Two GPS antennae interfere with each other

coloradobluesky

coloradobluesky

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coloradobluesky
Im going to need ADSB out someday and I intend to keep my KIng KLN90B (works fine and I can get updates). Anyway, if I put a WAAS GPS antenna on top, where the Kings antenna is, do I have to worry about them interfering with each other?
 
It shouldn't. I have 2 Garmin GPS antennas (GTN 650 and G3X) on the top of my plane and it's not an issue. Garmin specifies 9" center to center as the minimum separation distance with 12" being the target. Mine are about 18" apart, offset off the center line a few inches: forward one to the right and the aft one to the left. The install manual also calls for a 2 foot separation from any Com/Nav antennas.
 
The question about GPS receiver antennas deals more about footprint standing/space. The ADS-B fun begins when you have a transponder antenna and the ADS-B antenna and not enough real estate to properly separate 'em...o_O
 
GPS antennas are RX only... unless they're poorly designed or the feeds are poorly terminated, there shouldn't be any issue with proximity to one another. Any reason you can't just T the existing?

Proximity to anything transmitting is a far larger problem... but not necessarily as large as one may think.
 
Can you T the existing? These cables are waveguides, so I don't think so. That and the new one is WAAS. Doesn't a WAAS GPS require a different type of antenna?

Also, at least some GPS antenna are "active" antenna's. Not sure what that means, but if they transmit as well as receive there may be an issue.
 
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There are a few companies out there that make purpose built GPS splitters. One guy uses one made by GPS Networking (https://www.gpsnetworking.com/produc...dard-splitters) which he split a Gilsson GPS puck antenna mounted under his cowl with his GDL39 and NavWorx ADS600-EXP. He says it works fabulously. However there are certain failure modes you have to be aware of with these splitters, as they are not as redundant as having separate GPS antennas. Personally I'd just mount the 2nd antenna as I want the best performance I can get at the lowest risk.

As for spacing, I also wouldn’t disregard the min spacing requirements as spelled out by the manufacturer. Here’s what Garmin says in the GTN 650 Install manual:
“A 12 inch center-to-center spacing between GPS antennas is required to achieve the best possible low-elevation antenna gain by minimizing pattern degradation due to shadowing and near-field interaction….Spacing less than 9 inches center-to-center results in unacceptable antenna pattern degradation.”

Of course this applies to an IFR navigator so the tolerances are probably tighter than say for a VFR only install, but it would seem prudent to try and stick to the min spacing. YMMV....


 
mkosmo said:
GPS antennas are RX only... unless they're poorly designed or the feeds are poorly terminated, there shouldn't be any issue with proximity to one another. Any reason you can't just T the existing?

Proximity to anything transmitting is a far larger problem... but not necessarily as large as one may think.
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Splitters will introduce some loss in the received signal. I'm guessing along the lines of 3dB or so.
 
Putting T's in coax is a no no. I dunno, follow the mfgs recomendations. You can't just do anything that gets a connection. Need to do it right.
 
coloradobluesky said:
Can you T the existing? These cables are waveguides, so I don't think so. That and the new one is WAAS. Doesn't a WAAS GPS require a different type of antenna?

Also, at least some GPS antenna are "active" antenna's. Not sure what that means, but if they transmit as well as receive there may be an issue.
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WAAS uses the same frequencies as the regular GPS L1 signal.

Active GPS antennas don't transmit. The "active" means they have a built in RF amplifier right at the antenna, and require power down the coax from the radio to run that amp. The signal levels from the GPS satellites run in the range of around -157 dbW on L1 and it's just so tiny that having the amp right at the antenna helps.

Once the GPS Block III satellites are operational, both a civilian L2 and an essentially aviation only use L5 signal should make the need for the WAAS signal rapidly obsolete. Measurements of ionospheric delay should be possible in a so-called "dual receiver" user GPS directly and the augmentation provided by WAAS today to make the signal usable for precision approaches, should be able to be done inside the receiver.

Other improvements in the Block III systems include the ability for any satellite to announce that something's wrong with its own signal within six seconds, and piles of mathematical signal strength improvements. The military kids even get spot beams for about a -20 dB better margin to a specific area of the planet to help with counteracting jamming activity, amongst other things.

USAF is way behind schedule and massively over budget on the ground updates but are hopeful they'll get it done by the end of this year. OMB points out that they're not optimistic, considering the "mainframe style" original control system having only been decommissioned at Schreivener AFB in 2009 for the current ground control systems.

I think in layman' speech this equates to "We're going to need a lot more money." ;) (When don't they, really? Seen anything hit a timeline or budget in your lifetime?)

