This paper presents an analysis of radio direction finder bearings obtained by Pan American Airways (PAA) direction finder (DF) sites, at Wake Island, Midway Island, and Mokapu Point at Oahu, Hawaii, and by a temporary U. S. Coast Guard DF site at Howland Island, during the search for Amelia Earhart in July 1937. All bearings were taken on signals heard at night, on or near 3105 kHz, the Earhart night frequency.

The Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program¹ (ICEPAC) was used to model all aspects of high frequency (HF) signal propagation for this analysis.

The Low Frequency/Medium Frequency (LFMF) model²was used to model propagation in the medium frequency AM broadcast band.

The radiation pattern of the dorsal antenna on the Earhart aircraft, NR16020 was modeled with the Numerical Electromagnetics Code version 2 (NEC2),³ using physical configuration details of the antenna obtained from Lockheed Electra 10E configuration drawings,⁴ and photographs⁵ of the Electra cabin interior.

International Telecommunications Union (ITU) Recommendation F.339-6 specifies the required SNR values for various signal types and grades of service. An amplitude-modulated double-sideband signal, such as emitted by the NR16020 transmitter, is classified by F.339-6 as an A3E emission. The lowest acceptable grade of service specified in F. 339-6 for an A3E emission is 90 percent understandability of sentences (“just usable”) for non-diversity⁶ reception in fading conditions. The F.339-6 required input SNR for this grade of service is 51 decibels (dB) in a 1 Hertz band, which yields a 6 dB audio output SNR in a receiver with a 6 kHz noise bandwidth.

However, the F.339-6 results for analog systems are based on white Gaussian noise,⁷ and Spaulding⁸ has presented results showing that a given voice understandability can be achieved with a 6 dB smaller SNR in atmospheric noise, which is impulsive in nature. Applying this reduction would change the required SNR to 45 dB in a 1 Hertz band, for a 6 dB audio output SNR. This implies that a 39 dB input SNR would produce a 0 dB audio output SNR.

This analysis assumes that the background noise at the DF sites was predominantly atmospheric, and that the 6-dB SNR reduction for voice signals applies, based on the following considerations:

• Radio noise in the 3 MHz to 30 MHz band is a combination⁹ of galactic, man-made, and atmospheric noise, the characteristics of which are discussed in ITU Recommendation pp. 372-8, “Radio Noise.”
• Galactic noise did not affect reception of signals near 3105 kHz because the F2 layer critical frequency, the lowest frequency at which galactic noise energy can penetrate the ionosphere at night, was 4.7 MHz or greater during the period of interest.
• Manmade noise, which is produced by electrical equipment, was assumed to be minimal at the DF sites. All four sites were designated as “quiet rural,” the lowest of the manmade noise levels specified in P.372-8. Howland Island had no human population other than the small shore party stationed there during the search period. Wake Island and Midway Island were PAA Clipper bases, with small resident staffs and basic accommodations for passengers. PAA flight operations at each base consisted of one eastbound flight and one westbound flight each week. The Mokapu Point site, about 12 miles northeast of Honolulu, was in a sparsely populated farming region and was shielded from Honolulu electrical noise by the Koolau Mountain Range, more about which later.
• Atmospheric noise, which is generated by lightning discharges and can travel very long distances via the ionosphere, was the dominant noise factor, as indicated by comments in the radio logs of the Coast Guard cutter Itasca, and in the post-search summary reports of the PAA DF sites. This is consistent with the fact that atmospheric noise is most intense in a latitude band extending about 20 degrees north and south of the equator, particularly in summer, as shown by the worldwide atmospheric noise contour maps in ITU-R pp. 372-8.

Receiver selectivity, the ability to reject unwanted signals, must be considered when deciding whether signals can be heard on a frequency other than the one to which the receiver is tuned.

The radio equipment at each mid-Pacific Pan Am DF site¹⁰ included an RCA type AR-60 state of the art superheterodyne communication receiver. It is assumed that the AR-60 was used to listen for Earhart signals. Although the AR-60 selectivity characteristics are not available, communication receivers of the day typically operated at a bandwidth of 6 kHz when listening for voice signals. This analysis assumes that the AR-60 selectivity was essentially the same as that of the Hammarlund SP-110 superheterodyne receiver,¹¹ which was introduced in 1936 and is considered to be representative of the state of the art at the time. The SP-110 featured user-selectable bandwidth which, at 6 kHz, had a selectivity response that attenuated the output SNR of a signal on an unwanted frequency signal, relative to the SNR of a signal on the desired frequency, by 6 dB at a 3 kHz frequency difference, 23 dB at 5 kHz, 60 dB at 8 kHz, 90 dB at12 kHz, and 103 dB at 15 kHz. Accordingly, only signal source frequencies within 15 kHz of 3105, i.e., in the range 3090 kHz to 3120 kHz, were considered in this analysis.

The characteristics of the receiving antennas at the DF sites are not known, so this analysis assumes isotropic antennas. Similarly, it is assumed that the transmitting antennas of the potential signal sources, under consideration were isotropic, with the exception of the dorsal antenna on NR16020, and the broadcast band vertical tower radiators. The broadcast band antenna gain characteristics are provided in the LFMF model.

Aircraft

3105 kHz (A3) was an air-to-ground calling frequency for U.S. civilian aircraft.¹² Ground stations responded on 278 kHz.¹³

Since all DF bearings were obtained at night, areas where U.S. civilian aircraft were known to operate were evaluated for inclusion in this analysis, on the basis of ability to support night flight operations. Those areas were: the west coast of the United States, Hawaii, the Panama Canal Zone, and Venezuela.

Airfields in the vicinities of Seattle, San Francisco, Los Angeles, and San Diego supported night flight operations, so aircraft along the entire west coast of the United States were included in the analysis.

U.S. civilian aircraft operated in Hawaii in 1937, but no details of civilian airfield ability to support night operations have been discovered thus far. However, a December 7, 1941 post attack photo¹⁴ of Wheeler Field, a U.S. Army airfield on Oahu, shows the field was an open grass surface, without paved runways. This suggests that the field was not lighted for night flying operations, and further suggests that civilian, smaller, airfields were similarly unable to support night operations. Furthermore, local aircraft transmissions on 3105 kHz would have been heard by Navy radio personnel in Honolulu listening for Earhart signals, or by the PAA DF site at Mokapu. No such signals were reported, so it is concluded that there were no night flight operations in Hawaii, and aircraft in Hawaii were ruled out as potential signal sources.

The 1939 International Telecommunications Union (ITU) list¹⁵ of frequencies showed aircraft of the Caribbean Petroleum (CP) company as operating on 3105 kHz (A1,A3). However, neither the station location nor the date of commencement of operation was listed, suggesting that the capability was planned, but not yet operational in 1939. The 1938 edition of the list has no entry for Caribbean Petroleum , which implies that CP aircraft were not using 3105 kHz in 1937. Accordingly, C P aircraft are ruled as potential signal sources for this analysis.

The history of U.S. Army air defense preparations in Latin America,¹⁶ which began in 1938, provides strong evidence that there were no night flight operations in Latin America in 1937. Pan American Airways had by 1938 become the dominant commercial air service throughout the West Indian, Central American, and South American regions, and the Army considered using the Pan Am airfields as air defense bases. However, they were too small and not equipped for night flying, and thus unsuitable for military operations. Given that the Pan Am airfields were not equipped for night flying, it is reasonable to conclude that other civilian airfields did not support night flying either. This is consistent with anecdotal information in the TIGHAR archive¹⁷ from the former Chief Pilot of Creole Aviation, who operated in Venezuela from 1935 to 1958 and flew to wildcat oil well locations, landing on runways created by clearing off vegetation with a road grader and then oiling the surface. Accordingly this analysis assumes there were no night flight operations in Latin America.

