Update README for USB

README.md

@@ -1,31 +1,49 @@
 # Navpoint Desktop
 
-This is a simple Python program that will run two HTTP servers. One listens on
-the computer's local IP address (e.g. `192.168.x.x`), port 8888, to receive
-location updates from the [Navpoint
-Mobile](https://gallery.appinventor.mit.edu/?galleryid=2c18ee4d-4eed-452a-9228-de8e813820d1)
-Android app. The other listens on `127.0.0.1` (i.e. `localhost`), also on port
-8888, for connections from a digital globe program such as [Google Earth
-Pro](https://www.google.com/earth/about), allowing that program to track the
-user's location. (Navpoint is not developed or endorsed by Google.)
+Navpoint makes the user's current location available in KML format over
+a local HTTP server (listening on `127.0.0.1`; external devices cannot
+connect). Its main purpose is to allow GPS tracking from a digital globe
+program such as [Google Earth Pro](https://www.google.com/earth/about).
+(Navpoint is not developed or endorsed by Google.)
 
-Navpoint Desktop has a minimal GUI consisting only of a QR code. Simply scan
-the code in Navpoint Mobile to establish a connection. The phone and computer
-must be on the same Wi-Fi network; if a router is not available, enabling
-mobile hotspot on the phone and connecting the computer to that network should
-work well. An Internet connection is not needed; Navpoint works over a LAN.
-
-Navpoint Desktop makes the geographic data available in KML format at
+Navpoint makes the geographic data available in KML format at
 `http://127.0.0.1:8888/navpoint.kml`. The `navpoint_link.kml` file in this
 repository contains a link to this URL configured to update every three
 seconds; simply open this file in Google Earth or another compatible program to
 connect to Navpoint Desktop.
 
+## Connection modes
+
+Navpoint can obtain location information from two different sources. The first,
+recommended where possible, is from a USB GPS receiver that supports the [NMEA
+0183](https://en.wikipedia.org/wiki/NMEA_0183) protocol. (A handheld GPS
+receiver connected to a computer may or may not work; it does not work with my
+Garmin GPSMAP 64st. Any common USB GPS dongle, such as [this one available from
+Amazon](https://www.amazon.com/VK-162-G-Mouse-External-Navigation-Raspberry/dp/B01EROIUEW/)
+should work, although, of course, I cannot guarantee that any particular device
+will be compatible with Navpoint. On Windows, USB mode should work
+out-of-the-box with a compatible GPS receiver; on Linux, your user account
+needs to be a member of the `dialout` group. If you get "Permission denied"
+errors from Navpoint, run `sudo usermod -a -G dialout $USER`, then log out and
+back in.
+
+Alternatively, you may use the [Navpoint
+Mobile](https://gallery.appinventor.mit.edu/?galleryid=2c18ee4d-4eed-452a-9228-de8e813820d1)
+Android app. This uses an Android phone's GPS receiver to obtain location data;
+however, it is typically less accurate than a USB module, and more likely to
+have problems. Navpoint Desktop runs a second HTTP server listening on the
+computer's local IP address (e.g. `192.168.x.x`), and the phone connects to
+this server and sends location updates via HTTP requests. If you want to use
+Navpoint Mobile, choose that option when opening Navpoint Desktop, then scan
+the QR code from Navpoint Mobile. (Make sure that your phone and computer are
+on the same Wi-Fi network, or that your phone's hotspot is on and your computer
+is connected to it.)
+
 ## Licensing
 
 Navpoint Desktop is released under the [GNU General Public License
 v3.0](https://www.gnu.org/licenses/gpl-3.0.en.html), except the files
-`navpoint/get_ip.py` and `navpoint.fix_path`, which are released under
+`navpoint/get_ip.py` and `navpoint/fix_path.py`, which are released under
 [Creative Commons Attribution-ShareAlike 4.0
 International](https://creativecommons.org/licenses/by-sa/4.0/) due to use of
 code published on Stack Overflow