diff --git a/src/environment/global/gnss_satellites.cpp b/src/environment/global/gnss_satellites.cpp
index c3af068f2..21749c9aa 100644
--- a/src/environment/global/gnss_satellites.cpp
+++ b/src/environment/global/gnss_satellites.cpp
@@ -19,7 +19,7 @@ using namespace gnss;
 
 const size_t kNumberOfInterpolation = 9;
 
-void GnssSatellites::Initialize(const std::vector<Sp3FileReader>& sp3_files, const EpochTime start_time) {
+void GnssSatellites::Initialize(const std::vector<Sp3FileReader>& sp3_files, const time_system::EpochTime start_time) {
   sp3_files_ = sp3_files;
   current_epoch_time_ = start_time;
 
@@ -37,7 +37,7 @@ void GnssSatellites::Initialize(const std::vector<Sp3FileReader>& sp3_files, con
   if (nearest_epoch_id >= half_interpolation_number) {
     reference_interpolation_id_ = nearest_epoch_id - half_interpolation_number;
   }
-  reference_time_ = EpochTime(initial_sp3_file.GetEpochData(reference_interpolation_id_));
+  reference_time_ = time_system::EpochTime(initial_sp3_file.GetEpochData(reference_interpolation_id_));
 
   // Initialize orbit
   orbit_.assign(number_of_calculated_gnss_satellites_, orbit::InterpolationOrbit(kNumberOfInterpolation));
@@ -60,9 +60,9 @@ void GnssSatellites::Update(const SimulationTime& simulation_time) {
 
   // Get time
   UTC current_utc = simulation_time.GetCurrentUtc();
-  DateTime current_date_time((size_t)current_utc.year, (size_t)current_utc.month, (size_t)current_utc.day, (size_t)current_utc.hour,
-                             (size_t)current_utc.minute, current_utc.second);
-  current_epoch_time_ = EpochTime(current_date_time);
+  time_system::DateTime current_date_time((size_t)current_utc.year, (size_t)current_utc.month, (size_t)current_utc.day, (size_t)current_utc.hour,
+                                          (size_t)current_utc.minute, current_utc.second);
+  current_epoch_time_ = time_system::EpochTime(current_date_time);
 
   // Check interpolation update
   double diff_s = current_epoch_time_.GetTimeWithFraction_s() - reference_time_.GetTimeWithFraction_s();
@@ -74,10 +74,10 @@ void GnssSatellites::Update(const SimulationTime& simulation_time) {
   return;
 }
 
-math::Vector<3> GnssSatellites::GetPosition_ecef_m(const size_t gnss_satellite_id, const EpochTime time) const {
+math::Vector<3> GnssSatellites::GetPosition_ecef_m(const size_t gnss_satellite_id, const time_system::EpochTime time) const {
   if (gnss_satellite_id > number_of_calculated_gnss_satellites_) return math::Vector<3>(0.0);
 
-  EpochTime target_time;
+  time_system::EpochTime target_time;
 
   if (time.GetTime_s() == 0) {
     target_time = current_epoch_time_;
@@ -92,10 +92,10 @@ math::Vector<3> GnssSatellites::GetPosition_ecef_m(const size_t gnss_satellite_i
   return orbit_[gnss_satellite_id].CalcPositionWithTrigonometric(diff_s, math::tau / kOrbitalPeriodCorrection_s);
 }
 
-double GnssSatellites::GetClock_s(const size_t gnss_satellite_id, const EpochTime time) const {
+double GnssSatellites::GetClock_s(const size_t gnss_satellite_id, const time_system::EpochTime time) const {
   if (gnss_satellite_id > number_of_calculated_gnss_satellites_) return 0.0;
 
-  EpochTime target_time;
+  time_system::EpochTime target_time;
 
   if (time.GetTime_s() == 0) {
     target_time = current_epoch_time_;
@@ -109,9 +109,9 @@ double GnssSatellites::GetClock_s(const size_t gnss_satellite_id, const EpochTim
   return clock_[gnss_satellite_id].CalcPolynomial(diff_s) * 1e-6;
 }
 
