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5G versus commercial aircraft Radio Altimeters in the US, wtf?

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--- Quote from: Gyro on January 19, 2022, 07:00:09 pm ---
--- Quote --- Rule #1. Follow the money.

     I wonder how much the US cell phone carriers expect to make from the 5G market?  And what would it cost them out of pocket to replace it with something else if the 5G system was completely shutdown?
--- End quote ---

Well they (AT&T and Verizon) paid the FCC $69bn for the rights to the C band frequencies, so they obviously expect to make a fair bit.

It does feel as if the FCC were squeezing the pips a bit... South Korea use 3.5GHz, Europe use 3.7GHz and the US, 3.98GHz.

--- End quote ---

Eventually but for the first year at least they will only be using 3.7-3.8.  they will eventually have to make do with "only" a  220 MHz guard band but right now it is 400 MHz.  I get the political and historical baggage bit technically there is absolutely no reason this should be a problem.  Even a 220 MHz guard band is far more than is needed and 400 MHz is frankly obscene.

The prior operators in that spectrum are satellite video services.  The reshuffling moved all their operation into the upper half of the band, so they now operate in the 4.0-4.2 band between the 5G midband and the radar altimeters.  All those terminals are receiving extremely weak satellite signals and were designed to operate in the 3.7-4.0 band so deliberately had no filters.  Every operational system in the country had a filter installed.  They need to operate with only a 20 MHz guard band.

It's a lot more effort to get safety systems updated but there is absolutely no technical reason why a radar altimeter should have trouble filtering these signals.  I'm assuming at least some of the devices are ancient designs but if someone designed a safety critical radio receiver with that poor out of band rejection in this century I would fire them and decertify the device. 

The FAA has looked at radar altimeter interference before.  The #1 source of interference is radar altimeters.  They don't shut them off on the ground so flying over a busy airport you get reflections from dozens of aircraft taxiing around or just sitting.  It can be pretty strong too since the source is so close to the ground.  Probably not as much as a worst case 5G signal but it is also right in band and the same type of signal.  This is a real world problem that while has a low probability of causing problems it's not zero but the FAA (controlled by the airlines and Boeing...) decided it wasn't worth the cost to upgrade.  So I also don't have a lot of sympathy for their current concern.  And what do you know, now that everyone is looking at the FAA they are mandating testing and in 1 week they have tested the altimeters in 60% of aircraft and they all work.  If they had done that 2 years ago or 2 months ago they would be done by now with either clearance to fly or an actual problem.


--- Quote from: LaserSteve on January 20, 2022, 08:57:22 am ---Sorting the wheat from the chaff is difficult on other forumns,  but here goes: Dig hard enough in this thread you'll find links to European government   studies and tests,  and then some calculations by a poster named "satcom_guru" on pprune and the tw/itter platform.

FAA ADs re 5G interference
Started: 9th Dec 2021 by WillowRun 6-3
On PPRUNE forumn:

All I can tell is Faa/Fcc/CTIA are keeping tech data closely held. CEPT in Europe is not. Band in question is 4.0 to 4.4 Ghz. Cellular band edge in the US is 3.8 Ghz. No one seems to be talking about potential intermod or mixing products either. Ie Aviation Band + 5G = mixing products? Made all the more interesting by the fact that the US ERP per tower sector is twice that of the rest of the world in 5G bands. Most other nations left a larger guardband. Also a "third harmonic" product from the lower edge of 5G has been mentioned.

--- End quote ---

5G allocation in the US ends at 3.98.  RA band is 4.2-4.4 (worldwide).  The worst case 3rd order intermod product is 4.42.  Furthermore, a given RA doesn't use the whole band at once.  The standard setup is a swept frequency, and they are measuring delta-f between the transmit and receive due to time-of-flight.  The normal frequency difference is in the 10-100 kHz range.  So to get a 3rd order product at 4.42 you would need to be transmitting at the low end of the band and your IF filter should easily be able to block out the 220 MHz distortion product.  So inter-modulation shouldn't be a problem.  The only realistic problem is saturating the first stage LNA. 

Case Study from Estonia:

Only 1200 units of the RADALT model estimated to still be in service. However cost and timeframe of the redesign for a given aircraft install estimated at 3 years to obtain needed approvals.


Wow, thanks.  All of that seems to be pre-5G with transmitters between 800 MHz and 2.1 GHz which kind of boggles the mind. They say there have been 2300 cases since 2017. 

Honestly I can't see how such an instrument can even be considered usable in the modern world.  According to the FAA ( the primary radar system at an airport has 25 kW peak power at 2.7-2.9 GHz.  It's also deliberately designed to project towards approaching aircraft.  A device that can be disrupted by a cell tower transmitting <100 watt at 2.1 GHz is not a device that should be operating under any circumstances.

Since the FAA mandated manufacturers to test altimeters, in the last few weeks they have approved altimeters on 90% of commercial aircraft for use in low visibility landings near C band 5G towers:

well keep in mind how real life microwave filters end up looking like on a transfer function.. there could be peaks, modeing in waveguide, etc. I see alot of graphs that look like a cutoff then it bounces like a ball falling down a hill. I bet microstrip filters don't help too much with that.


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