by Joe Cerniglia
Lately I have been taking a new look at the TIGHAR Radio Signal Catalog. It is a masterpiece of forensic detail that, as a veteran data scientist, I can truly appreciate. Most worthy of consideration is Bob Brandenburg's landmark article called Time and Tide. Bob was the first person to understand that an accurate tidal hindcast of Nikumaroro could yield new insights into the relationship between the height of the water on the reef flat, the time of day, and the ability of the Lockheed Model 10E to transmit distress calls.
What Bob's graphical analysis did not attempt, however, was to unify the data into a single view, complete with the day-night cycles, the height of the tides, and the complete catalog of reported signals from Earhart. Instead, Bob showed 24-hour snapshots, small multiples of sets of radio messages graphed against the tidal patterns. Additionally, Bob limited his analysis only to the signals judged to be credible by TIGHAR. The non-credible and uncertain signals are a source of information as well, tenuous as they might be.
A new graph I have constructed of the timeline of radio signals reaffirms that the sun's ionization of the atmospheric D-Layer[1] during daytime on Nikumaroro prevented listeners from hearing Earhart's and Noonan's distress calls, even though credible evidence exists these calls were made.
In 2013, when the Time and Tide analysis was constructed, drawing professional-grade charts was a far more laborious process whose finished product, for all but those with access to the best platforms, hardware, and private software, was far from what one could envision. MATLAB and Adobe Illustrator were state-of-the-art tools, offering more sophisticated charting capabilities than the usual Excel and PowerPoint. Computers could easily crunch the numbers, but the weakest link was the output. Today, AI can easily ingest the data and create a Python program (and programs in many other languages) to actualize any design concept the user wants to implement. The process is far from instantaneous, and vetting and checking one's work is no less rigorous a process than it was back in 2013. This graph was built over the course of three solid days of effort, a stepwise refinement, a virtual conversation with AI, that required an advanced knowledge of Python and the Matplotlib library, which I have, as well as years of personal research of the radio signals themselves. The key point, however, is that the tools have advanced to meet the level of one's own ideas. Graphing becomes an extension of the mind rather than a laborious translation process with finicky tools and complex programming languages, which, even in the hands of an expert, can be extremely tricky.
Here is a new compendium graphic of radio catalog signals from July 2 - 8, 1937:
Note: This forensic timeline contains fine details; Pinch-zooming will aid in seeing these details more clearly.
The graph contains three horizontal lines acting as constraints on the mechanical operation of the radio and of the airplane. These constraints are:
1. Lower constraint: Optimal Engine Charging Zone (Water Depth <= 1.0 ft.)
1. Lower constraint: Optimal Engine Charging Zone (Water Depth <= 1.0 ft.)
This boundary existed because the reef on Nikumaroro is an extremely dynamic water environment. Currents and swells can easily exceed the tidal depth at given moments. The only way to ensure the propellers are clear and stay clear of the water so that they can spin is to run them when the tide is this low.[2]
2. Middle constraint: Propeller Clearance Limit (2.16 feet) - Generator cutoff
This is the absolute limit of the height of the water at which the batteries might be charged for brief periods, but the winds would need to be very low and the waves calm to charge.[3]
3. Upper constraint: Battery Submersion Ceiling (3.7 ft) — Terminal Short Circuit Threshold
This is the limit at which the airplane can keep the battery poles out of the water. Once this limit is exceeded, a catastrophic short circuit instantly discharges the entire electrical grid into the ocean, resulting in permanent system failure and making any further transmission an absolute physical impossibility.[4]
The heights that the water had to exceed are accurate for each constraint, given that we know the heights of various locations on the airplane. Still there is no way for anyone to know the precise height of the water relative to a parked airplane on Nikumaroro at a given moment in 1937. The reef on Nikumaroro is not a flat plain, nor is the water a calm swimming pool. However, the exact measurements for the specific conditions are not as important as illustrating that the constraints were present and to illustrate the effect they had on winnowing received signals, and on winnowing transmitted signals.
The graph does well in showing the rising frequency and urgency of transmissions only to show them tapering off toward the later dates as equipment began to fail. Restraint and relative calm in the early days of the transmissions, when more of them occurred at low tide, give way to an apparently greater urgency during the middle of the period, when riskier transmissions were tried at higher water levels. (See July 2, 3, and 4.)
Note also that the daytime periods on Nikumaroro have scant received transmissions, but there are a few notable exceptions from casual listeners (Nina Paxton on July 3, Betty Klenck on July 5, and Thelma Lovelace on July 6). This seems to indicate not that Earhart and Noonan gave up trying during the day, but rather that only harmonic multiples of their frequency could be picked up during daylight, bouncing off the ionosphere to land thousands of miles away on the continental United States.
