Much of his war time service was spent with RAF Coastal Command operating on convoy protection duties.
On the first serious test of the system in 1942, a Wellington piloted by Greswell, located two U-boats and seriously damaged both with depth charges and machinegun fire.
[2] On his return from a detachment in the United States, where he had overseen American manufacturers on the fitting of the Leigh Light system in Liberators and trained aircrew in its use, Greswell was promoted to wing commander and posted to No.
Upon recall in the 1950s he was promoted to group captain and his post war appointments included involvement in the air support planning for the British forces landings in Egypt during the Suez crisis in 1956.
[2] On promotion to air commodore in June 1964, Greswell was appointed as the 11th Commandant of the Royal Observer Corps during a period which saw the greatest reorganisation and upheaval in the organisation's history.