In order to take an accurate sextant sight, we need to have precise time, ideally within 1 second. The Earth revolves at 360deg/24hours = 15deg/1hour = 15min_arc/min_time = 1min_arc = 4secs_time. A shortwave receiver can be used to get this time using international time stations.
WWV UTC Shortwave Time Station
In Toronto, time stations CHU Ottawa (Ref.1) or WWV Boulder Colorado (Ref.2) can be received. CHU transmits on 3.33MHz/7.85MHz/14.67MHz and WWV on 2.5MHz/5.0MHz/10MHz/15MHz/20MHz/25MHz (experimental). In Costa Rica, WWV can be received. Figure.1 shows the web interface of NIST the US Government agency that runs WWV. Figure 2 shows the receive setup of the RTL-SDR version 2 receiver with HamItUp HF converter (Ref.3). Figure 3 shows one of the local residents listening to the broadcast.
#1. – “NRC Shortwave Station Broadcasts CHU”
#2. – “Radio Station WWV”
#3. – “RTL-SDR for HF 0-30MHz”