What is also interesting is that the radio catalog infrequently mentions tides. They were not the primary factor in ruling on the credibility of the various signals; therefore, the vertical axis of the graph, which shows winnowing based on water levels, functions as a kind of independent auditor of the overall credibility of the catalog.
The signals cluster on the graph where the tide is low AND the time is night. When the tide is high and it is nighttime, the signals drop in number, although they are still heard in a great number in the early morning of July 4 at a rising and high tide. When the tide is low and it is daytime, the signals attenuate almost to nothing. Many of the nights had both high and low tide. The best nights for transmission were the late evening of July 2, the late evening of July 3, the late evening of July 4, and just before and after 12 am of July 6 and July 7. There are only a handful of signals just before and after 12 am on July 6 and July 7, a good time to have transmitted. The last optimal period in the early morning hours of July 8 has no signals.
The day-night boundaries plotted on the graph are highly accurate, sitting within very close approximations of the actual astronomical times for Nikumaroro in July 1937.[5]
To figure out how high the ocean water was on the island reef each hour, I used two custom Python programs to create a realistic tidal simulator.
With the assistance of AI, I wrote two Python programs. The first is a script that combines the gravitational pull of the moon and the sun, matching the exact cycle speeds of five different ocean waves (the basic tidal constituents). The program adds these waves together, including a setting that mimics how tides get naturally higher or lower depending on the phase of the moon, and saved the results into a data file called
nikumaroro_1937_tides.csv.Second, my main charting program opens up that fresh data file and uses linear interpolation to fill in any gaps between the exact hours. This allows the program to automatically look up any radio signal's exact timestamp, figure out precisely how deep the water was at that exact minute, and plot all of the received signal times directly onto the timeline's wave curves. This is a simulation. Weather and wind at those exact times will have played a role in changing the currents and tides. Since I do not know exactly where an airplane would have parked near the Norwich City, this simulation is a good one in that it creates a realistic portrayal of the natural variability and push and pull of tides over the period of one week.
The long curved line on the graph illustrates the correspondence between the matching latitude and longitude that Ray Havens [6] (July 7) and Nina Paxton [7] (July 3) both gave in their accounts of receptions they stated were from Amelia Earhart. Since Ms. Paxton's account, which omitted compass headings, only came to public attention in the 1960s, there was no way for Mr. Havens to have copied his coordinates from Paxton. There had to be an independent third-party source for this information and the most likely source for it was the post-loss radio signals.
Havens' account is on the left and Paxton's private typescript is on the right.
173°W, 5°S is 107 miles ESE of Nikumaroro.
In the interest of reproducibility of research, a concern highlighted in my earlier paper in March of 2022, Machine Learning with Amelia, I am attaching the Python programs that created this graph.
Endnotes
[1] Britannica Editors, "D region," Encyclopædia Britannica, last updated by John P. Rafferty, accessed August 23, 2026, https://www.britannica.com/science/D-region.
[2] Brandenburg, Bob. 2013. "Time and Tide." TIGHAR Tracks 29, no. 1 (February): 53–64,
accessed August 23, 2026, https://tighar.org/Publications/TTracks/2013Vol_29/TTFeb2013.pdf
[3] Ibid., 53-64.
[4] Brandenburg, Bob. n.d. "Amelia's Batteries, Development and Application of a Computer Simulation Model of the Storage Batteries on Amelia Earhart’s Lockheed Electra, NR16020. TIGHAR Research Archive. Accessed August 23, 2026, https://tighar.org/Projects/Earhart/Archives/Research/ResearchPapers/Brandenburg/Batteries/batteries.html
[5] "Sunrise and Sunset Times in United States," Time and Date, accessed August 23, 2026, https://www.timeanddate.com/sun/@4030922.
[6] Havens Hears Message from Earhart Plane, The Independent-Observer, Conrad, Montana, 8 July, 1937, p. 1.
[7] Paxton, Nina L., "The Call of a Courageous Lady," undated typescript, Box 1, Folder 1, Nina L. Paxton Papers, 1937-1970, Southern Appalachian Archives, Mars Hill University, accessed August 23, 2026, https://southernappalachianarchives.org/items/show/227.
Note that the title of this paper borrows from that of an important work in the field of information displays and visual thinking, which I consulted often for guidance in setting up the graph:
Tufte, Edward R. Visual Explanations: Images and Quantities, Evidence and Narrative. Graphics Press, 1997.
