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diff --git a/pyatmos/.DS_Store b/pyatmos/.DS_Store
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diff --git a/pyatmos/StandardAtmosphere/StandardAtmosphere.py b/pyatmos/StandardAtmosphere/StandardAtmosphere.py
new file mode 100644
index 0000000..f49c896
--- /dev/null
+++ b/pyatmos/StandardAtmosphere/StandardAtmosphere.py
@@ -0,0 +1,103 @@
+import numpy as np
+
+# physical constants:
+g = 9.80665 # standard gravity acceleration in [m/s^2]
+R = 8314.32/28.9644 # gas constant for dry air in [J/kg/K] from the 1976 United States Standard Atmosphere(USSA) Model
+R_e = 6371000.8 # Earth's volumetric radius in [m]
+# temperature and pressure at the Mean Sea Level(MSL)
+t_msl = 288.15 # temperature in [K]
+p_msl = 101325 # pressure in [Pa]
+h_msl = 0 # altitude in [m]
+    
+def lapse_tp(t_lower, p_lower, lr, h_lower, h_upper):
+    '''
+    Calculate the temperature and pressure at a given geopotential altitude above base of a specific layer.
+    The temperature is computed by the linear interpolation with the slope defined by lapse rates. 
+    The pressure is computed from the hydrostatic equations and the perfect gas law. 
+    See the detailed documentation in http://www.pdas.com/hydro.pdf
+
+    Usage:
+    [t_upper, p_upper] = lapse_tp(t_lower, p_lower, lr, h_lower, h_upper)
+
+    Inputs:
+    t_lower -> [float] temperature in [K] at the lower boundary of the subset in the specific layer
+    p_lower -> [float] pressure in [Pa] at the lower boundary of the subset in the specific layer
+    lr -> [float] lapse rate in [K/m] for the specific layer
+    h_lower -> [float] geopotential altitude in [m] at the lower boundary of the subset in the specific layer
+    h_upper -> [float] geopotential altitude in [m] at the upper boundary of the subset in the specific layer
+    
+    Outputs:
+    t1 -> [float] temperature in [K] at the upper boundary of the subset in the specific layer
+    p1 -> [float] pressure in [Pa] at the upper boundary of the subset in the specific layer
+    
+    Reference: Public Domain Aeronautical Software(http://www.pdas.com/atmos.html) 
+               https://gist.github.com/buzzerrookie/5b6438c603eabf13d07e
+    '''
+    if lr == 0:
+        t_upper = t_lower
+        p_upper = p_lower * np.exp(-g / R / t_lower * (h_upper - h_lower))
+    else:
+        t_upper = t_lower + lr * (h_upper - h_lower)
+        p_upper = p_lower * (t_upper / t_lower) ** (-g / lr / R)
+
+    return t_upper,p_upper
+
+def isa(altitude,altitude_type='geometric'):
+    '''
+    Implements the International Standard Atmosphere(ISA) Model up to 86km. 
+    The standard atmosphere is defined as a set of layers by specified geopotential altitudes and lapse rates.
+    The temperature is computed by linear interpolation with the slope defined by the lapse rate. 
+    The pressure is computed from the hydrostatic equations and the perfect gas law; the density follows from the perfect gas law. 
+
+    Usage:
+    [temperature, pressure, density] = isa(altitude)
+    [temperature, pressure, density] = isa(altitude,'geopotential')
+
+    Inputs:
+    altitude -> [float] altitude in [m]
+
+    Parameters:
+    altitude_type -> [optional, string, default='geometric'] type of altitude. It can either be 'geopotential' or 'geometric'.
+    Relationship between the 'geopotential' and 'geometric' altitude can be found in http://www.pdas.com/hydro.pdf
+
+    Outputs:
+    temperature -> [float] temperature in [K] at the local altitude
+    pressure -> [float] pressure in [Pa] at the local altitude
+    density -> [float] density in [kg/m^3] at the local altitude
+
+    Note: the geopotential altitude should be in [610,84852] m and the geometric altitude should be in [-611,86000] m
+
+    Reference: U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C. 
