Hello all, new report.
5G C Band and Radio Altimeter Report
Simplified Report & Opinion
There is a concern in the USA, that the C Band spectrum, which happens to be the new 5G spectrum, is going to cause problems with the major airline’s Radio Altimeters. This brought up many questions.
- Why in the USA and not France or Japan?
- What is different in the USA?
- Is the Radio Altimeter the problem?
- What needs to change in the future?
- Is it really the BTS or the UE?
- How did the FAA approve 90% of RALTs so quickly?
- To be clear, this is not a 5G concern, but a C Band spectrum concern because 3.4GHz to 3.98GHz falls close to the Radio Altimeter spectrum used by Commercial Airline Radio Altimeters.
In case you’ve been following this, it’s been a very confusing situation. There has been a lot of speculation on what the problem really is. If you watch the news, they blame 5G for so many problems. Then they say that 5G will cause planes to crash. This is because the FAA put out warnings about 5G C Band spectrum. It’s very frustrating to watch.
The FCC pushed through the C Band auction per the executive office’s orders. They thought everything was in place and they could move forward. The NTIA didn’t raise any red flags. Some airline associations raised concerns, but the FAA and DOT were silent early on.
Then there are the last-minute complaints from the FAA. There seems to be a lot of pointing fingers. This was based on a report published by the RTCA. The FAA seemed to be upset that there wasn’t money to update the antiquated radio altimeters in use today.
The thing that frustrated me was that no one seemed to have any actual test results with actual radio altimeter models identified. Why was that? Also, other countries were using C Band successfully, yet in the USA it’s becoming a crisis. Why?
It seems that other countries have not been having any issues, yet, in the United States, (USA), the FAA raised a red flag which appears to be a last-minute thing. Now, if you look back at history, the FCC had earlier warnings from other groups, as far back as 2018. I point that out in this report. However, did the right agencies raise the right flags, or were they too vague? Did other government agencies make some mistakes? Could this have been handled better? Read on and decide.
However, the FAA didn’t seem to do much about it early on, which is strange. Then late in 2020, it became more important. Then in 2021, it became a crisis! Mainly because Verizon and AT&T deployed hundreds of sites of which they paid over $65B US Dollars for C Band spectrum. Think about it, over $65B for the right to use RF in the USA. All the money went directly to the US Government. I am not sure what the payment terms were, but that’s a lot of money. (Link here)
What is a Radio Altimeter (or Radar Altimeter)?
What is a Radio Altimeter? Also called a Radar Altimeter, is a device used on planes to measure the distance from the plane to the objects on the ground. In this report, I’ll call it RALT.
Primarily used below 5,000 feet. The RALT is reliable and is a relatively simple device. It is basically RADAR in the 4.2 to 4.5GHz range. Generally, like RADAR, (Radio Detection and Ranging) sweeping 400 to 440 MHz of spectrum to get an accurate measurement from the ground to the plane. It detects everything on the ground. It measures the actual distance using the delay of the RF leaving the antenna then coming back.
It works by shooting RF, (a beacon) down to the earth and reading the information it receives. Remember that RF travels at the speed of light, so this happens very quickly. So quick that the plan won’t go far before it has a response. The way it knows the distance is by looking at the delay from the time the beacon was sent until it’s received again. Generally, it can only see what is below it, not what’s ahead.
It uses the Frequency Modulated Continuous Wave (FMCW) scheme because it is capable of achieving the low altitude accuracy required with a simple and reliable system.
The NTIA put out a document on Radio Altimeters, which says that the power out of a RALT could be from 400mW to 5W. Very low power.
I tried to find the acceptable noise floor, but I could not find solid numbers.
Source: 2013 article, Next Generation Radar Altimeter Testing published in Microwave Journal, link.
A good overview of radio altimeters came from the NTIA, link here.
Another reference on the noise floor came from Honeywell on the FCC site, link here.
IEEE note on Radio Altimeter Noise found here.