-bool GnssSatellites::GetCurrentSp3File(Sp3FileReader& current_sp3_file, const EpochTime current_time) {
+bool GnssSatellites::GetCurrentSp3File(Sp3FileReader& current_sp3_file, const time_system::EpochTime current_time) {
   for (size_t i = 0; i < sp3_files_.size(); i++) {
-    EpochTime sp3_start_time(sp3_files_[i].GetStartEpochDateTime());
+    time_system::EpochTime sp3_start_time(sp3_files_[i].GetStartEpochDateTime());
     double diff_s = current_time.GetTimeWithFraction_s() - sp3_start_time.GetTimeWithFraction_s();
     if (diff_s < 0.0) {
       // Error
@@ -129,7 +129,7 @@ bool GnssSatellites::UpdateInterpolationInformation() {
   Sp3FileReader sp3_file = sp3_files_[sp3_file_id_];
 
   for (size_t gnss_id = 0; gnss_id < number_of_calculated_gnss_satellites_; gnss_id++) {
-    EpochTime sp3_time = EpochTime(sp3_file.GetEpochData(reference_interpolation_id_));
+    time_system::EpochTime sp3_time = time_system::EpochTime(sp3_file.GetEpochData(reference_interpolation_id_));
     double time_diff_s = sp3_time.GetTimeWithFraction_s() - reference_time_.GetTimeWithFraction_s();
     math::Vector<3> sp3_position_m = 1000.0 * sp3_file.GetSatellitePosition_km(reference_interpolation_id_, gnss_id);
 
@@ -226,9 +226,10 @@ GnssSatellites* InitGnssSatellites(const std::string file_name, const EarthRotat
   }
 
   //
-  DateTime start_date_time((size_t)simulation_time.GetStartYear(), (size_t)simulation_time.GetStartMonth(), (size_t)simulation_time.GetStartDay(),
-                           (size_t)simulation_time.GetStartHour(), (size_t)simulation_time.GetStartMinute(), simulation_time.GetStartSecond());
-  EpochTime start_epoch_time(start_date_time);
+  time_system::DateTime start_date_time((size_t)simulation_time.GetStartYear(), (size_t)simulation_time.GetStartMonth(),
+                                        (size_t)simulation_time.GetStartDay(), (size_t)simulation_time.GetStartHour(),
+                                        (size_t)simulation_time.GetStartMinute(), simulation_time.GetStartSecond());
+  time_system::EpochTime start_epoch_time(start_date_time);
   gnss_satellites->Initialize(sp3_file_readers, start_epoch_time);
 
   return gnss_satellites;
diff --git a/src/environment/global/gnss_satellites.hpp b/src/environment/global/gnss_satellites.hpp
index 0aa46ef38..77508f73b 100644
--- a/src/environment/global/gnss_satellites.hpp
+++ b/src/environment/global/gnss_satellites.hpp
@@ -54,7 +54,7 @@ class GnssSatellites : public ILoggable {
    * @param [in] sp3_files: List of SP3 files
    * @param [in] start_time: The simulation start time
    */
-  void Initialize(const std::vector<gnss::Sp3FileReader>& sp3_files, const EpochTime start_time);
+  void Initialize(const std::vector<gnss::Sp3FileReader>& sp3_files, const time_system::EpochTime start_time);
 
   /**
    * @fn IsCalcEnabled
@@ -87,7 +87,7 @@ class GnssSatellites : public ILoggable {
    * @param [in] time: Target time to get the GNSS satellite. When the argument is not set, the last updated time is used for the calculation.
    * @return GNSS satellite position at ECEF frame at the time. Or return zero vector when the arguments are out of range.
    */
-  math::Vector<3> GetPosition_ecef_m(const size_t gnss_satellite_id, const EpochTime time = EpochTime(0, 0.0)) const;
+  math::Vector<3> GetPosition_ecef_m(const size_t gnss_satellite_id, const time_system::EpochTime time = time_system::EpochTime(0, 0.0)) const;
 
