When do you have to fly a procedure turn?

flyingcheesehead said:
Those weren't DME-DME, they were VOR-DME. I don't know of anything that did DME-DME in small piston GA.

Speaking of DME-DME, how does it disambiguate between the two points that are generated by specific distances from two stations? :dunno:
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This is stuff I know well...

The rho-rho position solution works similar to how GPS works. Two DME stations form two intersecting spheres of radial distance information and are disambiguated by knowing the signal strength in dB and using rising values to determine a 2D Line Of Position (LOP).

By using a third DME, the additional sphere of distance provides two additional points of intersection and since only one of them can be above the Earth's surface, a 3-D position can be determined and an altitude above the datum can be calculated.

This rho-rho and also rho-rho-theta stuff is used for GPS denied navigation for "special people."

All this calculation is done in software and is included in certain DME transceiver units. Very expensive, for special purposes...
 
IK04 said:
This is stuff I know well...

The rho-rho position solution works similar to how GPS works. Two DME stations form two intersecting spheres of radial distance information and are disambiguated by knowing the signal strength in dB and using rising values to determine a 2D Line Of Position (LOP).

By using a third DME, the additional sphere of distance provides two additional points of intersection and since only one of them can be above the Earth's surface, a 3-D position can be determined and an altitude above the datum can be calculated.

This rho-rho and also rho-rho-theta stuff is used for GPS denied navigation for "special people."

All this calculation is done in software and is included in certain DME transceiver units. Very expensive, for special purposes...
Click to expand...
So in actuality DME/DME is a misnomer? It should really be called DME/DME/DME?
 
asicer said:
So in actuality DME/DME is a misnomer? It should really be called DME/DME/DME?
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It could be rho-rho-rho (DME/DME/DME) also, but then pilots would have to learn a new song or count to three. We all know how hard that is... :D
 
flyingcheesehead said:
Speaking of DME-DME, how does it disambiguate between the two points that are generated by specific distances from two stations? :dunno:
Click to expand...
The DME/DME feeds position updates to the FMS. GPS, if installed, does the same. Data also comes from the inertial reference system and air-data computers. The FMS takes all of it, figures out what's the most accurate, and produces its position. Each FMS unit comes up with its own position. Depending on the airplane, that could be one, two, or three FMS positions.

The DME/DME portion has a database of DME stations and their locations. It is constantly re-tuning the stations it is monitoring to get the best geometry to produce the most accurate fix. It's all automatic. We don't know what its doing unless we make an effort to pull up the FMS page which shows the current tunings.
 
IK04 said:
This is stuff I know well...

The rho-rho position solution works similar to how GPS works. Two DME stations form two intersecting spheres of radial distance information and are disambiguated by knowing the signal strength in dB and using rising values to determine a 2D Line Of Position (LOP).

By using a third DME, the additional sphere of distance provides two additional points of intersection and since only one of them can be above the Earth's surface, a 3-D position can be determined and an altitude above the datum can be calculated.

This rho-rho and also rho-rho-theta stuff is used for GPS denied navigation for "special people."

All this calculation is done in software and is included in certain DME transceiver units. Very expensive, for special purposes...
Click to expand...

I flew the early 767 prior to GPS. We had three IRUs for attitude and position and two or three scanning DME receivers. Only the two geometrically "best" DME stations were used for the rho position. Rho/Theta was the FMS fall back solution. We we flew oceanic we had to disable DME/DME and DME/VOR updating. SIDs and STARs today that authorize DME/DME are to accommodate very old 757/767s and very old Air Bus airplanes, like the one that almost landed on the taxiway at KSFO.
 
midlifeflyer said:
The song is already there.

"Rho, rho, rho your boat..."
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I'm glad somebody picked up on that. ;)

The reason I hijacked this thread with this news is because I just got notified of the NGA's plans to supplement the reduction of ground-based navaids with what are called "type 9" DME stations will probably end up being the only navaids left when they are done...

It sorta makes sense. decommission the VOR/TACAN/ILS/NDB, but leave the DME running...
 
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IK04 said:
I'm glad somebody picked up on that. ;)

The reason I hijacked this thread with this news is because I just got notified of the NGA's plans to supplement the reduction of ground-based navaids with what are called "type 9" DME stations will probably end up being the only navaids left when the are done...

It sorta makes sense. decommission the VOR/TACAN/ILS/NDB, but leave the DME running...
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Do you know if these type 9's can handle more interrogations the current DME's? I think its about 100 is the limit. Their Service Volume is based partly on expected interrogations. What is NGA?
 
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luvflyin said:
Do you know if these type 9's can handle more interrogations the current DME's? I think its about 100 is the limit. Their Service Volume is based partly on expected interrogations. What is NGA?
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That I don't know, yet. I'm pretty sure they are intended to handle a higher volume.

The National Geospatial-Intelligence Agency pretty much owns the ability to navigate on land, sea or air:

https://www.nga.mil/About/Pages/Default.aspx
 
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