If you want to see a good overview of radio altimeters, go here.
What is C Band?
C Band is spectrum that the 3GPP has approved for 5G use around the world. With that said, each country will allocate the spectrum the way they think it should be.
5G is a priority in the USA because broadband is considered a utility. To get it to more people cost-effectively wireless is being used. With C Band, the carriers can deploy more broadband than ever before.
C band is being used for 5G in South Korea, Japan, China, across Europe, and in the USA.
See the spectrum chart to understand who is using what.
What is the problem?
The FAA is concerned about how close the C Band spectrum is to the Radio Altimeter spectrum. In the USA, the FCC needed more bandwidth for 5G transmission. There’s quite a bit available in the C Band spectrum.
C BAND 5G in the USA transmits at a higher power than the rest of the world. It seems the FCC thought that a guard band would be enough to protect the RALTs. Seeing how there is over 200 MHz of guard band between the carrier’s C Band spectrum and the Radio Altimeters.
Normally RALTs would have little or no interference except maybe from an Earth Station it would fly over. Now that there is something near them, there could be a higher noise level or intermodulation that could interfere with the RALT’s receiver. The RALTs have a very sensitive front end.
Keep in mind the RALT has a low noise floor. The fear of the FAA is 2 fold. One, the noise floor could rise up and cause the RALT to fail. If it can’t hear the beacon, then it could cause the RALT to fail. This is the real fear.
The other worry is interference. It is less likely but could be an issue all the same. There is a type of interference called intermodulation, we’ll call it intermod for short. What happens is 2 or more bands could mix and cause a spur, an erroneous signal that could fall in the RALT spectrum. Or, the C Band radios could have an erroneous spur to cause problems, not likely, but a possibility. The telecom OEMs are very careful to put filters on their equipment to make sure this doesn’t happen.
It seems like there was little testing done until after the auction. I am not sure what was done before the auctions. The FAA and FCC normally rely on NTIA to do the testing and clear these things. NTIA works with IRAC, Interdepartmental Radio Advisory Committee, to get new spectrum. From their perspective, C Band looked like a very clean spectrum currently being utilized by Earth stations that communicate with satellites. All the FCC would have to do is work with the SAT providers to move to a new spectrum, not easy but not impossible. In fact, the FCC would protect those that could not move.
The FCC allocated $9.7B to help satellite companies move to new spectrum. Link here.
There are so many agencies involved yet, the FAA didn’t raise any alarms until Verizon and AT&T deployed hundreds of sites. It also seems that CEOs of airlines and airplane manufacturers waited for the FAA to say something before they joined in, which happened in 2020.
I’m not sure where they were in 2019 and 2020, but other groups did alert the FCC. According to Airlines.org, several groups alerted the FCC back in 2018. I see several groups but I don’t see the FAA saying anything. Link here to PDF.
The OEMs would assume that the standards in wireless telecom would be similar to that of satellite communications, but it’s not. I heard they wanted to do more testing but didn’t know how to get the RALT equipment or technical specifications. Trust me, I had to really dig to find information about radio altimeters, so it makes sense to me. I am not sure how to get technical information to see how sensitive the front ends are. I am sure it’s available somewhere.
I do know that satellite communications have wide-open front ends. What that means is they are very sensitive to interference.
However, RTCA put out a document that helped explain how the potential for interference was there. To counteract that document CTIA put out its own document that helped show it was slightly exaggerated. If you’re a pilot, you always want to be prepared for the worst-case scenario, I think that’s why RTCA points out the potential disasters that could happen.
Why only in the USA? They have C Band in France and Japan!
It is true, they are running C Band in France and Japan with great success and no problems in planes. In fact, this issue didn’t come up. I wonder why?
In France, it did come up and they put in large buffer zones. So if you’re in an airport, chances are you won’t get C Band coverage. They also have lower power levels.
