RNAV RNP

8 Answers

1. 
   

   
   

   Wes Beard on Aug 27, 2013

   Hi Dan,

   Due to the way DME-DME RNAV works, the lowest RNP value is 0.3NM. It can’t get lower.

   Lower RNP values requires a WAAS capable or the latest GLS capable GPS receivers. WAAS receivers are capable of category I ILS minimums while GLS receivers are in the process of being approved for category III ILS minimums.

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2. 
   

   
   

   Dan Chitty on Aug 27, 2013

   Wes,

   I am aware of the line of sight restrictions for DME equipment. However, I may have misread the note. Does it actually read/translate to “DME/DME and any RNP units 0.3 not authorized”?

   I ask because of the way it is written on the chart and I read it as ” Any DME/DME equipment only capable as low as RNP 0.3 not authorized”.

   Also, what does GLS mean?

   Let me know your thoughts and thank you for the feedback above.

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3. 
   

   
   

   John D Collins on Aug 27, 2013

   DME-DME RNAV systems are part of many FMS based navigation systems. They are generally suitable for use in the enroute and terminal navigation and not for approaches. Airlines and some corporate jets have this equipment. It is based on a database of DME stations and simultaneously using multiple DME’s at known locations to determine a position. They depend on having suitable DME stations within range at the enroute or terminal operation altitudes. At altitudes required by approaches, DME-DME reception is not typically suitable. The RNAV approaches with LNAV or LNAV/VNAV minimums are considered RNP 0.3 by ICAO definitions. Even if the DME/DME is capable of RNP 0.3, this equipment is not approved for use on RNAV approaches. AC 90-105 states this. You can read about DME-DME RNAV navigation in AC 20-138C, it contains some of the limitations of this technology. The note on the RNAV charts is merely there to advise the pilot that DME-DME RNP 0.3 capable FMS systems are not authorized for use on RNAV approaches. You will not encounter this type of navigation capability unless you have a job as an airline or corporate jet crew member. If you are an affected crew member, you will be trained by the training department on what you need to know to use this equipment. For me, I have no use for the knowledge.

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4. 
   

   
   

   Wes Beard on Aug 27, 2013

   John is right,
   DME-DME RNAV systems are on the decrease and they were only installed on corporate and airline jets. I am like you though and want to know why these notes and other things are the way they are.

   GLS stands for GBAS landing system. GBAS stands for ground based augmentation system. The previous name was LAAS or local area augmentation system. Currently, there are only a handful of airports that have GLS systems and FedEx is the only approved operator to fly these approaches from my knowledge… take it with caution.

   See AIM 1-1-20 for more information.

   On a side note, WAAS stands for wide area augmentation system and corrects GPS ionospheric errors over a wide area. LAAS (GBAS) on the other hand, corrects GPS ionospheric errors over a 20-30NM area.

   WAAS is one of several types of SBAS technology. SBAS stands for satellite based augmentation system and as the name implies there are no ground based systems transmitting data directly to the SBAS receiver. Other countries have developed SBAS systems. Southern Canada, all of the US and most of central America use WAAS for category I approaches. Most of Europe and parts of western Asia use the EGNOS system. Japan uses the MSAS system, India has a GAGAN system in development. There are other countries developing WAAS type of navigation. The coolest thing about this is ICAO has developed procedures so a WAAS system can use the systems from other countries if the need arises.

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5. 
   

   
   

   Dan Chitty on Aug 28, 2013

   Thank you John and Wes.

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6. 
   

   
   

   Ale on Sep 01, 2013

   Wes, GLS is the acronym for GNSS Landing System; GNSS is the acronym for Global Navigation Satellite System. GNSS is a generic name for any space based navigation system such as the USA GPS, the Russian GLONASS, or European GALIEO…etc.

   First let’s review some facts about RNAV:
   Your RNAV equipment is a computer that receives navigation signals from a specific navigation source. It then analyzes these nav signals and either accepts or rejects them based on integrity, accuracy, and a built-in logic.

   Depending on the RNAV equipment installed in your aircraft, the signals received may come from one or more of the following navigation sources:
   1. GNSS (e.g. the USA GPS, the Russian GLONASS, or European GALIEO…etc)
   2. IRS position (self contained navigation equipment installed in some aircraft)
   3. Radio position (the RNAV equipment selects and auto tunes certain ground-based nav aids in the following priority order:
   – DME/DME; if no suitable DMEs found, it search for a
   – VOR/DME, or
   – DME/ILS (if ILS approach selected), or
   – VOR/ILS (if ILS approach selected)

   Note: the RNAV equipment uses one or more of the above (GNSS, IRS, RADNAV) if available and creates a hybrid position called FM position (assuming the RNAV equipment used here is an FMS); it is also called RNAV position or FMGC position whatever the name is but it is a “mixture” of different nav sources positions mixed together to come up with a new more accurate position …etc.

   Back to your question; this is what the note is saying to you:
   1. You are permitted to use your RNAV equipment upto the IAF (i.g. you can fly a STAR)
   2. At and after the IAF (i.e. during the approach phase), you are permitted to use your RNAV equipment when the navigation sources “feeding” your RNAV equipment anything but DME/DME. So, if your RNAV equipment has a function by which you can DESelect the DME/DME from the nav options of your RNAV, then that is all you need to do to comply with the note. When you DECslect the DME/DME, your RNAV equipment will no longer use DME/DME navigation.

   When there is a “DME/DME NA” on an approach plate, it is most likely because that the procedure was test-flown using DME/DME only and found that the nav accuracy required (0.3 nm for the app phase) was not met.

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7. 
   

   
   

   Wes Beard on Sep 01, 2013

   Ale,

   AIM 1-1-20 specifically states that GLS stands for ground based augmentation system (GBAS) landing system.

   Most FMS’s will use GPS almost exclusively because of its high accuracy as compared to DME/DME or VOR/DME calculations. If the FMS is using GPS, there is no reason to turn off the DME on the FMS to make this approach. For all practical uses, the DME is in the calculation of its position.

   By what you said, if you deselect the DME you must also deselect the VOR because the VOR is much less accurate than DME. All you simply need to do is fly the approach, if GPS is not available as you approach the FAF, the FMS will tell you so.

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8. 
   

   
   

   John D Collins on Sep 01, 2013

   Ale,

   You wrote:

   “When there is a “DME/DME NA” on an approach plate, it is most likely because that the procedure was test-flown using DME/DME only and found that the nav accuracy required (0.3 nm for the app phase) was not met.”

   AC 90-105 is the operative FAA guidance on RNP 0.3 approaches in the US NAS and covers RNAV (GPS) approach procedures with LNAV and/or LNAV/VNAV lines of minima. These approaches are classified as RNP 0.3 by ICAO definition. It states:

   “NOTE: Multi-sensor systems that use DME/DME or DME/DME/IRU as the only means of RNP compliance are not authorized to conduct RNP approaches. However, operators still retain the option to develop Special RNP approaches using these systems.”

   Since the RNAV (GPS) procedures that are published for general use are not special approaches, the note is the default and it would take a special authorization for a specific operator to obtain approval for a RNP 0.3 approach that is based on DME-DME. In other words, it is not a flight test restriction for most of us. I personally have never seen a RNAV approach procedure that permitted DME-DME navigation because even if they exist, the general public would not have access to them.

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