases. Products of the survey include
calibrator source lists and fringe-rate and delay maps of wide areas --
typically a few degrees -- around each source. The density of sources with
significant correlated flux declines noticeably with baseline length over the
range 200--600~km, with good calibrators on the longest baselines appearing
only at the rate of 0.5 per square degree. Coherence times decrease from 1--3
minutes on 200-km baselines to about 1 minute on 600-km baselines, suggesting
that ionospheric phase variations contain components with scales of a few
hundred kilometres. The longest median coherence time, at just over 3 minutes,
is seen on the DE609 baseline, which at 227km is close to being the shortest.
We see median coherence times of between 80 and 110 seconds on the four longest
baselines (580--600~km), and about 2 minutes for the other baselines. The
success of phase transfer from calibrator to target is shown to be influenced
by distance, in a manner that suggests a coherence patch at 150-MHz of the
order of 1 degree.
