Milestones:The MU (Middle and Upper atmosphere) radar, 1984

Date Dedicated
Dedication #
Kyoto University, Koyama, Shigaraki-cho, Koka-city, Shiga, Japan
IEEE Regions
IEEE sections
Achievement date range


The MU (Middle and Upper Atmosphere) Radar, 1984


In 1984, Kyoto University built the MU (Middle and Upper atmosphere) radar as the first large-scale MST (Mesosphere, Stratosphere, and Troposphere) radar with a two-dimensional active phased array antenna system, with the collaboration of Mitsubishi Electric Corporation. The MU radar enabled continuous and flexible observation of the atmosphere, and has contributed to the progress of atmospheric science and radar engineering.

Street address(es) and GPS coordinates of the Milestone Plaque Sites

Shigaraki MU Observatory, Research Institute for Sustainable Humanosphere, Kyoto University, Koyama, Shigaraki-cho, Koka-city, Shiga 529-1812 Japan. Latitude of 136°06’32”E and longitude of 34°51’08”N.

Details of the physical location of the plaque

In the ground floor entrance hall.

How the intended plaque site is protected/secured

The plaque will be placed near the reception area at the floor entrance hall of Shigaraki MU Observatory. The observatory welcomes any visitors. Prior notification is required before a visit. The contact information the visitor will need is as follows.

Shigaraki MU Observatory

Tel +81-748-82-3211, Fax +81-748-82-3217

Historical significance of the work

The MU radar is the large-scale atmosphere radar where the active phased array mechanism was used for the first time in the world.

(1) Conventional observation methods and their limitations:

Before the MU radar, the rawinsonde (meteorological balloon) and rocket observed the stratosphere and the mesosphere, and the satellite observed the thermosphere and space. The former observations have a defect in that we can obtain the data only infrequently, and the latter also has one inasmuch as satellites do not give consecutive observations of a specific point. The radar offers a method of remote sensing of the atmosphere from the Earth’s surface. The Arecibo radar in Puerto Rico and the Jicamarca radar in Peru were developed as radars capable of observing the atmosphere and the ionosphere, and their immense utility has become clear. With time, the importance of three-dimensional wind velocity observations was recognized and this led to the development of the MU radar.

The U.S. implemented the Arecibo radar and the Jicamarca radar in the 1960’s as precursor atmospheric radars. They were, however, inappropriate for observing meteorological phenomena or atmospheric waves, since they could not perform high speed beam steering. As a new radar capable of overcoming this problem, the MU radar was designed. The active phased array technique was introduced as the key to satisfy this requirement in observations and research. This was the first such application of the active phased array technique to a large-scale system, and it has had a significant impact on the design of subsequent, similar radar systems, thus leading us to believe that the MU radar deserves recognition as an IEEE milestone.

(2) Importance of the observation of middle atmosphere dynamics:

The Earth is covered with an atmosphere which spans from Earth’s surface to outer space. The atmosphere consists of the troposphere (surface to 10 km altitude), the middle atmosphere (10 - 110 km, consisting of the stratosphere, the mesosphere, and the lower thermosphere), and the upper atmosphere (110 km and higher), the latter two including the ionosphere (60-500 km).

The ambient air motion in the troposphere and the stratosphere directly affects the global climate and daily weather, and the dynamics of the middle atmosphere plays critical roles in investigation of the behavior of various meteorological phenomena, including both temporary weather changes such as sudden downpours, typhoons, cold waves, and heat waves as well as long range phenomena such as seasonal winds.

Observation of the middle atmosphere is also important for environmental studies such those relating to global warming, depletion of the ozone layer, and the effects of CFC (chlorofluorocarbon) gas. CFC gas originating from sprays can reach the middle atmosphere and destroys the ozone layer which protects us from the ultraviolet rays of the sun. Observation of the middle atmosphere enables extensive studies in the field of abnormal weather caused by volcano eruptions, environmental pollution, effects of atmospheric gravity waves, and atmospheric tides based on solar radiation.

(3) Necessity of radar observations with fine temporal and spatial resolution, and of high speed beam steering:

In the middle atmosphere, there always exists a certain amount of turbulence. If radio waves are used to illuminate this turbulence, a small amount of the wave is scattered back to the radar on Earth. By measuring this scattered wave, we can obtain information about the turbulence.

In order to investigate atmospheric dynamics, especially in micro-scale and meso-scale processes occurring in the middle and the upper atmosphere, it is essential to observe the three-dimensional wind field, including a small vertical component, continuously both in time and space over a given domain, with fine temporal and spatial resolution. Fast and continuous beam steerability is necessary to determine the fine spatial structures of fast dynamic processes.

(4) Design and development of the MU radar with two-dimensional active phased array system:

The MU radar is the first large-scale two-dimensional active phased array radar developed through original technology [1, 3, 4, 11]. The MU radar can observe the lower, middle, and the upper atmosphere. It can observe continuously, regardless of weather condition.

