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

So the Air France discussion brought attention to the fact that when you get beyond the coastal regions, there’s not much ability for ATC to track planes on radar.

Well, we have some capacity for remote monitoring of distant oceanic regions, right?

Why not slap some radar sensing and uplink capability on these?

I’m imagining one answer is that power consumption for radar (needs a motor plus a source for the radar waves and pickups) is a lot more than for temperature probes and uplink. But couldn’t you have naval ships just visit the stations a bit more frequently to change batteries/refuel generators?

The buoys depicted in your link are a tiny fraction of the size necessary to support a search radar. Power requirements would be enormously higher. Storms and salt spray would be a seriously hostile environment for a radar antenna and electronics. A non-rigid platform would entail complications. An unmanned station would have to be much more reliable, and thus expensive (sort of like a poor man’s satellite). Providing maintenance would be hideously expensive.

IOW, lots of formidable challenges.

ATCs don’t regularly actually use their radar - but rather a computerized tracking and display of transponders.

There’s very little need for coverage over the ocean. Airplanes are spread far apart and can detect each other’s transponders. If a plane goes down relatively intact, it can use a satellite-detectable rescue buoy signal. If it crashes in a catastrophic way there’s no use for having radar there.

So essentially it would be quite a pain in the ass to set up and have little benefit.

My understanding is that a transponder responds to ground-based radar, rather than “broadcasting” independently. So ATC relies on primary radar to get a signal from the transponder.

Rather than using radar to find out where planes are over the ocean, would it be easier to have planes broadcast their location based on an onboard gps? Maybe it could make use of whatever type of connection a satellite phone uses, processed by some kind of internationally shared airplane locating service. There could be an automated alert that gets triggered if altitude drops below a certain point with emergency services put on standby whether a mayday was called or not.

Not to mention that IIRC, very few planes crash once achieving cruising altitude. It just doesn’t happen all that often and the expense and upkeep of a sea-based radar system would be enormous. Don’t most crashes occur during or shortly after takeoff or landing?

Actually, there are several types of transponder in use; you’re probably thinking of Mode C transponders, which respond to radar signals.

I’m currently conducting research involving a system to fill in geographical service gaps such as the ones over the ocean. These Mode-S transponders do in fact broadcast independent of radar. Canada now uses them for air traffic management in regions where radar is unavailable. The technology described on that page is Automatic Dependent Surveillance – Broadcast (ADS-B), which operates over Mode S transponders.

You’ve pretty much exactly described the operation of ADS-B.

The primary radar signal just gets a basic radar paint. To get the transponder reply you need secondary radar (SSR). SSR is not dependent on a primary radar signal.

Being out of radar coverage is not a big deal. About 80% of Australia does not have any radar coverage. Radar is used by ATC to get aeroplanes closer together, it’s very useful in terminal areas were you need to get a lot of aircraft in and out of one location, but for enroute use it is of less benefit unless the routes are very busy. It’s basically a collision avoidance tool for ATC. Radar coverage does not prevent in flight weather encounters.

ADSB is being used in parts of Australia to provide enroute “radar” coverage but the system still relies on ground stations.

There are also vast swathes of the US that have no radar coverage, most west of the Mississippi. Even where there is radar coverage available, general aviation pilots in good weather (VFR) are not required to use either it, air traffic control services, or even a radio over most of the US (although, because it is handy, most of us do to some extent or another).

Non-pilots tend to put much more emphasis on air traffic control and radar coverage than actual pilots do. Neither are *essential *for a safe flight, though clearly they are of such utility that various agencies spend tens of millions of dollars a year on support those systems.

It’s like this: you don’t need traffic lights to drive safely. In fact, a lot of roads don’t have traffic lights at all because they are so lightly traveled. Nontheless, in high traffic areas, signs, lights, and even traffic cops help things keep moving safely so much that municipalities are willing to spend a lot a money a year on those things where they really make a difference. It is not, however, cost effective to have elaborate street lights or traffic cops where a simple stop sign will do. Likewise, ATC and radar and stuff is most important at major travel hubs with a lot of traffic. Over the ocean is a lot like going down a country highway - sure, other people are out there, but you seldom see them, when you do it’s at a distance, and travel-hub level of traffic control won’t really make much difference regarding safety and performance so it’s not cost-effective to use it.

Exactly. GPS plus data uplink is the solution, not some high maintenance floating radar install. Radar is yesteday’s solutions to yesterday’s problems. I mean, we might as well start thinking about flying in zeppelins and wearing top hats.

And if the aviation authorities ever get their act to gether to deploy the “NextGen” ATC system, that *** is*** expected to be a major component of the scheme.

Yep, I think that was mentioned in the Air France thread (where I also threw in the perhaps-impractical idea of data uplink for the CVR in real time). Everyone agrees any form of GPS tracking system will cost billions, but would be pretty nifty in allowing tighter (faster, more fuel-efficient) routings, better safety tracking, etc.

Q.: Would GPS allow what we are looking for (in part) here – one of those “suddenly the aircraft descended from 35,000 feet to 10,000 feet, stayed at that level for four minutes, then dropped precipitously off the screen?” I.e., can GPS track altitude accurately? I would guess yes, as altitude is just another form of displacement and GPS does very well tracking that in the x-y coordinates.

Yes - a GPS receiver gives position in 3 dimensions (latitude, longitude, altitude) plus time.

GPS altitude is usually less accurate than horizontal position, but typical errors of around 10 meters make it quite useful.

You need more satellites visible to the aircraft to get altitude, however GPS altitude is not required as the barometric altitude can just be encoded in the uplink signal the same as it is with transponder data and secondary radar.

Unless you are at a known fixed altitude (e.g. sea level) a GPS receiver needs altitude to get a decent horizontal position. In theory this could be done by feeding pressure altitude to a GPS receiver, thus allowing a solution (possibly of reduced accuracy) with 3 instead of 4 satellites. In practice, sufficient satellites are nearly always in view, so the extra complexity and expense of this approach can be avoided.

Although there are some holes in the US radar coverage, they are relatively small. This is a map of radar coverage in Australia. Given that Australia is about the same size as the USA (not including Alaska), you can see that there are huge areas with no coverage at all. It’s not surprising that the radar coverage is concentrated around larger populated areas and this just illustrates that it is primarily a terminal tool rather than an enroute one. Enroute radar or similar would be nice to have but it needs a serious cost/benefit analysis on a case by case basis.

I do not think this is true. GPS works by triangulating in three dimensions by measuring the time differences from several satelites. I do not believe any receivers work by first assuming a certain altitude relative to the geoid.

This sounds like the perfect job for the “Return of the Airship” that is always on the cover of Popular Science but never happening.

The Bendix King KLN 89B is an example of a GPS receiver that can use baro altitude input to improve its horizontal accuracy. This is one of the earlier receivers commonly used in general aviation aircraft. Altitude input is mandatory if the receiver is to be used for GPS approaches, but optional otherwise.

Operating Manual (large PDF)

Check out the system diagram on page 1-0. Also, to quote from page 1-1,
“Altitude may be provided to the KLN 89(B) from an encoding altimeter
or blind encoder. Altitude is used as an aid in position
determination when not enough satellites are in view.”

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?