U.S. civilian aircraft operated in the Panama Canal Zone (CZ) from the 1920s, and by 1930 all commercial air services into the CZ used Army Airfields.¹⁸ France Field, near the Atlantic side of the CZ, was the principal Army airfield until the 1930s¹⁹ when its inferior landing surface, lacking a paved runway, resulted in the field being deemed unsafe for operation of the large military and commercial aircraft of the day. France Field served as Pan Am’s primary flying field until 1936,²⁰ when commercial service moved to Albrook Field, at the Pacific side of the CZ. Albrook also lacked a paved runway.²¹ A photo²² of Albrook taken on 21 August 1937 shows the field had only an open grass surface, and thus did not have a lighted runway. Construction of a paved runway was funded by Congress²³ in 1938 and was completed in April 1939. Accordingly, it is assumed for this analysis that there were no night flight operations in the CZ during July 1937. This assumption is supported by the July 15, 1937 timetable²⁴ of Panama Airways, showing 4 daily flights between France Field and Balboa – the site of Albrook Field – during daylight hours only. However, some signals heard at the DF sites occurred during daylight in the CZ, so aircraft there were considered as potential signal sources of those signals.

U.S. Army aircraft operated in the CZ, and 3100 kHz was listed²⁵ as a frequency used by Army aircraft, mobile stations, and portable stations. The U.S. Army station at Quarry Heights, at the Pacific side of the zone, was listed²⁶ as a multi-function station, serving as an aeronautical services station, a coast station, and a point-to-point communications station, on 3100 kHz. The coast station frequencies listed²⁷ for Quarry heights did not include 3100 kHz, which leaves communications with aircraft and mobile/portable stations as possibilities. Therefore, this analysis considers Quarry Heights as a potential source of signals heard at the DF sites, subject to the previously stated assumption that there were no night flight operations in the CZ.

Ships

The ITU listed²⁸ a total of 13 ships worldwide, 1 U.S. and 12 Soviet, capable of voice (A3) transmission on frequencies within 15 kHz of 3105 kHz. The U.S. ship and 11 of the Soviet ships operated on 3120 kHz. These ships were ruled out as possible DF signal sources, under the procedure described in Appendix A. The twelfth Soviet ship, the Magnitogorsk, had 3105 kHz among its assigned frequencies, but was ruled out as a possible DF signal source, under the procedure described in Appendix B.

Coast Stations

Coast stations provided two-way communications with ships at sea and also with ships on inland waterways. There were numerous coast stations,²⁹ but few were capable of A3 transmission on frequencies within 15 kHz of 3105 kHz.

Alaska.

There were 21 on Gulf of Alaska, 13 on operating on 3090 kHz and 8 on 3092.5 kHz. All were capable of A3 transmission. The King Cove station (3092.5 kHz, 100 watts), at 162.19 W, 55.04 N, had the highest power among these stations and was the closest to the DF sites, and is used in this analysis as proxy for the other Gulf of Alaska coast stations.

U.S. Pacific Coast, Hawai’i, and Guam.

There were 12 coast stations in this group, 9 on the U.S. Pacific Coast, 2 on the island of Oahu in Hawai’i, and one in Guam. All were assigned 3105 kHz as a calling frequency, and 3120 kHz as the primary working frequency. All operated in A1 (Morse code) mode, none in A3. All were ruled out as potential sources of A3 signals.

Soviet Union.

There were three A3-capable ocean coast stations: Anadyr Mys and Navarin Mys on the Soviet Pacific coast, and Billings on the Northern Maritime route. There also were 4 A3-capable coast stations on the inland waterway system, at Oust-Kiakhta, Oufa, Nijne-Angarsk, and Oulan-Oude.

Other Land Stations

Aeronautical Service.

There was an A3-capable aeronautical service station on 3088 kHz at Winslow, Arizona.

Point-to-Point Communication.

There was an A3-capable Soviet point-to-point communication station on 3090 kHz, at Voronej.

AM Band Broadcast.

There were two AM band broadcast stations operating on frequencies with harmonics near 3105 kHz. Station 3AR at Melbourne, Australia, operated on 620 kHz, at 4,500 watts. The fifth harmonic of 620 kHz is 3100 kHz. Station RW26, at Stalino, Soviet Union, operated on 776 kHz at 10,000 watts. The fourth harmonic of 776 kHz is 3104 kHz.

The output power spectrum of a transmitter designed without harmonic suppression circuitry can be estimated by assuming that the final power amplifier is designed according to the method of Terman³⁰ and Roake, and by numerically integrating the Fourier integral equations³¹ for the amplitudes of the output harmonic components.

Using this procedure, the 4th harmonic output of RW26 is estimated to be approximately 6 percent of the fundamental, or 600 watts. Similarly, the 5th harmonic of 3AR is estimated to be approximately 1.55 percent of the fundamental, or 70 watts.

It is highly doubtful that the Soviet government or the Australian government would have allowed significant harmonic radiation. RW26 was about 600 nmi from the Caspian Sea coastal station at Jilaia Kosa, which operated on 3105 (A1) at 70 watts. The 600-watt harmonic on 3104 kHz from RW26 would very likely interfere with Jilaia Kosa’s operations at night. The 3,150-watt second harmonic output of 3AR on 1240 kHz would very likely interfere with station 6IX in Perth, about 1500 nmi to the west, which operated at 500 watts on that frequency.

But since no evidence of harmonic suppression in either transmitter has been found thus far, this analysis assumes unrestricted harmonic radiation. However, any consideration of either station as a plausible signal source should be tempered by the practical realization that it is highly doubtful that either government would tolerate significant harmonic radiation.

The potential signal sources are compiled in Table 1.

Table 1: Voice (A3) Capable Stations Within 15 kHz of 3105 kHz.

Each source in Table 1 was tested using the method described in Appendix A, to determine the feasibility of reception at each DF site under hypothetical ideal propagation conditions. This test provides a conservative basis for deciding whether the source can be ruled out as the potential origin of signals at a given DF site. If the source signal could not produce the required receiver input SNR under the ideal conditions of the test, then it could not do so under realistic propagation conditions. The test results are shown in Table 2.

The receiver selectivity attenuation corresponding to the source frequency is shown, together with the source distance from each DF site, and the free-space SNR resulting from the test. Recalling that the threshold input SNR, for detecting the presence of an A3 signal without distinguishing any words is 41 dB for this analysis, the results shown in Table 2 can be sorted into three categories. The SNR for signals from some sources robustly exceed the threshold at some or all DF sites; some marginally exceed or fall below the threshold; and some fall far below the threshold.

Retaining the first two categories and deleting the third yields the following table of potential sources retained for further consideration in the analysis.

During the search the Itasca deployed an experimental high-frequency direction finder on Howland Island manned³² by Coast Guard Radioman Second Class Frank Cipriani. This equipment was used on July 5 to obtain an approximate bearing on a signal near 3105 kHz.

The PAA DF System

Sandretto’s description³¹ of the PAA Adcock DF system illustrates the importance of signal strength and duration. A signal bearing was indicated by an aural null. But instead of measuring the null bearing directly, the operator observed bearings on each side of the null where the signal level was high enough for accurate measurement, over a period of 2 to 3 minutes, and averaged those bearings to obtain the null bearing. The accuracy of the bearings on each side of a null, and thus the accuracy of the average bearing, would be adversely affected if a signal was weak or of short duration.

