A long hidden airport command line utility buried deep in Mac OS X can be used to scan for and find available wireless networks
To do this, the first thing you’ll want to do is create a symbolic link from the airport utility to /usr/sbin for easy access. Launch the Terminal and type the following command:
sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/sbin/airport
The above command must appear on a single line to work properly. Enter the administrator password to create the symbolic link, which functions as an alias would in the Finder. Now you can use the airport command without the lengthy path to access it.
Now, to scan for and find all wireless networks within range, type the following:
The list returned will show all available wifi networks and their router name (SSID), the router address (BSSID), signal strength (RSSI), channel, and security types used by the network.
By watching the output of airport -s and the RSSI strength, you could use the airport command line tool in a similar fashion to the Wi-Fi Diagnostics utility to optimize a wireless connection.
You can also get much of the same detailed information from the Wi-Fi menu by holding the Option key on click, although that will only show you details of one access point at a time.