steveq

[QUOTE=Michael;10693961]GPS can't indicate altitude but only coordinates.

Well, yeah, but z=0 = sealevel.

<snipped other interesting stuff>
Me, I suspect the bandwidth's there. I doubt you need huge amounts, because the update rate can be relatively low.

Michael

There has to be some reason that they didn't go with satellites for the proposal for the new ATC system since on the surface, that would seem to be a better and possibly even cheaper system. Obviously there is the problem with delays when using satellites and if the update rate is too low, maybe the system wouldn't be accurate enough around congested areas.

Another possibility may be a reliability issue. Possibly the airplanes GPS would have to keep the airplanes transmitting antenna pointing at the satellites at all times as well as switching between satellites and possibly frequencies as the plane flies out of range of one satellite. Maybe in bad weather, especially on takeoff and landing, the technology isn't currently available to keep the transmitting antenna pointing accurately as the plane bounces around. Also as the plane is climbing, landing, or turning GPS would likely be insufficient to keep the antenna pointing correctly and other onboard information would likely have to be used to correctly position the transmitting antenna.

With ground systems, the airplane transmits in all directions so there isn't a need to position the transmitting antenna but when transmitting thousands of miles, I would think an accurately positioned narrow beam would be needed to get enough transmitting power as well as the ability to get acknowledgements from the satelitte to be reliable. However maybe both the airplane and satellite are blasting enough power so that antenna positioning is not necessary to maintain reliability.

Maybe over oceans, they will continue to upgrade ACARS via SATCOM to give reasonable tracking information. Currently with ACARS, many commercial planes don't use SATCOM but use the less reliable HF if they even have ACARS at all.

I don't know the reasons why satellites aren't being used in the new proposed ATC systems but just speculating on some of the possible issues.

GeoffM

I know it could be achieved by other means but I would have thought the same triangulation used to calculate lat/lon would also be usable for altitude?

Remind me not to fly with you, particularly at low level!

Uncle_Bob

Reminded me of Air france 296. Experienced pilots (over 20 years) and a new airbus aircraft with computer systems that would ensure lift would always be available regardless of how the pilots handle the controls.

They did a fly by at an airshow with a plane loaded with passengers and it crashed at low level into a bunch of trees.

I havn't seen airbus do a low fly by with the new A380 loaded with passengers yet.

steveq

GPS gives effectively three co-ordinates, one of which is height related directly to the WGS84 reference geoid. If you're over the ocean, and your GPS is working you should be all set, if you're high enough for the measurement error to be negligible.

civilservant

Too many ifs in that sentence for my liking...

That was determined to be pilot error. They were at 40ft when approved for 100ft minimum, and the FDR indicated that the autothrottle and engines responded to the TO/GA command as per specifications. That said, I do agree that any demo fight with pax is just a terrible idea.

I think the fact the pilot went to prison for his actions really says it all.

steveq

So who do you blame there ? The report blamed the pilot.

Michael

I have heard that it is possible to get a not very accurate altitude reading from GPS but I don't understand how without something in addition to GPS. GPS satellites use atomic clocks and synchronize between each other so that they all transmit their IDs and time stamps at the same time. The GPS receiver receives the signals at different times and since it knows the coordinates of each of the satellites, the time difference allows for triangulation by the computer (basically a sophisticated version of LORAN used in WWII where triangulation was done by hand for ships). Since the triangulation is based on the time difference of the received signals, I don't see how GPS can determine altitude but instead possibly create a minor error in the calculated coordinates depending on the altitude.

Maybe there is a way to determine the size of the error to get an approximate altitude reading.

Michael

I just read about WGS84 and don't understand it at all. I suppose if I read the complete reference, it may possibly make some sense.

steveq

GPS is NOT done from only two satellites in the constellation, that's why. Altitude is only another co-ordinate from the system, and is knowable DIRECTLY to a high degree of position.

SURVEY grade GPS is good to fractions of a mm - but it can take a long time to get the numbers.

Steve

GeoffM

It seems it is possible to get an altitude reading, but one that's not as accurate as a lat/lon reading. Random Google link: http://gpsinformation.net/main/altitude.htm which I assume is reasonably factually correct.

According to the above link, it's around 1.5x the horizontal error. The description it gives would explain the varying altitude given in my Garmin satnav when stationery!

Michael

This article describes it better but I don't understand the concept of spheres especially for getting the third dimension.

http://airandspace.si.edu/gps/work.html