Bearing Ambiguity

The Adcock DF system produces a figure-eight antenna beam pattern, which is rotated via a goniometer to obtain a null, or signal minimum. The antenna pattern is symmetrical about the null axis, which allows bearing ambiguity because it is possible to get two null bearings 180 degrees apart. Adcock DF system designs of the 1930s usually incorporated a vertical sense antenna to eliminate bearing ambiguity. But some remarks in the PAA DF site logs indicate the possibility of bearing ambiguity, suggesting that PAA might have simplified their system design by omitting the sense antenna, since each DF site would know the general bearing of a Clipper flight. This is consistent with the recollection of Captain Almon A. Gray, USNR (ret), who was the Assistant Communications Superintendent for the PAA Pacific division during the Clipper era. Gray said of the PAA system: “The system was bi-directional hence one had to be ever mindful of the possibility of reciprocal bearings. That was not much of a problem however as one usually knew the general location of the aircraft.”³² This analysis assumes the PAA DF system was bi-directional and examines the possibility of a reciprocal bearing source in each case.

Distances

The distance from Wake Island to Midway Island is approximately 1,080 nautical miles (nmi), and the distance from Midway to Honolulu is approximately 1,150 nmi. Thus a flight on the route between Wake and Honolulu would never be more than about 600 nmi from the nearest DF site. In contrast, the distances from Gardner Island to Wake, Midway, and Honolulu are 1,825nmi, 1,990 nmi, and 1,850 nmi respectively.

Signal Strength Considerations

High frequency direction finding sites take bearings on skywaves, which travel long distances via refraction from the ionosphere. Short term variations in the ionosphere can cause signal strength fading, reducing the SNR and increasing the difficulty of getting an accurate bearing. If fading occurs on an already weak signal, the SNR can drop below the reception threshold, making it impossible to obtain a bearing. On the other hand, increasing atmospheric noise can cause the SNR of an already weak signal to drop below the threshold even without fading.

The PAA DF system³³ site locations, radio equipment, DF equipment, and operating procedures clearly were chosen to ensure accurate bearings would always be available for support of PAA flights.

The 70-watt output power of transmitters³⁴ on the Pan Am Clippers was roughly comparable to the 50-watt output of the transmitter on NR16020. However, as noted earlier, the distances from Gardner Island to the PAA DF sites were about 3 times the maximum distance from a Clipper to the nearest DF site on the route between Wake Island and Honolulu. Hence, the propagation loss for signals from Gardner Island was at least 10 times greater than for signals from a Clipper to the nearest DF site, making the SNR for any signals from Gardner Island correspondingly lower. And signals from the other potential sources, which were even further away, were likely to be much weaker than those expected from Clipper aircraft. The result, not surprisingly, was that bearings on potential post-loss signals were tenuous and approximate at best. That the DF site personnel were able to get any bearings at all in such conditions is a tribute to their skill and dedication.

Each PAA DF site³⁵ used two concentrically arranged sets of Adcock antennas, covering the frequency band 200 kHz to 6,000 kHz. Adcock antennas were used³⁶ because, although they are electrically equivalent to loop antennas, they are immune to the effects of polarization shifts³⁷ in radio waves refracted by the ionosphere.

A downcoming skywave, will have both vertically and horizontally polarized components which fade³⁸ independently of each other. The presence of a horizontally polarized component in a radio wave causes “night effect”³⁹ in a loop antenna by inducing voltages that do not cancel out when the plane of the loop is perpendicular to the bearing of the radio wave. This results in an inaccurate bearing, or an indistinct minimum, or both. The Adcock antenna, being insensitive to horizontally polarized waves, avoids this problem.⁴⁰

Notwithstanding its ability to obtain usable skywave bearings, the performance of the Adcock antenna is limited⁴¹ by the fact that its ability to extract energy from a passing radio wave is the same as that of a loop antenna with a single turn of wire, and hence is quite small. Consequently, even with the Adcock antennas at the DF sites, it was difficult to obtain usable bearings on weak signals.

Site Terrain Effects

Terrain interference of skywave signals at a DF site can reduce signal strength and cause bearing errors. The elevation angle of arriving skywave signals is a key consideration in this regard. This angle depends on the height of the ionospheric reflection point and its distance from the receiving site, which in turn depend upon date, time, signal frequency, and conditions in the ionosphere.

Wake Island, Midway Island, and Howland Island are essentially flat, so the DF sites there had no terrain interference. But the situation at the Mokapu Point DF site was quite different.

Mokapu Point Terrain

The Mokapu Point site was at approximately 21° 27′ 21″ North latitude, 157° 45′ 44″ West longitude, near the seaward edge of a large flat area in what later became the U.S. Marine Corps Air Station at Kaneohe Bay. The DF site coordinates were derived by correlating topographic map⁴² features with two photographs of the antenna array and associated buildings. One photo⁴³ was taken from offshore looking westward toward the Koolau mountain range and shows the DF site near the shoreline at a height of about 40 feet above the beach. The other photo⁴⁴ was taken from onshore looking north toward the sea.

The crest of the Koolau range on the great circle bearing of Gardner Island (214 degrees) is 6.75 statute miles from the DF site coordinates. The mean orientation of the ridge line⁴⁵ in that area is 340/160 degrees true, with some segments deviating from the mean by up to 50 degrees.

The southwest slope⁴⁶ of the Koolau range is a forested watershed with numerous ridges, valleys, and running streams. The terrain in this area, along the path of a signal arriving from Gardner Island, has a compound slope rising from sea level at Honolulu to about 1,800 feet over a distance of 6.3 miles at a vertical angle of approximately 3.1 degrees, and tilting downward to the right at a slope angle of approximately 10.8 degrees across the path to the DF.

The terrain on the northeast slope of the range drops from the ridge line to near sea level in about one mile, and remains essentially flat from there to the DF site, and beyond to the sea.

The elevation angle of the ridge line from the DF site coordinates, on the bearing of Gardner Island, is approximately 2.9 degrees, hence the DF site would be in the radio “shadow” of the Koolau range for skywave signals arriving with lesser elevation angles.

But even signals with somewhat greater elevation angles could encounter terrain effects. The lower edge of the wavefront would contact the terrain on the southwest slope, potentially sustaining directional skewing. Moreover, diffractive scattering from the ragged ridge line could cause signals to arrive at the DF site from multiple directions, making it difficult to obtain a reliable signal bearing.

The elevation angle at the DF site for signals on 3105 kHz arriving from Gardner Island and the Itasca ranged from 1.4 degrees to approximately 12 degrees at the times of the Mokapu bearings. Angles near the low end of this range were small enough to put the DF site in the radio shadow of the Koolau ridge. Angles near the middle of the range were small enough that, although the DF site was not in the radio shadow, there could have been significant terrain effects. Angles near the upper end of the range were large enough that terrain effects can be assumed to be minimal.

The bearing of the Itasca from the Mokapu DF site varied between 225 and 232 degrees on July 3 through July 6. The height of the Koolau ridge in this sector varies between 2,600 feet and 2,800 feet, and the average terrain elevation angle from the DF site is approximately 4.8 degrees. This may have been a factor in the unsuccessful attempt to get a bearing on the Itasca on July 6, discussed later.

The bearing of Midway from the Mokapu site coordinates is 295 degrees, and the Koolau ridge elevation angle on that bearing is 2 degrees. PAA flights on the Midway -Honolulu route operated only during daylight hours,⁴⁷ and their signals arrived at the DF site with elevation angles of at least 30 degrees. Hence, terrain was not a significant factor in bearing accuracy for normal PAA operations.

And the Mokapu site had an unobstructed “view” of the radio path of signals from PAA flights on the route to and from Alameda.

Clearly, the Mokapu DF site was well-positioned for obtaining accurate bearings for its designed mission of supporting flights to and from Midway and Alameda, but was operating at a potential terrain disadvantage in the search for post-loss Earhart signals. ICEPAC does not take into account the terrain interference effects described above, so the SNR values given in this paper for low-angle signals from Gardner Island or the Itasca should be regarded as upper-bound values, i.e., the actual SNRs could have been less than the values computed by the model.

