diff --git a/src/mynteye/device/channel/def.h b/src/mynteye/device/channel/def.h
index a903854..ec8b2b9 100644
--- a/src/mynteye/device/channel/def.h
+++ b/src/mynteye/device/channel/def.h
@@ -72,6 +72,8 @@ struct ImuSegment {
   std::uint32_t frame_id;
   std::uint64_t timestamp;
   std::uint8_t flag;
+  // Is external time source
+  bool is_ets;
   std::int16_t temperature;
   std::int16_t accel[3];
   std::int16_t gyro[3];
diff --git a/src/mynteye/device/standard2/channels_adapter_s2.cc b/src/mynteye/device/standard2/channels_adapter_s2.cc
index 13efb89..0c0157b 100644
--- a/src/mynteye/device/standard2/channels_adapter_s2.cc
+++ b/src/mynteye/device/standard2/channels_adapter_s2.cc
@@ -55,7 +55,8 @@ struct ImuData {
 void unpack_imu_segment(const ImuData &imu, ImuSegment *seg) {
   seg->frame_id = imu.frame_id;
   seg->timestamp = imu.timestamp;
-  seg->flag = imu.flag;
+  seg->flag = imu.flag & 0b0011;
+  seg->is_ets = (imu.flag & 0b0100 == 0b0100);
   seg->temperature = imu.temperature;
   seg->accel[0] = (seg->flag == 1) ? imu.accel_or_gyro[0] : 0;
   seg->accel[1] = (seg->flag == 1) ? imu.accel_or_gyro[1] : 0;