The operational frequency of the MU radar is 46.5 MHz, and its peak output power is 1 MW. It is composed of 475 crossed three-element Yagi antennas and an equivalent number of solid-state transmitter–receiver (TR) modules. Each Yagi antenna is driven by a TR module with peak output power of 2.4 kW. This system operates as an active phased array radar to achieve very rapid and almost continuous beam steering. The MU radar has the capacity to continuously monitor three-dimensional winds, waves, turbulence, and atmospheric instability over the wide range of altitudes found in the Earth’s atmosphere. Furthermore, it has a temporal resolution of approximately 1 min and an altitude resolution of approximately 100 m, unequalled by conventional instruments such as the radiosonde. Due to these resolutions it is possible to quantitatively investigate the small-scale atmospheric gravity waves that are considered to play important roles in the dynamics of the Earth’s atmosphere.

(5) The scientific impacts of the MU radar:

The atmospheric gravity waves produced in the lower atmosphere propagate to the middle and upper atmosphere and carry momentum. This plays a decisive role in determining the dynamical structure of the middle and upper atmosphere. The MU radar contributed to scientific progress by confirming this mechanism which had been theoretically predicted [2, 5, 6, 7, 8, 9, 10].

Furthermore, the later systems that followed the MU radar have contributed considerably to efforts to improve the precision of weather forecasts, particularly predictions of local meteorological phenomena, with great benefit to societies impacted by these phenomena.

(6) The social impacts of the MU radar

MU radar observed actual structures and mechanisms of various meteorological phenomena, from local extreme weathers to global atmospheric behaviors. Its observation served to explore the countermeasures to minimize damages of weather disasters and environment disruptions.

The Significance of the MU radar

- It can simultaneously capture the raindrop and the airflow when they are mixed, whereas the conventional meteorological radar can see only the raindrop. [A1]

- It observed the precise typhoon structure and the eye cross-section for the first time in the world. [A2]

- It revealed birth and growth processes of the violent raincloud causing the local downpour, etc. [A1]

- Its observation of the precise meteorological phenomena led precise weather forecasting. [A1] [A2]

- Its long-term observation of the vertical atmospheric movement led to quantifying the irregular atmospheric turbulence and its daily and seasonal fluctuation, for the first time in the world. The result revealed that the greenhouse gas diffusion is much slower than it had been believed. Its observation served as a concrete evidence for the global environmental sustainment measures. [A3]

- Its leading edge technologies of the active phased array antenna were utilized and followed by numerous atmospheric radars constructed later. [A4]

(7) The MU radar's influence on later systems:

The MU radar established the technological basis for a number of wind profiler systems and phased array meteorological radars that were later developed.

The MU archetype has been directly used for WINDAS (Wind Profiler Network and Data Acquisition System) operated by the Japan Meteorological Agency, which observes the three-dimensional wind from about 30 locations in Japan, EAR (The Equatorial Atmosphere Radar) at West Sumatra (Indonesia), and PANSY (Program of the Antarctic Syowa MST/IS radar) at Syowa Station (Antarctica). It has also influenced the MAARSY (The Middle Atmosphere Alomar Radar System) at Andøya (Norway).

The other active phased array radars constructed or planned after the MU radar include AMISR (the Advanced Modular Incoherent Scatter Radar funded by the National Science Foundation) at mobile sites in the United States and EISCAT_3D (next-generation radar project for atmospheric and geospace science conducted by the European Incoherent Scatter Scientific Association) in Tromsø (Norway), Kiruna (Sweden) and Sodankylä (Finland).

Features that set this work apart from similar achievements

There had been several previous large-scale atmospheric radars, such as the ones in Jicamarca and Arecibo.

The most outstanding feature of the MU radar is that there is no high-power transmitter in it. In conventional radar systems, a high-power transmitter feeds all phased array antenna elements via an appropriate cascading feeding network. The MU radar system, on the other hand, does not incorporate such a passive array connected to a high-power transmitter. Instead, each element of the phased array antenna is provided with its own solid state power amplifier, and all the amplifiers are coherently driven by low-level pulses in order to produce the desired peak output power. The peak output power of the individual antenna array is 2.4 kW. Since the total number of antennas is 475, the total peak output power becomes approximately 1MW allowing for antenna loss.

The main advantage of this active array system is that the phase of the signal transmitted from each antenna required for the beam steering is electronically controlled at low power level. Thus the MU radar system enables very fast transition (up to 2500 times per second) and almost continuous beam steering as well as various flexible operations made possible by dividing the antenna array into independent sub-arrays. The 475 antennas can be divided into 25 antenna sub-arrays, and can operate as at the maximum of four individual small radars [3, 4].

The MU radar was the first large-scale active phased array atmospheric radar, and the other radars constructed later followed the mechanisms of the MU radar. The MU radar was upgraded in 2004 and its contribution continues [12, 13].