The Bearings

Bearing reports and signal descriptions from source documents are presented and analyzed here, in chronological order.

The analysis of each bearing includes a table showing, for each potential signal source, the times of sunrise and sunset, the bearing – and its reciprocal – of the source from the DF site, the mean SNR for a signal from the source, and the probability that the SNR would exceed the reception threshold.

Most official messages and reports cited in this paper used Greenwich Civil Time (GCT), designated by the “Z” time zone and now known as Greenwich Mean Time (GMT), but some used Honolulu Standard Time (HST), which was 10.5 hours behind GCT. Both notations are retained in this paper, with each instance of HST time and date accompanied by the corresponding Z time and date to avoid confusion, since some events occurred on different HST and Z dates depending on the event time. For example, 4 PM (1600) HST on July 4 was 0230Z on July 5.

Bearing 1, 1512Z July 4, Mokapu Point

Honolulu radio station KGMB (1320 kHz) made a series of broadcasts to Earhart, beginning at 2000 HST July 3 (0630Z July 4) and continuing at intervals of about 15 minutes for several hours,⁴⁸ requesting that she indicate her general location by sending a combination of dashes from a list of options provided in the broadcasts. At 1512Z July 4 (0442 HST July 4), Mokapu obtained a bearing of 175 degrees on a signal presumably on 3105 kHz. This signal is not mentioned in the Mokapu supervisor’s post-search report⁴⁹ but was reported by the operator in charge at the Midway site,⁵⁰ who wrote “At 1512 GCT, a very faint broad signal apparently a phone was heard here but again was far too weak to take a bearing. Wake reported unheard while Mokapu reported taking a bearing on it which might be 175 approximately.”

Table 4 shows the source data for this signal. The “SSS” entries for sunset and sunrise at Billings signify that the site, being on the East Siberian Sea above the Arctic Circle, was continuously in daylight.

Analysis

1. The fact that this bearing was known to Midway but is not in the Mokapu report suggests that the Mokapu DF operator mentioned it on the inter-site radio circuit during the event, but the bearing was subsequently deemed by the Mokapu supervisor to be too unreliable for serious consideration.
2. The signal bearing passes near the Cook Islands and the Society Islands, but there were no known sources of signals on 3105 kHz in that area.
3. The possibility that this was an erroneous bearing on a signal from the Itasca can be ruled out because the ship did not transmit⁵¹ on either 3105 kHz or 6210 kHz between 0645Z and 2100Z.
4. The Quarry Heights site can be ruled out because of bearing incompatibility – it was in a bearing sector unaffected by terrain – and because the reception probability was so low as to be virtually zero.
5. The Melbourne broadcast station harmonic could not be heard at Mokapu, as shown in Table 3.
6. The reciprocal bearing of Navarin Mys, 166 degrees, was near the reported signal bearing, but the reception probability was about 4 million times lower than that for a signal from Gardner Island.
7. Although the Stalino broadcast station harmonic could theoretically be heard at Mokapu, and the reciprocal bearing of Stalino, 169 degrees, was near the reported signal bearing, the reception probability was so low as to be virtually zero. Furthermore, as stated earlier, it is highly doubtful that the Soviet government would have permitted significant harmonic radiation.
8. The reciprocal bearing of Oust-Kiakhta, 140 degrees, was 30 degrees off the reported signal bearing, and the reception probability was about 8 million times lower than that for a signal from Gardner Island.
9. West coast aircraft can be ruled out because of bearing incompatibility, since the entire west coast was in a DF site sector unaffected by terrain.
10. The reciprocal bearing of Billings, 169 degrees, was close to the reported signal bearing, but the reception probability was about 6 million times less than the probability for a signal from Gardner Island.
11. The reception probabilities strongly suggest that Gardner Island was the most likely source. But the computed vertical arrival angle for a signal from Gardner was 12.5 degrees, which argues against terrain effects sufficient to account for the 39-degree difference between the reported signal bearing and the bearing of Gardner. Notwithstanding the probability argument in favor of Gardner, the possibility, however remote, that one of the Soviet stations near the reciprocal bearing was the source, cannot be ruled out on the basis of the available evidence.

Bearing 2, 1523Z to 1530Z, July 4, Mokapu Point

The Mokapu supervisor’s post-search report⁵² states “Carrier again heard on 3105 – rough bearing only possible due to weakness and swinging of signals. Get bearing from Mokapu of approximately 213 degrees. Advised Coast Guard.”

Mr. G. W. Angus, the PAA Pacific Division Communication Superintendent, was at Midway when this bearing was taken, and later stated in his post-search report⁵³ “On Sunday morning about 5:30 a.m., local time at Mokapu, Mr. Paulson stated he heard signals in the vicinity of 3105 KC, although the frequency could not be accurately determined and on which he obtained an approximate bearing of 210 degrees. The signals Mr. Paulson heard were, undoubtedly, carrier signals modulated with voice although he could not understand the voice part of it. Although it is true several of the domestic lines use frequencies close to 3105 KC, it is doubtful if these signals would have carried to Honolulu at this particular time of day, the time then being 8:00 a. m., Pacific Standard Time. Usually at this time, stations on the West Coast have changed to day frequencies. I believe the signals heard by Mr. Paulson were from the Itasca. This information was passed on to the Coast Guard at Honolulu for whatever it was worth and we continued to listen to the two frequencies used by the plane.”

Table 5. Signal Data for Bearing 2.

Analysis

1. The bearing of 213 degrees reported by the Mokapu supervisor passes within 30 nmi southeast of Gardner Island. This bearing, evidently based on direct analysis of the DF operator’s readings, is considered more accurate than the value cited by Angus.
2. The elevation angle for a signal from Gardner at this time was 12 degrees, well above the terrain masking limit.
3. The time cited by Angus, 0530 HST July 4 (1600Z July 4), appears to be when Paulson mentioned the bearing on the inter-site circuit, rather than when he took the bearing (1523Z to 1530Z).
4. The bearing of the Itasca⁵⁴ at this time was 229.6 degrees, but the Itasca could not have been the source of this signal since the ship⁵⁵ did not transmit on 3105 kHz between 0645Z and 2100Z.
5. Other than Gardner Island, the only potential source near the reported bearing was AM broadcast station 3AR in Melbourne, which could not be heard at Mokapu.
6. Angus’ comment about signals from west coast aircraft: “…it is doubtful if these signals would have carried to Honolulu at this particular time of day, the time then being 8:00 a. m., Pacific Standard Time. Usually at this time, stations on the West Coast have changed to day frequencies.” indicates that the DF system was bi-directional, hence the possibility of a reciprocal bearing must be considered. He was correct about commercial aircraft changing to day frequencies, but he failed to consider general aviation aircraft, for which 3105 was the designated calling frequency.
7. The signal was heard approximately 3 hours after sunrise on the west coast, and about 30 minutes before sunrise at Mokapu. Hence nearly the entire propagation path was in daylight, and conditions at 3105 kHz were deteriorating. In contrast, the path from Gardner Island to Mokapu was still in darkness.
8. The reciprocal of the reported signal bearing was 033 degrees. The Seattle area, bearing 039 degrees, was the only source area close to the reciprocal bearing. The reception probability for a signal from Seattle was very low, but that value takes on added significance when compared to the probability for a signal from Gardner. The ratio of the two probabilities shows that a signal from Gardner was 71,000 times more likely to have been heard at Mokapu than a signal from the Seattle area.
9. Although not conclusive, the available evidence strongly suggests that Gardner Island was the most likely source of the signal.

Bearing 3, 0630Z July 5, Mokapu Point

On the evening of Sunday, July 4, KGMB (1320 kHz) conducted its second night of broadcasts, beginning at 8 PM (2000) HST (0630Z July 5), with instructions⁵⁶ for Earhart to turn on her transmitter for one minute for tuning purposes, then to send 4 long dashes, then wait for an acknowledgment by KGMB 15 minutes later.

