Expired Gps
- Thread starter CFICANFLY
- Start date
flyguy
Well-Known Member
The way I understand 1-1-19, it actually goes one step further in that even if the database is current, you still can only use GPS to substitute DME if the DME waypoint is in the database and there is some sort of backup in the event that the RAIM fails. Remember DME measure slant range distance, and GPS measures straight line distance. So to use GPS to substitute for DME you must navigate to the DME waypoint rather than to the navaid. On a LOC/DME, you must have an operable DME for range guidence. There is usually no cross radials for backup in the event the GPS RAIM fails, so even if the database is current and all of the waypoints are in the database, you still cannot substitude GPS for DME. If the RAIM were to fail there would be no way to identify the fixes. If it were any other approach that did not require DME but had DME distances on the chart, then you could use GPS to determine your position at the waypoint, if the database is current and includes the waypoints.
flyguy
Well-Known Member
It says the flight can be re-routed but I don't think this applies to an approach. In order to re-route and go to an alternate after an approach has started, you must go missed. If you have no range guidence whatsoever, how are you going to go misssed if you can't identify the MAP?
Eithr way, the question is does the database have to be current, and I think we both agree the answer is yes.
Eithr way, the question is does the database have to be current, and I think we both agree the answer is yes.
Wow...Excellent & informative posts. I hadn't given thought to the slant range vs. straight line distance issue.
The approach in qustion does have a GPS identifiable fix, however the aircraft that I fly GPS does not have it in it's database. Let's take it one step further. Suppose that I enter the coordinates into my GPS's database as a user defined waypoint?
'canfly
Yes, I know that this can be solved by updating the database, but that's a whole other story!
The approach in qustion does have a GPS identifiable fix, however the aircraft that I fly GPS does not have it in it's database. Let's take it one step further. Suppose that I enter the coordinates into my GPS's database as a user defined waypoint?
'canfly
Yes, I know that this can be solved by updating the database, but that's a whole other story!
pscraig
Well-Known Member
[ QUOTE ]
If a DME fix is in the GPS database, you can navigate to that fix rather than to the navaid. The coodinates for the fix take slant range into account.
[/ QUOTE ]
I disagree. I've flown aircraft with both IFR and GPS operating and seen differences of .1 to .2nm when flying directly to a fix. I've also noticed the difference in the ERJ between the DME and the FMS, which is driven by GPS.
If a DME fix is in the GPS database, you can navigate to that fix rather than to the navaid. The coodinates for the fix take slant range into account.
[/ QUOTE ]
I disagree. I've flown aircraft with both IFR and GPS operating and seen differences of .1 to .2nm when flying directly to a fix. I've also noticed the difference in the ERJ between the DME and the FMS, which is driven by GPS.
Ophir
Well-Known Member
[ QUOTE ]
The way I understand 1-1-19, it actually goes one step further in that even if the database is current, you still can only use GPS to substitute DME if the DME waypoint is in the database and there is some sort of backup in the event that the RAIM fails. Remember DME measure slant range distance, and GPS measures straight line distance. So to use GPS to substitute for DME you must navigate to the DME waypoint rather than to the navaid. On a LOC/DME, you must have an operable DME for range guidence. There is usually no cross radials for backup in the event the GPS RAIM fails, so even if the database is current and all of the waypoints are in the database, you still cannot substitude GPS for DME. If the RAIM were to fail there would be no way to identify the fixes. If it were any other approach that did not require DME but had DME distances on the chart, then you could use GPS to determine your position at the waypoint, if the database is current and includes the waypoints.
[/ QUOTE ]
Huh? That's not true. The use of GPS in lieu of DME is all refered to in the AIM. I don't have it in front of me but you can shoot a GPS approach with a IFR certified GPS without a backup DME. If you have a RAIM failure you are not supposed to continue the approach. But you can shoot it without a backup. It does say that you cannot file and alternate that does not have a non-GPS approach.
The way I understand 1-1-19, it actually goes one step further in that even if the database is current, you still can only use GPS to substitute DME if the DME waypoint is in the database and there is some sort of backup in the event that the RAIM fails. Remember DME measure slant range distance, and GPS measures straight line distance. So to use GPS to substitute for DME you must navigate to the DME waypoint rather than to the navaid. On a LOC/DME, you must have an operable DME for range guidence. There is usually no cross radials for backup in the event the GPS RAIM fails, so even if the database is current and all of the waypoints are in the database, you still cannot substitude GPS for DME. If the RAIM were to fail there would be no way to identify the fixes. If it were any other approach that did not require DME but had DME distances on the chart, then you could use GPS to determine your position at the waypoint, if the database is current and includes the waypoints.
