question: How can you pinpoint your location with GPS on your phone without service on the ground, but can’t do the same on an airplane?
comment: It works in plane too....if you have an unobstructed view of the sky with at least 4 satellites. Given cramped space and metal ceilinge of typical airliner body this is 50/50 chance of working if you're in window seat. But if you do it on a private small plane with a big plastic window, it works just fine.
passages: Many RTLS systems require direct and clear line of sight visibility. For those systems, where there is no visibility from mobile tags to fixed nodes there will be no result or a non valid result from locating engine. This applies to satellite locating as well as other RTLS systems such as angle of arrival and time of arrival. Fingerprinting is a way to overcome the visibility issue: If the locations in the tracking area contain distinct measurement fingerprints, line of sight is not necessarily needed. For example, if each location contains a unique combination of signal strength readings from transmitters, the location system will function properly. This is true, for example, with some Wi-Fi based RTLS solutions. However, having distinct signal strength fingerprints in each location typically requires a fairly high saturation of transmitters.
   The aircraft has three autopilot modes. The standard mode for en-route navigation is LNAV, which navigates along a great circle route between waypoints, adjusting the aircraft's heading to compensate for wind. Other modes will maintain the aircraft's heading—direction the nose is pointed (flight path will be affected by winds)—or the aircraft's track—direction the aircraft travels (flight path in a straight direction). The latter two modes are further affected by whether the aircraft used magnetic (normal reference) or true north (typically only used at high latitudes) as the reference for the autopilot. Since Flight 370 flew near waypoints VAMPI, MEKAR, NILAM, and possibly IGOGU—all along air route N571—while traversing the Malacca Strait, investigators considered whether Flight 370 followed any air routes or intersected any waypoints in the Southern Indian Ocean. Waypoints MUTMI and RUNUT were considered possible points that Flight 370 may have traversed, but tracks through these waypoints did not correlate well with paths generated from the BTO and BFO analysis.
   The technique used to analyse the BFO values was validated against 87 aircraft with the same SATCOM equipment operating in the region around the time of Flight 370's disappearance and against 9 previous flights operated by the same aircraft (9M-MRO). The sensitivity to error was calculated during the early phase of Flight 370 when the aircraft's location, track, and ground speed were known. This resulted in an uncertainty of ±28° heading and ±9° of latitude.
  
  Section::::Analysis by the Joint Investigation Team.:Combined analysis with flight dynamics.
   Two analysis techniques were used to combine the BTO and BFO results with flight parameters:
  
  BULLET::::- Data error optimisation – Candidate paths varied speed and heading at each handshake to minimise the error between the calculated BFO of that path versus the actual BFO recorded from Flight 370. These paths were not constrained by the behaviour of the aircraft's autopilot.
   With control plane locating, sometimes referred to as positioning, the mobile phone service provider gets the location based on the radio signal delay of the closest cell-phone towers (for phones without GPS features) which can be quite slow as it uses the 'voice control' channel. In the UK, networks do not use trilateration; LBS services use a single base station, with a "radius" of inaccuracy, to determine a phone's location. This technique was the basis of the E-911 mandate and is still used to locate cellphones as a safety measure. Newer phones and PDAs typically have an integrated A-GPS chip.
   A few deductions can also be made from the satellite communications. The first deduction that can be made from the satellite communications is that the aircraft remained operational until at least 08:19—seven hours after final contact was made with air traffic control over the South China Sea. The varying BFO values indicate the aircraft was moving at speed. The aircraft's SDU needs location and track information to keep its antenna pointed towards the satellite, so it can also be deduced that the aircraft's navigation system was operational.
   Location will never be reported "exactly", as the term "real-time" and the term "precision" directly contradict in aspects of measurement theory as well as the term "precision" and the term "cost" contradict in aspects of economy. That is no exclusion of precision, but the limitations with higher speed are inevitable.
  
