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Probing the Sky with Radio Waves

FROM WIRELESS TECHNOLOGY TO THE DEVELOPMENT OF ATMOSPHERIC SCIENCE

THE UNIVERSITY OF CHICAGO PRESS

Excerpt

CHAPTER 1

Introduction: From Propagation Studies to Active Sensors

On 26 October 2004, the Cassini Orbiter had its first close flyby of Titan, Jupiter's largest moon. Under the control of the Jet Propulsion Laboratory (JPL) at California Institute of Technology in Pasadena, the spacecraft had undertaken a seven-year cosmic odyssey since its launch from the Kennedy Space Center in Florida. The JPL staff and participating researchers around the world had waited for this day, since one of the project's major missions—arguably its primary task—was to explore Titan. As an exploratory platform, the Cassini boasted a dozen detectors, including two state-of-the-art cameras that had captured stunning images of Saturn and its rings and a cadre of spectrometers to monitor the chemical composition of any radiating celestial body.

But Titan posed a particular challenge to the instrument designers. It is the only satellite of the solar system possessing an atmosphere, and a thick, yellow haze of hydrocarbons almost perpetually blocks it to cameras and spectrometers. Even the onboard Huygens probe—the landing unit that the European Space Agency had made for exploring Titan's surface—was not the complete solution, as it could take measurements only near its landing site. To reveal the yellow moon's macroscopic geological characteristics, the Cassini team rested its hopes on imaging radar. Unlike cameras and spectrometers, which received light, energy, or particles that emanated from the observed object, the radar bounced microwaves off the object and timed their return. This apparatus did not disappoint the JPL staff. During the flyby, it scanned 1 percent of Titan's overall surface and relayed the echoed signals back to earth. Three days later, JPL's Media Relations Office proudly displayed the first radar images of Titan, which unveiled such novel features as the active surface, complex terrains, and the possible existence of lakes. The Titan radar had made its début.

The mapping of Titan's geology signified a mode of seeing that has permeated our world, ranging from the spectacular weather-radar images of ash out of the recently erupting Eyjafjallajõkull volcano in Iceland and the underwater sounding of the Titanic's debris in the North Atlantic, through the mundane altimeters every aircraft now carries and acoustic pulse-echo devices popular among oil-drilling stations, to the ubiquitous magnetic-resonance imaging (MRI), ultrasound, and X-ray machines that adorn modern hospitals. In none of these endeavors do instruments passively observe and measure objects in a nonintervening manner; rather, an acoustic wave, electronic beam, electromagnetic wave, or other form of energy or particle flow "pokes" the objects and then reconstructs their properties from their modification of the flow. This is the principle of the active sensor, one of the most powerful scientific instruments since the early twentieth century.

Instruments are never "just" instruments. Introducing a new instrument is not simply the addition of more advanced hardware to enhance human capacity. As history shows, it often accompanies a sea change of understanding and doing things: the telescope initiated the Scientific Revolution; the air pump nurtured laboratory science; the thermometer pioneered quantitative experimentation; the microscope redefined diseases; the particle accelerator made "big science"; the polymerase chain reaction heralded the genetic worldview. Likewise, the employment of active sensors represents a distinct approach to probing nature, the body, and artifacts that involves not only instrument design but also the making of theories and experimentation.

How did the approach of active sensing come into existence? What characterized this approach as it was developing? How did such a novel mode of seeing change the meanings of experimentation and the patterns of experimental practice? How did it affect the standard of legitimate evidence? How did theories of wave or particle propagation help form and refine active sensing? What kinds of epistemic functions did these theories aim to undertake? Why did this mode of seeing prevail?

While the complete answers to these questions require an overwhelming comparative analysis and synthesis of all active sensors that easily go beyond the scope of a monograph, investigation of an informative case may shed some light on such vexing puzzles. This book examines research on radio ionospheric propagation between 1900 and 1935. It is a story of mutual shaping between wireless technology and atmospheric science. After Guglielmo Marconi's first successful transatlantic test in 1901, scientists were curious about why and how radio waves could propagate over such a long distance without the earth's blocking them. From 1901 to 1925, European theoreticians and American engineers grappled with this problem. Its solution led to the discovery of an electrically active region in the upper atmosphere, which they named the "ionosphere."

