# Why do planes need line of sight for GPS to work?

Why is the below true when mobile devices such as mobile phones and designer watches with emergency beacons work without Line of sight?

Most commercial aircraft transmit their GPS-based position twice per second. This is part of their ADS-B broadcasts. The problem with providing world-wide receiver coverage for this system is that the frequency it uses only travels line via line of sight, so it won't travel past the horizon.

Providing coverage over large bodies of water would require a network of buoys, which would be quite expensive.

Another possibility is to put ADS-B receivers on satellites. This concept is being developed by Thales Alenia Space and Iridium (Aireon) at the moment. The first satellites will launch next year, the system is expected to be operational in 2018.

• I'm not sure what your question is (what you've posted seems like it wants to be a comment on something else), but you are incorrect about moble phones working "without line of sight" - they require (radiotransparent) line of sight to the cell tower (to make calls, send/receive data, etc.), and (radiotransparent) line of sight to GPS satellites to fix their position. PLBs (and ELTs) also require line of sight to the satellites monitoring for those signals: Put one under some concrete, or in deep enough water, and it's effectively useless. – voretaq7 Sep 10 '15 at 4:32
• Are you asking about how GPS works (your title) or how ADS-B works (question body)? Both require line-of-sight, but getting that to a GPS sat in a plane is normally easy. While getting it to an ADS receiver may be more difficult. – BowlOfRed Sep 10 '15 at 5:24
• mobile phones and designer watches with emergency beacons work without Line of sight who told you this? Your question is broken since this statement is wrong. – Simon Sep 10 '15 at 5:38
• I voted to repoen because the OP seems to be confusing ADS-B and GPS. We should clarify this point in answers to this question, since other people are likely to ask the same question. In fact a couple months ago, someone IRL asked me why GPS didn't work above the oceans for planes after they saw a local news report. – usernumber Sep 10 '15 at 8:35
• @mins Which would also be true for aicraft systems. It just so happens that most of the time, they have unitnterrupted line of sight. Additionally, the reflections are a minor part of the story. Far more important are the RF lines iof sight and what materials lie in the visible line of sight which cause attenuation. When you're in your house using your mobile, most of the signal is directly received. It may have passed through several buildings and your wall/window. Most things betweee you and the cell tower are lousy reflectors of RF. – Simon Sep 10 '15 at 12:47

## 3 Answers

tl;dr: Mobile devices and designer watches with (RF) emergency beacons don't work without line-of-sight to some kind of receiver. Most types of RF communication signals require a close approximation of line-of-sight between the transmitter and receiver to work.

All GPS receivers require line-of-sight to fix their position. However, this line of sight is up (because the satellites are in orbit,) which is usually not obstructed unless you're inside a building, under water, under a bridge, under ground, etc. Any GPS receiver will stop working under those circumstances. Airplanes are very rarely in situations where their GPS receivers don't have a working signal, except when they're in a hangar. Since they're normally above ground and outside, they normally have no problem getting a GPS signal.

Where the problem comes in is transmitting their location back to someone who's listening for it. This requires either radio communication to some ground-based system (or another aircraft,) such as the ADS-B receivers mentioned in the question. (Roughly) line of sight is required for this. The same is true of mobile devices. They also must be in range (which is often less than line-of-sight) of a receiver that is listening for their location transmission. In the case of cell phones, these receivers will just be cell towers. This is why, for example, we can't just track down the position of some crew or passenger's phone when looking for a boat or aircraft that has gone missing at sea (where there aren't any cell towers in range.)

Note that radio-frequency signals (such as ADS-B or cellular communications) can penetrate some objects, but this ability is very limited, especially as you move to higher frequencies. Thick, dense objects (like concrete parking garages, overpasses, and tunnel roofs) will reduce (attenuate) the signal faster and conductive objects (such as water or metals like copper or aluminum) will nearly eliminate the signal with only rather thin layers. Also, in general, higher-frequency RF signals will be attenuated much more quickly than lower-frequency ones. This is why your home Wi-Fi or a cell phone works through perhaps a few walls, but the new 60 GHz wireless networks wouldn't.

In the case of cell phones, the broadcast range is also intentionally limited so that more towers using the same channel can be packed into a smaller area to serve more people. If they didn't do this, there wouldn't be enough bandwidth to handle all of the cellular communication needs in densely-populated areas.

The Earth's surface happens to be composed primary of rather dense soil and water, so radio signals don't travel through the Earth much at all. In general, any form of radio communication must either have no land or water between the transmitting and receiving antennas or else cause the signal to reflect off of higher layers of the atmosphere or curve around the Earth. Neither of those things works well in the frequency bands used by cell phones. It does work at some of the lower frequencies used by amateur radio operators and such, though.

