No Aircraft Radar Mid-Ocean? Why Not Use Buoys?

[Xema]

@sailor

GPS works by triangulating in three dimensions by measuring the time differences from several satelites.

Actually, no triangulation is involved. The principle, as you note, is to measure the time difference in the arrival of signals from pairs of satellites. A line of constant time difference defines a hyperbola (in 3 dimensions, a “hyperbloid of revolution”); calculating the point of intersection of several of these yields a 3-D position.

[Broomstick]

@chorpler

I found maps of US radar coverage here, particularly sections 3 and 4. If I’m interepreting the maps correctly, it’s fascinating to see in section 4 how the coverage expands as you get higher; at 10km above ground, there are very few holes left – only the middle of Nevada, a bit of Oregon and northern Nevada, part of Arizona near the “Four Corners” region where Utah, Arizona, Colorado, and New Mexico meet, a bit of Montana/Wyoming, and a little bit of western Texas appear to be the only significant gaps. But at only 1km above the ground, the coverage is so small that it looks like you could fly from one coast to the other without being spotted (although those mountains in the middle might prove problematic if you’re trying to stay lower than 1km above the radar stations).

Am I reading them right?

Yes, you are. Below a certain height there is NO radar coverage in the US except fro terminal areas. East of the Mississippi there is more coverage at lower altitudes than in the west, as can be seen on the 1km and 3km maps. You certainly could fly coast to coast below the radar if you knew a little bit about planning such a trip.

As I said, there is a lot of the US airspace that has no radar coverage. Our Australian friend didn’t see it that way, but the difference is (apparently) that over much of Australia there is no radar coverage at any altitude, whereas in the US the non-radar covered areas are overlaid by airspace with radar coverage.

[Waffle_Decider]

@chorpler

I found maps of US radar coverage here, particularly sections 3 and 4. If I’m interepreting the maps correctly, it’s fascinating to see in section 4 how the coverage expandhttp://boards.straightdope.com/sdmb/newreply.php?do=newreply&p=11271989s as you get higher; at 10km above ground, there are very few holes left – only the middle of Nevada, a bit of Oregon and northern Nevada, part of Arizona near the “Four Corners” region where Utah, Arizona, Colorado, and New Mexico meet, a bit of Montana/Wyoming, and a little bit of western Texas appear to be the only significant gaps. But at only 1km above the ground, the coverage is so small that it looks like you could fly from one coast to the other without being spotted (although those mountains in the middle might prove problematic if you’re trying to stay lower than 1km above the radar stations).

Am I reading them right?

Note that that page depicts WSR-88D radar coverage. That is NEXRAD, or weather radar, not the radar used for air traffic control.

Overall though, yeah, coverage at the lower altitudes can sometimes be spotty even for ATC radar.

[billfish678]

@Xema

Actually, no triangulation is involved. The principle, as you note, is to measure the time difference in the arrival of signals from pairs of satellites. A line of constant time difference defines a hyperbola (in 3 dimensions, a “hyperbloid of revolution”); calculating the point of intersection of several of these yields a 3-D position.

GPS is inherently more accurate in x and y than z (altitude) just due to geometric considerations. You would not want to use GPS to fly 50 feet above the ground, but for keeping planes from crashing into each other and knowing roughly where they are the accuracy is more than suffiicient. Particularly for schmancy fancy systems.

[Richard_Pearse]

@Broomstick

As I said, there is a lot of the US airspace that has no radar coverage. Our Australian friend didn’t see it that way, but the difference is (apparently) that over much of Australia there is no radar coverage at any altitude, whereas in the US the non-radar covered areas are overlaid by airspace with radar coverage.

Yes. So in airline type operations where you cruise at high altitude you will stay in radar coverage for much of your flight in the US, as you descend to land you’re also a lot closer to the terminal area and so you still remain on radar. In Australia though, unless you are just flying along the east coast, no one gets any radar service at all, regardless of how high they are. Australia just has a lot more unpopulated areas and not enough traffic in a lot of the smaller towns to warrant a radar. In fact there are numerous towns up the west coast of Australia that have airline operations with B737 sized jets and no ATC at all. No tower, and no enroute ATC until 18000 feet. The jets just do their own thing as required to get on the ground.

@billfish678

GPS is inherently more accurate in x and y than z (altitude) just due to geometric considerations. You would not want to use GPS to fly 50 feet above the ground, but for keeping planes from crashing into each other and knowing roughly where they are the accuracy is more than suffiicient. Particularly for schmancy fancy systems.

I still think you’re better using barometric altitude for a GPS based “radar” such as ADSB. The information is already encoded into the transponder and the baro altitudes are all within a known error for all aircraft.

[Waffle_Decider]

@1920s Style "Death Ray"

Yes. So in airline type operations where you cruise at high altitude you will stay in radar coverage for much of your flight in the US, as you descend to land you’re also a lot closer to the terminal area and so you still remain on radar. In Australia though, unless you are just flying along the east coast, no one gets any radar service at all, regardless of how high they are. Australia just has a lot more unpopulated areas and not enough traffic in a lot of the smaller towns to warrant a radar. In fact there are numerous towns up the west coast of Australia that have airline operations with B737 sized jets and no ATC at all. No tower, and no enroute ATC until 18000 feet. The jets just do their own thing as required to get on the ground.

Getting a little off topic here, out of curiosity, how is ground-based NAVAID coverage in Australia? Are all the airways defined conventionally using VOR or NDB, or are there portions that may require long range NAV equipment?

[Richard_Pearse]

@Waffle_Decider

Getting a little off topic here, out of curiosity, how is ground-based NAVAID coverage in Australia? Are all the airways defined conventionally using VOR or NDB, or are there portions that may require long range NAV equipment?

You can use ground-based navaids to get around the country but there are some routes that are defined by RNAV* waypoints, these give a more direct track as well as allowing for multiple parallel tracks over busy routes away from ground-based aids. A good example is between the Melbourne-Adelaide and Perth. The segment between Adelaide and Perth is over water and there are about five parallel RNAV tracks for that route.

*RNAV being area navigation systems such as GPS, GLOSNASS, and inertial reference.

[Rumor_Watkins]

Another point to consider: what’s the point?

The Air France plane that went down went down really far off coast. Helicopters don’t have the range to get out there and back with emergency responses. Boats take a few days to get there. Airplanes can’t just drop some guy wire and whisk people away a-la Thunderball.

So we can spend a lot of money to know exactly (or at least a lot better) where the plane goes down in the ocean… what does that get us?

[Waffle_Decider]

@1920s Style "Death Ray"

You can use ground-based navaids to get around the country but there are some routes that are defined by RNAV* waypoints, these give a more direct track as well as allowing for multiple parallel tracks over busy routes away from ground-based aids. A good example is between the Melbourne-Adelaide and Perth. The segment between Adelaide and Perth is over water and there are about five parallel RNAV tracks for that route.

*RNAV being area navigation systems such as GPS, GLOSNASS, and inertial reference.

Got it. Thanks.

[JRDelirious]

@Rumor_Watkins

So we can spend a lot of money to know exactly (or at least a lot better) where the plane goes down in the ocean… what does that get us?

Actually, the point of GPS-based or any other over-the-horizon tech for a NextGen ATC tracking system would be primarily so you can know better where your plane and everyone else within X distance is, on all three axes, and how are they moving, in real- or near-real-time and without “blind spots” so you can more efficiently route or reroute your flights and prevent space-incursion incidents. THAT saves time & fuel and improves safety. It would be a secondary benefit that if you go down it pinpoints where, gaining valuable time to locate wreckage before it gets too battered by the elements and search for the onboard recorders if they’re even retrievable.