   /**
    * @fn GetGetClock_s
@@ -96,7 +96,7 @@ class GnssSatellites : public ILoggable {
    * @param [in] time: Target time to get the GNSS satellite. When the argument is not set, the last updated time is used for the calculation.
    * @return GNSS satellite clock offset at the time. Or return zero when the arguments are out of range.
    */
-  double GetClock_s(const size_t gnss_satellite_id, const EpochTime time = EpochTime(0, 0.0)) const;
+  double GetClock_s(const size_t gnss_satellite_id, const time_system::EpochTime time = time_system::EpochTime(0, 0.0)) const;
 
   // Override ILoggable
   /**
@@ -116,9 +116,9 @@ class GnssSatellites : public ILoggable {
   std::vector<gnss::Sp3FileReader> sp3_files_;   //!< List of SP3 files
   size_t number_of_calculated_gnss_satellites_;  //!< Number of calculated GNSS satellites
   size_t sp3_file_id_;                           //!< Current SP3 file ID
-  EpochTime reference_time_;                     //!< Reference start time of the SP3 handling
+  time_system::EpochTime reference_time_;        //!< Reference start time of the SP3 handling
   size_t reference_interpolation_id_ = 0;        //!< Reference epoch ID of the interpolation
-  EpochTime current_epoch_time_;                 //!< The last updated time
+  time_system::EpochTime current_epoch_time_;    //!< The last updated time
 
   std::vector<orbit::InterpolationOrbit> orbit_;  //!< GNSS satellite orbit with interpolation
   std::vector<math::Interpolation> clock_;        //!< GNSS satellite clock offset with interpolation
@@ -133,7 +133,7 @@ class GnssSatellites : public ILoggable {
    * @param [in] current_time: Target time
    * @return true means no error, false means the time argument is out of range
    */
-  bool GetCurrentSp3File(gnss::Sp3FileReader& current_sp3_file, const EpochTime current_time);
+  bool GetCurrentSp3File(gnss::Sp3FileReader& current_sp3_file, const time_system::EpochTime current_time);
 
   /**
    * @fn UpdateInterpolationInformation
diff --git a/src/math_physics/gnss/antex_file_reader.cpp b/src/math_physics/gnss/antex_file_reader.cpp
index 1547a3cf4..c36cdacc4 100644
--- a/src/math_physics/gnss/antex_file_reader.cpp
+++ b/src/math_physics/gnss/antex_file_reader.cpp
@@ -192,11 +192,11 @@ AntexPhaseCenterData AntexFileReader::ReadPhaseCenterData(std::ifstream& antex_f
   return phase_center_data;
 }
 
-DateTime AntexFileReader::ReadDateTime(std::string line) {
+time_system::DateTime AntexFileReader::ReadDateTime(std::string line) {
   size_t year, month, day, hour, minute;
   double second;
   sscanf(line.c_str(), "%zu %2zu %2zu %2zu %2zu %10lf", &year, &month, &day, &hour, &minute, &second);
-  return DateTime(year, month, day, hour, minute, second);
+  return time_system::DateTime(year, month, day, hour, minute, second);
 }
 
