Working on README

README.md

@@ -2,13 +2,19 @@
 
 **NEW:** Experimental support for Graw DFM-17 radiosondes added! Please test and report any issues!
 
+**NOTE:** **DFM-17 radiosondes require a GPS lock (and clear visibility to the sky) to calibrate its internal oscillator.**
+DFM-17 transmissions, especially APRS, may not decode correctly because of incorrect timing before the internal oscillator has been calibrated.
+
 **NOTE:** While this firmware has been tested (on RS41) with great success on a number of high-altitude balloon
 flights, it is still a work in progress and some features might not work as expected yet!
 In particular, the time sync (scheduling) features and use of an external Si5351 as a transmitter need more testing.
 
-This is a custom, amateur radio-oriented firmware for [Vaisala RS41](https://www.vaisala.com/en/products/weather-environmental-sensors/upper-air-radiosondes-rs41-rs41-e-models)
+## What is RS41ng?
+
+RS41ng is a custom, amateur radio-oriented firmware for [Vaisala RS41](https://www.vaisala.com/en/products/weather-environmental-sensors/upper-air-radiosondes-rs41-rs41-e-models)
 and [Graw DFM-17](https://www.graw.de/products/radiosondes/dfm-17/) radiosondes. These radiosonde models
 have very similar hardware, so that it is relatively easy to support both with the same codebase.
+It is unlikely that RS41ng could support any other radiosonde hardware for now.
 
 Some code is based on an earlier RS41 firmware project called [RS41HUP](https://github.com/df8oe/RS41HUP),
 but most of it has been rewritten from scratch. The Horus 4FSK code has been adapted from
@@ -16,7 +22,7 @@ the [darksidelemm fork of RS41HUP](https://github.com/darksidelemm/RS41HUP).
 
 ## Asking questions, filing feature requests and reporting issues
 
-* Please use [GitHub discussions](../../discussions) for asking questions and for sharing info about your RS41-based projects
+* Please use [GitHub discussions](../../discussions) for asking questions and for sharing info about your radiosonde-based projects
   * For example, questions about firmware configuration and connecting of external chips to the sonde belong here
 * Please use [GitHub issues](../../issues) to file new feature requests or issues that you have already identified with the firmware
   * However, please remember to post questions about usage to [GitHub discussions](../../discussions)
@@ -106,6 +112,15 @@ Features available on RS41 hardware only:
 * GPS NMEA data output via the external serial port pin 3 (see below). This disables use of I²C devices as the serial port pins are shared with the I²C bus pins.
   * This allows using the RS41 sonde GPS data in external tracker hardware, such as Raspberry Pi or other microcontrollers.
 
+Notes for DFM-17:
+
+* **DFM-17 radiosondes require a GPS lock (and clear visibility to the sky) to calibrate its internal oscillator.**
+  This is necessary, because the internal oscillator is not particularly accurate.
+  DFM-17 transmissions, especially APRS, may not decode correctly because of incorrect timing before
+  the internal oscillator has been calibrated.
+  * The RS41 radiosonde hardware uses an external oscillator, which is more stable, so RS41 does not
+    suffer from the same issue.
+
 ### Transmission modes
 
 On the internal Si4032 (RS41) and Si4063 (DFM-17) transmitters: