GPS vs Altimeter Altitude

[jetdriven]

Anybody trying to clear an obstruction by 100 feet at night or in IMC has a death wish no matter what altitude reference they are using.

If I remember my FARs if IMC you need to clear everything by 2000 feet for five miles either side of your course. That negates the difference between GPS and indicated altitude.

 

1000' normally, 2000' in mountainous terrain.

 

Looking at an altimeter, "high to to low, hot to cold, look out below"

[N201MKTurbo]

1000' normally, 2000' in mountainous terrain.

 

Looking at an altimeter, "high to to low, hot to cold, look out below"

 

 

Living in Arizona, 2000 normally, 1000 out east some where....

[bd32322]

The government has been measuring and mapping surface/ AGL with great accuracy from sattelites for years.

 

 

There are radar and maybe even LIDAR based mapping from satellites that will tell you terrain rise and fall - certainly that has been mapped.

 

But I was just saying that with pure GPS alone - you cannot get AGL - unless all obstacles and charts and terran databases also carry GPS altitudes for all terrain features

[bd32322]

Anybody trying to clear an obstruction by 100 feet at night or in IMC has a death wish no matter what altitude reference they are using.

If I remember my FARs if IMC you need to clear everything by 2000 feet for five miles either side of your course. That negates the difference between GPS and indicated altitude.

 

That is true - except for people scud running because some software gives them an "optimistic" altitude.

For IFR with today's situational awareness tools - the altitude difference is a moot point since the margins are pretty big like you said

[carusoam]

Bd.

The point is... The available data is accurate and readily usable by our Current GPS systems.

On the other hand, the density altitude system works globally, at a low initial cost, without electricity and is already installed and working.

I think the discussion here, is more about how the lapse rate works for aviation, actual vs. standard.

On a standard day, my GPS and altimeter will have the same answer to altitude...I hope!

Best regards,

-a-

[Bennett]

None one yet has mentioned the various datum databases used in cartography.  There are more than one in use.  While this has nothing to do with aviation, it does reflect a GPS problem.  About ten years ago I sailed up the east coast of Africa using British Admiralty charts and GPS.  These Admiralty charts were based  (according to the chart legend) on a French survey of 1865.  No wonder that the GPS locations  - reefs, points of land, prominent mountain peaks, etc. were miles off from the charted locations - sometimes more that three to four miles off.  Prudence dictated old fashioned "doubling of the points" and other marine pilotage methods, using GPS as backup for the voyage from Durban, South Africa to Kilifi, Kenya. None of the Garmin settable datum choices matched the unidentified Admiralty chart datum.  Of course the new WAAS GPS approaches are set to  "known" elevations and locations. 

 

Question for the current military pilots:  Isn't GPS used for low level terrain following.  I seem to recall some very detailed terrain models used by the Air Force, and with satellite mapping, I would expect some very accurate mapping for AGL low level missions. 

[jetdriven]

Airliners have GPS-based TAWS which charts all obstacles and inputs to the GPWS. 

[bd32322]

Airliners have GPS-based TAWS which charts all obstacles and inputs to the GPWS.

It is gps based but are you sure it doesnt use a radar altimeter along with gps? If not then all its charts and terrain database have GPS terrain values which would work. Basically all points on the chart or terrain database have to have gps altitude values.

To Bennett's question, if you search for tomahawk cruise missile on wikipedia, it shows its guidance system as GPS over sea, then a combination of radar altimeter and gps for flying low over terrain.

Heres a link:

http://en.wikipedia.org/wiki/Tercom - guidance system

http://en.wikipedia.org/wiki/Tomahawk_(missile)

[bd32322]

Regarding the cruise missile though, I dont see why the military cannot have their own charts with terrain mapped out in GPS altitude. And the reason they are using radar maybe for speed reasons (obstacles come up fast at mach 1, 20 feet agl) and to prevent GPS signal jamming.

[bd32322]

Alright I'll shut up after this but I got curious about how the terrain database was formed. Looks like the space shuttle did it for us by running a radar altimeter over the entire planet. So at each point the shuttle was overflying, it knew its gps coordinates and then it added the radar height readout. So if the shuttle's GPS altitude was 200,000 feet and radar altimeter was 199,000 then the gps alt height of the obstacle turns out to be 1000 in GPS altitude. So just GPS alone is again not sufficient, you need radar.