This revelation opened a new field in earth sciences, and, with the assistance of propagation studies, initiated a novel method of experimentation based on manipulating waves: sending radio waves to the ionosphere and detecting their return. Known as "radio sounding," this method transformed atmospheric studies from passive observation to active experimentation, undercutting the traditional distinction between field and laboratory sciences. From wireless to geophysics, the emergence of studies of radio ionospheric propagation occupies a significant position in the history of active sensing: it began this mode of seeing with electromagnetic waves and led directly to radar during World War II and various sensors in space exploratory programs since Sputnik and Apollo.

FROM PROPAGATION STUDIES TO ACTIVE SENSING: EXPERIMENT AND THEORY

Similar to the emergence of some other active sensors, the history of radio ionospheric propagation displays a transformation from studies of wave propagation to development of active-sensing systems. Looking at images or data from a lidar, radar, seismic sounder, sonar, or X-ray machine, we may assume that the stream of energy that the instrument sends to observed objects is a transparent medium that merely helps to illuminate the invisible, like a spotlight on a dark stage. But that is not the case. Far from being transparent, that medium is usually complex, entangles itself with imaging and measuring, attracts researchers' attention for its own sake, and thus has a rich history.

The origin of radio ionospheric sounding attests to the importance of wave-propagation research. What spurred radio echo-sounding probes of the ionosphere in the mid-1920s was not geoscientists' pressing need to measure the upper atmosphere, but physicists and engineers' desire to understand how radio waves propagated over long distances above the earth. Only after the discovery of the ionosphere and the invention of the sounding-echo scheme during wave-propagation research did scientists refocus from the waves to the upper atmosphere.

Along this axis of transformation, the history of radio ionospheric propagation epitomizes the challenges that active sensing has brought to our understanding of modern science and technology. In its first thirty years—from Marconi's wireless test to the establishment of the so-called magneto-ionic theory—which constitutes the scope of this book, such a history raises at least three major issues in experiment and theory: Is it possible to experiment outside laboratories? How do we define direct evidence? What role does theory play at different stages of research?

In a subsequent book, I will examine the development and ramification of the automatic ionospheric sounders in the 1930s based on the theoretical and experimental work on radio propagation, and the establishment of radio ionospheric forecasting services around the world during World War II. This forthcoming work will address more closely the issues of instrumentation and technology in radio ionospheric research.

The issues of experiment and theory raised in the development of active sensing were embedded in a broader context of changing senses of reality at the turn of the twentieth century. Historians have found that scientists during this period were increasingly concerned with the epistemic ground of various experiments, observations, and instruments that promised to make the invisible visible: Do scientific instruments uncover phenomena, or create them? What is the role of sensory experiences in the process of generating empirical knowledge? How does one make claims about microscopic or hidden entities based on macroscopic or observable effects? While the scientists' views on these questions diverged, they were all aware of the instrument-mediating character of scientific evidence, and the shaping force of instruments on experiment and theory.

Field Experiments and Direct Evidence

Above all, studies of radio ionospheric propagation in the early twentieth century broaden our historical understanding of experiment. The empirical investigations on wave propagation and the ionosphere, like the research and development relating to many other active sensors, had to take place outdoors. The scale could be as large as several thousand miles, and the objects of interest were geophysical in nature. These outdoor measurements and tests were by no means feasible in any laboratory. Therefore, we may not be able to understand them in terms of the laboratory studies that historians have explored in the past. For example, measurements of wave propagation and radio sounding of the ionosphere hardly followed what historians Steven Shapin and Simon Schaffer have called the "laboratory form of life": Control and manipulation of material conditions and relevant variables were often very challenging; replication was usually almost impossible; authoritative eyewitnesses of results were rare; and investigators aimed not so much to generate novel "matters of fact" or "scientific effects" as to figure out how those scientific effects interacted with large-scale nature.

Rather, the empirical work in this story resembled more the tradition of field sciences such as astronomy, botany, geology, geodetics, meteorology, and zoology. What characterized these sciences and radio ionospheric propagation alike were comprehensive and extensive fieldwork, careful preparation for expeditions, meticulous collection of data, and precise instrumentation for observations. The "Humboldtian approach" marked an apex of efforts to turn natural history into integrated, modern field science.

Nevertheless, calling radio ionospheric propagation Humboldtian may downplay its experimental features. Throughout the first half of the twentieth century, the physicists and engineers measuring wave propagation and sounding the ionosphere frequently called their activities "experiments." Much of their practice closely resembled experimentation rather than field observation: their instruments, not nature, produced the radio waves. Although they could not control the macroscopic geophysical structures that shaped the propagation of radio waves over distance, they could manipulate radio waves, including their frequencies, power, polarizations, and waveforms.