Mokapu obtained a bearing of approximately 215 degrees on a carrier signal at 3105 kHz immediately following the KGMB broadcast. The post-search report⁵⁷ by K. C. Ambler, the Mokapu supervisor, described the signal as “close to 3105 but signals so weak that it was impossible to obtain even a fair check. Average seems to be around 215 degrees – very doubtful bearing.” He further stated that although the KGMB broadcast was repeated at half hour intervals during the evening, no further signals were heard that appeared to correlate with the broadcasts. But he did mention that at 1225Z the same day, Wake obtained an approximate bearing of 144 degrees, which seemed “to tie in fairly well with our two bearings of 213 and 215.” The Wake bearing is discussed later in this paper.

Mr. Angus, the Pacific Division Communication Superintendent, stated in his report⁵⁸ “Arriving at Mokapu Sunday, I spent most of Sunday night at the radio station and we set up a watch on 3105 KC at the DF and the receiving station. At 7:30 p.m. local Honolulu time, the broadcast station KGMB arranged a special broadcast to the plane on their broadcast frequency, requesting the plane to transmit four long dashes on 3105 KC if they heard KGMB plane. broadcast. Immediately after the broadcast, Mr. Ambler and myself both distinctly heard four dashes on 3105 KC. We are certain of the frequency because the Coast Guard Cutter, Itasca, had previously set their transmitter on this frequency in an effort to contact the Shortly before, we had taken bearings on the Itasca on this frequency, obtaining an approximate bearing of 210 degrees. Upon hearing the four dashes mentioned above, we immediately called KGMB by phone and asked them to repeat the test. This was done and immediately after the second test, we again heard the same signals except at this time, only two dashes were received and the second dash trailed off to a weak signal as though the power supply on the transmitter had failed. Nothing was heard thereafter although a continuous watch was maintained on this frequency all night. During the time these dashes were heard, it was possible to obtain an approximate bearing of 213 degrees from Mokapu.”

Table 6. Signal Data for Bearing 3.

Analysis

1. The timing of this signal relative to the broadcast strongly suggests it was a response to the broadcast.
2. The bearing line passes approximately 30 nmi southeast of Gardner Island.
3. Angus had the event time wrong. The KGMB request for dashes was sent at 8:00 p.m. (2000) HST, not 7:30 p.m. At 6:30 p.m. (1830) HST (0500Z), and again at 7:30 p.m. (1930) HST (0600Z), KGMB broadcast⁵⁹ “Calling Earhart plane. Every effort being made to locate you. This station will call at 0630 GCT at which time please reply on 3105 kilocycles. Listen on 1320 as all communications originate here.” At 8 p.m. (2000) HST (0630Z), KGMB broadcast⁶⁰ “To the Earhart plane. We are using every means to establish communication with you. If you hear this broadcast please come in on 3105 kilocycles. Use key if possible, otherwise voice. If you hear this broadcast turn your carrier signal on one minute so we can tune you in, then turn your carrier signal on and off four times. Then listen for our acknowledgment.”[sic]
4. Angus’ statement “Shortly before, we had taken bearings on the Itasca on this frequency, obtaining an approximate bearing of 210 degrees’ is not consistent with known facts. The Mokapu supervisor’s report⁶¹ states that the DF watch on 3105 kHz started at 0600Z on July 5, and that the bearing of 215 degrees was obtained at 0630Z. Thus the “shortly before” bearing must have been taken between 0600Z and 0630Z. But the Itasca did not transmit⁶² on 3105 kHz between 0330Z and 0730Z. Hence, the Itasca could not have been the signal source for the bearing cited by Angus, although the ship was near the bearing line. The bearing of the Itasca⁶³ during the 24 hours preceding the 0630Z KGMB broadcast varied between 227 degrees and 230 degrees.
5. The “shortly before” bearing cited by Angus may have been taken on a signal misidentified as being from the Itasca. This would be consistent with a premature Earhart response to the 0600Z KGMB broadcast alerting her that a 0630Z broadcast would request her to reply at that time. The fact that Ambler did not mention this bearing in his report may indicate that he considered it too indefinite for serious consideration.
6. The only land source near the signal bearing, other than Gardner Island, was AM broadcast station 3AR in Melbourne, the 3100 kHz harmonic of which could not be heard at Mokapu. But even if 3AR could be heard, and even assuming the station staff heard the KGMB broadcast, it is inconceivable that 3AR would send dashes in response.
7. The elevation angle for a signal from Gardner at this time was approximately 2 degrees, which was below the Koolau ridge line elevation angle, and could explain the signal weakness and hence the difficulty in getting the bearing.
8. The fact that this signal appears to have been a response to KGMB raises the question of whether it could have originated on the west coast. A station responding to KGMB would first have to hear the broadcast. The KGMB signal was strong enough on the west coast to be heard if there was no interference. However, there were two 24-hour stations on 1320 kHz: KID in Idaho Falls, Idaho, and KGHF in Pueblo, Colorado. The interference by those stations was evaluated using the LFMF model, and the results are shown in Table 7.

Table 7. Interference With West Coast Reception of KGMB.

Since the background noise level at a given location was common to signals from KGMB, KID, and KGHF, the interference level can be determined by considering the field strength, in dB above one microvolt , arriving from each station. The table shows that the KGMB ground wave signal was far too weak to have been heard on the west coast, whereas the skywave signal was strong enough for reception. A similar situation existed for the signals from KID and KGHF. The skywave interference ratio for KID and KGHF at each of the four west coast areas is the difference, in dB, between the field strength from each station and the field strength from KGMB. Since the KID and KGHF signals could be heard simultaneously anywhere on the west coast, the combined interference must be considered. The combined interference ratio is the square root of the sum of the squared individual skywave interference ratios. The results, in dB and numerical form, are shown in the bottom row of the table. The maximum combined interference was 49.3 times the KGMB signal strength, at Seattle, and the minimum was 7.5 times the KGMB signal strength, at San Francisco, showing that the interference from KID and KGHF prevented reception of the KGMB signal anywhere on the west coast. Therefore, the signal heard at Mokapu could not have been a response to the KGMB broadcast by a station on the west coast.

Ruling out the Itasca, 3AR in Melbourne, and west coast stations as sources of a signal responding to the KGMB broadcast leaves only one plausible possibility, i.e., a voice-capable transmitter on 3105 kHz on the bearing of Gardner Island. KHAQQ on the Earhart aircraft is the only known source fitting that description.

Bearing 4, 0638Z July 5, Midway Island

A bearing of approximately 201 degrees was obtained on a signal at 3105 kHz described⁶⁴ by the Midway supervisor as “a faint wobbly fone” that was “of such a short duration that it was impossible to narrow it down properly.”

Table 8. Signal Data for Bearing 4.

Analysis

1. The fact that this bearing is well off the bearing of Gardner Island does not necessarily rule out Gardner as the signal source. The short duration and weakness of the signal could account for the difference. The PAA DF system was designed for strong signals of several minutes duration.
2. The bearing of station 3AR in Melbourne was near the reported bearing, but the 3100 kHz harmonic of the 3AR signal could not be heard at Midway.
3. There were no known voice-capable signal sources on 3105 kHz near the reported bearing or its reciprocal.
4. The bearings of Navarin Mys and Billings were near the reciprocal of the bearing of Gardner, and one of those stations could have been the source, although the reception probabilities suggest otherwise. A signal from Navarin Mys was 770,000 times less likely to be heard than a signal from Gardner, and a signal from Billings was 5.3 million times less likely to be heard.
5. The Itasca did not transmit⁶⁵ between 0330Z and 0730Z, and thus could not have been the source.
6. This signal was heard 8 minutes after the 0630Z KGMB broadcast to Earhart. The time proximity of this signal to the KGMB broadcast suggests it was a response to the broadcast, and west coast sources can be ruled out under the rationale used in the analysis of Bearing 3.
7. The Midway report indicates that they did not listen to KGMB prior to 0830Z, suggesting that they did not have first-hand knowledge of the 0630Z broadcast.
8. As in the case of Bearing 3, the only known voice-capable transmitter on 3105 kHz fitting the parameters of this case is KHAQQ on the Earhart aircraft.