In Japan, they have much lower power levels near the airports. They didn’t take any chances and the spectrum in Japan goes all the way up to 4,100MHz, higher than the 3,980 in the USA.
The USA has higher power BTS and UEs than other countries. Couple that along with going all the way up to 3,980 MHz which is 220 MHz away from the RALT spectrum. In reality, the power level near an airport will be much lower, not even half of full power, but in rural areas, it could go as high as 3,280 Watts. so I can see the concern here.
Add to that the RALTs have a very sensitive front end where any rise in the noise level may cause a problem, although the testing disproved most hypothetical doomsday predictions.
If you’ve ever done work with satellite earth stations, you might remember how sensitive the LNAs and LNBs were. I’m making the assumption that the front end of the RALT receiver was just as sensitive as a normal LNA/LNB.
To me, the real question is why did the FAA and DOT wait until the end of 2020 to raise concerns?
What is different in the USA?
Like I said above, it’s the power levels that have groups concerned. That, and our government is not working very well at the moment.
It seems to me, at least from the telecom perspective, that the FCC wired to the last minute to raise a red flag. This is what’s so perplexing.
From my perspective, the USA is different not because of the spectrum or power levels, but because our government has problems getting this checked out in a timely manner. That’s it in a nutshell.
How could things get so far without all the agencies knowing what is going on? You tell me and we’ll both be either very rich or taken out. I am not sure which.
The FCC didn’t hide anything. In fact, they published tons of documentation on the spectrum and details for years. The spectrum auction made headline news. The carriers spent over $65B USD to get it. It was a big thing. Maybe the other agencies were too busy to focus on it. I am in this business, so maybe I spent too much time focusing on it.
The pandemic had everyone’s attention, that could have been a distraction.
All I am saying is, it seems that it was a political thing.
What’s going on at the FAA?
To be honest, in a short time the FAA already cleared 90% of the RALTs. How did they do that in a short time? I am not sure, but they made the announcement. It’s impressive!
- Testing results link at https://www.faa.gov/5g and https://www.faa.gov/newsroom/faa-statements-5g.
- There’s a database of airports that have lower visibility, found at https://www.faa.gov/general/commercial-airports-low-visibility-approaches-5g-deployment
- It seems like in a few months, the FAA tested a lot of different RALT devices with 5G. They thank Verizon and AT&T for their cooperation.
- The real question is, “Why did the FAA wait so long?” Read on!
How does the RTCA look at this?
The RTCA is the Radio Technical Commission for Aeronautics, a private/public group that looks out for the airline industry. They put together a comprehensive report outlining the concerns from a technical point of view.
The 231-page report covers quite a bit of information. In section 6, the report pointed out by RTCA, (found here), points out some interesting facts.
- The BTS will have downtilt, so the plane’s antenna and the C Band BTS will both be pointing downward, lessening the chance of interference. The Carrier BTS if at full power could have an EIRP of 3,280W, 65.16dBm, although, near an airport, that is unlikely. They see this as a potential problem.
- It seems that the RTCA is very concerned about the UEs which have a power level of 1W, (30dBm). They showed a model they simulated with many UEs on the ground and thought that might be an issue. It turns out they weren’t going to be a problem.
- They also did a model that UEs were on the plane NOT in airplane mode. This was something they thought could become an issue.
- Speaking of UEs, they also ran a scenario where the UE in the plane could cause problems if not in Airplane mode. I am not sure how much protection the RALT antenna would have, it could be an issue.
- The RALT could get something from the far-off BTS, say something on the horizon, but I don’t see how the RALT would be hit with levels that would cause interference.
- To be honest, they did a thorough study, but it seems to be a lot of worst-case scenarios. That makes sense because when it comes to safety, you have to assume the worst case. It appears to be based on predictions and relevant tests, but I didn’t see them identify the make of the RALTs they tested with.
- They did respond to the wireless industry, which they published the Q&A in Appendix B.
