Radiosonde Replacement System
March 3, 1999
1.0 Background and Purpose
The National Weather Service (NWS) has begun an effort to replace its current network of obsolete radiosonde observing systems with a modern system that improves data availability and accuracy, operates within a reduced frequency spectrum, and is more efficient to operate and maintain. This network of 102 stations reaches from the Caribbean to Pacific islands, and from Alaska to south of the equator. The network includes systems located at NWS offices, contractor-staffed stations, stations staffed by other agencies, and stations staffed by other countries. Approximately 75,000 to 80,000 radiosondes are flown annually in twice-daily releases throughout the NWS-supported network. The observations are distributed on various domestic and international telecommunications systems. The data are also archived by the National Climatic Data Center in Asheville, North Carolina. The radiosonde data set is one of the most critical observational tools for obtaining atmospheric measurements required for numerical weather prediction, severe weather prediction, air pollution transport prediction, aviation operations, and the study and prediction of global climate change. The radiosonde data also serve as the benchmark for deriving estimates of temperature and moisture from satellite and ground-based thermodynamic profiler measurements.
The term radiosonde is a contraction for radio-sounding device. The instrument measures the ambient pressure, temperature, and moisture. Winds are determined from changes in the radiosonde position during the flight. All of this information is formulated into a rawinsonde observation. When attached to a weather balloon filled with a lighter-than-air gas, radiosondes can attain heights in excess of 30 kilometers. The radiosonde transmits its data to a ground-based telemetry system (antenna and receiver). This telemetry system receives the signals and forwards them to another module (signal processing system) to be decoded into meteorological units. Data are then passed to a computer for collection of data for the entire sounding and formulation of the observation products for general distribution. When the balloon reaches its elastic limit and bursts, a parachute slows the descent of the radiosonde to the ground. Recovered radiosondes can be returned to the factory for reconditioning and re-flown, thus reducing the overall cost of operating the program.
2.0 Assessment of Need
There are a number of issues threatening the continued operation of this critical component of NOAA's infrastructure. The main two areas relate to maintenance of the current system and its requirement for a large radio-frequency band.
The mechanical portions of the ground-based system were installed in the 1950s and certain electronic components were implemented in the early 1980s. More than 90% of the system parts are currently obsolete, making the system more difficult to maintain. The numerous integrated circuit chips used throughout the system are particularly difficult to replace. Due to the age of the equipment, failures have increased and will become routine if the system is not replaced. The number of parts repaired has risen by more than 100% in the past five years. The supply of parts necessary to maintain the outdated IBM PC/XT computer is only sufficient to last two years, and the software must be rewritten before use on a newer computer. At some stations, additional ground equipment (transponders) provide special tracking of the radiosondes when they are carried over the horizon in high-wind situations. This transponder equipment uses vacuum tubes which are no longer available. In 1990, 33 stations had transponder units. Currently, only two stations have the equipment still functioning. As a result high-altitude wind data are lost and the placement of the jet stream in upper level analyses can be in error. These occasional errors degrade numerical prediction models and also degrade the application of the data for aviation users projecting aircraft fuel usage.
The current generation of radiosondes use a single-stage transmitter design demanding large portions of the frequency spectrum, namely 1670 through 1690 MHz. This design uses a greater portion of the frequency spectrum compared to more expensive components capable of maintaining a more stable transmission frequency. The current ground receiving equipment receives signals across the same large range, matching the radiosondes. As a result of the Omnibus Budget Reconciliation Act of 1993, the Government has reallocated 5 MHz (1670-1675 MHZ) to the private sector as of January 1, 1999. The current radiosonde ground systems will be very susceptible to interference from new transmitters operating in this broad frequency band. This would result in lost data during portions of the radiosonde flights and cause an inability to operate the system at some locations. Furthermore, the radiosonde transmitters will be required to be tuned to a higher frequency to prevent interference with new receivers operating in the 1670-1675 MHz frequency band. Use of higher frequencies in the approved range for radiosonde operations will increase the existing interference between radiosondes and data transmissions from meteorological satellites to various users across the country.
