Friday, August 06, 2010

Local Positioning Signal

I was chatting online with some non-pilots who knew I was a pilot and one, whom I have dubbed Rhubarb, asked me, "Why doesn't each airport have a local positioning signal?" They kind of do, so I asked what he meant.

<Aviatrix>   What is a local positioning signal?
<Aviatrix>   A lot of them have a green and white flashing beacon, if that counts.
<Rhubarb>    heh
<Rhubarb>    Well, like GPS, but local.
<Rhubarb>    A collection of transmitters whose signals can be recieved and processed the same way.
<Aviatrix>   A lot of them also have an AM radio broadcast signal in morse code from quite nearby. You can't put it ON the runway because the antenna would be inconvenient.
<Rhubarb>    Sure.
<Aviatrix>   Some have a transmission in the range 108-117.9 MHz with directional information that helps you align with the runway.
<Rhubarb>    What I'm proposing would undoubtedly involve a whole 'nother receiver, and I guess adding more instruments is not an easy proposition. :-P
<Aviatrix>   And some even have information in that signal that tells you how far away the runway is, and/or your deviation from an ideal glideslope to the runway.

Rhubarb kind of answered his own question, by recognizing that every technology needs a receiver, before I got into historical techniques like A-N beacons, painting the name of the airport in big letters, or lighting bonfires leading to the airport. A local positioning signal is a very good idea, and has been implemented in many ways. Maybe when I'm in a researchy mood I'll enumerate the dates and instigators of all the various local positioning signals that have been used over the last hundred years.

If another blogger wants to steal that idea, and backlink it here for the traffic, it's yours for the taking.


Colin said...

A little of what he is talking about (at least a local transmitter to give you a more accurate position report, requiring additional receiver equipment) is covered with WAAS. I wrote an entry on making the decision to upgrade ("WAAS the Big Deal?").

A Squared said...

WAAS isn't exactly a "local" positioning signal, in a nutshell, it is just a refinement to the accuracy of the global signals. Also, WAAS doesn't use a local transmitter to broadcast the refinements. Those are transmitted by geostationary satellites. You may be thinking of LAAS, which *does* use local transmitters to broadcast similarly derived signal corrections.

At one point in the evolution of GPS implementation, it was proposed to use transmitters in fixed terrestrial locations, broadcasting the same positioning signal as the satellites, to be received and included in the GPS position solution computed by the receiver. They were called "pseudolites" because mathematically, they fit into the GPS solution in the same way as the satellite signal, but obviously, were not satellites.

That concept lost favor and as far as I know was never implemented operationally.

I suspect that this was because making a "pseudeolite" that accurately mimics the rather complex GPS signal structure would be pretty expensive, whereas it really isn't all that difficult or expensive (comparatively) to compute localized corrections and apply them to the solution. You can buy commercially available systems which do exactly that for somewhere in the tens of thousands of dollars range.

Frank Ch. Eigler said...

Maybe he's trying to reinvent the Transponder Landing System.

Anonymous said...

There's differential GPS which uses earthbound transmitter-repeaters to refine the GPS signal for increased accuracy.
Cellphone (and now car) GPS solutions typically also take in cellphone tower signals and triangulate those to provide extra accuracy on top of civilian GPS (or even in its stead, which of course means they're no longer GPS but the term has stuck in the public mind).

A Squared said...

"....signals and triangulate those to provide extra accuracy...."

Triangulation is done by measuring angles. Solving for a position by measuring distances (which is what GPS and cell phones do) is Trilateration.

Sorry, pet peeve from someone who used to do both triangulation and trilateration for a living.

GeorgeG said...

So, from an avation-ignorent passenger type: What information enables a pilot to know how to get from point A to B? I know about map and compass navigation, but airspeed and crosswinds have to be taken into account, obviously. Different planes must have different levels of sophistication, but maybe someone who understands this stuff could offer a simplistic explanation of what info the pilot gets for navigation and how it is processed?

D.B. said...

I will try and answer George's question.

There are multiple navigation methods. You can simply fly a heading and airpseed, and adjust for expected winds. That is known as "ded" or "dead" reckoning, and was used in WW2 for night or weather flying. Private pilots in the USA are taught this, plus "pilotage", which is the use of ground landmarks to confirm the course and position.

Instrument pilots are taught to use radio aids, or which there are several. The oldest that is currently in use in the Automated Direction Finder (ADF), which is a needle that points or "homes" towards a beacon radio, called a "Non-direction beacon". The ADF/NDB combination is good enough to be used for instrument approaches to airports, mostly in rural areas or in developing countries.

More precision is available using "Variable Omni Range" (VOR) stations. They not only provide direction, but also send out signals that the receiver in the cockpit can use to identify specific radials, which are used to define airways. Add "Distance Measuring Equipment" (DME), and you know, for example, that you are directly East of the station and 25 miles out. Tighten up the tolerance, add horizontal guidance too, and you have the Instrument Landing System, or ILS. That allows precision approaches, and even 100% blind landings in fog.

Now we have GPS, which is very simple to use, and relies on satellites only (no ground radios). Simply enter where you want to go, and the system puts up a map display with the straight line in nice magenta/purple color. Enter "waypoints" (a set of locations defined by the FAA/CAA), and the line becomes a route. Couple the autopilot to the GPS, and the aircraft can fly itself to the destination.

Finally, add "Wide Area Augmentation Systems" (WAAS), and you have a satellite based, very accurate system for performing approaches to landing, even at airports with no ground facilities at all.

With GPS/WAAS the guess work is taken out. I learned to fly instruments without GPS because I wanted to know the grass roots, and in many ways I think GPS makes it too easy. Whatever kind of approach it is, you just follow the purple line. But then the olde time pilots probably thought the same thing about VOR/DME and ILS.

paul said...

I'm floored. You know what an AN beacon is?

Do you know the story of the development of GPS--the Doppler shift noted with Sputnik? Fascinating stuff.


A Squared said...

That is known as "ded" or "dead" reckoning, and was used in WW2 for night or weather flying.

both NDB/RDF and A-M Range navigation were in use during WWII.

More precision is available using "Variable Omni Range" (VOR) stations.

VOR stands for Very high frequency OmniRange

majroj said...

The competition for frequencies makes new services harder to start up, and once done, they need infrastructure, oversight and standards and all that.
We can't even unsnarl the USA's air traffic control system now as it is.
How about getting the military's terain databases for cruise missiles, install that hardware into the aircraft, and then let it fly itself?
Oh, yeah, they might wind up heading for Moscow or Domodedovro instead of Oakland or Manitoba...and at a hundred feet altitude at that.

Thomas said...

We actually have local positioning here. It is a research project that consists of pseudolites around our runway. These send the same Signal as Galileo Satellites and could be used for navigation :)

A Squared said...

We actually have local positioning here.

Interesting, where is "here"? I haven't heard discussion of the use of pseudolites for a few years, but them, I haven't been directly involved in positioning at the technical level in a few years. Interesting to hear the concept is still alive.

Thomas said...

"here" is around BWE. The pseudolites are not used for navigation but only for development and testing of Galileo receivers. For Navigation, we have a GBAS research project.