Notes from Report by RTCA:
RTCA has a report out with 5G Mitigation notes. I’ll summarize here, taken from the report “FSMP-WG11-IP07_RTCA_Report”.
- 5G power spectral flux density levels should not exceed radio altimeter interference threshold limits.
- Reduce 5G base station conducted spurious emissions limits into the radio frequency band 4200-4400 MHz to -48 dBm/MHz.
- Limit effective isotropic radiated power (“EIRP”) total power of User Equipment (“UE”) operating while onboard aircraft to -16 dBm.
- Reduce 5G UE conducted spurious emissions limits across the 4200-4400 MHz band to -30 dBm/MHz for UEs operating on the ground and -57 dBm/MHz for UEs onboard aircraft.
- Per section 11.1.1, Findings and conclusions, they state, “5G base stations present a risk of harmful interference to radar altimeters across all aircraft types, with far-reaching consequences and impacts to aviation operations.” So you can see why the industry panicked!
- Section 11.1.3 mentions the UE devices on the plan could be an issue as well.
- Appendix A outlines the test setup and simulations used.
- Appendix B has a Q&A for the questions they answered back to the wireless industry.
How does the Wireless Industry look at this?
They didn’t take it lightly, in fact, they did what testing they could. The group, 5G Americas, put together a night white paper showing some actual test results.
The results were interesting and real. I was impressed with the presentation.
Verizon put together a letter to the DOT and FAA trying to explain the separation and the power levels. Link here. I think the letter does a good job showing how they will not put C Band within 2,100 meters in line with the landing path and 910 meters on either side along with lower power settings of 58 dBm within 2,640 meters in line with the landing path and 1,450 meters on either side.
Notes from CTIA and 5G Americas reports:
CTIA is the Wireless Industry Association that looks out for the Wireless industry. 5G Americas is a 5G association supporting the 5G Industry in the Americas.
The CTIA and 5G Americas put out publications to help clarify the actual problems.
- RF path losses, Free Space Path Loss, were not accounted for properly in the RTCA report. Normally, the higher in frequency, the higher the path loss. It’s why when your smartphone performance degrades when you don’t see any cell towers, the signal drops or goes away.
- CTIA put out several white papers explaining how the BTS and UE signals should not cause interference based on the calculations and initial testing done. In other words, there are flaws in RTCA’s findings. I think they did a great job putting it into a realistic perspective. Reports found here and here.
- 5G Americas did a great job putting out several papers helping to explain the problem. One in particular, link here, tied with this slide deck, here.
What the CTIA and 5G Americas papers say:
- The 5G Americas paper goes into detail showing the path loss and actual RALT models they used to do testing. They point out how the RALT OEMs were not identified. It points out the flaws in the RTCA study and the lack of detailed documentation. It broke down the study and validated the details. In other words, 5G Americas paper was “in the weeds” looking at each detail and making technical analysis that made sense. While you may or may not agree with this approach, it seems to align with the real-world test results that the FAA is seeing as they already passed 90% of RALTs on commercial airlines.
- The CTIA has worked to get the testing done and 5G Americas provided sufficient information. Now the FAA seems satisfied for 90% of the planes.
What’s going on at the FCC?
The FCC is cooperating and trying to work with the FAA to find a safe resolution that both airline and wireless industries can agree on that is safe by looking at the facts and test results, not hearsay.
- Link to letter from FCC to limit deployments near airports, link here.
- The FCC is working to resolve this as soon as possible but did not see the immediate issue, which the FAA seems to be supporting after testing the Altimeters.
- The Committee on Transportation and Infrastructure, (yeah, that’s a thing, also called T&I Committee), chaired by PETER A. DeFAZIO, a U.S. representative for Oregon’s 4th congressional district since 1987 (35 years), as recently as Jan 3rd wrote another letter to the FCC Director Jessica Rosenworcel asking for a longer stay. Link here. To be fair, this group pointed out their concerns back in 2019 to the FCC.
