GPS approach designators

[GravityHater]

What is the specific difference between these two approaches?
Thanks:

a) "GPS 34"
b) "RNAV (GPS) 34"

these are fabricated examples, not real approaches. just wondering why they are designated as such.

 

[AK47]

What is the specific difference between these two approaches?
Thanks:

a) "GPS 34"
b) "RNAV (GPS) 34"

these are fabricated examples, not real approaches. just wondering why they are designated as such.

It's all in the manual (Instrument Procedures Handbook)

"The naming convention used for stand-alone GPS approaches is “GPS RWY XX.” The coding for the approach in the database does not accommodate multisensor FMSs because these procedures are designed only to accommodate aircraft using GPS equipment. These procedures will eventually be converted to RNAV (GPS) approaches."
​

 

[FL420]

What is the specific difference between these two approaches?
Thanks:

a) "GPS 34"
b) "RNAV (GPS) 34"

these are fabricated examples, not real approaches. just wondering why they are designated as such.

In the first example("GPS 34",) it is a stand-alone(as opposed to an overlay) GPS approach that was published before ground augmentation(in the form of WAAS) was available. It only contained minima for Straight-in(i. e. - S-34) or Circling approaches.

In the second example("RNAV (GPS) 34",) it is a similar(perhaps identical) approach published in the latest and greatest format decided upon by the FAA. In addition to having minima published for an unaugmented Straight-in approach(i. e. - LNAV,) it also may have Straight-in minima published for a WAAS or certified Baro-VNAV augmented GPS approach(i. e. - LNAV/VNAV.) This type of approach provides the pilot and/or coupled autopilot horizontal AND vertical steering information down an ≈3° glidepath to minimums currently no lower than 250' HAT and ½ sm visibility.

Some runways(usually previously surveyed for ILS) have been certified to meet a whole host of conditions re: GPS reception, WAAS reception, runway length, width and marking criteria, runway lighting systems, approach lighting systems, parallel taxiways and taxiway distance from landing runway criteria, etc. You may see an additional line of minima labeled LPV with minima as low as 200' HAT and visibility minima of ½ sm.

If you see a minima line labeled GLS or GLS N/A, disregard it. The FAA has decided not to use that terminology. That line will eventually disappear or will be replaced by the LPV label when the LPV approach is certified for that runway.

What can you shoot?
If you have a Garmin GNS 430 or 530 or a King KLN 89 or 94 with an AFM supplement saying your GPS is IFR-certified for enroute, terminal and approach IFR operations, you may shoot a GPS approach(assuming it's in your GPS database) to minima labeled either(e. g. - S-34R or LNAV.)

If you are lucky enough to have one of the new Garmin GNS 480 boxes, it is WAAS-enabled and if the AFM supplement supports it, you can also shoot GPS approaches to minima labeled LNAV/VNAV. You may also shoot this type of approach with an IFR approach certified GPS or an FMS/IRS with a certified Baro-VNAV system subject of course to restrictions on the plate such as temperature restrictions or DME/DME availability.

LPV approaches are coming when they get a couple more WAAS satellites launched and in service or they get some LAAS stations in service. LPV minima to suitable runways based on LAAS are expected to eventually go as low as CAT IIIc.

The newest FAA format for GPS IAPs will have minima lines(from lowest to highest) that look like this:
LPV
LNAV/VNAV
LNAV
Circling

LNAV = Lateral Navigation
VNAV = Vertical Navigation
WAAS = Wide Area Augmentation System, a SBAS DGPS
LAAS = Local Area Augmentation System, a GBAS DGPS
SBAS = Space Based Augmentation System(i. e. - satellites transmitting corrected DGPS signals directly to aircraft)
GBAS = Ground Based Augmentation System(i. e. - ground stations transmitting corrected DGPS signals directly to aircraft within 25-30 miles of the airport)
DGPS = Differential Global Positioning System(i. e. - the satellite signal corrected for timing and pseudorange errors transmitted to aircraft via a SBAS or GBAS to refine the aircraft's computed position enough to safely shoot an instrument approach to very low weather minima)

Sorry to dump on you like this. I didn't mean to "build you a watch" when you only asked for the time but as you can see, the answer to your question isn't really all that simple. I recently had a CFII oral with a Fed. He made the mistake of asking me to explain GPS approaches. An hour later he said he thought he had a handle on GPS but he learned a few things he had never even considered before.

 

[GravityHater]

Actually I found all that fascinating and will probably have more questions!
Thanks!

 

[GravityHater]

1. What other types of LNAV/VNAV equipment other than "baro augmented VNAV" is there?

2. Some LNAV-only units ask you to input the alt setting.... and their manuals say they use altitude data to aid the gps in confirming position. If we don't put in an altitude, and only insert the alt setting, how is it getting our altitude?