+               Public Domain Aeronautical Software(http://www.pdas.com/atmos.html) 
+               https://gist.github.com/buzzerrookie/5b6438c603eabf13d07e
+               https://ww2.mathworks.cn/help/aerotbx/ug/atmosisa.html
+    '''
+    # the lower atmosphere below 86km is separated into seven layers 
+    geopotential_alt = [-610,11000, 20000, 32000, 47000, 51000,71000,84852] # Geopotential altitudes above MSL in [m]
+    geometric_alt = [-611,11019, 20063, 32162, 47350, 51413, 71802,86000] # Geometric altitudes above MSL in [m]
+    lr = np.array([-6.5, 0, 1, 2.8, 0, -2.8, -2])/1e3 # Lapse rate in [K/m]
+    
+    t0,p0,h0 = t_msl,p_msl,h_msl
+
+    if altitude_type == 'geopotential':
+        if altitude < geopotential_alt[0] or altitude > geopotential_alt[-1]:
+            raise Exception("geopotential altitude should be in [-0.610, 84.852] km")
+    elif altitude_type == 'geometric':   
+        if altitude < geometric_alt[0] or altitude > geometric_alt[-1]:
+            raise Exception("geometric altitude should be in [-0.611, 86.0] km") 
+        # convert geometric altitude to geopotential altitude   
+        altitude = altitude*R_e/(altitude+R_e)
+    else:
+        raise Exception("The type of altitude should either be 'geopotential' or 'geometric'")       
+
+    for i in range(len(lr)):
+        if altitude <= geopotential_alt[i+1]:
+            temperature, pressure = lapse_tp(t0, p0, lr[i], h0, altitude)
+            break
+        else:
+            # if altitudes are greater than the first several layers, then it has to integeate these layers first.
+            t0, p0 = lapse_tp(t0, p0, lr[i], h0, geopotential_alt[i+1])
+            h0 = geopotential_alt[i+1]
+
+    density = pressure / (R * temperature)
+    return {'temperature':temperature,'pressure':pressure,'density':density}
\ No newline at end of file
diff --git a/pyatmos/StandardAtmosphere/__init__.py b/pyatmos/StandardAtmosphere/__init__.py
new file mode 100644
index 0000000..b136d90
--- /dev/null
+++ b/pyatmos/StandardAtmosphere/__init__.py
@@ -0,0 +1,13 @@
+'''
+pyatmos StandardAtmosphere subpackage
+
+This subpackage defines the following functions:
+
+# =================== Standard Atmosphere Model================= #
+
+isa - Implements the International Standard Atmosphere(ISA) Model up to 86km. 
+
+# ===================== utility functions ==================== #
+
+lapse_tp - Calculate the temperature and pressure at a given geopotential altitude above base of a specific layer.
+'''              
\ No newline at end of file
diff --git a/pyatmos/__init__.py b/pyatmos/__init__.py
index 31a90cc..395a39d 100644
--- a/pyatmos/__init__.py
+++ b/pyatmos/__init__.py
@@ -1,6 +1,5 @@
 '''
 pyatmos package
 
-This package is an archive of scientific routines that can be used to
-perform the estimation of papameters for various atmosphere models.  
+This package is an archive of scientific routines that implements the estimation of atmospheric properties for various atmosphere models.  