What about the carriers?
Think about this, Verizon paid $45 Billion and AT&T paid $$23 Billion for this spectrum. Do you understand how frustrated they may be? Do you think they would have liked to know about this issue prior to December of 2021?
Yeah, when you look at it that way they got screwed. In fact, they had to pay that money or at least commit to it, before installing one site. They had to put money up just so they could add spectrum for their customers.
You can see why they thought the government agencies should work out themselves. Of course, they didn’t, and Verizon and AT&T assisted the FAA any way they could. Something they would have been happy to do in testing 2 to 3 years earlier.
Did anyone think of Massive MIMO?
I am not sure if you’re aware, but Massive MIMO would be a problem because of how it concentrates the signal
If the FAA really wanted to be safe they may have thought to mention massive MIMO as restricted near airports.
OK, a quick lesson on Massive MIMO.
This is where we have more active elements in the antenna; this could be 16T16R, 32T32R, 64T64R, 128T128R, or even higher. The T means transmit elements, and the R means receive elements. Today most massive MIMO is TDD, so that means that the transmit and receive elements are the same elements. If it were FDD, then you would have a set of transmitting elements and another set of receive elements.
It’s an active antenna with great control over the beamwidth. Vertical and horizontal beamwidths provide a very concentrated beam to the UE. This is so helpful in throughput and spectrum reuse.
Massive MIMO is the perfect midband solution for heavy traffic areas because of all the value it brings carriers. In midband, the antennas are small and powerful. The spectrum allows them to be smaller than low band.
The fact that TDD allows the same small antenna to be used for transmit and receive. This also makes the antenna smaller and more efficient.
However, it’s not cheap, it’s more than a typical RRH and antenna, but so much easier to install.
Massive MIMO is a technology that has created solid beamforming. The beams of RF greatly improve band reuse and concentrate transmit power where it has to be. It also allows the received signal to focus on a specific area.
That’s the issue, it can concentrate power into a specific area. That could be a problem for RALTs, so I think they should do a lot more testing with RALT before putting massive MIMO at an airport.
All the drama!
OK, as you look at the government issues, you don’t know who is to blame but you do see a lot of drama.
Obviously, no one wants any planes to crash, that’s a given. So safety really is job one.
You have so many people speculating on what could happen. That was frustrating. I am not sure why the NTIA didn’t do more testing prior to the auction, that seems like a huge miss.
I don’t know how we got so far into deploying C Band until the FAA and DOT stepped up.
I see how politicians are nervous about picking sides. On one hand, you want to be sure flights are safe. On the other hand, you know that business-wise and for the USA to deploy 5G, after all, the internet is considered a utility. We all want to be connected and 5G is key in making that happen for everyone.
Also, think how many billions the deployment of 5G systems, not just C Band, but all the other spectrum that DISH and T-Mobile are deploying brings to the table. It’s a huge economic boost. All wireless carriers along with fiber companies are putting a lot of money into the economy to ensure broadband is everywhere.
Groups did point out the potential problems as early as 2018, but they didn’t offer a resolution, only more delays.
Who looks good here?
- RTCA for looking into it from a technical perspective, albeit a biased one, but they did the leg work. I think they did a pretty good job.
- 5G Americas for actually doing detailed studies and reviewing in great depth what RTCA did. .
- I think CTIA did a good job to diffuse the situation based on the 5G Americas report.
- I believe the FCC remained cool and didn’t overreact.
- The carriers, Verizon and AT&T, are working with the FAA to resolve this issue. They had a lot at stake, especially after paying the government over $60B to get a chunk of air, sorry, I mean spectrum, before they deployed one site.
- When the FAA started to actually do some testing on the RALTs, that was great, and the fact they got 90% approved by the end of January was amazing! Even though it felt like a last-minute decision.
- The A4A and ASRI for sending the FCC their concerns in 2018.