Communications systems that need to work far away from land or otherwise in remote places where receivers can't reasonably be located within line-of-sight range on the surface will normally use satellites. This is because Earth is not between you and a satellite that is above you and, thus, isn't blocking your signal. Even connections to satellites don't normally work when the satellite is below the horizon (again, due to the Earth attenuating the signal,) but satellite communication systems are normally set up in constellations that are designed to keep at least a certain number of satellites above the horizon in coverage areas at all times or else are set up in geostationary orbit where they will always remain directly above the same surface position.

• What if they get the ADS transcievers to themselves act as routers for traffic? The packet size shouldn't be too large since I doubt these messages are bandwidth hungry. At any given time the likelihood that another plane is in line of sight must be quite good? At least better than having a ground station in line of sight. – curious_cat Sep 10 '15 at 5:42
• @curious_cat Aside from needing to redesign ADS, the biggest problem I see with that immediately is that it would be chaos in busy airspace. You couldn't just blast back out every ADS-B broadcast you received because then you'd have infinite retransmissions. You'd pretty much have to set up a mesh network, but that would become intractable quickly with the physical locations of the nodes moving relative to each other at well over Mach 1. You'd probably jam the channel just with routing configuration messages. – reirab Sep 10 '15 at 5:49
• @curious_cat It might work a little better if you only enabled it, say, on trans-oceanic (or otherwise very remote) flights. – reirab Sep 10 '15 at 5:53
• Exactly. The protocol could distinguish between land based receivers and airborne routers. If a land based receiver is contactable then a handshake happens and things proceed the usual way. If no land based receiver is found then a different kind of packet is sent to which airborne receivers respond. I'm sure the protocol can be made robust enough to avoid infinite re-transmissions etc. with a TTL field, tweaked routing protocols etc. – curious_cat Sep 10 '15 at 8:25
• The airborne routing transcievers with the most viable / robust path to a land based station could respond and then a fixed route is followed until that path starts fading and then a new route discovery packet is sent. Mach 1 sounds fast but that's relative to sound. Relative to EM transmission I'm sure Mach 1 is insignificantly slow. The tricky issue might be how soon line of sight gets lost at Mach 1. i.e. How unstable would be the discovered routes and how often would route discovery packets (i.e. overhead) need to be sent. – curious_cat Sep 10 '15 at 8:28

You are asking about two very different, unrelated technologies.

GPS is the technology that determines the receiver's position based on signals from satellites. Those satellites are overhead, so line-of-sight is pretty easy.
(airplanes usually don't fly in caves, buildings or other places where the overhead view is blocked)

ADS-B is the technology that broadcasts the position determined by GPS to other airplanes and ground receivers at regular intervals. It also requires line-of-site between the sender and receiver. (more-or-less: slightly indirect routes will work if the signal is not degraded too much)

You also claim that cell phones and other devices don't require line-of-sight. That is only true to the extent that the direct line-of-sight isn't terribly obstructed, which is usually related to how nearby and how powerful the transmitter is. You don't always get cell phone signals in the interior of large concrete and metal buildings, or deep in parking garages. It is also why cell-towers are mounted up high and in locations where they can see a lot of terrain.

GPS satellites transmit a very weak signal. About 60 Watts are irradiated at the satellite, arriving at earth's surface with just miliwatts of power. so anything in the line of sight between the receiver and the satellite will block the signal. A mere mili meter tin foil will do it. This is mostly a problem in the ground when surrounded with skyscrapers or tall mountains, because there are usually 31 operational GPS satellites in the world, with a full view of the sky we typically receive 11-13 satellites. So even with the first 5 degrees from the horizon blocked we should still typically get something like at least 7 satellites, when just 4 are essential and 5-6 help improve the quality of the fix substantially. But if the bottom 30 degrees (or more) from the horizon are covered in every direction we start running into the possibility of not having enough satellites to properly calculate a 3D fix (4 satellites). This doesn't happen all the time. For aviation GPS this isn't a problem for en route, cruise climb/descent situations. But during taxing, initial climb and critically instrument approaches there are airports where GPS can't be relied upon. In the future we'll use a mix of USA GPS, Europe Galileo, Russian Glonass and other GNSS constellations, ensuring proper reliable coverage 24x7 even in airports surrounded by mountains. With 4x as many ranging sources, there are always enough satellites above a 45 degree angle that you'll always get a good fix.

• a few sources would be nice. – Federico Feb 9 '18 at 6:51
• The information presented is the result of day by day usage of GPS receivers, some engineering knowledge and a few things I read here and there. I don't have sources, because they are my own conclusion. Many numbers are estimates. – Marcelo Pacheco Feb 10 '18 at 8:04