 '''    
\ No newline at end of file
diff --git a/pyatmos/atmosclasses/.DS_Store b/pyatmos/atmosclasses/.DS_Store
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diff --git a/pyatmos/atmosclasses/__init__.py b/pyatmos/atmosclasses/__init__.py
deleted file mode 100644
index 4356368..0000000
--- a/pyatmos/atmosclasses/__init__.py
+++ /dev/null
@@ -1,10 +0,0 @@
-'''
-pyatmos atmosclasses subpackage
-
-This subpackage defines a class that facilitates the parameter estimation for various atmosphere model 
-
-Class structure:
-
-    Coordinate
-'''
-from .coordinate import Coordinate
\ No newline at end of file
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diff --git a/pyatmos/atmosclasses/__pycache__/timespace.cpython-37.pyc b/pyatmos/atmosclasses/__pycache__/timespace.cpython-37.pyc
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diff --git a/pyatmos/atmosclasses/coordinate.py b/pyatmos/atmosclasses/coordinate.py
deleted file mode 100644
index f4f9168..0000000
--- a/pyatmos/atmosclasses/coordinate.py
+++ /dev/null
@@ -1,110 +0,0 @@
-from astropy.time import Time
-
-from ..msise.nrlmsise00 import nrlmsise00 
-
-class Coordinate(object):
-    '''
-    space-time coordinate class
-
-    The coordinate of this class can be initialized using the following
-    constructor method:
-
-    x = coordinate(t,lat,lon,alt)
-
-    Once initialized, each class instance defines the following class
-    attributes:
-
-    t   : time, default in UTC
-    lat : latitude, default in degrees
-    lon : longitude, default in degrees
-    alt : altitude, default in km
-
-    Each class instance provides the following methods:
-
-    nrlmsise00() : Estimate the atmosphere parameters at a specific coordinate using the nrlmsise00 model 
-    '''
-    def __init__(self,t,lat,lon,alt):
-        self.t = t
-        self.lat = lat
-        self.lon = lon
-        self.alt = alt
-
-    def __repr__(self):
-    
-        return 't = {:s} UTC\nlat = {:f} deg\nlon = {:f} deg\nalt = {:f} km'.format(self.t,self.lat,self.lon,self.alt)
-
-    def nrlmsise00(self,sw_obs_pre,omode='Oxygen',aphmode='NoAph'):
-        '''
-        Estimate the atmosphere parameters at a specific coordinate(time and location) using the nrlmsise00 model.
-
-        Usage: 
-        para_input,para_output = st.nrlmsis00_data(sw_obs_pre,[omode,aphmode])
-
-        Inputs: 
-        st -> [Coordinate class instance] It can be initialized by defining a specific set of time and location
-        sw_obs_pre -> [2d str array] space weather data
-        omode -> [str, optional, default = 'Oxygen'] If 'Oxygen', Anomalous Oxygen density will be included. If 'NoOxygen', Anomalous Oxygen density will be excluded.
-        aphmode -> [str, optional, default = 'NoAph'] If NoAph', 3-hour geomagnetic index will not be used. If Aph', 3h geomagnetic index will be used. 
-
-        Outputs: 
-        para_input -> [dictionary] parameters for time, location, solar radiation index, and geomagnetic index
-        para_output -> [dictionary] parameters for atmospheric density and temperature
-
-        Examples:
-        >>> from pyatmos.msise import download_sw,read_sw
-        >>> from pyatmos.atmosclasses import Coordinate
-
-        >>> # Download or update the space weather file from www.celestrak.com
-        >>> swfile = download_sw() 
-        >>> # Read the space weather data
-        >>> sw_obs_pre = read_sw(swfile) 
-        >>> 
-        >>> # Test 1
-        >>> # Set a specific time and location
-        >>> t = '2015-10-05 03:00:00' # time(UTC)
-        >>> lat,lon = 25,102 # latitude and longitude [degree]
-        >>> alt = 70 # altitude [km]
-        >>> # Initialize a coordinate instance by a space-time point
-        >>> st = Coordinate(t,lat,lon,alt)