Once the ground systems are done and the Block III satellites (and some of the Block II that can also handle some of the new signal types get to play also) are aloft, it's anyone's guess as to how long before FAA and other aviation authorities certify the dual receive systems. It looks like it will be a good long while. Unknown also as to what the market will do as far as utilizing Glonass and Galileo systems simultaneously with GPS signals. No particular technical challenges in using more than one, other than cost, including certification, at the user end of things. Probably not a lot of benefit, either. But they're there. Or getting there.

Even with all these new toys, better signal strength and accuracy, the system still won't be able to do Cat-III approach levels of accuracy. There hasn't been much news on the LAAS (Local Area Augmentation System) front to make GPS receivers accurate enough for that. Probably because there's an economic breakpoint where an ILS just is cheaper and easier and still works with everything a Cat-III equipped aircraft already has on board.

ILS looks like it'll continue to have a long and happy life for a very long time. At least in Civil aviation.

^^ None of the above includes the new military signal which probably has some nifty tricks up its sleeve, but isn't generally going to be used by the public. The spot beam antennas are a nifty trick, considering they have to move and track, unlike what most folks think of spot beams from high gain antennas in geosynchronous orbits like the TV folks use. Those generally don't move. They're fixed. The Block III sats have to do some nifty antenna tricks to point those antennas at a particular place or places on the planet as they fly by. Nothing NRO hasn't been "speculated" to be doing for quite a while, but it's the first really large scale project where it's been mentioned in public as a major feature. Even if it is only targeted (see what I did there?) at the mil crowd.

Heck, in MilAv, at the rate avionics get swapped out and upgraded, the drones will have all these new receivers on board, long before the human flown assets. Logically the spot beams and higher signal strengths and more anti-jamming tech, points right straight to the "needs" on board a drone for command and control. As do the new message blocks in the frames.

Skynet needs reliable Comm AND Nav. Heh. Go figure. :)
 
Anecdotally, I've see two devices, both receiving, interfere with each other.

An XMRadio Roady would hammer the bars on the #2 Garmin in my Cirrus down to zero. Something about an unshielded chipset in the Roady causing issues.

I also recall if two AM radios were rotated just so, they would interfere with each other.

So the idea is not so farfetched.
 
So, after considering quantum effects, time dialation and black hole considerations, as well as solar wind, plasma effects, coriolis effects and electromagnetohydrodyamic considerations, will I need another GPS antenna for my ADSB transponder or can I use the one that my KLN90B is using?
 
coloradobluesky said:
So, after considering quantum effects, time dialation and black hole considerations, as well as solar wind, plasma effects, coriolis effects and electromagnetohydrodyamic considerations, will I need another GPS antenna for my ADSB transponder or can I use the one that my KLN90B is using?
Click to expand...

Key word there "Transponder" - this means it is not only a receiver but also a transmitter. You will need to use a separate antenna. The transmit function of the ADSB transponder will not allow you to share the antenna with any other receiver, the first time the transponder sends a 250W pulse out the antenna it will cook the receiver that is also connected to that antenna.
 
airguy said:
Key word there "Transponder" - this means it is not only a receiver but also a transmitter. You will need to use a separate antenna. The transmit function of the ADSB transponder will not allow you to share the antenna with any other receiver, the first time the transponder sends a 250W pulse out the antenna it will cook the receiver that is also connected to that antenna.
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Hu? If he intends on keeping the KLN90B he will need another GPS antenna if the transponder has an internal GPS receiver. The transmitting portion of the transponder is routed to a belly mounted transponder/DME antenna not the roof mounted GPS antenna.

Even if he ditches the KLN90B and wants to use the existing antenna to run an internal GPS receiver in a capable transponder, the old antenna may not meet minimum performance requirements and require replacement.

 
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I have never seen a diplexer to allow two GPS receivers to share one GPS antenna, in a certified aircraft. If such a device existed, it would probably cost as much as a GPS antenna.

Even then I have to wonder if the transponder (with GPS) would induce something that would knock out the other receiver attached to a shared GPS antenna. The transponder and it's GPS receiver should be immune to the transponder's output but its hard to say if the other GPS receiver would work.
 
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coloradobluesky said:
Im going to need ADSB out someday and I intend to keep my KIng KLN90B (works fine and I can get updates). Anyway, if I put a WAAS GPS antenna on top, where the Kings antenna is, do I have to worry about them interfering with each other?
Click to expand...

My professional opinion is probably not, they won't interfere with each other as long as meet minimum spacing requirements in the installation documents. That being said we aren't looking at the whole picture are we? Where are the com antennas? Where is the ELT antenna? What ELT is installed? The installation documents should cover spacing requirements from the other antennas too and any other considerations.

There's a lot more to consider than just two GPS antennas.
 
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