 }  // namespace gnss
diff --git a/src/math_physics/gnss/antex_file_reader.hpp b/src/math_physics/gnss/antex_file_reader.hpp
index e730b09c2..13e126c55 100644
--- a/src/math_physics/gnss/antex_file_reader.hpp
+++ b/src/math_physics/gnss/antex_file_reader.hpp
@@ -203,12 +203,12 @@ class AntexSatelliteData {
    * @fn SetValidStartTime
    * @param[in] valid_start_time: Valid start time
    */
-  inline void SetValidStartTime(const DateTime valid_start_time) { valid_start_time_ = valid_start_time; };
+  inline void SetValidStartTime(const time_system::DateTime valid_start_time) { valid_start_time_ = valid_start_time; };
   /**
    * @fn SetValidEndTime
    * @param[in] valid_end_time: Valid end time
    */
-  inline void SetValidEndTime(const DateTime valid_end_time) { valid_end_time_ = valid_end_time; };
+  inline void SetValidEndTime(const time_system::DateTime valid_end_time) { valid_end_time_ = valid_end_time; };
   /**
    * @fn SetNumberOfFrequency
    * @param[in] number_of_frequency: Number of frequency
@@ -235,12 +235,12 @@ class AntexSatelliteData {
    * @fn GetValidStartTime
    * @return Valid start time
    */
-  inline DateTime GetValidStartTime() const { return valid_start_time_; };
+  inline time_system::DateTime GetValidStartTime() const { return valid_start_time_; };
   /**
    * @fn GetValidEndTime
    * @return Valid end time
    */
-  inline DateTime GetValidEndTime() const { return valid_end_time_; };
+  inline time_system::DateTime GetValidEndTime() const { return valid_end_time_; };
   /**
    * @fn GetNumberOfFrequency
    * @return Number of frequency
@@ -256,8 +256,8 @@ class AntexSatelliteData {
  private:
   std::string antenna_type_;                             //!< Antenna type
   std::string serial_number_;                            //!< Serial number or satellite code
-  DateTime valid_start_time_;                            //!< Valid start time
-  DateTime valid_end_time_;                              //!< Valid end time (The latest data does not have the end time)
+  time_system::DateTime valid_start_time_;               //!< Valid start time
+  time_system::DateTime valid_end_time_;                 //!< Valid end time (The latest data does not have the end time)
   size_t number_of_frequency_ = 1;                       //!< Number of frequency
   std::vector<AntexPhaseCenterData> phase_center_data_;  //!< Phase center data for each frequency
 };
@@ -342,7 +342,7 @@ class AntexFileReader {
    * @param[in] line: A single line in ANTEX file
    * @return Read date time
    */
-  DateTime ReadDateTime(std::string line);
+  time_system::DateTime ReadDateTime(std::string line);
 };
 
 }  // namespace gnss
diff --git a/src/math_physics/gnss/sp3_file_reader.cpp b/src/math_physics/gnss/sp3_file_reader.cpp
index 8106ba9cc..7fbf0e831 100644
--- a/src/math_physics/gnss/sp3_file_reader.cpp
+++ b/src/math_physics/gnss/sp3_file_reader.cpp
@@ -15,9 +15,9 @@ namespace gnss {
 
 Sp3FileReader::Sp3FileReader(const std::string file_name) { ReadFile(file_name); }
 
-DateTime Sp3FileReader::GetEpochData(const size_t epoch_id) const {
+time_system::DateTime Sp3FileReader::GetEpochData(const size_t epoch_id) const {
   if (epoch_id > epoch_.size()) {
-    DateTime zero;
+    time_system::DateTime zero;
     return zero;
   }
   return epoch_[epoch_id];
@@ -69,7 +69,7 @@ bool Sp3FileReader::ReadFile(const std::string file_name) {
     size_t year, month, day, hour, minute;
     double second;
     sscanf(line.substr(3, 28).c_str(), "%zu %2zu %2zu %2zu %2zu %12lf", &year, &month, &day, &hour, &minute, &second);
-    epoch_.push_back(DateTime(year, month, day, hour, minute, second));
+    epoch_.push_back(time_system::DateTime(year, month, day, hour, minute, second));
 
     // Orbit and Clock information
     for (size_t satellite_id = 0; satellite_id < header_.number_of_satellites_; satellite_id++) {
@@ -117,7 +117,7 @@ bool Sp3FileReader::ReadFile(const std::string file_name) {
   }
 
   // Test
-  DateTime test = epoch_[0];
+  time_system::DateTime test = epoch_[0];
   test = epoch_[1];
   std::vector<Sp3PositionClock> test_p = position_clock_[0];
   test_p = position_clock_[1];
@@ -126,7 +126,7 @@ bool Sp3FileReader::ReadFile(const std::string file_name) {
   return true;
 }
 
-size_t Sp3FileReader::SearchNearestEpochId(const EpochTime time) {
+size_t Sp3FileReader::SearchNearestEpochId(const time_system::EpochTime time) {
   size_t nearest_epoch_id = 0;
 
   // Get header info
@@ -134,10 +134,10 @@ size_t Sp3FileReader::SearchNearestEpochId(const EpochTime time) {
   const double interval_s = header_.epoch_interval_s_;
 
