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Non-pilot question on dual gps

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This might be a silly question, but as a non-pilot it isn't obvious to me! I've been flying in fs2004 with dual garmins from Reality XP. I'm still learning how to use both units together, and I got wondering, what is the real purpose of having two GPS units? Redundancy? Having each unit at a different zoom level? Having a direct-to for a nearby airport in the event of a failure? All of the above?

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Donny AKA ShalomarFly 2 ROCKS!!!The only stupid question is the one you don't ask.All of the above may apply... butfor true redundancy, be prepared to fall back on another system such as VOR, radar vector, pilotage etc...just in case a major solar storm takes out the system for a while.The biggest problem with GPS accuracy is that rouge nations can target you with it. So it isn't as accurate as it could be- if it could replace an ILS it would be very usefull to North Korea too.Best Regards, Donny:-wave

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I know a couple of real world pilots that load 2 flight plans. When they get their clearance, they cross feed the one that they're going to be flying into the other unit. Enroute, they have one range set to cover the entire route of flight, and the other one set so that they can see the next 10 miles ahead.Another use for 2 units is to just put "direct to" your destination in one unit and the route in the other. That way, you know how far it is until you touch down.

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>Donny AKA Shalomar>Fly 2 ROCKS!!!>>for true redundancy, be prepared to fall back on another>system such as VOR, radar vector, pilotage etc...>just in case a major solar storm takes out the system for a>while.>>The biggest problem with GPS accuracy is that rouge nations>can target you with it. So it isn't as accurate as it could>be- if it could replace an ILS it would be very usefull to>North Korea too.Yes, we'll all have to do with a few feet of potential error instead of a few inches I suppose. Other than that, GPS will easily get you lined up with the runway. I have enough belief in todays GPS systems combined with a sectional, that I could care less if VOR's disappeared tomorrow. In the short future, it will be GPS that opens many additional airports to future IFR approaches, that don't have ILS's now.IMO, there are much too many "myths" regarding GPS failure. Chances are, you have a better chance of getting an out of service VOR, than a GPS failure; especially two seperate GPS's with two seperate antenna systems.L.Adamson

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GPS's such as the Garmin 430 are todays new NAV/COM. For newer aircraft, and those who can afford the upgrade, it is no different than putting 2 KX-155's in the panel, as used to be done. It is for obvious redundancy and convenience.We're not exactly worried about "rouge nations", units such as the Garmin 430/530 also have VHF nav capability.Ultimately, the VOR will follow the ADF into extinction in the USA, perhaps within the next 10 years. Ultimately, GPS is far more precise, especially with WAAS, which will allow GPS to even perform precision approaches without the need for a conventional localizer or glide slope. :-)Regards,http://www.dreamfleet2000.com/gfx/images/F...R_FORUM_LOU.jpg

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Donny AKA ShalomarFly 2 ROCKS!!!I do stand partially corrected. However, if it can get a bomb on target it should get you on the glidesope and tell you your deviation. It was a major concern during building of the GPS system, for a while they toyed with the idea of "random induced inacuracy" as a possible solution.Best Regards, Donny:-wave

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One reason to have two is that the Garmin GPS 430 and 530 units also have NAV/COM radios. It's pretty standard for most planes have two nav and com radios.I like to having two GPS units so I can use them in separate ways and I personally prefer not to have the Garmin cross-feed feature turned on.I put my active flight plan in the top GPS. I then use the bottom GPS to:Get information on nearby airports (should I need to divert)Display information about my destinationLoad the ATIS frequency for my destinationPerform VNAV calculationsCalculate winds aloftDisplay additional, obscure data fieldsInvestigate shortcut routings that I might want to ask forGet information on special use airspace that I might be passing nearPerform a RAIM calculationIf two people are on board, the person not flying can always play with the #2 GPS to help pass the time. ;-) I did just that on a long instructional flight last night in a plane with dual Garmins.John

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I don't know about rogue nations, but I over the years I have seen several GPS receivers fail for a variety of reasons.Just last week, the King unit in the airplane I was flying occasionally said it had a RAIM calculation error and that I should verify my position. On subsequent IFR flights over the next few days, the frequency of RAIM failures increased to the point the unit became unusable. The plane went in for maintenance and the symptom was low GPS satellite signal strength. I don't know if this was an antenna problem or what.My point? I rely on GPS every day for my work, but I recognize that it is not infallible. That's why I also tune the underlying VORs and even NDBs."Trust, but verify ..."John

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Larry, Like John, I work with GPS every day too. In fact, I am in the navigation business. We use GPS as an adjunct to Ring Laser Gyro based Inertial Navigation Systems. GPS' do fail, they do malfunction and they can be spoofed and jammed. Dual systems are typically for redundancy purposes, but when combined with a nav computer, can provide a higher combined accuracy - but you probably would not find that functionality in a GA aircraft. As a mariner, I would never rely on a single nav source, and as a pilot, I would certainly have all my options available - not relying on a single point of failure such as GPS. Someday that may change, but as long as the sensor (in this case GPS) relies on the radio spectrum, is subject to the varagries of the atmosphere, weather, and the less than perfection of man, you won't find me being solely dependent upon it.

