Search Marshall


Text Size

Fact sheet number: FS-1995-04-006-MSFC
Release date: 04/95

Optical Transient Detector

The Optical Transient Detector (OTD) is a small, highly sophisticated system which, from Earth orbit, is able to detect and locate lightning over large regions of the globe. As such, it is one of the early elements of NASA's Earth Observing System within the Mission to Planet Earth program, which is aimed at gaining a better understanding of how the Earth functions as a system.

In recent years, scientists have become increasingly aware of the key role played by lightning in the dynamic interplay of forces occurring in the Earth's atmosphere. Research has indicated, for instance, that lightning may be a very good indicator of the strength of large-scale convective storm systems. The OTD has contributed to the discovery of potential lightning indicators for application to more timely hazardous weather and tornado warnings, and for improved forest fire and wild-land fire management; to the use of lightning as a proxy for detecting intense atmospheric convection; to the production of the most complete and detailed maps of the global lightning distribution; and to the discovery that the global flash rate is approximately 40 flashes per second, less than half of the widely accepted estimates dating back to 1925.

The OTD project has also provided a concrete demonstration of how the development process for new technology systems can be greatly streamlined from the traditional model. Development of the OTD was formally launched at the Marshall Center in June 1993. The finished, tested and calibrated system was completed only nine months after that start date.

System Description

The OTD is a highly compact combination of optical and electronic elements. It was developed as an in-house project at NASA's Marshall Space Flight Center in Huntsville, Ala. The name, Optical Transient Detector, refers to its capability to detect the momentary changes in an optical scene which indicate the occurrence of lightning. The OTD instrument is a major advance over previous technology in that it can gather lightning data under daytime conditions as well as at night. In addition, provides much higher detection efficiency and spatial resolution than has been attained by earlier lightning sensors.

At the heart of the system is a solid-state optical sensor similar in some ways to a TV camera. However, in overall design and many specific features, OTD had to be uniquely designed for the job of observing and measuring lightning from space. Like a TV camera, the OTD has a lens system, a detector array (serving a function somewhat analogous to the retina in the human eye), and circuitry to convert the electronic output of the system's detector array into useful data.

The sensor system (camera) is approximately eight inches in diameter and 18 inches high, while the supporting electronics package is about the size of a standard typewriter. Together, the two modules weigh approximately 18 kilograms (40 pounds). The total weight of the satellite placed on orbit is 68 kilograms (150 pounds).

OTD Operations

Under an agreement between NASA and the Orbital Sciences Corporation, the Optical Transient Detector was carried as a secondary payload on a Pegasus, an Orbital Sciences Corporation air-launched rocket. The Pegasus launch on April 3, 1995, delivered the OTD into an Earth orbit of approximately 710 kilometers (446 miles) altitude, with an inclination of 70 degrees. With that orbit, and OTD's wide 100-degree field of view, it will be able to survey virtually all areas of the globe where lightning normally occurs.

OTD was expected to be in operation for only two years, but still continues collecting data on the occurrence and worldwide distribution of lightning. The data are transmitted on a daily basis from OTD to a ground station in Fairmont, W.Va. From there the data are sent to the Global Hydrology and Climate Center in Huntsville, Ala., for processing, analysis and distribution to the scientific community. The ODT data has provided past field experiment support to the NASA PEM-TROPICS chemistry mission. In the summer of 1997, OTD data will be used to support mission planning during the NASA SONEX mission.

The OTD data and associated scientific research has confirmed its importance in contributing to the understanding of atmospheric and precipitation processes. This contribution justifies the need for similar observations from geosynchronous orbit.


Two doctorate degrees have been completed making use of OTD data, and a total of 10 masters's and doctorate degrees are now under way which incorporate OTD observations into experimental and theoretical studies. OTD data has been used in a cooperative project between the Marshall Center and the Huntsville City Schools involving five high school students.

Using a unique vantage point in space, the Optical Transient Detector promises to expand scientists' capabilities for surveying lightning and thunderstorm activity on a global scale. At the same time, it will help prepare the way for the future, when more systematic monitoring of a wide range of indicators will allow scientists to better track our Earth's vital signs.