diff --git a/LightAPRS-pico-balloon/LightAPRS-pico-balloon.ino b/LightAPRS-pico-balloon/LightAPRS-pico-balloon.ino
index bd783f5..0b59032 100644
--- a/LightAPRS-pico-balloon/LightAPRS-pico-balloon.ino
+++ b/LightAPRS-pico-balloon/LightAPRS-pico-balloon.ino
@@ -75,6 +75,7 @@ uint32_t GEOFENCE_no_tx               = 0;
 
 boolean radioSetup = false;
 boolean GpsFirstFix=false;
+boolean ublox_high_alt_mode_enabled = false; //do not change this.
 
 static char telemetry_buff[100];// telemetry buffer
 uint16_t TxCount = 1;
@@ -161,6 +162,7 @@ void loop() {
       updateTelemetry();
       
       GpsOFF;
+      ublox_high_alt_mode_enabled = false; //gps sleep mode resets high altitude mode.
       GpsFirstFix=true;
       //APRS frequency isn't the same for the whole world. (for pico balloon only)
       if (!radioSetup) {
@@ -209,13 +211,19 @@ void aprs_msg_callback(struct AX25Msg *msg) {
 }
 
 void sleepSeconds(int sec) {  
-  if(GpsFirstFix)GpsOFF;//sleep gps after first fix
+  if (GpsFirstFix){//sleep gps after first fix
+      GpsOFF;
+      ublox_high_alt_mode_enabled = false;//gps sleep mode resets high altitude mode.
+    } 
   RfOFF;
   RfPttOFF;
   Serial.flush();
   wdt_disable();
   for (int i = 0; i < sec; i++) {
-    if(readBatt() < GpsMinVolt) GpsOFF;  //(for pico balloon only)
+    if (readBatt() < GpsMinVolt){
+      GpsOFF;
+      ublox_high_alt_mode_enabled = false;//gps sleep mode resets high altitude mode.
+    } 
     LowPower.powerDown(SLEEP_1S, ADC_OFF, BOD_ON);   
   }
    wdt_enable(WDTO_8S);
@@ -561,6 +569,18 @@ int getDifference(Date dt1, Date dt2)
 static void updateGpsData(int ms)
 {
   GpsON;
+
+  if(!ublox_high_alt_mode_enabled){
+      //enable ublox high altitude mode. increase ublox max. altitude limit from 12.000 meters to 50.000 meters.
+      delay(100);
+      setGPS_DynamicModel6();
+      #if defined(DEVMODE)
+        Serial.println(F("ublox DynamicModel6 enabled..."));
+      #endif      
+      ublox_high_alt_mode_enabled = true;
+      
+   }
+  
   while (!Serial1) {
     delayMicroseconds(1); // wait for serial port to connect.
   }
@@ -701,4 +721,80 @@ static void printStr(const char *str, int len)
 #endif
 }
 
+//following GPS code from : https://github.com/HABduino/HABduino/blob/master/Software/habduino_v4/habduino_v4.ino
+void setGPS_DynamicModel6()
+{
+ int gps_set_sucess=0;
+ uint8_t setdm6[] = {
+ 0xB5, 0x62, 0x06, 0x24, 0x24, 0x00, 0xFF, 0xFF, 0x06,
+ 0x03, 0x00, 0x00, 0x00, 0x00, 0x10, 0x27, 0x00, 0x00,
+ 0x05, 0x00, 0xFA, 0x00, 0xFA, 0x00, 0x64, 0x00, 0x2C,
+ 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x16, 0xDC };
+ 
+ while(!gps_set_sucess)
+ {
+ sendUBX(setdm6, sizeof(setdm6)/sizeof(uint8_t));
+ gps_set_sucess=getUBX_ACK(setdm6);
+ }
+}
 
+void sendUBX(uint8_t *MSG, uint8_t len) {
+ Serial1.flush();
+ Serial1.write(0xFF);
+ delay(500);
+ for(int i=0; i<len; i++) {
+ Serial1.write(MSG[i]);
+ }
+}
+boolean getUBX_ACK(uint8_t *MSG) {
+ uint8_t b;
+ uint8_t ackByteID = 0;
+ uint8_t ackPacket[10];
+ unsigned long startTime = millis();
+ 
+// Construct the expected ACK packet
+ ackPacket[0] = 0xB5; // header
+ ackPacket[1] = 0x62; // header
+ ackPacket[2] = 0x05; // class
+ ackPacket[3] = 0x01; // id
+ ackPacket[4] = 0x02; // length
+ ackPacket[5] = 0x00;
+ ackPacket[6] = MSG[2]; // ACK class
+ ackPacket[7] = MSG[3]; // ACK id
+ ackPacket[8] = 0; // CK_A
+ ackPacket[9] = 0; // CK_B
+ 
+// Calculate the checksums
+ for (uint8_t ubxi=2; ubxi<8; ubxi++) {
+ ackPacket[8] = ackPacket[8] + ackPacket[ubxi];
+ ackPacket[9] = ackPacket[9] + ackPacket[8];
+ }
+ 
+while (1) {
+ 
+// Test for success
+ if (ackByteID > 9) {
+ // All packets in order!
+ return true;
+ }
+ 
+// Timeout if no valid response in 3 seconds
+ if (millis() - startTime > 3000) {
+ return false;
+ }
+ 
+// Make sure data is available to read
+ if (Serial1.available()) {
+ b = Serial1.read();
+ 
+// Check that bytes arrive in sequence as per expected ACK packet
+ if (b == ackPacket[ackByteID]) {
+ ackByteID++;
+ }
+ else {
+ ackByteID = 0; // Reset and look again, invalid order
+ }
+ }
+ }
+}