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Hence why at elevated alert levels the GPS accuracy sometimes is worse.As it stands, I think you can get 3 feet of accuracy with aviation GPSs, while my Garmin RINO (Radio Intigrated with Navigation for the Outdoors) 120 handheld I use for naviagation while hiking and hunting gets around a 40-60 foot accuracy in 3D, possibly because the radio antenna and GPS antenna are about a 1/4" away from each other and probably interfering to some degree. So accuracy also depends on the unit.----------------------------------------------------------------John MorganReal World: KGEG, UND Aerospace Spokane Satillite, Private ASEL 141.2 hrs, 314 landings, 46 inst. apprs.Virtual: MSFS 2004"There is a feeling about an airport that no other piece of ground can have. No matter what the name of the country on whose land it lies, an airport is a place you can see and touch that leads to a reality that can only be thought and felt." - The Bridge Across Forever: A Love Story by Richard Bach

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It is WAAS-capable, John? Just a thought."but as long as the sensor (in this case GPS) relies on the radio spectrum, is subject to the varagries of the atmosphere, weather, and the less than perfection of man, you won't find me being solely dependent upon it."So then, Tom, aside from VOR and ADF (both of which rely on RF), and along with the mag compass / pilotage, etc. What does one rely on as a backup in the average GA aircraft if they cannot rely on RF?We rely on a second GPS unit, or also have the VHF stuff installed. Again, some form of RF. As to an ADF, we found a Strike Finder and XM Satellite radio serves better in its place. ;-)Regards,http://www.dreamfleet2000.com/gfx/images/F...R_FORUM_LOU.jpg

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>Like John, I work with GPS every day too. In fact, I am in the>navigation business. We use GPS as an adjunct to Ring Laser>Gyro based Inertial Navigation Systems. GPS' do fail, they do>malfunction and they can be spoofed and jammed. Dual systems>are typically for redundancy purposes, but when combined with>a nav computer, can provide a higher combined accuracy - but>you probably would not find that functionality in a GA>aircraft. As a mariner, I would never rely on a single nav>source, and as a pilot, I would certainly have all my options>available - not relying on a single point of failure such as>GPS. Someday that may change, but as long as the sensor (in>this case GPS) relies on the radio spectrum, is subject to the>varagries of the atmosphere, weather, and the less than>perfection of man, you won't find me being solely dependent>upon it.In reality, failure of an airborne GPS is almost nill these days, except for bad units & less than desirable antenna locations. With 24 satellites in orbit, you can count on a 12 channel system to usually pick up at least eight satellites. The GPS only needs two of these for pinpointing location, and three for a vertical readout. Nearly all new aviation recievers are capable of WAAS which corrects for atmospheric disturbance, orbit errors, and timing. WAAS also provides accuracy of 10' or less.If you're to look up "GPS failure" with search functions on the internet, you'll find most negative information dates clear back to the late 90's. Why, because these day's they hardly ever go off line.New solid state auto-pilots that have become very popular within the experimental/kitbuilt aircraft market use GPS signals for heading data instead of a larger motor driven gyro. The solid state "gyro" fit's within a quarter inch cube, is very reliable, and will easily fly a heading such as an airway within 50' or less. You hardly ever hear about GPS signal error effecting these units. The plane I flew 29 hours of cross-country in last year, only suffered a few moments of GPS failure when I placed a "Flight Guide" on top of the glaresheild mounted antenna. It was flown 104 hours by a friend of mine, with no GPS failure before being transported to it's owner.I also have a habit of asking other GPS/GPS auto-pilot owners, as well as a couple of Cirrus & Cessna glass panel demo pilots about onboard GPS navigation failure. The answer is always the same; as failure is very remote these days, and last's for a few seconds at best, if it occurs at all. On one of my aviation forums, a pilot talked of flying 600 hours with an old "one channel" reciever and only had three momentary pauses in the whole time, which he blamed on a lousy antenna location on a Cessna 172.For IFR operations, GPS only, still isn't legal, so that's a moot point. For someone like me, a color moving map GPS backed up by a second cheaper hand-held and a hand-held NAV/COM capable of receiving VOR signals works great for two sources of added redundency.One thing for sure; by using a color moving map GPS, combined with a sectional chart, the pilots situational awareness, is greatly improved, especially in stressful conditions. And it's now being realized, that thanks to improved situational awarness, more time is spent looking "out" the windscreen, even with the new glass panel setups. In other words, you don't have your eyes fixated on a sectional trying to compute a position with reference to a couple of VOR's, while that bird, mountain, or other plane is on a collusion course!And FWIW, some of todays hand-held, panel mounts, and glass systems for the GA pilot, offer more than what many airline pilots get. We have up-linked satellite weather which easily displays weather systems for hundreds of miles in all directions, as well as TFR's, weather at the destination airport, and every fuel stop in between. Not to mention restricted areas, Class B rings, etc. And then there is terrain and terrain warning features. Some GPS units even have a 3D topography display, that's much like flying low level with flight simulator.Certainly a GPS reciever is prone to mechanical or antenna malfunctions. But these days, I think they're just as reliable as other land based navigation systems which are prone to malfunctions in their own right. Yet the GPS offers MUCH more "life saving" information than has ever been available in the past.L.Adamson