  BULLET::::- Steady location error
  
  Recognizing a reported location steadily apart from physical presence generally indicates the problem of insufficient over-determination and missing of visibility along at least one link from resident anchors to mobile transponders. Such effect is caused also by insufficient concepts to compensate for calibration needs.
   The BTO analysis was able to determine the distance between the satellite and aircraft with a relatively high degree of accuracy, while the BFO analysis was able to estimate the heading and speed of the aircraft, but is sensitive to small changes in input data. To determine the final location of Flight 370, the BTO and BFO analyses were considered in combination with aircraft performance limitations, such as altitude, airspeed, and wind. The BFO analysis was able to isolate the Doppler shift between the aircraft and determine the relative motion of the aircraft to the satellite, which is reduced by the limited range of speeds at which the aircraft can fly and thus a limited set of speed/direction combinations exist that correlate with the calculated Doppler shifts.
   In 2016, the ICAO adopted a standard that, by November 2018, all aircraft over open ocean report their position every 15 minutes. In March, the ICAO approved an amendment to the Chicago Convention requiring new aircraft manufactured after 1 January 2021 to have autonomous tracking devices which could send location information at least once per minute in distress circumstances.
   By combining the distance between the aircraft and satellite, speed, and heading with aircraft performance constraints (e.g. fuel consumption, possible speeds and altitudes), investigators generated candidate paths that were analysed separately by two methods. The first assumed the aircraft was flying on one of the three autopilot modes (two are further affected by whether the navigation system used magnetic north or true north as a reference), calculated the BTO and BFO values along these routes, and compared them with the values recorded from Flight 370. The second method generated paths which had the aircraft's speed and heading adjusted at the time of each handshake to minimise the difference between the calculated BFO of the path and the values recorded from Flight 370. A probability distribution for each method at the BTO arc of the sixth handshake of the two methods was created and then compared; 80% of the highest probability paths for both analyses combined intersect the BTO arc of the sixth handshake between 32.5°S and 38.1°S, which can be extrapolated to 33.5°S and 38.3°S along the BTO arc of the seventh handshake.
   At 00:42 MYT, Flight 370 took off from runway 32R, and was cleared by air traffic control (ATC) to climb to flight level 180—approximately —on a direct path to navigational waypoint IGARI (located at ). Voice analysis has determined that the First Officer communicated with ATC while the flight was on the ground and that the Captain communicated with ATC after departure. Shortly after departure, the flight was transferred from the airport's ATC to "Lumpur Radar" air traffic control on frequency 132.6 MHz. ATC over peninsular Malaysia and adjacent waters is provided by the Kuala Lumpur Area Control Centre (ACC); Lumpur Radar is the name of the frequency used for "en route" air traffic. At 00:46, Lumpur Radar cleared Flight 370 to flight level 350—approximately . At 01:01, Flight 370's crew reported to Lumpur Radar that they had reached flight level 350, which they confirmed again at 01:08.
   Additional factors that affect the BFO are a translation made at the ground station between the reception and processing of the signal (which is monitored and can be factored in) and a fixed bias in the aircraft and satellite oscillators due to drift and ageing (which can be calibrated by measures recorded when the aircraft's location and speed were known). At 2:40 and 6:14, ground-to-aircraft phone calls were made that were unanswered by the cockpit but acknowledged by the SDU. The signals associated with these calls could not be analysed to generate a BTO value, but BFO values of these signals can be considered in the analysis with the other BTO and BFO data.
   A few deductions can be made from the satellite communications. The first is that the aircraft remained operational until at least 08:19 MYT—seven hours after final contact was made with air traffic control over the South China Sea. The varying burst frequency offset (BFO) values indicate the aircraft was moving at speed. The aircraft's SDU needs location and track information to keep its antenna pointed towards the satellite, so it can also be deduced that the aircraft's navigation system was operational.
   BULLET::::- Constrained autopilot dynamics – The aircraft is assumed to be flying under the control of one of the autopilot modes. Candidate paths were generated using each mode. BTO and BFO values of each path were calculated and compared against the recorded values from Flight 370.
   BULLET::::- location signature uses "fingerprinting" to store and recall patterns (such as multipath) which mobile phone signals are known to exhibit at different locations in each cell
  
  The first two depend on a line-of-sight, which can be difficult or impossible in mountainous terrain or around skyscrapers. Location signatures actually work "better" in these conditions however. TDMA and GSM networks such as Cingular and T-Mobile use TDOA.
  
  CDMA networks such as Verizon Wireless and Sprint PCS tend to use handset-based radiolocation technologies, which are technically more similar to radionavigation. GPS is one of those technologies.
   To combat these effects most recently, great emphasis is placed upon software solutions. It is highly likely that one of those software algorithms was the proximate cause of a mid-air collision recently, as one airplane was reported at showing its altitude as the pre-flight paper filed flight plan, and not the altitude assigned by the ATC controller (see the reports and observations contained in the below reference ATC Controlled Airplane Passenger Study of how radar worked).
  
  See the reference section below for errors in performance standards for ATCRBS transponders in the US.
   Section::::Timeline.
  