Significant references

There are over five hundred papers and articles related to the MU radar. The most significant papers among them are listed below.

[1] Kato, S., T. Ogawa, T. Tsuda, T. Sato, I. Kimura, and S. Fukao, The Middle and Upper Atmosphere Radar: First Results Using a Partial System, Radio Sci., 19, 1475-1484, 1984.

[2] Fukao, S., K. Wakasugi, T. Sato, S. Morimoto, T. Tsuda, I. Hirota, I. Kimura, and S. Kato, Direct Measurement of Air and Precipitation Particle Motion by Very High Frequency Doppler Radar, Nature, 316, 712-714, 1985.

[3] Fukao, S., T. Sato, T. Tsuda, S. Kato, K. Wakasugi, and T. Makihira, The MU Radar with an Active Phased Array System: 1. Antenna and Power Amplifiers, Radio Sci., 20, 1155-1168, 1985.

[4] Fukao, S., T. Tsuda, T. Sato, S. Kato, K. Wakasugi, and T. Makihira, The MU Radar with an Active Phased Array System: 2. In-House Equipment, Radio Sci., 20, 1169-1176, 1985.

[5] Matuura, N., Y. Masuda, H. Inuki, S. Kato, S. Fukao, T. Sato, and T. Tsuda, Radio acoustic measurement of temperature profile in the troposphere and stratosphere, Nature, 323, 426-428, 1986.

[6] Tsuda, T., T. Inoue, D. C. Fritts, T. E. VanZandt, S. Kato, T. Sato, and S. Fukao, MST Radar Observations of a Saturated Gravity Wave Spectrum, J. Atmos. Sci., 46, 2440-2447, 1989.

[7] Yamamoto, M., S. Fukao, R. F. Woodman, T. Ogawa, T. Tsuda, and S. Kato, Mid-Latitude E-Region Field-Aligned Irregularities Observed with the MU Radar, J. Geophys. Res., 96, 15943-15949, 1991.

[8] Nakamura, T., T. Tsuda, M. Yamamoto, S. Fukao, and S. Kato, Characteristics of Gravity Waves in the Mesosphere Observed with the Middle and Upper Atmosphere Radar: 1. Momentum Flux, J. Geophys. Res. 98, 8899-8910, 1993.

[9] Fukao, S., M. D. Yamanaka, N. Ao, W. K. Hocking, T. Sato, M. Yamamoto, T. Nakamura, T. Tsuda, and S. Kato, Seasonal variability of vertical eddy diffusivity in the middle atmosphere, 1. Three-year observations by the middle and upper atmosphere radar, J. Geophys. Res. 99(D9), 18973-18987, 1994.

[10] Sato, K., H. Hashiguchi, and S. Fukao, Gravity Waves and Turbulence Associated with Cumulus Convection Observed with the UHF/VHF Clear-Air Doppler Radars, J. Geophys. Res., 100(D4), 7111-7119, 1995.

[11]Chiba, I., Y. Konishi, T. Nishino, Progress of Phased Array Systems in Japan, 2010 IEEE International Symposium on Phased Array Systems and Technology (ARRAY), 19-28, doi:10.1109/ARRAY.2010.5613395, 2010.

[12] Sureshbabu, V.N., V.K. Anandan, T. Tsuda, J. Furumoto, and S.V. Rao, Performance Analysis of Optimum Tilt Angle and Beam Configuration to Derive Horizontal Wind Velocities by Postset Beam Steering Technique, IEEE Transactions on Geoscience and Remote Sensing, 51, 520-526, doi:10.1109/TGRS.2012.2200256, 2013.

[13] Kumar, S., V.K. Anandan, T. Tsuda, J. Furumoto, and C.G. Reddy, Improved Performance in Horizontal Wind Estimation Using a Spaced Antenna Drift Technique and Signal Processing Approaches, IEEE Transactions on Geoscience and Remote Sensing, 51, 3056-3062, doi:10.1109/TGRS.2012.2214442, 2013.

Additional References of the MU radar's social impacts

[A1] The Raincloud Development Process Revealed, Kyoto Univ. Group Uses VHF Radar, Simultaneous Measurement of the Atmospheric Movement and the Raindrop, Nikkei Newspaper, Oct. 2, 1985.

[A2] Caught the Typhoon-eye (Kyoto Univ. Group), Yomiuri Newspaper, Tuesday Evening, Dec. 13, 1994.

[A3] Air Pollutant Diffuses Slowly, Freon (CFC) Resides ten times longer than the established value, Kyoto Univ. Revealed the Turbulence Movement, Yomiuri Newspaper, Tuesday Evening, Aug. 12, 1993.

[A4] "Eye" for Abnormal Weather Clarification, Kyoto Univ. Build a radar in Sumatra, Kyoto Newspaper, March 23, 2001.

Supporting materials



Loading map...