FMS and DME Slant Range Error

M

meritflyer

Well-Known Member
For you guys using FMS, I read that that DME is measured in slant range. So naturally, does the slant range errors affect even airplanes that have advanced avionics (DME).?
 
You don't have to worry about that when using an FMS because for every navaid and airport, it knows what the elevations are are does the math. Some airports have regular STARs and RNAV stars that use the same fixes but the distances are different for this reason.
 
Huh.. interesting. Does the DME in a RJ work the same in any other airplane? If it does, I'd think that slant range error would be a definate possibility.
 
meritflyer said:
Huh.. interesting. Does the DME in a RJ work the same in any other airplane? If it does, I'd think that slant range error would be a definate possibility.
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The only time RJ's would use raw-data DME would be when using "green needles" shooting an approach or when the FMS is mel'd. 99.9999999999999% of the time in cruise you are using "white needles" which derives its guidance from the FMS, which doesn't have slant range error in it (to my knowledge at least).
 
I wouldn't think FMS or GPS would have any slant range error due to the fact that they calculate distances based on geographical coordinates. If you are over N47.23.5 W118.15.6, you are over that exact coordinate regardless of your altitude. Altitude is the only reason for slant range error in DME.
 
Doesn't the FMS have a database of navigational data (ID, frequency, coordinates, ect.), regardless of associated navigational systems? Thats why you see the coordinates under the FIX or NAVAID ID if there are duplicates in the database.

EDIT:
Just came across this study that appears to have been done by the National Aerospace Laboratory in Europe. It is hosted on a server based in the Netherlands, so it takes a minute or two to load up.
http://www.nlr.nl/id~2940/lang~en.pdf
 
meritflyer said:
Not every FMS has a GPS attached to it. Several RJs have FMS but no GPS. However that works out..
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I haven't flown one that doesn't have one. If the GPS is not functioning, that is a different story. There are several different levels that the FMS can operate on. The primary source of nav info is going to be the GPS. After that, DME triangulation. After that, VOR-DME triangulation. Then there are a few more levels depending if you have IRS or not. The last resort for the FMS is going to be a dead reckoning mode (approximating your position based on the last known position).
 
Foxcow said:
I haven't flown one that doesn't have one. If the GPS is not functioning, that is a different story.
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How so? Just because there isn't a GPS input being made into the FMS doesn't mean it can't determine it's lat/long by another means. Hence, no slant range error unless you are in green needles.
 
In Auto-tune, the FMS is drawing from up to six VORS as well...


Stop thinking too hard Merit:). If you're looking for something to do, read up on exemption 3585.
 
There isn't anything magical about an FMS, it is nothing more than a single point of entry and output for flight data and navigation. Some FMS do not have a GPS input and get their "RNAV" capability from DME inputs, VOR inputs, NDB inputs, INS inputs etc. Those will have slant range error. But slant range error, isn't really an error as the word makes it sound. If you fly a GPS overlay approach and you are at the correct altitude the GPS and DME distance will line up over a waypoint since that waypoint has a LAT/LON coordinate. That LAT/LON coordinate is derived from where the waypoint exists physically over the ground in 3 dimensions. So if the VOR Approach has a waypoint at 5 DME at 2100 feet on the 360 radial namedy BIGGYBOOBY, at BIGGYBOOBY the GPS distance will read 0 and the DME distance will read 5.
 
The FMS I fly uses a blend or functions to find it's position. You can actually watch the data it is pulling and normally it is a combination of DME/DME, VOR/DME and GPS. There are times where it goes into GPS ONLY mode, but that is rare.

Not related to DME slant range, but there are a few times when we are required to use green needles (ground based nav stations) and not the FMS white needles. We can force the FMS to have an RNP of .01, where as with DME the best we can get is 0.1. However, our POI is insisting that anytime we fly a non RNAV departure that has a DME fix for a turn we have to do it in green needles. Rather frustrating (and less accurate) but this has become on of her pet projects.
 
wheelsup said:
How so? Just because there isn't a GPS input being made into the FMS doesn't mean it can't determine it's lat/long by another means. Hence, no slant range error unless you are in green needles.
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I did not mean it like that. The GPS not functioning comment was in relation to me not having flown one without a GPS.
 
A subtle, but important distinction is that the FMC "PRESENT POSITION" is not the same thing as "GPS". Admittedly, I don't get out much, but I'd be surprised if a 121 flight would be dispatched with an expired FMC nav database (correct me if I'm wrong, I think MEL is an 80's band). To wit, for a particular manufacturer:

Computation of Position
Each FMGC computes its own aircraft position (called the "FM position") from a MIX IRS position, and a computed radio position or a GPS position.

The FMGS selects the most accurate position considering the estimated accuracy and integrity of each positioning equipment.

GPS/INERTIAL: is the basic navigation mode provided. GPS data are valid and successfully tested. Otherwise NAVAIDS plus inertial only are used.

FM Position
At flight initialization, each FMGC displays an FM position that is mix IRS/GPS position (GPIRS). It is subsequently updated:
  • At takeoff, when the FM position is updated to the runway threshols position as stored in the database, possibly corrected by the takeoff shift entered in the [takeoff performance page].
  • In flight, the FM position approaches the radio position or the GPS position at a rate depending upon the aircraft altitude.
FMPostiion.jpg

Bias
Each FMGC computs a vector from its MIX IRS position to the radio or GPIRS position. This vector is called the "bias."
Each FMGC updates its bias continuously as a radio position or a GPIRS position is available.
If an FMGC looses its radio/GPIRS position, it memorizez the bias and uses it to compute the FM position, which equals the mix IRS position plus the bias.
Until the radio or the GPIRS position is restored, the bias does not change.
The crew can update the FM position manually. This also updates the bias.

MIX IRS Position
Each FMGC receives a position from each of the three IRSs and computes a mean ewighted average callled the "MIX IRS" position. If one of the IRSs fails, each FMGC uses only one IRS (onside or IRS3). Each IRS position and velocity is continuously tested. Is the test fails, the corresponding IRS is rejected.
MixIRS.jpg
When the MIX IRS position differs from the radio position by more than 12 NM [a light comes on and an angel gets its wings].

GPS Position
duh...

Radio Position
Each FMGC uses onside navaids to compute its own radio position. ...The navaids it can use are:
  • DME/DME
  • VOR/DME
  • LOC
  • DME/DME-LOC
  • VOR/DME-LOC
[The FMGC] uses LOC to update the lateral position using LOC beam during an ILS approach.
LOC is also used for quick updates when in GPS/IRS mode (if GPS installed).
If one or more navaids fail, each FMGC can use its offside navaids to compute a VOR/DME or DME/DME radio position.
RadioPos.jpg
Click to expand...
 
We get sent with an expired database all the time. The MEL states that you can do it and still use the database as long as each waypoint can be verified by other means (IE, VORs). We end up filing /K (I think) instead of /Q. We can't do RNAV stuff, but we can use white FMS needles to get around. The CRJ anyways was NOT designed to be flown enroute with green VOR needles. It tends to bracket a good deal.
 
Ours don't. When we had Dorks a few (old AWAC I think) did, but none of our CRJs have them. They are expensive as heck and people never wait the required time for them to spin up and stabilize so they don't work to well. Our Director of Ops was on a kick for a while about getting them put in but this is a company that won't spend $10 to replace a seal. There is no way they are going to coin up to put in IRUs in all the airframes. A few regionals may have them, but I don't know.
 
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