Bearing 5, 1105Z, July 5, Midway Island

A bearing of 175 degrees on a signal on 3105 kHz described as “a strong carrier”and a “steady unmodulated carrier” that “continued for over two hours.” The Midway report further stated that this signal “proved to be some unidentified station probably in South America or Russia and was later definitely disregarded as a possibility.”

Table 9. Signal Data for Bearing 5.

Analysis

1. This bearing line passes through Gardner Island.
2. A 2-hour transmission by KHAQQ would be plausible although it would have been necessary to keep the starboard engine running to avoid depleting the battery charge.
3. The Itasca did not transmit⁶⁶ on 3105 kHz between 0815Z and 1213Z, and thus could not have been the source of this signal.
4. There appears to have been a discrepancy between the clocks at Midway and on the Itasca. The Midway report⁶⁷ states that at 1235Z the Itasca was heard “on 3105 KC calling the Earhart plane giving instructions.” The radio log of the Itasca shows a transmission to Earhart at 1223Z requesting dashes. The next transmission to Earhart was at 1240Z and was logged as “CALLED EARHART FONE//.” If this is a condensed reference to a transmission requesting some action by Earhart, and was the signal that Midway reported hearing at 1235Z, then there was a 5 minute discrepancy between Midway and the Itasca. Unfortunately, there is not enough information available to resolve whether the error was in one or both of the clocks.
5. The PAA basis for deciding the source was an unidentified station in South America or Russia was not stated. But there were no known stations on 3105 in South America. There were 3 Russian stations near the reciprocal bearing: Stalino, Navarin-Mys, and Billings. The reception probabilities for signals from Stalino and Billings were too low for serious consideration. A signal from Navarin-Mys would be plausible, though very unlikely.
6. On balance, there is no clear weight of evidence for or against this signal originating at Gardner Island.

Bearing 6, 1105Z, July 5, Howland Island

As noted earlier, the Itasca deployed an experimental direction finder on Howland Island, manned by Radioman Second Class Frank Cipriani. Leo Bellarts, the Itasca’s chief radioman in July 1937, described⁶⁸ this direction finder as a portable Navy unit using a loop antenna, that had neither slip rings connecting the loop to the output signal wires, nor limit stops to prevent over-twisting the signal wires. According to Bellarts, the wires were badly twisted and broken when the unit was returned to the Itasca.

The Howland station log entry⁶⁹ for 1105Z on July 5 stated: “Weak carrier on 3105. No call given. Unilateral bearing impossible due to night effect. Using small pocket compass to determine relative direction. Bearing only approximate SSE or NNW.”

Howland sent a message⁷⁰ to the Itasca at 1455Z stating: “At 0035 HST, obtained bearing on a continuous wave of unknown origin indicating south southeast or north northwest on magnetic compass. Unable to obtain unilateral bearing due to night effect. No call given. Frequency is slightly above 3105 KCS.”

Table 10. Potential Signal Sources for Bearing 6.

Analysis

1. 0035 HST was 1105Z the same day.
2. The radio message to the Itasca paraphrased the Howland log entry, omitting the key facts that the signal was weak, and the compass used was a small pocket compass.
3. The computed reception probability for a signal from Gardner Island at the time was nearly 100 percent, contradicting Cipriani’s description of the signal strength, if the signal he heard was in fact from Gardner. However, a strong signal from Gardner could have been perceived as weak if the DF unit’s wires were already twisted and broken when the signal was heard.
4. It is not surprising that Cipriani was unable to get a unilateral bearing. The unsuitability of a loop antenna for direction finding of skywave signals was discussed earlier.
5. The Itasca was not the source of this signal. The ship did not transmit⁷¹ on 3105 kHz between 0815Z and 1213Z.
6. The lack of a stated signal duration precludes direct comparison with the signal reportedly heard at the same time by Midway in the Bearing 5 case.
7. Converting a magnetic compass bearing to a true bearing requires knowing the local magnetic variation and the magnetic deviation of the compass. The local variation might have been available from a navigational chart. But magnetic deviation depends on the mass and specific location of ferrous metal near the compass, and it is highly doubtful that Cipriani’s pocket compass had been calibrated for deviation. Therefore, the bearing axis reported by Cipriani must not be considered as only a very general approximation.
8. The potential signal sources generally northwest of Howland were Navarin Mys, Stalino, Oust-Kiakhta, and Billings. With the exception of Navarin Mys, the reception probabilities for signals from those sites are too low for serious consideration. Navarin Mys is plausible, but highly unlikely.
9. With the exception of Gardner Island, there were no known sources generally southeastward of Howland.
10. The reception probability for a signal from Gardner was 58 percent.
11. Given the available evidence, Gardner Island is the most plausible source of this signal.

Bearing 7, 1223Z, July 5, Wake Island

This was a bearing of approximately 144 degrees on a signal at 3105 kHz, described by R. M. Hansen, the Wake Island supervisor⁷² as a “very unsteady voice modulated carrier,” with characteristics identical to those of a signal heard (without a bearing) at 1215Z the previous night, July 4, “an intermittent fone of rather wobbly characteristics,” and “voice modulated with male voice although unreadable through noise,” with signal strength that “started in at a carrier strength of QSA 5 and at 1236, when this transmission stopped it had gradually petered out to QSA 2 during the intervals when it was audible.”

Hansen also said: “While no identification call letters were distinguished in either case, I was positive at that time that this was KHAQQ. At this date, I am still of this opinion,” and “There is not the slightest question in my mind the signals I have reported on could have been those of a maladjusted CG phone, because (1) the abnormal and erratic characteristics of these signals as compared to the steady operating CG phones. And (2), the fact that no CG boats were, to the best of my knowledge, anywhere near the line of the Wake DF bearing of 144 degrees appx.”

Ambler, the Mokapu Point supervisor mentioned this signal in his report,⁷³ stating at 1225Z: “signal strength rises sufficiently for Wake to obtain an approximate bearing of 144 degrees. This seems to tie in fairly well with our two bearings of 213 and 215. At same time Midway reports obtaining a very poor bearing – due to swinging – of 201 degrees;” and at 1350Z: “Carrier heard again on 3105 but by time we could open DF signal off again.”

Angus, The Pacific Division Communication Superintendent, was not present at Wake Island, but stated in his report⁷⁴ “Wake reported signals in the vicinity of 3105 KC, obtaining an approximate bearing of 140 degrees. However, I do not believe the signals that Wake heard were from the Earhart plane inasmuch as they were unheard at Mokapu at this time. The signals heard at Wake were a continuous carrier for several minutes at a time and we were of the opinion that possibly these signals emanated from somewhere in Japan.”

Table 11. Potential Signal Sources for Bearing 7.

