diff --git a/firmware/inc/drift.h b/firmware/inc/drift.h
new file mode 100644
index 0000000..2680b03
--- /dev/null
+++ b/firmware/inc/drift.h
@@ -0,0 +1,36 @@
+/*
+ * Monitors xosc drift while transmitting
+ * Copyright (C) 2016  Richard Meadows <richardeoin>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#ifndef DRIFT_H
+#define DRIFT_H
+
+#include "samd20.h"
+#include "si_trx.h"
+
+int16_t drift_get_channel_offset(void);
+void drift_set_fconfig(struct si_frequency_configuration* config);
+void drift_measure_start(void);
+void drift_measure_stop(void);
+
+#endif  /* DRIFT_H */
diff --git a/firmware/inc/xosc.h b/firmware/inc/xosc.h
index cc77c47..052c8ba 100644
--- a/firmware/inc/xosc.h
+++ b/firmware/inc/xosc.h
@@ -28,8 +28,8 @@
 #include "samd20.h"
 
 enum xosc_measurement_t {
-  XOSC_MEASURE_GCLK0,
   XOSC_MEASURE_TIMEPULSE,
+  XOSC_MEASURE_LFTIMER,
 };
 
 typedef void (*measurement_result_t)(uint32_t result);
@@ -54,7 +54,8 @@ void gclk1_init(void);
 void gclk2_init(void);
 
 /** Measurement */
-void measure_xosc(enum xosc_measurement_t measurement_t, measurement_result_t callback);
+void measure_xosc(enum xosc_measurement_t measurement_t, measurement_result_t callback, uint8_t oneshot);
+void measure_xosc_disable(enum xosc_measurement_t measurement_t);
 
 /** LF Timer */
 void lf_tick_start(void);
diff --git a/firmware/src/data.c b/firmware/src/data.c
index edf7ec3..35e71be 100644
--- a/firmware/src/data.c
+++ b/firmware/src/data.c
@@ -55,8 +55,9 @@ void collect_data_async(void)
 #endif  /* GPS_TYPE_UBX */
 
   /* Measure XOSC against gps timepulse */
-  //measure_xosc(XOSC_MEASURE_TIMEPULSE, xosc_measure_callback);
+  /* single shot, ongoing until two rising edges on GPS TIMEPULSE */
   /* NOT IN USE CURRENTLY */
+  //measure_xosc(XOSC_MEASURE_TIMEPULSE, xosc_measure_callback, 1);
 
