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I've recently watched the 1993 movie Alive, "based on" the real events of Uruguayan Air Force Flight 571. I don't know how accurate it was, but the basic idea is that the pilots did a mistake and started going down with the plane, so that its tail hit a mountain top and the entire back part of the plane was cut off and people got sucked out mid-air. Then both the wings were cut off and the remaining part of the plane crashed into the snow with mountains all around in/near Peru.

GPS was apparently started in 1978. So I get that it didn't have GPS-based signals.

However, did they really not have some sort of automatic tracking signal used to know where the plane is at all times? Even if all electronics were destroyed in the crash, would it not have sent several signals back to the airports (or whatever is/was tracking air planes) so that they would know almost exactly where the crash occurred?

In one scene, they find a little pocket radio and are able to hear on it that the rescue mission/search party has been "called off"; that they have given up looking for them. I found this very strange. Surely they would not just "give up" like that before finding them, even if they are presumed dead? If only to retrieve the "black box" and learning more about what happened?

And, again, why did they call it off in the first place since they must have received signals about its near-final location? Were air craft in 1972 really so primitive that they relied entirely on humans to communicate with their voices over the radio their current position regularly? That sounds more like something that would be done in the 1920s or something. The more I learn about aircrafts in WW2, they seemed to have all kinds of mind-boggingly advanced and automatic equipment even then. But this took place in 1972!

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    $\begingroup$ Such technology doesn't even exist today. See MH370. $\endgroup$ Dec 1 '21 at 23:08
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Short answer

In 1972, there was no automatic position reporting, this had to be done by voice when the aircraft was not in radar contact. Distress transmitters, which range can reach 150 km in favorable circumstances, were already used by military at some places, and were being generalized to commercial aircraft. However distress signals were not monitored by satellites like they are today, but from ground stations (including by ham radio-operators who since a long time play a significant role in disaster communications and localization), from airborne aircraft and from ships.

If the search team couldn't find the aircraft, it can be because no distress beacon was on the aircraft, because they searched at the wrong place, or because for some reason it was not active. Beacons of that era could fail simply because their antenna was destroyed, or shielded by metal debris. Automatic beacons could also fail to activate, and the team may be unable to trigger it manually because they cannot recover it.

You mention GPS, be aware GPS is of no help in position reporting. Like other positioning systems, GPS can be used by the crew to determine their position. They still have to report it to ground control by some means.



Details

Position determination vs. reporting

It is important not to not mix two different things: Position determination and position reporting. Today you may read everything is "GPS tracked", but this is inaccurate and misleading. While many systems track us in real time, like the cellular phone operators, this is not the case of GPS, because a GPS receiver is a just a receiver, it doesn't send any information nor interact with satellites.

In the case of aircraft, ground control using a radar is able to determine the aircraft location and can share it with the crew. Radars have a limited range, don't work well in mountainous area, and are not implemented in oceans. When no radar can be used, the aircraft crew themselves determine the position. Then this position must be reported to ground control to make them aware.

Position determination and reporting by aircraft

  • Position determination. Today includes GNSS (GPS, Galileo, Glonass, etc). At the time this happened GNSS didn't exist, but other techniques were used, most still used today: Inertial system, Loran, VOR, TACAN, etc.

  • Position reporting. At the time, when out of radar coverage, reporting was done by the crew by voice communication, at predetermined reporting points along the route. The need to report the position is part of regulation, e.g. for FAA in FAR 71.5:

    § 71.5 Reporting points. The reporting points listed in subpart H of FAA Order JO 7400.11F (incorporated by reference, see § 71.1) consist of geographic locations at which the position of an aircraft must be reported in accordance with part 91 of this chapter.

    Today there are multiple techniques to automatically report the position, like CPDLC, the most used is ADS-B. ADS-B is coupled with a GNSS receiver. The GPS position is broadcast blindly by the ADS-B transmitter and can be received both by control for tracking and by other aircraft for collision avoidance.

    enter image description here

    Current integration of SSR (radar) and ADS-B/TIS-B, source

As ADS-B is not encrypted, actually anyone can receive the aircraft position, and this is what ADS-B tracking sites use first.

There must be ADS-B ground stations closer than radio horizon. So ADS-B suffers the same range limit than radar. But this is going to evolve quickly, in the next 3 to 5 years ADS-B will be received by satellites, and satellites will relay the reportings to ground control. See details below.

When the aircraft position cannot be tracked continuously from the ground, another possibility can be used when a crash occurs: A distress radio signal can be transmitted on a special frequency.

