Check in your power settings - many computers have an option to disable USB ports or other ports automatically when entering a lower power state. Disabling this option should do it. Or you can set the Windows power scheme to Always On which keeps the CPU and everything running at full speed at all times and disables any power saving, I'm pretty sure. Setting to Home/Office or Laptop allows the CPU to scale down and may start disabling ports if it thinks not in use or is set to time out.
If you don't see the option to stop USB from sleeping in power, and are running Windows XP:
Go to Start, Run and type devmgmt.msc, hit enter.
The Device Manager should open. Scroll down to the USB at the bottom.
Open "USB Root Hub" properties (there are probably several of these listed).
Look for the Power tab and note the devices it shows attached.
On one of the root hubs you should see the Airnav Radar Box listed.
Now go over to the Power Management tab on that hub and clear the check mark in "Allow the computer to turn this device off to save power."
Close Device Manager and it should prevent XP from putting that USB connection to sleep. Not sure if you need to reboot but probably do for it to take effect.
Note that this may shorten battery life slightly on a laptop but can do no other harm.