 

[FL420]

1. What other types of LNAV/VNAV equipment other than "baro augmented VNAV" is there?

WAAS-enabled IFR approach certified GPS receivers can be approved for LPV and LNAV/VNAV approaches in addition to LNAV approaches, stand-alone(e.g.-GPS) and overlay(e.g.-VOR/DME or GPS) GPS approaches. The advantages of shooting a LNAV/VNAV approach with WAAS instead of using Baro-VNAV for the same approach are that remote altimeter settings can be used and there are no temperature limitations restricting the use of the approach.

Unfortunately, the Garmin GNS 480 is the only WAAS-enabled IFR approach certified receiver available to the small aircraft General Aviation operator at present. I think some higher-end receivers are just becoming available for bizjets. This caveat is subject to change at almost any moment.

The Garmin GNS430/530 series receivers were designed to be WAAS upgradeable in the future. Garmin is working on it but I read recently they are running into problems with certification. The Honeywell/Bendix/King KLN-89 and 94 were not designed to be WAAS upgradeable.

2. Some LNAV-only units ask you to input the alt setting.... and their manuals say they use altitude data to aid the gps in confirming position. If we don't put in an altitude, and only insert the alt setting, how is it getting our altitude?

Some GenAv GPS receivers like the GNS 430/530 and KLN94 can calculate a Vertical Descent Path for "advisory information only." To do this, the receiver must have some kind of altitude information as an input. If the receiver is receiving at least 4 satellites with adequate integrity it is capable of 3D navigation, the third dimension being calculated altitude. If only 3 or fewer satellites are being received, the altitude input is from a blind encoder corrected by the altimeter setting you input to the receiver.

Feel free to practice(in VFR conditions) LNAV approaches using the "advisory only" VNAV steering information from your GPS. Check you GPS receiver supplement in your POH or AFM. You still may not legally use the "precision-like" DA minimums published for the LNAV/VNAV approach. Instead, you must use the non-precision MDA minimums published for the LNAV approach(or straight-in MDA for the stand-alone or overlay GPS approach, if applicable.)

 

[time builder]

What's the typical error on a GPS postition, 30 feet? Is it the same vertically? If it is, that's better than my altimeter which is typically off by 50 feet when I land.

 

[FL420]

What's the typical error on a GPS postition, 30 feet? Is it the same vertically? If it is, that's better than my altimeter which is typically off by 50 feet when I land.

I have read all kinds of numbers on GPS accuracy so don't feel comfortable quoting any of them.

Re. your altimeter. How can you tell how far off it is unless you know you are on a part of the airport that has been precisely surveyed and the elevation is published? Are you on the correct part of the touchdown zone to use TDZE? Are you at the highest point of all the usable runways to use the published airport elevation? See the problem?

 

[time builder]

I have read all kinds of numbers on GPS accuracy so don't feel comfortable quoting any of them.

Re. your altimeter. How can you tell how far off it is unless you know you are on a part of the airport that has been precisely surveyed and the elevation is published? Are you on the correct part of the touchdown zone to use TDZE? Are you at the highest point of all the usable runways to use the published airport elevation? See the problem?

Yes, but at the same time, I fly out of one airport whose altimeter setting is consitently .04, or 40 feet higher than the airport 10 miles down the road.

The point is that you have error either way, if one source is questionable, you can use another to back it up. I don't think we've even mentioned radar altimeter yet.

 

[FL420]

Yes, but at the same time, I fly out of one airport whose altimeter setting is consitently .04, or 40 feet higher than the airport 10 miles down the road.

Sorry. I don't understand the relevance of your statement. Local barometric pressure can vary widely over even very short distances. For instance, flying from the coastal mountains in California eastbound out over the hot, dry desert you often can notice a very rapid change in indicated altitude and quite different altimeter settings from one microclimate to the other.

The point is that you have error either way, if one source is questionable, you can use another to back it up. I don't think we've even mentioned radar altimeter yet.

My point was that you are comparing the elevation indicated on your altimeter with some known published elevation. The two most common published elevations are the "airport elevation" and the "touchdown zone elevation." Since no airport is perfectly level you must be at precisely the point where the "airport elevation" or the "touchdown zone elevation" is measured to be able to really ascertain your altimeter error, assuming the exact barometric pressure is set.

Trying to precisely compare a GPS altitude to an indicated barometric altitude is a fruitless endeavor. Heck, even the elevation of a particular spot on an aircraft carrier's flight deck varies with how heavily loaded the ship is with consumables; whether the deck is pitching or rolling in a sea state or how much the ship is heeling over in a high speed turn.

Re. the radar altimeter. It's information isn't really useful for precise measurement until you have passed the runway threshold on a precision approach unless there is a precisely graded and flat clearway before the threshold. Certified CAT II and III runways will have such a clearway so the radar altimeter is useful for determining arrival at the DH(not DA) on a CAT II or III approach. The radar altimeter is absolutely necessary for autoland to reduce the power and initiate the flare at the appropriate HAT.

I always get a kick out of flying an approach to minimums at an airport with a runway sticking out into the water or up on a mesa with someone who is not familiar with the airport. They are usually startled by the rapid unwinding of the radar altimeter and the Low Altitude alert as you pass over the cliff.