Below link is about how celestial navigation works.  https://www.airspacemag.com/flight-today/how-things-work-celestial-navigation-2640112/

      On September 3rd, LEEYOUNG SND, developer of Korea digital sextant and Incheon National Maritime High School had a ceremony for donating educational equipment(Korea Digital Sextant) and signing an industry-academic cooperation agreement. It was posted on the website of Incheon Maritime High School as below link. http://inm.icehs.kr/boardCnts/updateCnt.do?boardID=220211&viewBoardID=220211&boardSeq=28858032&lev=0&action=view&searchType=null&statusYN=W&page=1&m=0604&s=inm

  Below link is about the Ian Rivers who is a former SAS soldier. https://www.bbc.com/news/uk-england-hereford-worcester-57126645 He aims to raise £500,000 by rowing the Atlantic Ocean alone. He decided not to use GPS and rely on celestial navigation with stars and sextants.

       VIQ7.0.05_OCIMF (18 Feb.2019) 4.17 Are the master and deck officers aware of the requirements of Electronic Chart Display and Information System (ECDIS) and does the system fitted meet SOLAS and flag state requirements? Frequent checks should be made of the ECDIS position fixing system (normally GPS) by the use of other means. Such checks should include: - Parallel indexing and use of clearing bearings; - Use of radar to check the accuracy of the charted position by comparing the location of the radar target against the charted symbol, including the use of radar overlay if fitted. - Visual cross bearings; 4.20 Was a comprehensive berth to berth passage plan available for the previous voyage and  were the deck officers aware of position fixing requirements including the use of parallel indexing both at sea and during pilotage? The following should be marked on the chart, where it enhances safe navigation: • Parallel indexing (not from floating objects unless they have been first

  VIQ 6 (2014 Rev 1.2) established by OCIMF dated on 1 st February, 2016   Chapter 4 (Navigation) 4.26 Was position fixing including the use of parallel indexing satisfactory throughout the previous voyage and the frequency of plotted fixes in accordance with the passage plan? Good navigational practice demands that the officer of the watch: ■ Cross-checks position-fixes using independent sources of information: this is particularly important when electronic position-fixing systems, such as GPS are used as the primary means of fixing the position of the ship; ■ Uses visual navigation aids to support electronic position-fixing methods , i.e. landmarks in coastal areas and celestial navigation in open waters ; ■ Does not become over reliant on automated navigational equipment, including electronic chart systems, thereby failing to make proper navigational use of visual information; Notes: At least two methods of position fixing should be charted, where possible. Vi

    Below link is about the article for the Apollo sextant in space. https://astronomy.com/news/2018/06/the-story-of-the-apollo-sextant The Sextant was used during the Apollo program. The Sextant does not require power and operates independently of other navigation systems, so it can be used as a fail-safe in case of power and/or communication failures.  The Apollo Sextant did just this. This device was used repeatedly throughout the Apollo program.  

Below link is about the article when satellites don't work. https://www.bbc.com/news/uk-politics-57440787 When satellites are not working, there is no proper communication system to replace them. It also mentions jamming and spoofing as risks to using satellite systems. To protect ships, alternatives to this should be prepared.