   /* Analogue Measurements */
   start_adc_sequence();
diff --git a/firmware/src/drift.c b/firmware/src/drift.c
new file mode 100644
index 0000000..4b3c624
--- /dev/null
+++ b/firmware/src/drift.c
@@ -0,0 +1,146 @@
+/*
+ * Monitors xosc drift while transmitting
+ * Copyright (C) 2016  Richard Meadows <richardeoin>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include <math.h>
+
+#include "samd20.h"
+#include "hw_config.h"
+#include "si_trx.h"
+#include "xosc.h"
+#include "drift.h"
+
+/**
+ * Initial reference, in Hz
+ */
+uint32_t initial_xosc_frequency;
+/**
+ * Current offset, in Hz at the xosc output
+ * +ve if xosc too fast, -ve if xosc too slow
+ */
+float current_frequency_offset;
+/**
+ * Current frequency configuration
+ */
+struct si_frequency_configuration* _fconfig = NULL;
+/**
+ * Current offset, in rf channels for the current frequency configuration
+ * +ve if xosc too fast, -ve if xosc too slow
+ */
+volatile int16_t current_channel_offset;
+
+
+/**
+ * Moving average
+ */
+#define MOVING_AVERAGE_N	(8)
+int32_t frequency_moving_average[MOVING_AVERAGE_N];
+uint32_t frequency_moving_average_index = 0;
+
+
+/**
+ * The result is the xosc frequency in Hz
+ */
+void xosc_drift_measure_callback(uint32_t result)
+{
+  int32_t offset;
+
+  if (initial_xosc_frequency == 0) { /* Initial reference */
+    initial_xosc_frequency = result;
+    offset = 0;
+    frequency_moving_average[0] = 0;
+    frequency_moving_average_index++;
+  } else {
+    offset = result - initial_xosc_frequency;
+
+    /* add to moving average */
+    frequency_moving_average[(frequency_moving_average_index % 8)] = offset;
+
+    /* calculate average */
+    uint32_t n = (frequency_moving_average_index > 8) ? 8: frequency_moving_average_index;
+    float sum = 0;
+    for (uint32_t i = 0; i < n; i++) {
+      sum += frequency_moving_average[i];
+    }
+
+    /* record result */
+    current_frequency_offset = sum / n;
+    frequency_moving_average_index++;
+  }
+
+  if (_fconfig != NULL) {       /* if we seem to have a fconfig */
+    float rfband_offset_hz = ((float)current_frequency_offset * _fconfig->frequency)
+      / XOSC_FREQUENCY;
+
+    float channel_offset_f = rfband_offset_hz / _fconfig->lsb_tuning_resolution;
+    current_channel_offset = (int16_t)round(channel_offset_f);
+  }
+}
+
+/**
+ * Returns the current channel delta caused by drift.
+ *
+ * The subtract this from the intended channel to compensate.
+ */
+int16_t drift_get_channel_offset(void)
+{
+  return current_channel_offset;
+}
+
+
+/**
+ * Set the current frequency configuration.
+ *
+ * drift_get_channel_offset will always return zero if this is not set
+ */
+void drift_set_fconfig(struct si_frequency_configuration* config)
+{
+  _fconfig = config;
+  current_channel_offset = 0;
+}
+/**
+ * Start xosc drift measurements
+ */
+void drift_measure_start(void)
+{
+  initial_xosc_frequency = 0;
+  current_frequency_offset = 0;
+  current_channel_offset = 0;
+  frequency_moving_average_index = 0;
+
+  /* start ontinuous measurement (oneshot=0) */
+  measure_xosc(XOSC_MEASURE_LFTIMER, xosc_drift_measure_callback, 0);
+}
+/**
+ * Stop xosc drift measurements
+ */
+void drift_measure_stop(void)
+{
+  /* clear offsets to zero while we're stopped */
+  initial_xosc_frequency = 0;
+  current_frequency_offset = 0;
+  current_channel_offset = 0;
+
+  /* stop measurement */
+  measure_xosc_disable(XOSC_MEASURE_LFTIMER);
+}
diff --git a/firmware/src/init.c b/firmware/src/init.c
index 9e7c571..a001dba 100644
--- a/firmware/src/init.c
+++ b/firmware/src/init.c
@@ -111,7 +111,6 @@ void init(enum init_type init_t)
   gclk0_to_hf_clock();          /* and switch, clock at 8MHz */
   gclk1_init();
   gclk2_init();
-  rtc_init();
 
   /* Clock up to 14MHz with 0 wait states */
   system_flash_set_waitstates(SYSTEM_WAIT_STATE_1_8V_14MHZ);
diff --git a/firmware/src/rtc.c b/firmware/src/rtc.c
index 019d3c9..53c3d9f 100644
--- a/firmware/src/rtc.c
+++ b/firmware/src/rtc.c
@@ -46,7 +46,7 @@ void rtc_init(void)
   for (uint8_t i = 0; i < RTC_NUM_OF_COMP16; i++) {
     config_events.generate_event_on_compare[i] = false;
   }
-  for (uint8_t i = 0; i < 7; i++) {
+  for (uint8_t i = 0; i < 8; i++) {
     config_events.generate_event_on_periodic[i] = false;
   }
   config_events.generate_event_on_periodic[7] = true;
diff --git a/firmware/src/telemetry.c b/firmware/src/telemetry.c
index 25b667b..34400d1 100644
--- a/firmware/src/telemetry.c
+++ b/firmware/src/telemetry.c
@@ -33,6 +33,7 @@
 #include "aprs.h"
 #include "ax25.h"
 #include "pips.h"
+#include "drift.h"
 #include "si_trx.h"
 #include "si_trx_defs.h"
 #include "system/gclk.h"
@@ -455,6 +456,9 @@ void telemetry_init(void)
 
