Update README to reflect added gravity model

Author: cmorrison31

README.md

@@ -2,24 +2,36 @@
 [![Release](https://github.com/cmorrison31/TerraFrame/actions/workflows/release.yml/badge.svg)](https://github.com/cmorrison31/TerraFrame/actions/workflows/release.yml)
 
 # TerraFrame
+TerraFrame is a library designed to provided key Earth related calculations and 
+associated functionality to modeling & simulation software. Primarily, 
+TerraFrame provides Earth orientation and gravity routines.
 
-TerraFrame is a library designed to provide earth-orientation functionally to
-modeling and simulation software by calculating the transformation 
-tensor between the Geocentric Celestial Reference System (GCRS) and the 
-International Terrestrial Reference System (ITRS). This transformation accounts for 
-precession, nutation, and polar motion using the IAU 2006/2000A 
-precession-nutation theory and IERS data files.  
+## Earth Orientation
+TerraFrame provides an implementation of the IAU 2006/2000A precession-nutation 
+model which accounts for precession, nutation, and polar motion. Specifically, 
+this implementation provides a transformation tensor between the Geocentric 
+Celestial Reference System (GCRS) and the International Terrestrial Reference 
+System (ITRS).
 
-Additionally, support is provided for converting from geodetic latitude, 
-longitude, and height above ellipsoid to geocentric cartesian coordinates or 
-vice versa. The WGS84 spheroid and a simple spherical earth are provided as 
-built-in options.
+IERS precession and nutation model data files are shipped with TerraFrame. 
+Utility code is also provided which automates the downloading of IERS data for 
+polar motion, UTC, and UT1 offsets.
 
-TerraFrame provides robust datetime and timescale conversion functionality that
-is fully leap second aware. Conversions between UTC, UT1, TT, and TAI are
-provided. The user is encouraged to not work in UTC directly to avoid
-leap second ambiguity. Conversion to UTC from TT or TAI can be safely done
-in post-processing.
+## Gravity
+Gravity calculations are provided by an implementation of the EGM2008. A double 
+Clenshaw summation approach is used to evaluate the spherical harmonics which 
+yields high numerical stability and computational efficiency. TerraFrame is 
+numerically capable of evaluating EGM2008 to it’s complete degree and 
+order: 2190.
+
+By default, only data for degree and order 200 is shipped with TerraFrame for 
+space efficiency. Most near earth modeling & simulation applications will not 
+need to exceed degree and order 100 per WGS84 guidance.
+
+## Ancillary Functionality
+Support is provided for converting from geodetic latitude, longitude, and height above ellipsoid to geocentric cartesian coordinates or vice versa. The WGS84 spheroid and a simple spherical earth are provided as built-in options.
+
+TerraFrame also provides robust datetime and timescale conversion functionality that is fully leap second aware. Conversions between UTC, UT1, TT, and TAI are provided. The user is encouraged to not work in UTC directly to avoid leap second ambiguity. Conversion to UTC from TT or TAI can be safely done in post-processing.
 
 ![Animation of CGRS to ITRS Transformation](https://raw.githubusercontent.com/cmorrison31/TerraFrame/main/Animations/GCRS_to_ITRS.gif)
 
@@ -51,6 +63,8 @@ provided by the following sources:
 - Gérard Petit and Brian Luzum (Eds.). IERS Conventions (2010), IERS Technical
   Note No. 36, Frankfurt am Main: Verlag des Bundesamts für Kartographie und
   Geodäsie, 2010. ISBN: 3-89888-989-6.
+- Deakin, R.E., 1998, 'Derivatives of the earth’s potentials'. Geomatics 
+  Research Australasia, No.68, June, 1998, pp. 31-60.
 
 # Acronyms and Abbreviations
 
@@ -60,6 +74,7 @@ provided by the following sources:
 | CIP  | Celestial Intermediate Pole                                |
 | CIRS | Celestial Intermediate Reference System                    |
 | CEO  | Celestial Ephemeris Origin                                 |
+| EGM  | Earth Gravitational Model                                  |  
 | GCRS | Geocentric Celestial Reference System                      |
 | IAU  | International Astronomical Union                           |
 | IERS | International Earth Rotation and Reference Systems Service |
@@ -71,4 +86,6 @@ provided by the following sources:
 | TT   | Terrestrial Time                                           |
 | UT1  | Universal Time                                             |
 | UTC  | Coordinated Universal Time                                 |
+| WGS  | World Geodetic System                                      |
+