By 1984, more than a decade had passed since the National Aeronautics and Space Administration (NASA) weather and climate program had won approval for a new research mission. There was concern that it would be difficult to justify the budget of the program, so ideas were requested for a new research mission aimed at advancing our understanding of the weather and/or climate. More than a dozen proposals were submitted, including one by North, Wilheit, and Thiele for a mission to observe rainfall directly from space. They called it the Tropical Rainfall Measuring Mission (TRMM).

Studies were conducted to demonstrate that the proposal was feasible by deploying airborne versions of the proposed precipitation radar, microwave radiometer, and visible-infrared radiometer over carefully documented ground-based observations of rainfall. Sampling studies were undertaken to assure that one satellite could adequately sample precipitation events, and advanced mission studies were undertaken to define the mission as well as its cost.

When it became obvious that the cost of the mission would severely limit chances of winning approval, it was decided to invite an international partner to share the cost. With the support of Dr. Bert Edelson, the NASA associate administrator, and through the cooperation of Dr. Nobuyoshi Fugono of Japan, it was possible to study the mission as a joint enterprise. Although the one-year joint mission study concluded that the mission was feasible, obtaining the funding in both countries was anything but simple. When Dr. North decided to leave NASA, Dr. Simpson was suggested as his successor as project scientist. Dr. Simpson's energy and determination were key to winning approval of TRMM by the U.S. Congress. Dr. Simpson had, as President of the American Meteorological Society, briefed Congressman Green of New York on the enormous potential scientific benefits of TRMM. The fiscal year 1991 NASA budget was amended, mandating a new start for TRMM. Once NASA had approval for the mission, Japan agreed to share the costs, and the rest is history. TRMM was launched in 1997 and continues to acquire unprecedented rainfall data on a global scale.