   /* Setup fconfig for telemetry. This is static */
   si_trx_get_frequency_configuration(&telemetry_fconfig, TELEMETRY_FREQUENCY);
+
+  /* We only compensate drift for telemetry, set fconfig for this */
+  drift_set_fconfig(&telemetry_fconfig);
 }
 
 
diff --git a/firmware/src/xosc.c b/firmware/src/xosc.c
index ef323ee..f7dabbb 100644
--- a/firmware/src/xosc.c
+++ b/firmware/src/xosc.c
@@ -35,6 +35,7 @@
 #include "tc/tc_driver.h"
 #include "hw_config.h"
 #include "xosc.h"
+#include "rtc.h"
 #include "watchdog.h"
 
 enum measure_state_t {
@@ -42,6 +43,7 @@ enum measure_state_t {
   MEASURE_MEASUREMENT,
 } measure_state = MEASURE_WAIT_FOR_FIRST_EVENT;
 enum xosc_measurement_t _measurement_t;
+uint8_t _measurement_oneshot;
 measurement_result_t _callback;
 
 /**
@@ -358,17 +360,38 @@ void timepulse_extint_event_source_disable(void) {
   extint_disable();
 }
 
+/**
+ * Configure 1Hz events from the LF Timer
+ */
+void lftimer_event_source(void)
+{
+  /* Start the RTC */
+  rtc_init();
+
+  /* Route the RTC PER7 event to event channel 0 */
+  events_allocate(0,
+                  EVENTS_EDGE_DETECT_NONE, /* Don't care for async path */
+                  EVENTS_PATH_ASYNCHRONOUS,
+                  0xB, /* RTC PER7 event */
+                  0);
+}
+void lftimer_event_source_disable(void) {
+}
+
+
+
 /**
  * Triggers a measurements the number of cycles on XOSC
  *
  * A callback from the timer interrupt is available. Obviously don't dwell here too long.
  */
 void measure_xosc(enum xosc_measurement_t measurement_t,
-                  measurement_result_t callback) {
+                  measurement_result_t callback, uint8_t oneshot) {
 
   measure_state = MEASURE_WAIT_FOR_FIRST_EVENT;
   _measurement_t = measurement_t;
   _callback = callback;
+  _measurement_oneshot = oneshot;
 
   /* Timer 2 runs on GLCK1 */
   bool t2_capture_channel_enables[]    = {true, true};
@@ -408,11 +431,11 @@ void measure_xosc(enum xosc_measurement_t measurement_t,
 
   /* Configure an event source */
   switch (measurement_t) {
-  case XOSC_MEASURE_GCLK0:
-//    osc8m_event_source();     // osc8m issues events
-    break;
   case XOSC_MEASURE_TIMEPULSE:
-    timepulse_extint_event_source(); // timepulse issues events
+    timepulse_extint_event_source(); /* timepulse issues events at 1Hz */
+    break;
+  case XOSC_MEASURE_LFTIMER:
+    lftimer_event_source();     /* lftier issues events at 1Hz */
     break;
   }
 
@@ -423,12 +446,12 @@ void measure_xosc_disable(enum xosc_measurement_t measurement_t) {
   tc_disable(TC2);
 
   switch (measurement_t) {
-  case XOSC_MEASURE_GCLK0:
-//    osc8m_event_source_disable();
-    break;
   case XOSC_MEASURE_TIMEPULSE:
     timepulse_extint_event_source_disable();
     break;
+  case XOSC_MEASURE_LFTIMER:
+    lftimer_event_source_disable();
+    break;
   }
 }
 
@@ -449,17 +472,17 @@ void TC2_Handler(void) {
       measure_state = MEASURE_MEASUREMENT; /* Start measurement */
       break;
     case MEASURE_MEASUREMENT:
-      /* Measurement done. Read off data */
+      /* Measurement made. Read off data */
       capture_value = tc_get_capture_value(TC2, 0);
 
       /* Calcuate the frequency of GLCK1 relative to this source */
       switch (_measurement_t) {
-      case XOSC_MEASURE_GCLK0:
-        source_freq = capture_value * XOSC_GCLK_DIVIDE;
-        break;
       case XOSC_MEASURE_TIMEPULSE:
         source_freq = capture_value * XOSC_GCLK_DIVIDE * GPS_TIMEPULSE_FREQ;
         break;
+      case XOSC_MEASURE_LFTIMER:
+        source_freq = capture_value * XOSC_GCLK_DIVIDE;
+        break;
       }
 
       /* Callback if we have one */
@@ -467,8 +490,10 @@ void TC2_Handler(void) {
         _callback(source_freq);
       }
 
-      /* Disable measurement system */
-      measure_xosc_disable(_measurement_t);
+      if (_measurement_oneshot != 0) { /* single shot */
+        /* Disable measurement system */
+        measure_xosc_disable(_measurement_t);
+      }
     }
   }
 }