   // Check range
-  EpochTime start_epoch(epoch_[0]);
+  time_system::EpochTime start_epoch(epoch_[0]);
   if (start_epoch > time) {
     nearest_epoch_id = 0;
-  } else if ((EpochTime)(epoch_[num_epoch - 1]) < time) {
+  } else if ((time_system::EpochTime)(epoch_[num_epoch - 1]) < time) {
     nearest_epoch_id = num_epoch - 1;
   } else {  // Calc nearest point
     double diff_s = time.GetTimeWithFraction_s() - start_epoch.GetTimeWithFraction_s();
@@ -171,7 +171,7 @@ size_t Sp3FileReader::ReadHeader(std::ifstream& sp3_file) {
   size_t year, month, day, hour, minute;
   double second;
   sscanf(line.substr(3, 28).c_str(), "%zu %2zu %2zu %2zu %2zu %12lf", &year, &month, &day, &hour, &minute, &second);
-  header_.start_epoch_ = DateTime(year, month, day, hour, minute, second);
+  header_.start_epoch_ = time_system::DateTime(year, month, day, hour, minute, second);
   header_.number_of_epoch_ = std::stoi(line.substr(32, 7));
   header_.used_data_ = line.substr(40, 5);
   header_.coordinate_system_ = line.substr(46, 5);
@@ -199,7 +199,7 @@ size_t Sp3FileReader::ReadHeader(std::ifstream& sp3_file) {
     return 0;
   }
   // Read contents
-  header_.start_gps_time_ = GpsTime(std::stoi(line.substr(3, 4)), std::stod(line.substr(8, 15)));
+  header_.start_gps_time_ = time_system::GpsTime(std::stoi(line.substr(3, 4)), std::stod(line.substr(8, 15)));
   header_.epoch_interval_s_ = std::stod(line.substr(24, 14));
   header_.start_time_mjday_ = std::stoi(line.substr(39, 5));
   header_.start_time_mjday_fractional_day_ = std::stod(line.substr(45, 15));
diff --git a/src/math_physics/gnss/sp3_file_reader.hpp b/src/math_physics/gnss/sp3_file_reader.hpp
index e904b6050..15e4566dc 100644
--- a/src/math_physics/gnss/sp3_file_reader.hpp
+++ b/src/math_physics/gnss/sp3_file_reader.hpp
@@ -51,16 +51,16 @@ enum class Sp3OrbitType {
 struct Sp3Header {
   // 1st line information
   // version -> not implemented yet
-  Sp3Mode mode_;                   //!< position or velocity
-  DateTime start_epoch_;           //!< Time of start epoch
-  size_t number_of_epoch_ = 0;     //!< Number of epoch in the SP3 file
-  std::string used_data_;          //!< Used data to generate the SP3 file
-  std::string coordinate_system_;  //!< Coordinate system for the position and velocity data
-  Sp3OrbitType orbit_type_;        //!< Orbit type
-  std::string agency_name_;        //!< Agency name who generates the SP3 file
+  Sp3Mode mode_;                       //!< position or velocity
+  time_system::DateTime start_epoch_;  //!< Time of start epoch
+  size_t number_of_epoch_ = 0;         //!< Number of epoch in the SP3 file
+  std::string used_data_;              //!< Used data to generate the SP3 file
+  std::string coordinate_system_;      //!< Coordinate system for the position and velocity data
+  Sp3OrbitType orbit_type_;            //!< Orbit type
+  std::string agency_name_;            //!< Agency name who generates the SP3 file
 