Distress transmitters

The distress transmitters have generally an automatic switch detecting the large deceleration associated with a crash. This radio signal doesn't need to transmit the position which can be determined by several techniques, from triangulation on the ground to Doppler evaluation from satellites.

Distress transmitters are known under different appellations, the one used by aviation is emergency locator transmitter or ELT.

ELT

At the time this accident happened, ELT were used by US military aircraft, perhaps not by Uruguayan military aircraft. In the early 70s the system was also required on commercial aircraft.

At this time ELT transmitted on the distress frequency of 121.5 MHz could be received by stations closer than the radio horizon. That could be ground stations or airborne aircraft, mainly those crossing the oceans. In non populated areas, a beacon triggered within a radius of say 150 km from a flying aircraft could still be detected and the information relayed to search and rescue organizations.

enter image description here

121.5/243 MHz ELT, source

To locate the ELT, search teams used triangulation techniques similar to direction finders used both by ground control and aircraft.

ELT have evolved to a detection from satellites which permanently monitor distress signals at any place. Triangulation techniques have been replaced by Doppler sensing based on the satellite velocity relatively to the searched beacon.

Such ELT transmit an identifier at time intervals on 406 MHz, a frequency constantly monitored by Cospas-Sarsat, a system made of two constellations of satellites, one in geostationary orbit at high altitude, and one at low altitude but constantly moving.

Most 406 MHz ELT are coupled with a GPS receiver to autonomously determine their position. The GPS-based position is transmitted on the 406 MHz signal, double-checked by Doppler sensing, and reported to the search and rescue organization.

enter image description here

ELT using GPS and Cospas-Sarsat, source

They still send a continuous signal on the old 121.5 MHz frequency in order to allow the ground search team to use short-range direction finders to reach the ELT.

ELT limitation

While ELT may replace continuous tracking for the purpose of search and rescue, this is still unsatisfactory, as both MH370 and AF447 accidents have shown:

  • In some crashes, ELT were not triggered.
  • ELT cannot work underwater
  • ELT may be destroyed in the crash or their signal shielded by debris.

There are some ELT which can be released before ground contact, especially on helicopters. However technology allows for better solutions, but this assume a significant change in existing air traffic control organization, switching from ground-based control to space-based control.

ATC switching to space-based ATC

Current aviation traffic control relies mostly on ground stations. When the aircraft flies over oceans the communication is broken (or at least unreliable).

To solve this problem with modern technologies, plans are being implemented, both in Europe and North America, to use satellites to track aircraft and these plans include a generalized use of ADS-B transmission by aircraft.

enter image description here

Thales / Aireon solution for space ATC, source

In anticipation of this deployment some operators are already using satellites to communicate their position at regular interval. They don't sent it to ATC since ATC is not ready to process it, but to their own operation control center. This was a lesson learnt from MH370.

MH370 had all the technology required to determine and report its position:

  • GPS and inertial navigation system for position determination.
  • ACARS to report its position.
  • a satcom link to transmit over water areas (the satcom transmitter position was determined by Doppler sensing, this led to searching water areas).

However the ACARS was configured to transmit only to ACARS land stations using VHF links, and was prevented to use the satcom link, for economical reasons as the flight was supposed to happen mostly over land.

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    $\begingroup$ @JonCuster: 121.5 MHz ELTs still exist and 121.5 is still an international distress frequency, but 121.5 is no more monitored by Corpas-Sarsat. Current 406 MHz ELTs generally have also a 121.5 transmitter which is used as a homing beacon by the ground rescue team. $\endgroup$
    – mins
    Dec 2 '21 at 14:48
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    $\begingroup$ I agree that Corpas-Sarsat stopped monitoring 121.5, however there are plenty of non-satellites listening for it - it just may take a bit (or a lot) longer to be noticed depending on where it is going off. The ones in hangers at the airport tend to be noticed fairly quickly... $\endgroup$
    – Jon Custer
    Dec 2 '21 at 15:00
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    $\begingroup$ Again, I fully understand the 406 system and how it is better than 121.5, but 121.5 is still out there and will be for a long long time. In addition, 406 was not used in 1972, the timeframe of the OP's question, and it is quite likely that 121.5 was not being used much outside of the US. $\endgroup$
    – Jon Custer
    Dec 2 '21 at 15:27
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    $\begingroup$ There was an incident maybe 5 years back when a scrapped airplane still had it's elt in it. Some kids got inside (iirc) and tripped the transmitter. Quite a whoohaa in SAR when they were trying to find out who had crashed and where... $\endgroup$
    – Jpe61
    Dec 2 '21 at 18:32
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    $\begingroup$ @Jpe61: I can't remember who she was, but maybe 20 years ago a solo round-the-world sailor triggered her Argos beacon at her hotel prior to her departure. At this time Argos was also used for distress. Same mess. $\endgroup$
    – mins
    Dec 2 '21 at 20:14
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The Navstar program that we know as GPS today did in fact get its start in 1978 but was not considered fully operational for a decade or so later. Even then the small receivers that we know today didn't come out until the 1990s. The civilian use of GPS with the accuracy improved by removal of "selective availability" for aircraft navigation didn't exist until the year 2000 or so. The need for more accurate navigation by civil aviation was made clear by a few international incidents of aircraft drifting into places they should not have been. Before that GPS was considered a purely military asset by the US government, though non-military applications were not barred as far as I know. Before GPS there were a number of satellite and land based radio navigation systems with varying degrees of availability and accuracy. Over the horizon LORAN navigation would have accuracy of several miles and line-of-sight VOR would be accurate to several hundred feet. These land based radio navigation systems should be able to keep people from flying into mountains. Satellite navigation systems used and maintained by the US Army and Navy were for military use, and not all that accurate. It turns out that close counts with horseshoes, hand grenades, and nuclear ordnance.

