Add datasheet links

Author: kanhereashwin

docs/source/reference/reference.rst

@@ -26,6 +26,29 @@ in learning more about GNSS:
    a visually-appealing and interactive blog post about some of the
    basic principles of GNSS positioning.
 
+Reference Documents for GNSS Standards
+--------------------------------------
+
+GNSS constellations and receivers use standardized file formats to transfer
+information such as estimated receiver coordinates, broadcast ephemeris
+parameters, and precise ephimerides.
+The parsers in ``gnss_lib_py`` are based on standard documentation for
+the systems and file systems, which are listed below along with their use
+in ``gnss_lib_py``.
+
+  * *Rinex v2.11* (`version format document <https://geodesy.noaa.gov/corsdata/RINEX211.txt>` __
+    retrieved on 2nd July, 2023): for parsing broadcast navigation ephimerides.
+  * *Rinex v3.05* (`version format document <https://files.igs.org/pub/data/format/rinex305.pdf>` __
+    retrieved on 2nd July, 2023): for parsing broadcast navigation ephimerides.
+  * *Rinex v4.00* (`version format document <https://files.igs.org/pub/data/format/rinex_4.00.pdf>` __
+    retrieved on 2nd July, 2023): currently not supported by ``gnss_lib_py``.
+    *NMEA* (`reference manual <https://www.sparkfun.com/datasheets/GPS/NMEA%20Reference%20Manual-Rev2.1-Dec07.pdf>` __
+    retrieved on 23rd June, 2023): for parsing NMEA files with GGA and RMC messages.
+  * *SP3*: used to determine SV positions for precise
+  * *GLONASS ICD* (retrieved from this `link <https://www.unavco.org/help/glossary/docs/ICD_GLONASS_4.0_(1998)_en.pdf>` __
+    retrieved on 27th June, 2023): for determining GLOASS SV states from
+    broadcast satellite positions, velocities, and accelerations.
+    
 Package Architecture
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