Hi Jay!

In Nov 1940 the Submarine Tracking Room was fed with reports from
DF Stations located at 7 widely seperated sites,from the Shetlands
to Land´s End,and also at Gibraltar.These were later supplemnted by
new stations in Iceland and the Azores.With a good spread of bearings
from the majority of sites a transmitting U-boat could be located to within
about 25 miles.

Later in the war,after entry of USA,one might think every thing should go
smother.But the HF/DF stations in England and Africa provided better fixes
than those in Canada and USA,because the American coastline is roughly
parallell to the transatlantic convoy route that the U-boats travelled as well.

The Canadians developed a devise whereby bearings were recorded semi-
automatically.As a further improvement USN developed a machine wich plotted
the bearings geometrically.

A good example of effectiviness of land-based HUFF/DUFF is from 30 June -42.
At noon that date,DF-ranges were obtained by the stations at Bermuda,
Hartlant Point,Kingston and Georgetown.Operations officers at N.O.B.Bermuda
plotted the bearings showing the U-boat tobe at 33* N,-67*30´W. about 130
miles WSW of St Georges.
Ltnt Richard E Schreder USNR in a Mariner of Sqdr VP-74 found U-158 within
ten miles of the predicted position,and sank her,leaving no survivors.

In the spring of 1942,more and more ships in bouth RN and USN,obtained
shipborne HF/DF.If only one ship in a convoy had HF/DF,she was made the
rescue ship and sailed in the last place of the middle column.Later it was
possible to equip several ships with it in each convoy they disposed,so as to
obtain cross-bearings on near-by U-boats making transmissions.

Sources:
Padfield:War beneath Sea.Page:104.
Morison:Hist of US naval operations in WW II.Vol.I.Pages:226-228.
Best regards/Anders.