-        >>> 
-        >>> para_input,para_output = st.nrlmsise00(sw_obs_pre)
-        >>> print(para_input)
-        {'doy': 278, 'year': 2015, 'sec': 10800.0, 'alt': 70, 'g_lat': 25, 'g_long': 102, 'lst': 9.8, 'f107A': 150, 'f107': 150, 'ap': 4, 'ap_a': array([4, 4, 4, 4, 4, 4, 4])}
-        >>> print(para_output)
-        >>> {'d': {'He': 9100292488300570.0, 'O': 0, 'N2': 1.3439413974205876e+21, 'O2': 3.52551376755781e+20, 'AR': 1.6044163757370681e+19, 'RHO': 8.225931818480755e-05, 'H': 0, 'N': 0, 'ANM O': 0}, 't': {'TINF': 1027.3184649, 'TG': 219.9649472491653}}
-        >>> 
-        >>> # Test 2
-        >>> t = '2004-07-08 10:30:50' 
-        >>> lat,lon,alt = -65,-120,100 
-        >>> st = Coordinate(t,lat,lon,alt)
-        >>> para_input,para_output = st.nrlmsise00(sw_obs_pre)
-        >>> print(para_input)
-        {'doy': 190, 'year': 2004, 'sec': 37850.0, 'alt': 100, 'g_lat': -65, 'g_long': -120, 'lst': 2.5138888888888893, 'f107A': 109.0, 'f107': 79.3, 'ap': 2, 'ap_a': array([2.   , 2.   , 2.   , 2.   , 2.   , 3.125, 4.625])}
-        >>> print(para_output)
-        {'d': {'He': 119477307274636.89, 'O': 4.1658304136233e+17, 'N2': 7.521248904485598e+18, 'O2': 1.7444969074975662e+18, 'AR': 7.739495767665198e+16, 'RHO': 4.584596293339505e-07, 'H': 22215754381448.5, 'N': 152814261016.3964, 'ANM O': 1.8278224834873257e-37}, 't': {'TINF': 1027.3184649, 'TG': 192.5868649143824}}
-        >>>
-        >>> # Test 3
-        >>> t = '2010-02-15 12:18:37' 
-        >>> lat,lon,alt = 85,210,500 
-        >>> st = Coordinate(t,lat,lon,alt)
-        >>> para_input,para_output = st.nrlmsise00(sw_obs_pre,'NoOxygen','Aph')
-        >>> print(para_input)
-        {'doy': 46, 'year': 2010, 'sec': 44317.0, 'alt': 500, 'g_lat': 85, 'g_long': 210, 'lst': 2.310277777777779, 'f107A': 83.4, 'f107': 89.4, 'ap': 14, 'ap_a': array([14.   ,  5.   ,  7.   ,  6.   , 15.   ,  5.375,  4.   ])}
-        >>> print(para_output)
-        {'d': {'He': 3314507585382.5425, 'O': 3855595951659.0874, 'N2': 19285497858.028534, 'O2': 395599656.3119481, 'AR': 146073.85956102316, 'RHO': 1.2650700238089615e-13, 'H': 171775437382.8238, 'N': 38359828672.39737, 'ANM O': 5345258193.554493}, 't': {'TINF': 776.3155804924045, 'TG': 776.3139192714452}}
-        >>> 
-        >>> # Test 4
-        >>> t = '2019-08-20 23:10:59' 
-        >>> lat,lon,alt = 3,5,900 
-        >>> st = Coordinate(t,lat,lon,alt)
-        >>> para_input,para_output = st.nrlmsise00(sw_obs_pre,aphmode = 'Aph')
-        >>> print(para_input)
-        {'doy': 232, 'year': 2019, 'sec': 83459.0, 'alt': 900, 'g_lat': 3, 'g_long': 5, 'lst': 23.51638888888889, 'f107A': 67.4, 'f107': 67.7, 'ap': 4, 'ap_a': array([4.   , 4.   , 3.   , 3.   , 5.   , 3.625, 3.5  ])}
-        >> print(para_output)
-        {'d': {'He': 74934329990.0412, 'O': 71368139.39199762, 'N2': 104.72048033793158, 'O2': 0.09392848471935447, 'AR': 1.3231114543012155e-07, 'RHO': 8.914971667362366e-16, 'H': 207405192640.34592, 'N': 3785341.821909535, 'ANM O': 1794317839.638502}, 't': {'TINF': 646.8157488121493, 'TG': 646.8157488108872}}
-        '''
-        para_input,para_output = nrlmsise00(Time(self.t),self.lat,self.lon,self.alt,sw_obs_pre,omode,aphmode)
-        return para_input,para_output   
-
-
-            
diff --git a/pyatmos/msise/__init__.py b/pyatmos/msise/__init__.py
index bab9148..8577228 100644
--- a/pyatmos/msise/__init__.py
+++ b/pyatmos/msise/__init__.py
@@ -3,6 +3,10 @@ pyatmos msise subpackage
 
 This subpackage defines the following functions:
 
+# ==================== nrlmisise-00  model =================== #
+
+nrlmsise00  - Implements the NRLMSISE 00 model
+
 # =================== spaceweather functions ================= #
 
 download_sw - Download or update the space weather file from www.celestrak.com