   // 2nd line information
-  GpsTime start_gps_time_;                        //!< Start time of orbit
+  time_system::GpsTime start_gps_time_;           //!< Start time of orbit
   double epoch_interval_s_ = 1.0;                 //!< Epoch interval (0.0, 100000.0)
   size_t start_time_mjday_;                       //!< Start time of the orbit data (44244 = 6th Jan. 1980) [Modified Julian day]
   double start_time_mjday_fractional_day_ = 0.0;  //!< Fractional part of the start time [0.0, 1.0) [day]
@@ -172,19 +172,19 @@ class Sp3FileReader {
   inline Sp3Header GetHeader() const { return header_; }
   inline size_t GetNumberOfEpoch() const { return header_.number_of_epoch_; }
   inline size_t GetNumberOfSatellites() const { return header_.number_of_satellites_; }
-  inline DateTime GetStartEpochDateTime() const { return header_.start_epoch_; }
-  inline GpsTime GetStartEpochGpsTime() const { return header_.start_gps_time_; }
+  inline time_system::DateTime GetStartEpochDateTime() const { return header_.start_epoch_; }
+  inline time_system::GpsTime GetStartEpochGpsTime() const { return header_.start_gps_time_; }
   // Data
-  DateTime GetEpochData(const size_t epoch_id) const;
+  time_system::DateTime GetEpochData(const size_t epoch_id) const;
   Sp3PositionClock GetPositionClock(const size_t epoch_id, const size_t satellite_id);
   double GetSatelliteClockOffset(const size_t epoch_id, const size_t satellite_id);
   math::Vector<3> GetSatellitePosition_km(const size_t epoch_id, const size_t satellite_id);
 
-  size_t SearchNearestEpochId(const EpochTime time);
+  size_t SearchNearestEpochId(const time_system::EpochTime time);
 
  private:
-  Sp3Header header_;             //!< SP3 header information
-  std::vector<DateTime> epoch_;  //!< Epoch data list
+  Sp3Header header_;                          //!< SP3 header information
+  std::vector<time_system::DateTime> epoch_;  //!< Epoch data list
 
   // Orbit and clock data (Use as position_clock_[satellite_id][epoch_id])
   std::map<size_t, std::vector<Sp3PositionClock>> position_clock_;                                  //!< Position and Clock data
diff --git a/src/math_physics/time_system/date_time_format.cpp b/src/math_physics/time_system/date_time_format.cpp
index d3efc41ca..069c7247f 100644
--- a/src/math_physics/time_system/date_time_format.cpp
+++ b/src/math_physics/time_system/date_time_format.cpp
@@ -11,6 +11,8 @@
 #include <iomanip>
 #include <sstream>
 
+namespace time_system {
+
 DateTime::DateTime(const std::string date_time) {
   sscanf(date_time.c_str(), "%zu/%zu/%zu %zu:%zu:%lf", &year_, &month_, &day_, &hour_, &minute_, &second_);
 }
@@ -52,3 +54,5 @@ std::string DateTime::GetAsString() const {
   std::string output = stream.str();
   return output;
 }
+
+}  // namespace time_system
diff --git a/src/math_physics/time_system/date_time_format.hpp b/src/math_physics/time_system/date_time_format.hpp
index 1e7d69d59..25084e979 100644
--- a/src/math_physics/time_system/date_time_format.hpp
+++ b/src/math_physics/time_system/date_time_format.hpp
@@ -10,6 +10,8 @@
 
 #include "epoch_time.hpp"
 
+namespace time_system {
+
 class EpochTime;
 
 /**
@@ -60,4 +62,6 @@ class DateTime {
   double second_;  //!< Second [0.0, 60.0)
 };
 
+}  // namespace time_system
+
 #endif  // S2E_LIBRARY_TIME_SYSTEM_DATE_TIME_FORMAT_HPP_
diff --git a/src/math_physics/time_system/epoch_time.cpp b/src/math_physics/time_system/epoch_time.cpp
index b0bcda350..b3b6ec730 100644
--- a/src/math_physics/time_system/epoch_time.cpp
+++ b/src/math_physics/time_system/epoch_time.cpp
@@ -7,6 +7,8 @@
 
 #include <cmath>
 
+namespace time_system {
+
 EpochTime::EpochTime(const DateTime date_time) {
   // No leap second calculation
   const int doy[] = {1, 32, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335};  //!< Day of Year for the 1st day of each month
@@ -73,3 +75,5 @@ EpochTime EpochTime::operator-(const EpochTime& right_side) const {
   EpochTime result(time_s, fraction_s);
   return result;
 }
+
+}  // namespace time_system
diff --git a/src/math_physics/time_system/epoch_time.hpp b/src/math_physics/time_system/epoch_time.hpp
index b783c674c..7f7dfe871 100644
--- a/src/math_physics/time_system/epoch_time.hpp
+++ b/src/math_physics/time_system/epoch_time.hpp
@@ -10,6 +10,8 @@
 