Analysis

1. “QSA” ratings are subjective operator estimates of signal strength on a scale of 1 to 5. “QSA 5” means “very good signal strength,” and “QSA 2” means “weak signal strength.“
2. This bearing line passes approximately 70 nmi southwest of Gardner Island.
3. The bearing of the Itasca at this time was 140 degrees, very close to the reported signal bearing.
4. The statement by Hansen that the signal characteristics were identical to those heard at 1215Z on July 4 provides an important clue. The Itasca could not have been the source of that signal. The ship’s radio log shows no transmissions on 3105 between 0645Z and 2100Z that day. Therefore, if the characteristics of the signal of Bearing 7 were identical to those of the signal heard on July 4, the signal of Bearing 7 was not from the Itasca.
5. The Itasca sent a signal to Earhart at 1223Z, logged as “ANOTHER CALL AS ABOVE // 3105,” but the log does not state the transmission mode. However, the signal referred to as “AS ABOVE” was sent in Morse code at 1213Z, asking Earhart to “INDICATE RECEPTION BY FOUR LONG DASHES AND THEN GIVE BEARING HOWLAND NORTH OR SOUTH.” Therefore, it is assumed that the 1223Z signal was sent in Morse code.
6. There is a 1230Z entry in the Itasca radio log stating: “HEARD THREE DASHES / VERY RAGGED / OFF FREQ.” Since the Itasca was listening for signals on 3105 kHz at this time, it can be assumed that this signal was close to 3105. Although the character of this signal is different from that reported by Wake, this reception by the Itasca might be a serendipitous clue, unrelated to the signal of Bearing 7, in that it could have been an Earhart response to the 1223Z Itasca request for dashes. The computed SNR for the Itasca request shows that the signal could have been heard loud and clear at Gardner Island. And the computed mean SNR at the Itasca for a response from Gardner was just at the minimum level required to hear dashes, with a 50 percent chance of exceeding that level due to randomness in the propagation environment.
7. Notwithstanding Angus’ assertion, failure to hear the signal at Mokapu does not imply that it wasn’t from Earhart. As for Angus’ suggestion that the signal could have originated in Japan, there were no transmitters in Japan operating on 3105 kHz, or on a frequency with a harmonic near 3105 kHz.
8. Of the signal sources in Table 11, only the Siberian station at Oust-Kiakhta was near the reciprocal of the signal bearing, and the probability of hearing a signal from that station was about 900 billion times lower than for a signal from Gardner Island.
9. The available evidence strongly suggests that Gardner Island was the source of the reported signal.

Bearing 8, 0947Z, July 6, Mokapu Point

The Itasca sent a message⁷⁵ at 0815Z on July 6 (2145 HST July 5) to the Coast Guard Commander of the Hawaiian Section (COMHAWSEC): “For purpose determining corrective factor on Earhart bearing request Honolulu take bearing on Itasca on 3105 KCS at 2215 and 2315 Honolulu time also Wake at 2315 Honolulu time.” The times of the requested bearings corresponded to 0845Z and 0945Z, respectively, on July 6.

COMHAWSEC replied⁷⁶ at 0840Z confirming the requested arrangements for the bearings at Mokapu, and advising that Wake Island would not be ready until 1145Z. The Itasca radio log shows transmissions to Earhart at 0843Z, 0945Z, 1000Z, and 1400Z. The failure to transmit at 1145Z for Wake turned out to be of no consequence because Wake⁷⁷ was unable to hear signals after 0700Z “owing to the presence over and near Wake of a severe electrical storm.”

COMHAWSEC⁷⁸ informed the Itasca at 1208Z that Mokapu had obtained a bearing of approximately 196.5 degrees, “Believed to be on Itasca phone transmitter,” at 2317 HST (0947Z on July 6), but provided no information about signal strength, bearing accuracy, or whether the signal was heard by Coast Guard or Navy facilities in the Honolulu area. The bearing of the Itasca from Mokapu at 0947Z was 232 degrees.

Analysis

1. The Itasca radio log for July 6 shows a transmission to Earhart on 3105 kHz at 0945Z and that the transmission lasted until 0948Z.
2. The bearing of the Itasca was 232 degrees at 0945Z.
3. The computed elevation angle of a signal arriving from the Itasca, was approximately 5 degrees. Given the 4.8-degree Koolau ridge line elevation angle on the bearing of the Itasca, terrain interference could have skewed the signal arrival direction and reduced the SNR below the level required for Mokapu to recognize that the signal was from the Itasca, and to get an accurate bearing.
4. The computed elevation angle for a signal from Gardner Island was 1.5 degrees, which was below the 2.9-degree ridge line elevation angle on the bearing of Gardner, so terrain interference could have skewed the signal arrival direction and reduced the SNR below the level required for Mokapu to recognize that the signal was from the Itasca, and to get an accurate bearing.
5. The only entry for July 6 (GMT) in the Mokapu supervisor&rsdquo;s report⁷⁹ states: “Our ship enroute to Alameda therefore all our facilities in use guarding flight. Nothing heard from Coast Guard. After turning over plane guard to Alameda at 1030 3105 KC was guarded as much as possible but nothing of interest was heard,” which appears to conflict with the COMHAWSEC message.
6. The apparent conflict between Ambler and COMHAWSEC can be explained by the following hypothetical scenario: Ambler saw that the Itasca transmissions were scheduled at times when no bearings on the Pan Am flight to Alameda flight would be required, so Mokapu could listen briefly for the Itasca signals without jeopardizing flight safety. Nothing was heard at 0845Z, but an unreadable voice signal was heard at 0947Z and a bearing of 196.5 degrees was obtained. The signal was reported to COMHAWSEC as “Believed to be on Itasca phone transmitter”based on its time proximity to the expected 0945Z signal from the Itasca. Ambler did not know the Itasca’s location at the time, and thus did not question the bearing. During the next several days prior to his report on July 10, Ambler learned the Itasca’s location at 0947Z on July 6 and concluded that the signal that was heard could not have been from the Itasca, and that the timing of the signal was coincidental. Thus, having decided that the only signal heard, at 0947Z, was not from the Itasca, Ambler wrote “Nothing heard from Coast Guard” in his report.
7. The design of this test was flawed in that it failed to take into account the likely interaction of signal propagation geometry and Koolau range terrain interference. Consequently, the test results were inconclusive.

Conclusion

The attempt to obtain DF bearings on possible signals from Earhart was undertaken in extremely difficult conditions and yielded a total of 8 bearings. The Pan Am Adcock DF sites, which obtained 7 of the bearings, were searching for signals with SNRs far lower than those for which the DF system was designed. The DF system at Howland Island, an experimental Navy portable unit of doubtful reliability, obtained the eighth bearing.

All of the bearings were approximate at best, due to weakness and short duration of signals, and potential terrain interference effects in the case of the Mokapu site. And none of the signals included conclusive evidence as to the source identify.

Notwithstanding the lack of specific source identification, analysis of multi-dimensional circumstantial evidence has yielded important insights. The evidence associated with Bearings 2, 3, and 7 strongly supports the TIGHAR hypothesis that Earhart landed at Gardner Island and transmitted radio signals from there. The evidence associated with Bearings 1, 4, and 6 moderately supports the hypothesis, and the evidence associated with bearings 5 and 8 is inconclusive.

In sum, the weight of available evidence strongly supports the TIGHAR hypothesis.

