New details about communications between the missing and an Inmarsat satellite show an additional, partial “ping” occurred eight minutes after the final hourly contact between the aircraft and spacecraft.
Using an analysis of satellite data furnished by London-based Inmarsat, Malaysian government officials said March 25 that Britain’s Air Accidents Investigation Branch (AAIB) had furnished evidence of a final, partial signal between the MH370’s onboard L-band terminal and an Inmarsat gateway Earth station via the Inmarsat-3F1 commercial communications satellite in the early morning hours of March 8, when the aircraft disappeared en route from Kuala Lumpur to Beijing.
Investigators said after primary radar and Aircraft Communications and Reporting System (Acars) transmissions with MH370 ceased, the aircraft’s satellite antenna continued communicating hourly pings to the spacecraft, a protocol that ensures an idle aircraft terminal remains connected to the network.
But a chart showing Doppler-shift data released by the Malaysian government March 25 indicates three earlier pings were recorded during the plane’s take-off and ascent, between 12:30 a.m. and 1 a.m. local time. The plane then made a sharp turn left shortly after it was lost from Malaysian radar coverage at 2:22 a.m., followed by three pings occurring in rapid succession in the span of a few minutes, just before 2:30 a.m.
After that, hourly pings, or “handshakes” occurred at 3:40 a.m., 4:40 a.m., 5:40 a.m., 6:40 a.m. and 8:11 a.m. A final, partial ping between the aircraft and the ground station was recorded at 8:19 a.m., eight minutes after the final full ping.
In a March 25 statement issued by the Malaysian government, investigators said a subsequent ping was slated to occur at 9:15 a.m., but “no response was received from the aircraft,” indicating it was no longer logged on to the network.
Analysts note that the pings that occurred close together between 2:22 a.m. and 2:30 a.m. may have been transmitted for some reason other than that of the regular, hourly pings, or the more frequent earlier “handshakes” recorded after take off. The final incomplete ping suggests the transmission may have occurred when the plane lost power.
“The pings on the Doppler chart weren’t regularly spaced and the irregular ones appeared to coincide with the period of rapid altitude changes, after initial turn,” says Tim Farrar, an analyst with TMF Associates, a consulting firm based in Menlo Park, Calif. “That would imply the terminal on the plane potentially tried to reacquire the signal after it lost the satellite signal during a sharp maneuver. If that supposition is correct, then the last partial ping is potentially due to the same issue (dive/stall etc). Therefore one might conclude tentatively that the plane crash was at or close to that time.”
In the initial days following the aircraft’s disappearance, Inmarsat used the pings to assist in determining the likely direction of the aircraft, leading investigators to establish arcs north and south of the equator that indicated a range of possible locations for the missing plane. More recently Inmarsat used an innovative technique that took into account the velocity of the aircraft relative to the satellite and the resulting change in signal frequency, known as the Doppler Effect.
Inmarsat analyzed the difference between the frequency that its ground station expected to receive and the one actually measured. Inmarsat then compared its predictions with six other777 aircraft flying on the same day in various directions, establishing a pattern that narrowed MH370’s final path as traveling south over the Indian Ocean, in the vicinity of where search teams have been working for more than a week.
However, Malaysian officials said the evidence of a final, partial ping, or “handshake” between the aircraft and the ground station would require further investigation.
“At this time this transmission is not understood and is subject to further ongoing work,” the statement said. Sometime between 8:11 a.m. and 9:15 a.m. “the aircraft was no longer able to communicate with the ground station,” timing that investigators said is consistent with the maximum endurance of the aircraft.
“This analysis by Inmarsat forms the basis for further study to attempt to determine the final position of the aircraft,” the AAIB said. “Accordingly, the Malaysian investigation has set up an international working group, comprising agencies with expertise in satellite communications and aircraft performance, to take this work forward.”
A map accompanying the March 25 statement depicts two possible flight paths in a large swath of sea in the southern Indian Ocean where the plane is likely to have crashed, assuming it was traveling at a ground speed between 400-450 kt.
So far, sightings of debris in the southern Indian Ocean 2,500 mi. west of Perth, Australia, have not been linked to MH370’s disappearance.