Midway through his 20-day sail last month, Midshipman (MID) David Teo, who had only a few minutes to call home, almost missed talking to his mother.

Already, the full-time national serviceman on a training deployment had been away from home for three weeks, including one week of isolation in camp before sailing off as part of Covid-19 safety measures.

His mother, unaware he was calling, told her pre-teen son who answered the call that she did not want to be disturbed.

"My 12-year-old brother told her: 'It's kor-kor (Chinese dialect for "older brother"), he has only got a few minutes to talk to you!' and my mother immediately ran to the phone and asked how I was doing," the 19-year-old recounted, as he reflected on how much he missed his family and home.

"The few minutes speaking to my brother and mum on the phone, that was when it hit me - what we sacrifice to keep our nation safe," he told The Straits Times on Monday, a day after returning from the landing ship tank (LST) RSS Endeavour.

LSTs are the largest vessels in the Singapore navy, which has at least 30 vessels, including frigates, which are also used for training midshipmen.

The weeks at sea under the Midshipmen Sea Training Deployment (MSTD) is part of MID Teo's nine-month training to be a naval officer in the Singapore Armed Forces.

There are about 40 of them in his batch, one of three that typically undergo the training deployment each year.

The deployment is among the navy's training and operations that have continued throughout the pandemic, but with some changes to the programme to ensure safety amid Covid-19.

The safety measures include the Endeavour not docking at any port, and, with no shore leave, the deployment was shortened from 28 to 20 days: Nov 9 to 29.

The send-off for the midshipmen was also virtual.

Instead of parents and loved ones sending them off at Changi Naval Base, this time, MID Teo and his fellow trainees sent messages to their families via video, and these were shown on Nov 9 on the Singapore navy's Facebook.

MID Teo laughingly told his family he was on a "cruise to nowhere".

Course commander Major Arens Ong, 32, was quick to stress that training standards have not been compromised. Despite the changes, "they were still able to see that the navy continues to be at the forefront, continues to be ready and engages our foreign partners in exercises", he said.

The midshipmen also took part in two foreign exercises: the Singapore-India-Thailand Maritime Exercise in the Andaman Sea and the Singapore-India Maritime Bilateral Exercise in the Andaman Sea and Indian Ocean.

While on board the Endeavour, the crew do not wear masks as they had undergone an isolation period and tested negative before setting sail.

Their daily routine includes carrying out their duties alongside the ship's crew, who guide them in the operations at sea, such as the firing of small arms and how to respond to contingencies.

Military Expert 2 Chong Hong Qin, 29, navigation chief on the Endeavour, said that by the end of the deployment, he was proud to see that the midshipmen were proficient in skills such as using the navigation radar and keeping a lookout for dangers in surrounding waters.

MID Nisha Sunder, 19, who became a regular after her A levels, said the most challenging part was the packed programme, with only 12 days for practical lessons before going on the foreign exercises, which lasted six days.

"The MSTD gave me a better insight into how practical all the skills and lessons that I learnt were.