Question about RNAV, GPS, INS navigation

I have a couple questions about RNAV, GPS, and INS navigation.

First of, I know most airliners are equipped with RNAV. As I understand it, RNAV can use VOR/DME, GPS, INS, etc to determine your position, correct? My question is, when you are flying an RNAV route, do you have to manually tune in the VORs for the RNAV to use VOR/DME as it’s means of determining your position? Or does the RNAV unit do it’s own VOR tuning, independently of the nav radios? logic tells me that it must, since you can be flying an RNAV STAR and tuning your nav radios to the localizer freq or whatever…

next question, when flying an RNAV route, does the RNAV get most of it’s information from GPS, from VOR/DME, or from the INS? what about when flying over oceans where no VOR/DME is available. is the INS or the GPS used primarily?

That’s correct

The FMS or FMZ or FMC depending on manufactures, will select the VOR and/or DME’s it wants to use. No input is required from the pilot.

Usually GPS (I don’t fly INS, but I think INS is secondary)


I’ll add this, most of the new RNAV(GPS) approaches (I think all) have a statement that say “RNP .03 DME/DME NA” That means you can’t use the FMS(Z)(C) to fly the approach if it is using DME/DME to locate it’s position. I think that all the FMS’s out there have that coded ito the database, so that if you are going to fly the approach and your GPS dropped off line (it could happen) then you would not get an APPROACH light and could not continue on the approach

That is correct…but for the purpose of symantics, if I’m reading your questions correctly, let’s talk in terms of FMS equipped aircraft that are RNAV (Area Navigation) capable, as it is the FMS or navigation system of whatever color providing the computational power to conduct RNAV.

There are alot of flavors of FMS navigation systems out there that use different prioritization of sensors to navigate. But for the purpose of your questions, if a nav system uses DME/DME for its primary means of positioning it will interrogate the DME station independently.

Again it depends on the architecture of the particular system. For example, the Rockwell Collins FMS that I use navigates with DME/DME as primary then GPS, and sometimes a composite of the two. Most Universal and Honeywell FMS’ use GPS as primary or a composite of all sources. Other GA units from Garmin and Bendix/King are GPS primary.

Yes, overwater or over remote areas GPS/INS would become the primary means of navigation.

:unamused: Quick Draw McGraw, I am not tonight… :wink:

I thank you for your replies. It appears I was a little unclear with one of my questions though. When I said “VOR/DME” I was referring to the usage of simply radial and distance for position determination (whether it be a VOR/DME or VORTAC), not the use of two DMEs to triangulate your position.

Does the FMS independently tune the VOR frequency (I think from your replies that it does, but I just want to make sure my question was completely clear).

Most modern systems will only use DME stations because it provides better accuracy. More specific to your question, these systems don’t use the VOR radial info, they use triangulation.

Yes, it will independently tune as it calculates distance from as many stations as it can receive.

I’d suspect the map would be quite cluttered if it displayed all 360 radials from every VOR in the vicinity. I have a Garmin 430 in my plane and it only displays the VOR.

It has a distance feature, but I am lead to believe it’s not the same as DME.

Going back on memory, DME is slant range and GPS is ground range.


I couldn’t believe that I was able to jump on an answer finally. I’ve been waittttting

A purely secondary bit of info on the VOR/DME tuning done by the FMS/FMC/FMZ. The FMS tunes in these frequencies independently, that is to say that the primary radio tuning unit doesn’t necessarily reflect the freq that the FMS is using. Unless, of course, you have selected the NAV function to auto, and then you spend most of your flight wondering “What in the hell is CZI VOR?”.

Also, some units (UNS1 for example) might prompt you to tune a certain frequency if it wants to verify DME.

IDX key, page 2, Database, IDENT: CZI. :wink:
I use it a lot.

Someone on topic, two different days last week out west, we (as well as numerous other aircraft in the area) lost GPS signal completely. There was no NOTAM about possible outages, so it came as a bit of a surprise. Funny thing that happened though, both times, that as the signal was slowly being lost, the data being received got all crappy. Our position reports from the GPS were crap, so the plane thought it was the wind. The direction indicator starting going nuts and the wind speed went anywhere from calm to 165 knots. That caused the FD to try to correct for the drift by turning, which was our first clue as to something screwy going on. Once the GPS signal was lost completely it would work fine using DME/DME and VOR. A check of the GPS STATUS confirmed that we were receiving zero satellites. Oddly enough, the first time happened on 9-11. The other time was the 13th.
Weird. Musta been aliens.

There was a NOTAM about this, but it wasn’t easy to find or decipher. I had the same experience.

That page sure helps to fight the monotony an those long cross country’s…It’s great for “Hey, what’s that airport down there?”

The aliens do their secret squirl stuff out here all the time…notam’d or not…the little G-men have carte blanc.

Question and food for thought though…Our FMS-6100 is Collins defaulted DME/DME…although they’re essentially the same architecture is the FMS-3X00 that you use set up differently? We haven’t experienced that…and we get RAIM msg’s all of the time out west…I’m just wondering if with the different crews going from airplane to airplane, some one might have changed a setting…like selected ‘NO’ for VOR and DME usage on IDX page 1/2 VOR/DME CTL. If it was selected ‘NO’, GPS would navigate as primary until it lost RAIM completely then revert to VOR/DME rather than combining both for positioning… Just thinking out loud…

All sources are “ON”. But the satellites didn’t blink out all at once. It was searching for satellites and must have gotten some crap data while the men in black were messing with stuff.
Pretty amazing actually how they can flip a switch and shut down the satellites in an instant. So maybe they saw your P180 and mistook it for one of their own and were trying to communicate…