You can track trade by watching the density of radio traffic from the shipping lanes. This is in some ways a better indicator than the Baltic Dry Index. I was

You can track trade by watching the density of radio traffic from the shipping lanes. This is in some ways a better indicator than the Baltic Dry Index. I was trying to do what these guys did 10 years ago - but the night images data was expensive and came off a defense meteorological satellite. Found a guy at NOAA who had done the work before me and was distributing it as a lights at night study. He thought this technique of estimating GDP had issues because different regions have rules and special conditions that cause a broad difference in light levels (e.g. Denmark has particularly strict light pollutions regulations) that do not always compose at the national level. I'll be interested in these guys results to see how they addressed the problems.