Dual GPS Receiver

[170C]

If I have an iPad with built in GPS is there any reason to have a Dual Universal GPS Receiver?

[Bruce Fenstermacher]

If you care the Dual is a WAAS receiver and therefor is more accurate. It is capable of placing you on the center line of the runway rather than just off one side. Most pilots can't tell the difference.

Another reason you might want the Dual is it can be located in view of the sky while your IPad, you might not be able to. Not a big deal in w 170. My IPad can see the satellites almost everywhere I want to put it. There are a few dead spots however.

[gfeher]

To give you some metrics, when I use Foreflight with my GPS IPad, it typically shows a GPS accuracy of 10 meters, sometimes 25 meters. When i use the same IPad with my Dual Bluetooth GPS connected, it typically shows 1 meter accuracy. For navigation though, 10 or even 25 meters is good enough. I've switched between the two while navigating (VFR) and didn't notice a difference in the location or behavior of the moving airplane cursor, although the reported accuracy was different.

[170C]

Thanks for the advise.

[GAHorn]

To give you some metrics, when I use Foreflight with my GPS IPad, it typically shows a GPS accuracy of 10 meters, sometimes 25 meters. When i use the same IPad with my Dual Bluetooth GPS connected, it typically shows 1 meter accuracy. For navigation though, 10 or even 25 meters is good enough. I've switched between the two while navigating (VFR) and didn't notice a difference in the location or behavior of the moving airplane cursor, although the reported accuracy was different.

Do not confuse satellite reception quality (constellation geometry) with actual accuracy.
Just because your unit indicates 1 meter accuracy does not mean you are actually getting that sort of performance. It only means the satellite constellation/reception MAY allow that kind of accuracy.

WAAS uses a ground-based transponder to communicate with satellites to "accurize" the signals by removing satellite/atmospheric and timing errors. However, your unit's prediction of capability based upon signal strength/quality is not a cross-check of actual performance.

[gfeher]

George, i guess i don't completely understand what you are saying. While i doubt that the GPS "accuracy" shown in Foreflight is truly correct in absolute terms (in other words that the accuracy may not truly be 1 meter even though it says that), I thought that my comparison of the two GPS systems would be valid in relative terms given that the same IPad and Foreflight application was used. Is that incorrect given what you say about how WAAS works?

[Bruce Fenstermacher]

George, what you said can not be disputed. While signal strength good constellation geometry effect accuracy I don't believe what Foreflight is telling you is you could have the accuracy. It is telling you you do have it. However it is important to note that what you said is not only true of the $99 Dual WAAS GPS receiver and an IPad running Foreflight or even Duals monitoring utility, but also of a $10K Garmin product.

I believe the Dual is just as accurate a Garmin when both say they have 1 meter accuracy.

[170C]

I am glad I don't necessarily need a 1 meter accuracy for the type of flying I do. I bought both of my iPad mini's with internal gps's since I ditched my old, obsolete gps's. Somehow I got the thought that I might need the Dual unit to provide me with additional data. Thanks to your input I am returning my $93 Dual unit to Amazon and all it cost me was about $6 in shipping. I will be more careful with future purchases and not be so trigger happy. I am one of many who love my iPads and ForeFlight. Just need to learn more of its capabilities.

[GAHorn]

Unless the GPS being used is an IFR certified unit installed properly, I would not be comfortable believing any accuracy claims.

I have personally witnessed a 60-mile error using a Garmin 196 that claimed I was over my home when my Mark I eyeballs was looking straight down on Fredricksburg, TX. I was doubtless within 15 meters of where it thought I was, but not where my eyeballs thought I was.

I had to power-off and re-boot the GPS and eventually it found itself again.

[170C]

George that gps would have been correct if you had a green 170, but your red one was 60 miles slow

[GAHorn]

George that gps would have been correct if you had a green 170, but your red one was 60 miles slow

I shouldn't respond to this ...but in fact... we were in a blue and white Debonair! (AND...BTW... you guys with non-certified GPS installations/laptops.. HEADS UP... the owner of that Deb began to notice a failure of BOTH his GPS units (Garmin and Apollo) when his NARCO 810 comm unit was tuned and only listening on certain frequencies. This is a known issue and he had to install a grounding-shield to block some sort of "sideband" interference between the LED/Gas-Plasma display and the GPS units.)

I know some of us feel pretty confident about our so-called "portable" GPS units while making a "Bandito-One" arrival to some airfields...but BEWARE. You can end up in the tops of some of those non-charted, un-lighted cell-phone towers.

He actually placarded the offending frequencies which he later found published elsewhere as known problems. I'll try to locate that info and post it.*

(I realize few, if any of us operate Phenom 300 aircraft, but as a side-note, there have actually been NOTAMS issued suggesting that some aircraft avoid certain areas where GPS signal propagation can adversely affect flight controls in such aircraft.)
Most recently was in June 2016 in the southwestern U.S. over quite a large area.

The point is... do not become complacent in trusting GPS. I was dismayed to realize that a great many of the pilots I recently trained at my simulator job were completely ignorant of basic navigational techniques and became totally disoriented if I failed GPS, even in VFR conditions.

Stay situationally-aware. Research for NOTAMS prior to flights..even short flights. Even over routes you think are common. (Do I need to tell about the time I nearly shot thru a pop-up TFR near my home when I didn't follow my own advice?)

[GAHorn]

Blatant Plagiarism <edited> (Thx to original author- Stan Protigal of Seattle Community Network):

The Interference

This primarily affects handheld GPS with antennas inside the aircraft. It is caused by the IF local oscillator (LO) used in superheterodyne receivers. The problem occurs when the nav/com is in the receive mode. That is blatantly obvious to EE's but counter-intuitive to others. If the radio is turned off, it will not transmit the LO signal. The standby frequency does not cause problems.

The signal primarily leaks from the faceplate although it could theoretically leak from the back or through the sides.

The worst offenders seem to be the Narco digitals, although this artificial "selective availability" has been reported in King digitals and King 170's.




The Frequencies

Digital Narco
Narco Mk 12D/E, Narco Com810/811, Narco Com810+/811+, Narco 824/825
com 119.30 109.70
nav 131.20 115.50 ***

*** The NAV frequency of 115.50 is the default NAV frequency, which comes up every time you turn on the radio. So select two nav stations on preflight like you're supposed to!

Narco Mk 16

Narco Mk 16 tuned to any 115 or 109 channel was shown to jam a hand-held GPS


King KX 155/165

com 119.275* 119.9 131.825
nav 109.55 116.15

119.275 may be an error; interference was noted on 119.9 from one source. It isn't likely that both 119.275 and 119.9 would cause interference. Also note the precise frequencies below. I plan to test it to find out. Note: KX 155/165 transmitting on 118.15 was shown to jam an external-mounted antenna, but this should be nonconsequential.

King KX 170/175
(should also apply to Mac conversions)
com 122.275 130.175
nav 113.65

Collins Microline

com 120.775 132.75
nav 109.45 116.05
Narco Mk12/D/E & Nav 824/825 if not wired with memory keep-alive or upgraded to Plus configuration will default to 115.5 in the active channel and will jam any GPS receiver while on this frequency.
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