There were systems for automated sending of distress signals for a long time before 1972. On ships of the era they might still see use of the old Morse code telegraph systems that would send a distress signal by use of a kind of clockwork mechanism to send the dits and dahs. These could exist on aircraft too, though with some electronics instead of clockwork.

Around this time there was work on replacing the old distress systems that were based on decades old analog and shortwave radio with systems that had more automation and use of satellites. They really didn't get any traction until the 1990s and are still hanging on in some places of the world. GMDSS is the standard now but even in the 1980s you'd see communications equipment much like that used in World War 2. Radios cost a lot of money and if it ain't broke it's hard to get people to fix it. Also, getting a fancy new radio locator beacon won't do much good unless the people looking know you have it and have the hardware to look for it. It's a classic chicken or egg problem.

The US military worked on radio location and navigation considerably during and after World War 2 but that didn't work its way into civil aviation for some time. I suspect that a lot of this had to do with people getting so good with celestial navigation, dead reckoning, charts, compasses, and so on that they didn't see a need for anything else. These methods work so well that we are seeing the US military training more people in them. Helpful in case someone launches missiles at your satellites. But I digress.

When it comes to the decision to give up the search there's only so long that people would expect to find survivors. This would have been spring time in that part of the world and waiting for summer to continue the search would have made their work looking for the flight data recorder easier. More than a week looking for survivors would likely be far longer than many would expect to find anyone alive. Delays in the search for wreckage in the anticipation of better weather would be common then and now.

I also find myself amazed at times at the technical capabilities of aircraft and electronics from the time. The problem is that this technology cost a lot of money then so it left people living in the 1970s with a world that to some looked more like the 1930s. I wish I could recall where I heard this but it seems appropriate, "The future is now, it's just not evenly distributed."

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    $\begingroup$ Both the question and this answer seem to have missed the fact that GPS signals are only transmitted by satellites. @mins explains it. $\endgroup$ Dec 2 '21 at 14:39
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    $\begingroup$ Small nitpick: civilian use of GPS with the accuracy needed for aircraft navigation didn't exist until the year 2000 or so. We were logging GPS tracks in gliding competitions already in 1995. An error of 50 meters is not a big deal when you are moving at 50m/s. $\endgroup$ Dec 2 '21 at 15:21
  • $\begingroup$ Did I correct the nitpick with my edit? $\endgroup$
    – MacGuffin
    Dec 3 '21 at 0:00
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    $\begingroup$ @MacGuffin Not really, and it's more than a nitpick. For terrain/airspace avoidance, "close" isn't good enough, sure. But for SAR, "close" is absolutely all you need. Narrowing down to the nearest 1km is fine, and even 10km would give you a manageable search box. To the nearest 100m, which was the "official" civilian accuracy with selective availability, puts you in direct line of sight. The issue has never been the accuracy of positioning - it's always been the problem of getting that position back to SAR services. $\endgroup$
    – Graham
    Dec 3 '21 at 12:57
  • $\begingroup$ "in the 1980s you'd see communications equipment much like that used in World War 2" Your recall of 1980 is a bit distorted. Let's compare a portable transceiver: from WW II and from 1976 (SCR-300-A with tubes vs. Icom IC-2AT with IC synthesizer). $\endgroup$
    – mins
    Dec 3 '21 at 15:58

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