ESIR Historical Ionogram Analysis

NBS Ionosonde Development

Early US ionosonde development was by the National Bureau of Standards (NBS) and consisted of experimental hardware prior to 1945. In late 1945, design of the NBS Central Radio Propagation Laboratory (CRPL) Model C ionosonde was begun. The C-1 model was based on earlier experimental designs by Gilliland and was intended to provide a "standard" ionosonde that could be produced commercially. The C-1 lacked frequency and height markers, and used a lady's wristwatch as the time stamp; only one prototype was built.

The C-2 model improved the design by adding frequency/height markers and established the standard frequency range of 1-25 MHz. It was first deployed in Brazil for a solar eclipse study in 1947. It became the first production sounder to be deployed at multiple NBS field stations.

The C-3 model was introduced in 1950, and the C-4 model was placed in service in 1957 for IGY. The C-4 sounder and its master clock are shown below.

All three models (NBS C-2, C-3, and C-4) were in use for IGY at numerous US-operated sites and continued to be used into the late 1970s, when digital models began taking over. These "standard" models operated at up to 10 kW peak power, covered 1-25 MHz with a logarithmic sweep (7.5, 15, or 30 second sweep times) with 1 MHz frequency markers. Height marks were selectable at 50 or 100 km. Virtual height ranges of 0-500 km, 0-1000 km, or 0-4000 km could be selected. The antenna was typically a crossed-delta (similar to the CADI antenna.) The differences between the models were small; the main improvement in the C-4 was the use of crystal-controlled oscillators for height markers, pulse repetition rate, etc.

Considerable adjustment and calibration was required. The IGY Instruction Manual (1956) has recommendations for ionosonde operation:

The bottom edge of the ground pulse must be aligned with the zero-height marker to avoid height offsets
Height marks that are not crystal-controlled (C-2, C-3) are liable to drift and must be checked against a 3 kHz reference. Photographic recording of the calibration is recommended.
Frequency marker accuracy should be checked weekly, using either a crystal-controlled oscillator or a standard frequency station such as WWV. A warning: "Cases have occurred where the first frequency mark on ionograms was 2 Mc but was assumed to be 1 Mc, so that critical frequencies have been scaled 1.0 Mc too low." Oops.
It is important to record all measured errors during calibration and forward the information with operating logs.
The lowest trace frequency fmin should be monitored for long-term drifts that could indicate problems with the sounder hardware (transmitter power, antenna performance, receiver gain)
Receiver gain can affect the readability of traces. Changing the gain can affect f_min (used as a rough absorption measure) and fE_s.
At least 1 sweep per hour must be performed, on the hour, based on local standard time. Sweeps at 10 or 15 minute intervals are highly recommended. Continuous sounding is desirable if practical.
Recommend performing at least one sweep per hour at 0-500 km range setting, so range coverage may vary.

The manual includes the following recommended maintenance steps:

Weekly check of receiver settings; frequency markers; height markers; transmitter output.
Monthly check mechanical parts (cams, relays), antenna, and feedline condition.

Ionosonde engineer John Pitts is shown below adjusting a C-3 sounder at the Boulder field station in 1974. [Photo courtesy of Dr. Robert Hunsucker.]

The manual also has a section on the handling of the photographic film used to record the ionograms, and it is clear that the film type, exposure, and processing has a great impact on the quality of the ionograms. Indeed, variations in these factors is probably responsible for much of the month-to-month variation in the overall appearance of the ionograms available for the current analysis. The scanning process adds some variability. Some of the original films were highly-flammable nitrate stock, and were converted to modern safety film at some point in the 1960s, so there are several image transfers involved between the original ionogram and those available for analysis.

Reference

Beynon, W. J. G. and G. M. Brown, eds (1956), Annals of the International Geophysical Year, volume III, IGY instruction manual: the ionosphere, pp. 69-87, New York: Pergamon Press.

Hunsucker, R. D. (1991), Radio techniques for probing the terrestrial ionosphere, pp. 67-93, New York: Springer-Verlag.

Snyder, W. F. and C. L. Bragaw (1986), Achievement in radio: seventy years of radio science, technology, standards, and measurement at the National Bureau of Standards, pp. 413-435, Boulder: National Bureau of Standards.

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