Thursday, May 12, 2005

GPS VNAV

As I noted a couple of days ago, the entire A.I.P. section on GPS approaches with vertical guidance has been rewritten. Both the the technology that takes advantage of GPS and the GPS system itself keep improving. Cutting a few minutes off a flight by coming at a runway more directly saves more money than denying everyone headsets or in-flight meals, so companies are very interested in taking as much advantage of these technolgies as possible.

The theory is, that while GPS can tell you how high up you are, it doesn't do it with the accuracy and fail-safeness of traditional instrument landing systems, so GPS can't be called a precision approach. The non-precision method of approaching a runway you can't see involves timing how long it's been since you passed a particular point and deciding when you've gone too far based on airspeed and reported windspeed. That's still allowed, and is at many airports the only option. It has to be admitted that vertical guidance from GPS is a bit more accurate than that, so ought to be allowed.

There are two ways of determining altitude from GPS. One is called the wide area augmentation system (WAAS) and works because the Americans have installed extra GPS satellites called SBAS that monitor the other satellites to bully and interrupt them if they don't think they are doing a good job. The term SBAS is said to stand for Satellite-Based Augmentation System, not actually to sound like "Yes, Boss." The WAAS system is optimized for the USA, but they can't help it from splashing out over the border into Canada, so we could use it too. New reference stations are even being added in Canada, possibly to facilitate US invasion, but apparently WAAS will be approved for vertical guidance in Canada sometime later this year.

Meanwhile, BARO VNAV, i.e. barometric vertical navigation, based on computers examining changes in air pressure with altitude, has been in use in Canada for about twenty years. The Americans may make many of our computers, but they have no control over our barometric pressure. BARO VNAV has never been considered reliable enough for precision approaches, but combined with GPS, and taking into account the temperature at the airport, properly equipped aircraft can now descend on a calculated glideslope where an RNAV GPS approach is published.

The computer involved is called an FMS. Some FMS are capable of accepting temperature data for the aerodrome and others aren't. This matters because the colder the air is, the closer the air molecules are together, hence the closer you are to the ground if you're using barometric pressure for altitude information. If your FMS cannot accept temperature information then you must not conduct a BARO VNAV approach if the temperature is below the limiting temperature Tlim published on the approach plate. There's even a little chart in the A.I.P. showing how your safe three percent glideslope could diminish to a tree, rock, and caribou-whacking two and a half percent, when the temperature is minus thirty-one.

Also, you can't play Minesweeper on an FMS.

4 comments:

Stu said...

I still remember the day I decided to look up how altitude is measured. Boy did I wish I'd never asked :)

Lord Hutton said...

You watch those Yanks don't invade by stealth.

Anoynmous said...

SBAS doesn't involve "extra GPS satellites". Like any other GPS Augmentation System, it uses ground-based GPS receivers at precisely known locations, which then compute correction values to apply to the GPS-derived coordinates. The first letter in SBAS indeed stands for Satellite, but it refers to a geosynchronous relay satellite used to broadcast the correction messages to a Wide Area (The US WAAS works roughly anywhere DirectTV or XM Radio can be received). Other SBAS implementations are in the works around the world, using communication relay satellites over areas besides the Americas.

There's also LAAS, a Local Area Augmentation System, using individual VHF transmitters at each reference location to provide the correction values to nearby GPS receivers. The generic non-US term for similar schemes is GBAS, with Ground taking the place of Satellite in the name.

Ray Nixon said...

Actually, we DO exert a significant measure of control over the barometric pressure of Canada. We do it with an ingeniously conceived device we call Florida. Here's how it works: Every winter, when 2/3 of the population of Ontario and Quebec flee the ice and snow, a gigantic suction is created, resulting in a monstrous cyclone that can actually be seen from space! So let's just hope, for your sakes, that SPECTRE doesnt' get their hands on it!