  The aircraft, a Boeing 777-200ER operated by Malaysia Airlines, last made voice contact with air traffic control at 01:19 MYT, 8 March (17:19 UTC, 7 March) when it was over the South China Sea, less than an hour after takeoff. It disappeared from air traffic controllers' radar screens at 01:22 MYT, but was still tracked on military radar as it deviated westwards from its planned flight path and crossed the Malay Peninsula, until it left the range of the military radar at 02:22 while over the Andaman Sea, northwest of the island of Penang in northwestern Malaysia.
   At 03:30, Malaysia Airlines' operations centre informed Kuala Lumpur ACC that the locations it had provided earlier were "based on flight projection and not reliable for aircraft positioning." Over the next hour, Kuala Lumpur ACC contacted Ho Chi Minh ACC asking whether they had signalled Chinese air traffic control. At 05:09, Singapore ACC was queried for information about Flight 370. At 05:20, an undisclosed official contacted Kuala Lumpur ACC requesting information about Flight 370; he opined that, based on known information, "MH370 never left Malaysian airspace."
   While the BTO is able to determine the "distance" between the satellite and the aircraft at the time of each handshake, it was still necessary to determine "where" along the BTO arcs the aircraft was. To accomplish this, an analysis was performed on another attribute of received signals that was recorded by the ground station: the burst frequency offset (BFO)—the difference between the expected and actual frequencies of the signal received from the aircraft. The BFO is primarily caused by the Doppler shift—a shift in frequency caused by the relative movement of the aircraft, satellite, and ground station—along with several other factors which can be calculated and removed, allowing the Doppler shift between the aircraft and satellite to be isolated. The Doppler shift between the aircraft and satellite indicates the relative motion of the aircraft relative to the satellite, although multiple combinations of aircraft speed and heading exist that match a given Doppler shift value.
   Hybrid surveillance does not make use of ADS–B's aircraft flight information in the TCAS conflict detection algorithms; ADS–B is used only to identify aircraft that can safely be interrogated at a lower rate.
  
  In the future, prediction capabilities may be improved by using the state vector information present in ADS–B messages. Also, since ADS–B messages can be received at greater range than TCAS normally operates, aircraft can be acquired earlier by the TCAS tracking algorithms.
   With the advent of cellphones and the Internet a separate or integrated SDU can be used to offer telephone and Internet services to passengers.
  
  Logs of satellite communication have been used to inform search and rescue agencies of locations of missing aircraft, notably that of Malaysia Airlines Flight 370 whose position was unknown due to loss of radar contact and other communications. Automated SATCOM transmissions suggested it flew about off its designated flight path having flown approximately south-southwest rather than the intended approximately north-northeast.
   Crowdsourced Wi-Fi data can also be used to identify a handset's location. The poor performance of the GPS-based methods in indoor environment and the increasing popularity of Wi-Fi have encouraged companies to design new and feasible methods to carry out Wi-Fi-based indoor positioning. Most smartphones combine Global Navigation Satellite Systems (GNSS), such as GPS and GLONASS, with Wi-Fi positioning systems.
  
  Section::::Technology.:Hybrid.
   BULLET::::- A holder for a portable GPS device was found in the cockpit, but the device itself was not found. The investigation report, based on CVR recordings, states that the pilots may have relied on the portable GPS device to understand their location.
  
  BULLET::::- The report suggests that during the final minutes of the flight, all three pilots were focused on correcting the horizontal deviation from the landing course and did not pay attention to the altitude.
  
  Section::::Final investigation report.:Cause.
  
  The report states the following reason of the crash:
   At the time that the transponder stopped functioning, military radar showed Flight 370 turning right, but then beginning a left turn to a southwesterly direction. From 01:30:35 until 01:35, military radar showed Flight 370 at on a 231° magnetic heading, with a ground speed of . Flight 370 continued across the Malay Peninsula, fluctuating between in altitude. A civilian primary radar at Sultan Ismail Petra Airport with a range made four detections of an unidentified aircraft between 01:30:37 and 01:52:35; the tracks of the unidentified aircraft are "consistent with those of the military data". At 01:52, Flight 370 was detected passing just south of the island of Penang. From there, the aircraft flew across the Strait of Malacca, passing close to the waypoint VAMPI, and Pulau Perak at 02:03, after which it flew along air route N571 to waypoints MEKAR, NILAM, and possibly IGOGU. The last known radar detection, from a point near the limits of Malaysian military radar, was at 02:22, after passing waypoint MEKAR (which is from Penang) and northwest of Penang airport at an altitude of .
   Records of signals sent between the aircraft and a communications satellite over the Indian Ocean revealed that the aircraft had continued flying for almost six hours after its final sighting on Malaysian military radar. Initial analysis of these communications determined that Flight 370 was along one of two arcs—equidistant from the satellite—when its last signal was sent; the same day this analysis was disclosed publicly, 15 March, authorities announced they would abandon search efforts in the South China Sea, Gulf of Thailand, and Strait of Malacca to emphasize their efforts on the two corridors. The northern arc—from northern Thailand to Kazakhstan—was soon discounted, for the aircraft would have had to pass through heavily militarised airspace and those countries claimed their military radar would have detected an unidentified aircraft entering their airspace.
  
presuppositions: You cannot pinpoint your location with GPS on your phone on an airplane. You can't use GPS on an airplane.
corrections: It is possible to pinpoint one's location with GPS on an airplane, if there is an unobstructed view of the sky with at least 4 satellites. You can use GPS on an airplane, but does not work as well due to the metal ceiling of a typical airliner.
raw_presuppositions: You cannot pinpoint your location with GPS on your phone on an airplane. You can't use GPS on an airplane.
raw_corrections: It is possible to pinpoint one's location with GPS on an airplane, if there is an unobstructed view of the sky with at least 4 satellites. You can use GPS on an airplane, but does not work as well due to the metal ceiling of a typical airliner.