Add support for automatic frequency calibration on server side

README.md

@@ -35,7 +35,10 @@ All work was done on ESP32-WROOM with custom made LoRa shield, if your ESP32 boa
 - consider minimum decode level based on on BW + SF ![alt text](images/bandwidth_vs_sf.jpg)
 - use 80 MHz ESP32 frequency in Arduino, it will prolong battery life when operating portable, higher CPU speed is not required, there are no CPU intensive operations
 - uses LoRa **built-in checksum** calculation to drop broken packets
-- note, that there a is **significant frequency drift** on temperature changes for different modules, you need to use **external TCXO** if you are planning to use modules for narrow bandwidths less than 125 kHz or calibrate clients based on server frequency drift report by changing **LORAPRS_FREQ**, for example, let client and server run for an 30-60 minutes and if server reports err: -1500, then set client frequency to about 1000 kHz less, e.g. instead of 433.775 set it to 433.774, this will give couple of additional dB
+- note, that there a is **significant frequency drift** on temperature changes for different modules
+  - you need to use **external TCXO** if you are planning to use modules for narrow bandwidths less than 125 kHz 
+  - or calibrate clients based on server frequency drift report by changing **LORAPRS_FREQ**, for example, let client and server run for an 30-60 minutes and if server reports err: -1500, then set client frequency to about 1000 kHz less, e.g. instead of 433.775 set it to 433.774, this will give couple of additional dB
+  - alternatively automatic calibration could be done on server side by enabling automatic frequency correction by setting **LORAPRS_FREQ_CORR** to **true**, might be suitable for experiments where only one client is operating
 
 # Test Results
 ![alt text](images/setup.png)

esp32_loraprs.ino

@@ -21,12 +21,14 @@
 #define LORAPRS_WIFI_KEY    ""
 #define LORAPRS_LOGIN       "NOCALL-0"
 #define LORAPRS_PASS        "00000"
+#define LORAPRS_FREQ_CORR   false
 #else
 #define LORAPRS_BT_NAME     ""
 #define LORAPRS_WIFI_SSID   "<mywifi>"
 #define LORAPRS_WIFI_KEY    "<key>"
 #define LORAPRS_LOGIN       "NOCALL-0"
 #define LORAPRS_PASS        "00000"
+#define LORAPRS_FREQ_CORR   false
 #endif 
 
 LoraPrs loraPrs(
@@ -35,7 +37,8 @@ LoraPrs loraPrs(
   LORAPRS_WIFI_SSID, 
   LORAPRS_WIFI_KEY,
   LORAPRS_LOGIN,
-  LORAPRS_PASS);
+  LORAPRS_PASS,
+  LORAPRS_FREQ_CORR);
 
 auto watchdogLedTimer = timer_create_default();
 

loraprs.cpp

@@ -1,9 +1,10 @@
 #include "loraprs.h"
 
-LoraPrs::LoraPrs(int loraFreq, const String & btName, const String & wifiName, 
-  const String & wifiKey, const String & aprsLoginCallsign, const String & aprsPass) 
+LoraPrs::LoraPrs(long loraFreq, const String &btName, const String &wifiName, const String &wifiKey, 
+  const String &aprsLoginCallsign, const String &aprsPass, bool autoCorrectFreq) 
   : serialBt_()
   , loraFreq_(loraFreq)
+  , autoCorrectFreq_(autoCorrectFreq)
   , btName_(btName)
   , wifiName_(wifiName)
   , wifiKey_(wifiKey)
@@ -27,7 +28,7 @@ void LoraPrs::setup()
   setupBt(btName_);
 }
 
-void LoraPrs::setupWifi(const String & wifiName, const String & wifiKey) 
+void LoraPrs::setupWifi(const String &wifiName, const String &wifiKey) 
 {
   if (wifiName.length() != 0) {    
     Serial.print("WIFI connecting to " + wifiName);
@@ -78,7 +79,7 @@ void LoraPrs::setupLora(int loraFreq)
   Serial.println("ok");  
 }
 
-void LoraPrs::setupBt(const String & btName)
+void LoraPrs::setupBt(const String &btName)
 {
   if (btName.length() != 0) {
     Serial.print("BT init " + btName + "...");
@@ -107,7 +108,7 @@ void LoraPrs::loop()
   delay(10);
 }
 
-void LoraPrs::onAprsReceived(const String & aprsMessage)
+void LoraPrs::onAprsReceived(const String &aprsMessage)
 {
   if (WiFi.status() == WL_CONNECTED) {
     WiFiClient wifiClient;
@@ -149,7 +150,7 @@ String LoraPrs::decodeCall(byte *rxPtr)
   return result;
 }
 
-String LoraPrs::convertAX25ToAprs(byte *rxPayload, int payloadLength, const String & signalReport)
+String LoraPrs::convertAX25ToAprs(byte *rxPayload, int payloadLength, const String &signalReport)
 {
   byte *rxPtr = rxPayload;
   String srcCall, dstCall, rptFirst, rptSecond, result;
@@ -228,6 +229,7 @@ void LoraPrs::onLoraReceived(int packetSize)
 
   float snr = LoRa.packetSnr();
   float rssi = LoRa.packetRssi();
+  long frequencyError = LoRa.packetFrequencyError();
 
   String signalReport = String(" ") +
     String("rssi: ") +
@@ -237,9 +239,12 @@ void LoraPrs::onLoraReceived(int packetSize)
     String(snr) +
     String("dB, ") +
     String("err: ") +
-    String(LoRa.packetFrequencyError()) + 
+    String(frequencyError) +
     String("Hz");
 
+  if (autoCorrectFreq_)
+    loraFreq_ -= frequencyError;
+
   String aprsMsg = convertAX25ToAprs(rxBuf, rxBufIndex, signalReport);
 
   if (aprsMsg.length() != 0) {

loraprs.h

@@ -11,16 +11,16 @@
 class LoraPrs
 {
 public:
-  LoraPrs(int loraFreq, const String & btName, const String & wifiName, 
-    const String & wifiKey, const String & aprsLoginCallsign, const String & aprsPass);
+  LoraPrs(long loraFreq, const String &btName, const String &wifiName, const String &wifiKey, 
+    const String &aprsLoginCallsign, const String& aprsPass, bool autoCorrectFreq);
   
   void setup();
   void loop();
 
 private:
-  void setupWifi(const String & wifiName, const String & wifiKey);
+  void setupWifi(const String &wifiName, const String &wifiKey);
   void setupLora(int loraFreq);
-  void setupBt(const String & btName);
+  void setupBt(const String &btName);
 
   void reconnectWifi();
   
@@ -30,7 +30,7 @@ private:
   
   void kissResetState();
 
-  String convertAX25ToAprs(byte *rxPayload, int payloadLength, const String & signalReport);
+  String convertAX25ToAprs(byte *rxPayload, int payloadLength, const String &signalReport);
   String decodeCall(byte *rxPtr);
   
 private:
@@ -77,7 +77,8 @@ private:
   const String CfgAprsHost = "rotate.aprs2.net";
 
 private:
-  int loraFreq_;
+  long loraFreq_;
+  bool autoCorrectFreq_;
   String btName_;
   String wifiName_;
   String wifiKey_;