 #include "date_time_format.hpp"
 
+namespace time_system {
+
 class DateTime;
 
 /**
@@ -67,4 +69,6 @@ class EpochTime {
   double fraction_s_;  //!< Fraction of second under 1 sec [0, 1)
 };
 
+}  // namespace time_system
+
 #endif  // S2E_LIBRARY_TIME_SYSTEM_EPOCH_TIME_HPP_
diff --git a/src/math_physics/time_system/gps_time.cpp b/src/math_physics/time_system/gps_time.cpp
index a9ceabeb8..cb3a5fec9 100644
--- a/src/math_physics/time_system/gps_time.cpp
+++ b/src/math_physics/time_system/gps_time.cpp
@@ -5,6 +5,8 @@
 
 #include "gps_time.hpp"
 
+namespace time_system {
+
 const DateTime GpsTime::kEpochOfGpsTimeInDateTime_ = DateTime("1980/1/6 00:00:00.0");
 const EpochTime GpsTime::kEpochOfGpsTimeInEpochTime_ = EpochTime(kEpochOfGpsTimeInDateTime_);
 const EpochTime GpsTime::kLeapSecondAheadFromUtc_ = EpochTime(18, 0);  //!< Leap second ahead from UTC @ May 2023
@@ -23,3 +25,5 @@ void GpsTime::CalcEpochTime() {
 
   epoch_time_ = kEpochOfGpsTimeInEpochTime_ + time_diff;
 }
+
+}  // namespace time_system
diff --git a/src/math_physics/time_system/gps_time.hpp b/src/math_physics/time_system/gps_time.hpp
index 2a2f657ee..f86242754 100644
--- a/src/math_physics/time_system/gps_time.hpp
+++ b/src/math_physics/time_system/gps_time.hpp
@@ -11,6 +11,8 @@
 #include "date_time_format.hpp"
 #include "epoch_time.hpp"
 
+namespace time_system {
+
 /**
  * @class GpsTime
  * @brief A class to define GPS time expression
@@ -95,4 +97,6 @@ class GpsTime {
   void CalcEpochTime();
 };
 
+}  // namespace time_system
+
 #endif  // S2E_LIBRARY_TIME_SYSTEM_GPS_TIME_HPP_
diff --git a/src/math_physics/time_system/test_date_time_format.cpp b/src/math_physics/time_system/test_date_time_format.cpp
index 8ac7a60cd..4ac1a3840 100644
--- a/src/math_physics/time_system/test_date_time_format.cpp
+++ b/src/math_physics/time_system/test_date_time_format.cpp
@@ -2,6 +2,8 @@
 
 #include "date_time_format.hpp"
 
+using namespace time_system;
+
 /**
  * @brief Test Constructor with value
  */
diff --git a/src/math_physics/time_system/test_epoch_time.cpp b/src/math_physics/time_system/test_epoch_time.cpp
index 6ade9cad2..6cb5f0c1b 100644
--- a/src/math_physics/time_system/test_epoch_time.cpp
+++ b/src/math_physics/time_system/test_epoch_time.cpp
@@ -3,6 +3,8 @@
 #include "date_time_format.hpp"
 #include "epoch_time.hpp"
 
+using namespace time_system;
+
 TEST(EpochTime, ConstructorNominal) {
   EpochTime time(1000000000, 0.250);
 
diff --git a/src/math_physics/time_system/test_gps_time.cpp b/src/math_physics/time_system/test_gps_time.cpp
index c3aeabe24..425556774 100644
--- a/src/math_physics/time_system/test_gps_time.cpp
+++ b/src/math_physics/time_system/test_gps_time.cpp
@@ -2,6 +2,8 @@
 
 #include "gps_time.hpp"
 
+using namespace time_system;
+
 /**
  * @brief Test Constructor
  */