This is called the SRTM database currently used by Jeppessen and accurate to 15 - 30 meters, not quite WAAS accurate (limitations with radar), but good enough.

Before this terrain databases were offby as much as 1800 feet! So the solution was to raise all terrain in the inaccurate region by that much !

Info on srtm

http://proceedings.esri.com/library/userconf/proc08/papers/papers/pap_1391.pdf

In short, if using a terrain database, you will be accurate to within 15 meters or so. If comparing chart MSL values to GPS altitude, you will be off

[jetdriven]

It is gps based but are you sure it doesnt use a radar altimeter along with gps? If not then all its charts and terrain database have GPS terrain values which would work. Basically all points on the chart or terrain database have to have gps altitude values.

To Bennett's question, if you search for tomahawk cruise missile on wikipedia, it shows its guidance system as GPS over sea, then a combination of radar altimeter and gps for flying low over terrain.

Heres a link:

http://en.wikipedia.org/wiki/Tercom - guidance system

http://en.wikipedia.org/wiki/Tomahawk_(missile)

 

AFAIK it is active below 2500' radar altitude and it looks ahead for obstacles in the GPS database. It knows where you are at, and where you are going. If it has a collision risk within 90 seconds it sounds the caution terrain or one of the other appropriate modes.

[Awful_Charlie]

GPS altitude can vary from from the actual altitude for a variety of reasons, the main variations depending on whether the GPS ellipsoid is accounted for

 

A good introduction can be found in http://www.pplir.org/using-the-rating/360-rnav-training-manual-v18 on pages 49-54, if you read one page only, page 52 is the one you want (and page 3 of course! )

[Bennett]

Very interesting reading.  Thanks for the link.  Older marine GPS units allowed for the choice of datums to work with various paper charting datums. It appears that everyone is headed for a common datum, although not yet achieved. 

[AndyFromCB]

Two more cents to add: earth's shape and MSL is actually very predicable without any complex databases. It's a oblate spheroid. If you know your x and y, the z basis is really to figure out for convertible ESL to MSL. Like I said, I've only been doing mapping software since 1997 ;-)

[carusoam]

Astel,

Thought you would have gone with a spherical coordinate system( r, theta, phi....) in place of (x, y ,z)...

Best regards,

-a-

[M016576]

None one yet has mentioned the various datum databases used in cartography. There are more than one in use. While this has nothing to do with aviation, it does reflect a GPS problem. About ten years ago I sailed up the east coast of Africa using British Admiralty charts and GPS. These Admiralty charts were based (according to the chart legend) on a French survey of 1865. No wonder that the GPS locations - reefs, points of land, prominent mountain peaks, etc. were miles off from the charted locations - sometimes more that three to four miles off. Prudence dictated old fashioned "doubling of the points" and other marine pilotage methods, using GPS as backup for the voyage from Durban, South Africa to Kilifi, Kenya. None of the Garmin settable datum choices matched the unidentified Admiralty chart datum. Of course the new WAAS GPS approaches are set to "known" elevations and locations.

Question for the current military pilots: Isn't GPS used for low level terrain following. I seem to recall some very detailed terrain models used by the Air Force, and with satellite mapping, I would expect some very accurate mapping for AGL low level missions.

We had DTED in the super hornet (both FA-18E and F) with a GPWS. It was based on a multi-source approach of using both the DTED and the RADALT. We would fly it down to 100' AGL in the CONUS under FAA regs. At 100' and 600kts, you feel like you're going pretty fast! Typical single seat ops are a min alt of 500' AGL, and a minimum of 350 kts... Any slower and you don't have enough energy to perform a max performance avoidance maneuver (ie- birds, wires, random cessnas, etc).

The F-15C doesn't have either. We use a regular old pressure altimeter to fly low level. But we aren't down there that often, as we're primarily up in the 40's for our initial intercepts in training. Of course, if we need to go lowere to interest an air breather, we do. At times like that, I miss the RADALT from the hornet, and the GPWS....

Edit- WGS-84 is the datum of choice these days....

[M016576]

Another thing: "GPS Altitude" is actually referred to as "Height above Ellipsoid" or HAE. Or at least it was a couple years ago!

[fantom]

The F-15C doesn't have either. We use a regular old pressure altimeter to fly low level. But we aren't down there that often, as we're primarily up in the 40's for our initial intercepts in training.

 

Ah...air-to-air

 

Wonder if the F-15E has it for their air-to-ground role?