Such delicate control encouraged them to tinker with devices, redesign procedures, coordinate measurements, and manipulate signals. For example, military engineers tested wireless equipment between warships, radio amateurs demonstrated long-range radio communications with coordinated voluntary actions all over Europe and the Americas, physicists explored scientific effects between ground and sky at particular experimental sites, and geoscientists acquired data from networks of observing stations (which resembled labs) and interpreted them. These were not laboratory but rather field experiments in our eyes and in theirs. Instead of working indoors, they experimented outdoors and turned nature into a laboratory.

How credible was their empirical evidence? While wave propagation studies had suggested the possibility of an electrically active upper atmosphere, general acceptance of the ionosphere's existence occurred only after the sounding-echo experiments in the 1920s. Why? Many scientists believed that the sounding-echo experiments generated "direct" evidence for the ionosphere. But what was direct about the evidence produced by this particular approach? Control in field experiments, I believe, offers the answer: unlike propagation experiments, which only changed the transmitting radio waves' power and frequency, the sounding-echo tests relied on more elaborate control of waveforms. Instead of sinusoids modulated by Morse code dots and dashes, radio waves now could be chirps, pulses, or other patterned undulations, which scientists designed so that their return, scattering, or deflection from an unknown entity would exhibit their properties more clearly.

Here Nancy Cartwright and Ian Hacking's concept of entity realism may help us: a scientific object is real if we can manipulate it. Sounding-echo experimenters in the 1920s could not modify the ionosphere, yet they could tinker with the transmitting radio waveforms as malleable signals and observe the corresponding changes at the receivers. The introduction of waveform control in propagation experiments made the ionosphere seem more "real" and transformed propagation studies into active sensing.

Epistemic Status of Theories

A central desideratum of research on radio ionospheric propagation was to understand how radio waves traveled above the ground, across water, within the atmosphere, around geographical obstacles, or in any other open environment on the earth. Through the first half of the twentieth century, scientists and engineers proposed, elaborated, and fought over several theories: surface diffraction, atmospheric reflection, ionic refraction, and magneto-ionic refraction. The contest between these theories spurred studies of radio ionospheric propagation, especially up to 1930.

However, it is misleading to interpret the story as one theory replacing another, like Hempel-style expansion of covering laws or Kuhnian paradigm change. Scientists devised these theories for different purposes, and they served different functions. Although they did generate mutually incompatible predictions on some empirical questions, and although researchers fiercely debated such forecasts, more often they operated within their own realms and were either irrelevant or marginal to others.

This plurality becomes clear as we examine the epistemic status of wave-propagation theories. At least six questions are germane here. What was a theory's aim and function? What was its most important intellectual virtue? What was the empirical knowledge essential to the theory? What were the central questions it meant to answer? What was its method for tackling these questions? What sort of answers did scientists expect?

The two dominant theories of wave propagation between 1900 and 1920, for instance, differed in nature, even though they both attempted to explain the possibility of long-distance radio. Consistent with Pierre Duhem's two types of scientific theories, the hypothesis of surface diffraction aimed at formal representation of an empirical fact, whereas atmospheric reflection proposed causal explanations for a broader set of wireless phenomena.

Mathematical physicists worked on surface diffraction and sought a mathematical model to represent long-distance propagation of radio waves along the earth's curvature. Their model comprised a wave equation and a simple boundary condition and gained a life of its own. It became more and more a platform to develop approximating techniques in solving differential equations instead of a reference point for empirical observations. In other words, mathematics was replacing physics.

By contrast, radio engineers were the main explorers of atmospheric reflection. The theory's mathematical structure was much cruder and simpler than surface diffraction before 1919. It boasted no differential equations, no Bessel functions, no asymptotic approximations; it worked with just naive ray tracing and geometric optics. But formal refinement was never the point. Rather, radio engineers sought to explain numerous wireless phenomena from daily practice—not only long-distance propagation, but also diurnal, geographical, and seasonal variations of ambient noise from the atmosphere. Even though the explanations that they generated were only partly quantitative, their broad but rough theory of atmospheric reflection explained their field observations much better than the precise but narrow theory of surface diffraction.
(Continues...)

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Excerpted from Probing the Sky with Radio Waves by Chen-Pang Yeang. Copyright © 2013 by The University of Chicago. Excerpted by permission of THE UNIVERSITY OF CHICAGO PRESS.
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