The standard (analog) ionosonde produces photographic records known as ionograms, which show the variations of the virtual height of reflection as a function of the radio frequency. The frequency band normally used is from about 1 MHz to about 20 MHz though some ionosondes operate down to about 0.20 MHz and up to 30 MHz. The ionograms actually show the time of travel of the pulse signal from the transmitter to the ionosphere and back to the receiver. A given station typically generates a sweep-frequency ionogram in about two minutes. Normally four ionograms are taken per hour, one at each quarter hour. As of May 1994, there are approximately 16,140 rolls of ionogram film at our Data Center. Most of these photographic records (ionograms) were recorded on 35mm film, but there are some 16mm ionogram film. One month of 35mm ionograms averages about 800 feet of film. These rolls of film are kept off site but can be retrieved when needed. If you would like to study specific ionograms, researchers are welcome at the NOAA National Centers for Environmental Information (formerly National Geophysical Data Center), but are requested to give adequate advance notice of their visit so that appropriate arrangements may be made for their work space at the Data Center. Some ionograms were digitized and are stored in TIF G4 format.