I definitely would go for Level 5 of the Terminal B parking garage. From up there you can see all runways, all activity. If the weather is too cold they have the elevator waiting areas with windows, that are usually a bit warmer than outside. During the Spring and Summer, if the wind is blowing in the right direction the planes will be landing on Runway 15R (and you're not driving) got to one of the terminals and take the #77 bus to the satellite car park. It is next to the threshhold of Runway 15R. The best time to spot is between 11.30-19.00 during the summer. You will see virtually ALL international traffic. The Satellite Car Park is also near Terminal E (international). The only problem is that it is a car park, so there are no toilets, and if you have to go, the bus ride is about 5-10 min, and it is VERY isolated. These are the BEST places to spot. Castle Island is good, but it is a bit too far for my liking.
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