End Notes

1. Developed by the U.S. Department of Commerce Institute for Telecommunication Sciences, Boulder, CO.
2. Ibid.
3. An extensive literature on NEC2 is available on the Internet.
4. TIGHAR archives.
5. Ibid.
6. In non-diversity reception, a radio receiver has a single antenna.
7. Spaulding, A. D., and F. G. Stewart, “A Critique of the Reliability & Service Probability Calculations for the Ionospheric Communication and Prediction Program – IONCAP,” NTIA Report 93-297, U.S. Department of Commerce, National Technical Information Service, August 1993.
8. Spaulding, A. D., “Atmospheric noise and its effects on telecommunication systems,” Chapter 6, Handbook of Atmospherics, Ed. H. Vollard, (CRC Press, Boca Raton FL, 1982).
9. Spaulding, A. D., and F.G. Stewart, “An updated noise model for use in IONCAP,” NTIA Report 87-212, U. S. Department of Commerce, National Technical Information Service, January 1987.
10. Roberts, H. W. “Radio Network and Practice of Pan American Airways” Parts 1 and 2, Aero Digest, January and February 1938.
11. Http://www.nostalgiaair.org/Resources
12. Federal Radio Communication Radio Service Bulletin, February 1932.
13. Federal Radio Commission General Order 94, June 1930.
14. www.bluejacket.com/ww2_12-07-41_index.htm
15. Liste des Frequence publiée par le Bureau de L’Union Internationale des Telecommunications, 9^th Edition, Fevrier 1939, Berne.
16. Stetson Conn and Byron Fairchild, United States Army in World War II, The Western Hemisphere, THE FRAMEWORK OF HEMISPHERE DEFENSE, Center of Military History, United States Army, Washington, D. C., 1989.
17. Email from “Charlie” on 11 May 2002.
18. Dr. Susan I. Enscore, et al, “Historical And Architectural Documentation Reports for Albrook Air Force Station and Howard Air Force Base, Former Panama Canal Zone” for the Air Combat Command, Langley Air Force Base, Virginia, September 1997.
19. Ibid.
20. Ibid.
21. Ibid.
22. Ibid.
23. Ibid.
24. www.timetableimages.com
25. Liste des Frequences Publiée par le Bureau de L’Union Internationale des Telecommunications. 9^th Edition, Fevrier 1939, Berne.
26. Ibid.
27. “List of Coast Stations and Ship Stations,” 10^th Edition, March 1938, published by the International Telecommunication Union, Berne.
28. Ibid.
29. Ibid.
30. Terman, F.E. and W.C. Roake, “Calculation and Design of Class C Amplifiers” Proc. I.R.E., vol. 24, pp. 620–632, April 1936.
31. Terman, F.E. and J.H. Ferns, “The Calculation of Class C Amplifier and Harmonic Generator Performance of Screen -Grid and Similar Tubes”, Proc. I.R.E. ,vol. 22, pp. 359–373, March 1934.
32. Long, Elgin M. Transcript of interview of Lieutenant Leo G. Bellarts, U.S. Coast Guard (retired), April 11, 1973. Bellarts was the chief radioman on USCGC Itasca during the Earhart search. The transcript is in the TIGHAR archive.
33. Sandretto, P.C., Principles of Aeronautical Radio Engineering, (McGraw-Hill, 1942), p. 253.
34. Article in the Sparks Journal, December 20, 1983, The Society of Wireless Pioneers, P.O. Box 86, Geyserville, CA.
35. Roberts, H. W. “Radio Network and Practice of Pan American Airways,” Parts 1 and 2, Aero Digest, January and February 1938.
36. Ibid.
37. Sandretto, 253.
38. Ibid, 244.
39. Terman, F.E., Radio Engineering, Third Edition, (McGraw-Hill, 1947), 629.
40. Ibid, 652.
41. Ibid, 822.
42. Ibid.
43. Ibid.
44. U.S. Geological Survey map, “Mokapu Quadrangle,” 7.5 minute series, scale 1:24,000, edited 1983.
45. Weems, P.V.H., Air Navigation, second edition, McGraw-Hill, 1938, Figure 151, p. 226.
46. Gray, A., Capt USNR (Retired), “Amelia Didn’t Know Radio,” Naval History Magazine, December 1993.
47. U.S. Geological Survey Map, “Kaneohe Quadrangle,” 7.5 minute series, scale 1:24,000, edited 1983.
48. U.S. Geological Survey Map, “Honolulu Quadrangle,” 7.5 minute series, scale 1:24,000, edited 1983.
49. Roberts.
50. Honolulu Star Bulletin, July 4, 1937.
51. Ambler, K.C., memorandum report to PAA Pacific Division Communications Superintendent, July 10, 1937, TIGHAR archives.
52. Miller, G.H., memorandum report to PAA Pacific Division Communications Superintendent, July 11, 1937, TIGHAR archives.
53. TIGHAR Earhart Project Research CD, Volume 1, which includes a comprehensive collection of radio messages, radio logs, ship deck logs, navigational data, and weather data, compiled and organized by Randy Jacobson, Ph.D., TIGHAR #136. Hereinafter cited as “TIGHAR CD.”
54. Ambler.
55. Angus, G.W., memorandum to PAA Chief Communication Engineer, July 10, 1937.
56. Computed from Itasca navigation data on the TIGHAR CD.
57. TIGHAR CD
58. Honolulu Star Bulletin, July 5, 1937.
59. Ambler.
60. Angus.
61. Honolulu Star Bulletin, July 5, 1937.
62. Ibid.
63. Ambler.
64. TIGHAR CD.
65. Computed from Itasca navigation data on the TIGHAR CD.
66. Miller.
67. TIGHAR CD.
68. TIGHAR CD.
69. Ibid.
70. Long.
71. TIGHAR CD radio message data base, record #1499.
72. TIGHAR CD radio message data base, record #441.
73. TIGHAR CD.
74. Hansen, R.M., memorandum report to PAA pacific Division Communication Superintendent, July 10, 1937, TIGHAR archives.
75. Ambler.
76. Angus.
77. TIGHAR CD, record #808.
78. TIGHAR CD, record #454.
79. Hansen.
80. TIGHAR CD, record #465.
81. Ambler.

APPENDIX B

Ruling out the Magnitogorsk

The Soviet ship Magnitogorsk was listed in the March 1938 edition of the ITU List of Coast Stations and Ship Stations as having a 50-watt transmitter capable of A1 (CW) or A3 (voice) emission, and was authorized to use 3105 kHz, 4140 kHz, 6180 kHz, and 6210 kHz. But merely having A3 capability does not mean that it was used on all assigned frequencies.

The Magnitogorsk was the only non-U.S. ship listed as authorized to operate on 3105 kHz. Numerous U.S.ships and coast stations were authorized to operate on 3105 kHz, but not in A3 mode. Consequently, there were no known ships or coast stations with which the Magnitogorsk could communicate on 3105 kHz in A3 mode, and the Magnitogorsk can be ruled out as a possible source of voice signals on 3105 kHz.

A search of the ITU list for shore stations with frequencies matching those of the Magnitogorsk found that only 2 Soviet coast stations, Jilaia Kosa and Donbas, used 6180 kHz. Both stations were on the Caspian Sea.

Jilaia Kosa, with a 70-watt transmitter (A1 only) was assigned 3105, 4140, 4170, and 6180 , and was the only Soviet coast station operating on 3105. Donbas, with a 50-watt transmitter (A1 and A3) was assigned 4170 and 6180, but not 3105.

The fact that two of the Magnitogorsk’s four frequencies, 3105 and 6180, were used only by Jilaia Kosa and Donbas suggests that the Magnitogorsk operated in the Caspian Sea. The ship also apparently operated on other inland seas and on inland waterways, as indicated by the fact that the other two frequencies assigned to the Magnitogorsk, 4140 and 6210 kHz, were used only by coast stations in those areas. Specifically:

1. The Astrakhan station on the Caspian Sea at the Volga River delta: 4140 kHz (A1 & A3);
2. The Rostov-Don station at the Don-Volga canal junction with the Sea of Azov: 4140 and 6210 (A1);
3. The Kerch station at the passage from the Sea of Azov to the Black Sea: 4140 and 6210 (A1 & A3);
4. The Yalta station on the Black Sea: 4140, 5625, 6210, and 6290 (A1 & A3).
5. The Sevastopol station on the Black Sea: 4140 and 6210 (A1 & A3);
6. The Novorossiisk station on the Black Sea: 4140 and 6210 (A1 & A3).

Since Jilaia Kosa was the only station with which the Magnitogorsk could communicate on 3105 kHz, and since Jilaia Kosa could only operate in A1 mode, the Magnitogorsk could not have been a source of A3 signals heard on 3105 kHz.