Expert System for Ionogram Reduction (ESIR)

Historical Ionogram Analysis

Automatic ionosondes were developed in the late 1940s that recorded ionograms on film. In the United States, the National Bureau of Standards (NBS) developed a series of automatic ionosondes known as the C-3 and C-4 models that were deployed around the world for the International Geophysical Year (IGY), 1957-58.

Large quantities of ionosonde film have been stored at World Data Centers for decades, but it is rarely used due to the difficulty of viewing and analyzing the film reels. The National Geophysical Data Center (NGDC) in Boulder, Colorado began a project to scan the films into ionogram images.

SEC's Expert System for Ionogram Reduction (ESIR) software has been modified to analyze NBS C-3/C-4 ionograms scanned from film, and the results of a pilot project to analyze the historical ionograms is described here.

Pilot Project: WA938

For the pilot project, ionograms from the ionosonde WA938 were selected, since it was the original test site for NBS development with films dating back to 1948. WA938 was located at Ft. Belvoir, just south of Washington DC. The original site of time station WWV was north of Washington DC, so the 2.5 MHz WWV transmission is available as a reliable frequency marker. When WA938 was shut down, soundings became available from Wallops Island (WP937) that continue to the present time. Thus the WA938/WP937 sounding record is one of the longest in the world, running from 1948 to the present.

The Ft. Belvoir Site

When the NBS Central Radio Propagation Laboratory (CRPL) was established in 1946, it was based in Washington DC along with other NBS departments, and inherited earlier NBS ionosonde work. One of its first tasks was developing a standard ionosonde design for production use. The Radio Propagation Laboratory was established in Sterling, Virginia at that time, but CRPL was relocated to Boulder, Colorado in 1954, and was eventually transferred to the Environmental Science Services Administration (ESSA) in 1965, remaining in Boulder.

With this background, it is obvious why the "Washington DC" ionosonde WA938 was the primary CRPL operating site prior to 1954. It was located at Ft. Belvoir in Fairfax, VA, which was also home to the Army Engineer School and the Army Engineer Research and Development Laboratory during that era. Some of the scanned WA938 notes refer to "Belvoir" and the film ionograms have the label "FB" or "BE" (after 1955) in front of the time stamp referring to this location.

NGDC's SPIDR lists data availability from 1957 to 1968 for WA938, though film ionograms are available back to 1948, with a few earlier samples that were probably from NBS test systems prior to and including the C-series. SPIDR lists Wallops Island WP937 data availability beginning in 1967, though the currently-available scanned images are from the late 1970s. Thus the transfer from WA938 to WP937 took place at about the same time as the WWV move to Boulder and marks the end of NBS radio operations in the Washington DC area. This also means that strong WWV signals should be available for frequency comparison through the WA938 era.

WA938 ( 38.7 N, -77.1 E) is approximately 165 km northwest of the Wallops Island sounder WP937 (37.9353 N, -75.4717 E). They should thus see the same ionosphere, though propagation conditions may be different at Wallops Island due to its proximity to the ocean.

For more information, see:


We wish to acknowledge the support of the NGDC, NSF #AGS-1041939, and original development support through AFRL SBIR #FA8718-04-C-0002. Historical materials were provided by Dr. Robert Hunsucker. Portions of ESIR are covered by U.S. Patent No. 7,541,967.

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