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I agree with a lot of what you say, so perhaps you can add my observations to what you've heard from other pilots.I've seen RAIM failures with three different brands of IFR-certified units, in several different aircraft over the years. While I fly around 500 hours per year, these failures probably account for less than 1% of my flight time. Several of the failures occured while I was in instrument conditions.My "life saving" advice? Perform a RAIM check, compare the GPS data against other navigation information, and always, always have a plan B.John

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No, the RINO isn't certified for air navigation.----------------------------------------------------------------John MorganReal World: KGEG, UND Aerospace Spokane Satillite, Private ASEL 141.2 hrs, 314 landings, 46 inst. apprs.Virtual: MSFS 2004"There is a feeling about an airport that no other piece of ground can have. No matter what the name of the country on whose land it lies, an airport is a place you can see and touch that leads to a reality that can only be thought and felt." - The Bridge Across Forever: A Love Story by Richard Bach

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>I agree with a lot of what you say, so perhaps you can add my>observations to what you've heard from other pilots.>>I've seen RAIM failures with three different brands of>IFR-certified units, in several different aircraft over the>years. While I fly around 500 hours per year, these failures>probably account for less than 1% of my flight time. Several>of the failures occured while I was in instrument conditions.>>My "life saving" advice? Perform a RAIM check, compare the GPS>data against other navigation information, and always, always>have a plan B.>Regarding GPS's, much has changed in the last few years. As an example, newer hand-helds will keep a lock on GPS signals better than older IFR certified panel mounts. Though antenna locations, still make a difference for best performance. WAAS was incorporated into aviation hand-held's before IFR certfied panel mounts. WAAS systems perform their own integrity checks, and overall, perform with much better reliability than the none WAAS, RAIM only recievers. Therefor, when we're talking RAIM failures of IFR certified units over a period of years, it's a lot different, than today's new technology IFR certified GPS recievers. And for Tom, WAAS capable recievers are excellent for both marine & auto use in the United States and surrounding waters, where WAAS coverage is high.L.Adamson

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I just was reassigned the plane that had the GPS failure I mentioned earlier. Maintenance replaced the receiver with a new unit and it worked fine. So this was apparently a receiver malfunction, not an antenna issue.I agree that many hand-held units offer greater accuracy that the older, IFR-certified units. I still recommend to pilots that I instruct that they understand the factors that can affect GPS reliability, regardless of the vintage of the receiver or whether it is IFR-certified or WAAS capable.There are currently only two IFR-certified WAAS GPS receivers that I know of - the Garmin (nee UPS/AT) GNS 480 and one produced by Chelton Flight Systems.Even a WAAS-enabled GSP receiver can have reliability issues and pilots need to check for WAAS NOTAMs when they get a briefing. WAAS NOTAMs can be issued for a particular airport or for a wider area. A WAAS airport NOTAM usually give a time range during which LNAV/VNAV and LPV approach minimums will not be available. WAAS Area NOTAMs are issued for larger geographic areas if large ionosphere disturbances are predicted or if a ground-based WAAS Reference Station is out-of-service.GPS is great, but it's important to know the limits of any navigational equipment.

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