3.0 Project Objective
To meet the continuing requirement of a broad base of users for atmospheric sounding data, the NWS will replace its entire current network of radiosonde systems with a modern design to:
1. Maintain or increase system availability and data accuracy,
2. Reduce the use of radio-frequency spectrum,
3. Require less operator interaction and maintenance,
4. Provide a complete high-resolution data set to users,
5. Provide a balloon inflation and launch shelter capable of controlled balloon launches for use at NWS facilities that have space limitations, and
6. Provide consistent and accurate measurement of surface weather parameters at the point of balloon release.
The NWS must implement and operate an economical and efficient upper air observing system over the next 20 to 25 years to continue meeting the data availability requirements of weather forecasters and climate researchers. The new system must make efficient use of allocated radio frequencies in order to coexist with other users in the same or nearby spectrum band. The system must operate as much as possible in an automated mode, yet allow operator intervention when appropriate.
The current radiosonde system provides for the distribution to users of only a small portion of the data collected. These data sets do not contain sufficient detail to fully meet the needs of today's numerical prediction models. The replacement system will distribute high-resolution data sets and give users access to the full set of observed data collected by the system. The newer generations of numerical prediction models are now able to apply this high- resolution data. The data sets will include both processed and unprocessed data. The processed data will meet user requirements for the highest quality data (corrected for solar radiation effects on sensors). Unprocessed values will be provided for climate researchers, who require the capability to reprocess old data using newer algorithms to maintain data continuity.
The modernization and restructuring of the NWS has relocated many facilities. Construction of large balloon inflation buildings supporting the current antenna system are impractical in certain cases. Balloon launches from these current structures are performed manually by the observer, who carries the balloon to a suitable launch point to avoid having the radiosonde crash into the launch shelter. Smaller balloon inflation shelters can be used to provide the capability to automatically release the balloon, saving observer time and requiring less obstruction-free real estate surrounding the release site. These new shelters will be used on a case by case basis at locations lacking suitable conventional inflation buildings. Rooftop installation of these shelters will be possible in cases of extreme land restrictions and urban environments.
Radiosonde soundings require the accurate determination of surface weather parameters (temperature, humidity, pressure, and wind) to successfully process the data from the complete sounding. Surface data are used in the calculation of the height of the radiosonde (using pressure measurements from the surface and radiosonde). Small errors in surface pressure, for example, can lead to large height errors later in the sounding. Most NWS automated surface observing equipment (Automated Surface Observing System) is positioned more than a quarter mile from the balloon release point, and is not representative of the upper air site environment. A dedicated suite of instruments will be deployed for consistent measurement of surface weather data supporting the radiosonde operation.
4.0 Acquisition Approach and Schedule
The replacement radiosonde system will consist of several components which will be acquired in separate acquisitions. The telemetry subsystem (antenna and receiver) is to be a generic design which will allow the ground system to work with different brands of radiosondes. This is critical for promoting competition in the acquisition of the radiosondes, minimizing costs throughout the lifetime of the system. The radiosondes will be purchased in combination with a signal processing subsystem that will decode the proprietary signals from the radiosonde into a standardized meteorological data stream which is passed to the Government's computer. The computer will be a Pentium computer running Windows NT, which will in turn be connected to the AWIPS local area network. Surface weather observing equipment and balloon inflation and launch shelters will be purchased separately.
The acquisition of these various system components is beginning in FY 1999. The evaluation of the prototype radiosondes and ground systems will be completed, and the initial production contract will be awarded during FY 2000. Operational implementation of the new systems at field sites would occur beginning early FY 2002 through FY 2004. The implementation has been phased across three years in order to keep the necessary annual funding at lower levels.
For the past several years, development of the software for the Government workstation has been underway. The implementation of this software is planned in three stages: 1.) To replace the IBM PC/XT currently used with the existing ground equipment, 2.) Attach the new upper air workstation to the AWIPS LAN, and 3.) To integrate the new radiosonde ground system. This supports a phased-in approach to implementing the replacement system equipment. Implementation of the new workstations (interfaced to the current radiosonde ground equipment) would begin in FY 2000, and be completed in FY 2001.
Key Program Milestones
|FY 1999||Develop Upper Air PC Software.
Begin systems acquisitions.
|FY 2000||Begin replacement of IBM PC/XT systems.
Complete radiosonde/ground system prototype evaluation.
Award production contract for radiosonde ground system.
Award contracts for surface instrumentation and launch shelters.
|FY 2001||Award production contract for radiosondes.
Complete replacement of IBM PC/XT systems.
|FY 2002-04||Implement new radiosondes and associated ground equipment.|