Recent and Upcoming Changes to the National Weather Service's Upper Air Network

The National Weather Service (NWS) Upper Air Observation Program oversees the radiosonde observations completed at the 92 upper air sites across the Continental United States (CONUS), Pacific and Alaska Regions, along with supporting the 10 upper air stations located in the Caribbean. The radiosonde observations form the backbone of weather forecasts, National Hurricane Center (NHC) forecasts and upper air analyses, which support the NWS mission to protect life and property.  The radiosonde observations also serve as the "ground truth" from which satellite temperature and moisture retrievals are derived. In 2005, the Radiosonde Replacement System (RRS) was first implemented into field operations to replace its antiquated Microcomputer Automatic Radiotheodolite (MicroART) system, which had been in operation since the late 1980s. Both of these systems operated at the 1680 MHz center frequency, which was the dominant frequency within the NWS network. In recent years, the NWS has made strides to change the operational frequencies of its systems over to the 403 MHz frequency range, which is utilized by the majority of upper air stations across the world.

The first instance of the changeover was with the Cooperative Hurricane Upper Air Stations (CHUAS) Radiosonde Observations System (CROS) during the winter of 2015-2016.  Since the changeover, the sites have become more reliable as there are fewer parts with the potential for failure, and an upgrade in the functionality of the software being utilized. In addition to the frequency changeover, the CHUAS network switched over from radiosondes with Radio Direction Finding (RDF) generated winds to radiosondes with Global Positon System (GPS) generated winds.  Within the past year, the NWS has been required to transition additional upper air stations to 403 MHz systems due to the launching of the GOES-R satellite. Multiple upper air stations are located in close proximity to the GOES-R downlink stations, and it was determined the current operational radiosondes could pose some interference to the satellite downlink.  This prompted the NWS to develop the Transitional Radiosonde Observing System (TROS) which will be deployed during the fall of 2016 at eight sites within the NWS upper air network.

 This paper will discuss the operational observations conducted at the sites which have been transitioned to the 403 MHz center frequency and the processes which occurred in order for this transition to ensue. It will also confer the additional updates to the NWS Upper Air network to 403 MHz in the future.