TAMDAR (Tropospheric Airborne Meteorological Data Reporting) is a weather monitoring system that utilizes sensors mounted on ordinary commercial aircraft for data gathering. It is developed by a company called AirDat, and deployed in the United States.
In response to a government aviation safety initiative, NASA, in partnership with the FAA, NOAA, and private industry, sponsored the development and evaluation of a proprietary multi-function in-situ atmospheric sensor for aircraft. AirDat (formerly ODS of Rapid City, SD), located in Morrisville, North Carolina and Lakewood, Colorado, was formed in 2003 to develop and deploy the Tropospheric Airborne Meteorological Data Reporting (TAMDAR) system based on requirements provided by the Global Systems Division (GSD) of NOAA's Earth System Research Laboratory, the FAA, and the World Meteorological Organization (WMO).
System capabilities 
The TAMDAR sensor offers a broad range of airborne data collection capabilities, including icing and turbulence data that is critical to both aviation safety and operational efficiency of the National Airspace System (NAS). TAMDAR sensors are installed on commercial airliners, where they continuously transmit atmospheric observations via a global satellite network in real-time as the aircraft climbs, cruises, and descends.
TAMDAR observations not only include icing and turbulence, but also temperature, pressure, winds aloft, and relative humidity (RH). Additionally, each observation includes GPS-derived horizontal and vertical (altitude) coordinates, as well as a time stamp to the nearest second. With a continuous stream of observations, TAMDAR provides much higher spatial and temporal resolution compared to the Radiosonde (RAOB) network, as well as a more complete data set than ACARS, which lacks RH. Current upper-air observing systems are also subject to large latency based on obsolete communication networks and quality assurance protocol. TAMDAR observations are typically received, processed and quality controlled, and available for distribution or model assimilation in less than 15 seconds from the sampling time. The sensor requires no crew involvement; it operates automatically, and sampling rates and calibration constants can be adjusted by remote command from the AirDat operations center in Morrisville, NC.
Numerous third-party studies have been conducted by NOAA-GSD, the National Center for Atmospheric Research (NCAR), and various universities to verify the accuracy of TAMDAR against weather balloons and aircraft test instrumentation, as well as quantifying the TAMDAR-related impacts on numerical weather prediction (NWP). Ongoing data denial experiments suggest that the inclusion of TAMDAR data can significantly improve forecast model accuracy with the greatest gains realized during more dynamic events.
The TAMDAR system has been in continuous operation on regional airliners over the central United States since December 2004. AirDat is currently in the process of equipping over 425 aircraft that serve the continental United States and Alaska, which will provide over 5000 daily soundings.