A session lasts twenty minutes. A session doesn't start until you click on the map to request a line-of-sight computation. (So you can work on your tower list, for example, without starting a session.)
You can use the trial sessions with any number of towers.
After you've used those sessions,
if you have 20 or fewer
towers, buy more sessions for $1 per session (minimum purchase $10)
if you have 100 or fewer towers,
a subscription at $100 per month
if you have more than 100 towers, please
If you have 100 or fewer towers, you can skip the trial offer and set up a PayPal subscription. The price is $100/month.
If you have five or fewer towers, you can use the site
for free. Towers will be stored in a cookie on your machine.
Use the URL