[/ QUOTE ]
Huh? That's not true. The use of GPS in lieu of DME is all refered to in the AIM. I don't have it in front of me but you can shoot a GPS approach with a IFR certified GPS without a backup DME. If you have a RAIM failure you are not supposed to continue the approach. But you can shoot it without a backup. It does say that you cannot file and alternate that does not have a non-GPS approach.
flyguy
Well-Known Member
[ QUOTE ]
[ QUOTE ]
If a DME fix is in the GPS database, you can navigate to that fix rather than to the navaid. The coodinates for the fix take slant range into account.
[/ QUOTE ]
I disagree. I've flown aircraft with both IFR and GPS operating and seen differences of .1 to .2nm when flying directly to a fix. I've also noticed the difference in the ERJ between the DME and the FMS, which is driven by GPS.
[/ QUOTE ]
A DME fix is defined by what the DME will read at a given altitude on a given radial, and the DME is slant range, therefore the coordinates of the DME fix will take slant range into consideration.
Is it a DME fix you were flying to, or an intersection of 2 radials that happens to be identifiable by DME? If that is the case, I think you are right that they don't take the slant range into consideration, because you can be at a variety of altitudes, each giving a different slant range, but still probaly has less than .1 error due to the distance to the navaid.
If we are both talking about a DME fix, the ones we find on a DME approach, I think the difference is due to the difference in accuracy of the DME, GPS and altimeter. .1 to .2 is pretty small error as we're only talking about a few hundred feet.
[ QUOTE ]
If a DME fix is in the GPS database, you can navigate to that fix rather than to the navaid. The coodinates for the fix take slant range into account.
[/ QUOTE ]
I disagree. I've flown aircraft with both IFR and GPS operating and seen differences of .1 to .2nm when flying directly to a fix. I've also noticed the difference in the ERJ between the DME and the FMS, which is driven by GPS.
[/ QUOTE ]
A DME fix is defined by what the DME will read at a given altitude on a given radial, and the DME is slant range, therefore the coordinates of the DME fix will take slant range into consideration.
Is it a DME fix you were flying to, or an intersection of 2 radials that happens to be identifiable by DME? If that is the case, I think you are right that they don't take the slant range into consideration, because you can be at a variety of altitudes, each giving a different slant range, but still probaly has less than .1 error due to the distance to the navaid.
If we are both talking about a DME fix, the ones we find on a DME approach, I think the difference is due to the difference in accuracy of the DME, GPS and altimeter. .1 to .2 is pretty small error as we're only talking about a few hundred feet.
flyguy
Well-Known Member
[ QUOTE ]
If you have a RAIM failure you are not supposed to continue the approach. But you can shoot it without a backup. It does say that you cannot file and alternate that does not have a non-GPS approach.
[/ QUOTE ]
Not continue the approach? You mean go missed? If you are shooting an approach in IMC and have no range guidence, when do you begin the missed approach? You won't know where the missed approach point is.
AIM 1-1-19 f.(b)(3)
[ QUOTE ]
Waypoints, fixes, intersections and facility locations to be used for these operations must be retrieved from the GPS airborn database. The database must be current. If the the required positions cannot be retrieved from the airborn database, the substitution of GPS for ADF and/or DME is not authorized.
[/ QUOTE ]
AIM 1-1-19 f.(b)(7)
[ QUOTE ]
Procedures must be established for use in the event that GPS integrity outages are predicted or occur (RAIM annunciation). In these situations, the flight must rely on other approved equipment; this may require the aircraft to be equipped with operational NDB and/or DME recievers. Otherwise the flight must be rerouted, delayed, canceled or conducted VFR.
[/ QUOTE ]
If you have a RAIM failure you are not supposed to continue the approach. But you can shoot it without a backup. It does say that you cannot file and alternate that does not have a non-GPS approach.
[/ QUOTE ]
Not continue the approach? You mean go missed? If you are shooting an approach in IMC and have no range guidence, when do you begin the missed approach? You won't know where the missed approach point is.
AIM 1-1-19 f.(b)(3)
[ QUOTE ]
Waypoints, fixes, intersections and facility locations to be used for these operations must be retrieved from the GPS airborn database. The database must be current. If the the required positions cannot be retrieved from the airborn database, the substitution of GPS for ADF and/or DME is not authorized.
[/ QUOTE ]
AIM 1-1-19 f.(b)(7)
[ QUOTE ]
Procedures must be established for use in the event that GPS integrity outages are predicted or occur (RAIM annunciation). In these situations, the flight must rely on other approved equipment; this may require the aircraft to be equipped with operational NDB and/or DME recievers. Otherwise the flight must be rerouted, delayed, canceled or conducted VFR.
[/ QUOTE ]