- The AVSI and Committee on Transportation and Infrastructure for sending the FCC their concerns in 2019. While they wanted to delay the auction, I think they should have been asking for testing to be done.
- The CTIA, AIA, and A4A came together in December of 2021 to resolve this issue together.
So, who looks bad? (Who should we blame?)
- The FCC for releasing spectrum and having the auction and raising over $60B? I don’t think so.
- The FCC for not listening to the industry associations, (A4A, ASRI, AVSI, T&I Committee)), who asked for more discussions in 2018 and 2019? Maybe.
- Why was the FAA silent for so long? Why did they wait until December of 2020 to petition the FCC? Why not ask about testing the Radio Altimeters? Or asking the carriers and FCC for cooperation? Yeah, I see some blame here.
- The NTIA for not testing thoroughly? Hmmm, maybe. Why didn’t they? I read that they also had trouble getting RALT equipment. Strange.
- The carriers for not knowing this was an issue? They don’t know what they don’t know, so no.
- The wireless OEMs for not testing earlier? Maybe, but who would they test with, they actually were trying to do more testing but didn’t get cooperation from the airline industry.
- The airline industry for not testing with anyone until the last minute? Maybe, I see some blame there but I don’t know the whole story. They probably relied on their OEMs or the FAA to take care of it.
- For all the groups that petitioned the FCC, why didn’t they also petition the FAA or DOT to take action? Did they? I didn’t see anything. It seems like the two big boys could have worked something out sooner.
I think we’re all surprised how far this got before the FAA raised a red flag. There are a lot of questions here which I tried to answer. The only one that goes unanswered is why the FAA and FCC didn’t do any testing earlier. I don’t understand.
We need the 5G spectrum. The carriers have 5G spectrum and it still takes years to build out. It’s not magic and doesn’t happen overnight.
It is a very long and expensive process. Tower climbers literally die doing tower work every year. It’s not easy work. It’s not quick. It takes testing, planning, management, deployment, commissioning, integration, approvals, and more. Verizon and AT&T will spend billions of US dollars to deploy the equipment just to get it on the air.
Based on a letter the DOT and FAA sent a letter to the NTIA to talk to the FCC to delay the auction. They sent this on Dec 1 202 and the auction was Dec 8, 2020. Link here. Luckily, they gave plenty of time, at least 7 days, asking the NTIA to talk to the FCC about delaying the auction. Is that crazy or what? It seems like it was last minute. Also, they asked NTIA “We also expect that the cost of replacement or retrofit of radar altimeters will be substantial. The Commission does not appear to have taken these factors into account in its decision-making process.”
Does anyone think this was a bit last minute? Did the FAA have a plan in place? Do they know what they would replace them with? Did they do any testing to see if the replacement was necessary? What do you think?
So let’s look at some notes.
- RTCA study was done Oct 2020
- It seems strange that RALT faces straight down and the C Band antennas that normally have 1 to 3-degree downtilt, that they would cause each other issues.
- What is the last time aviation upgraded these devices? Would LIDAR or video work? Seems antiquated.
- It doesn’t surprise me that the front end of the RALT could be affected by a signal 200MHz away, this is technology that was invented in the 20s and has not seen great improvement since the 60s.
- One thing we have to be very careful of, and that’s the use of Massive MIMO radios near airports. They have a very concentrated RF signal. That’s not good for RALTs.
- FAA & DOT wait until 8 days before the auction to ask for money to replace the radio altimeters. Also, replace them with what?
What needs to change going forward?
This is a key question. What are the lessons learned?
The FCC and FAA work well together, or at least they have in the past. Remember the drone situation in the USA? The FCC and FAA worked together on that issue.
The issue was that the FCC knew the telecom industry wanted to use drones so that there were fewer climbs on a tower. However, the FCC didn’t want drones to become a problem for commercial and private planes.
They were able to put rules in place, rather quickly, allowing professional drone pilots to do their job. This took a while, I think over 2 years, but it happened without much drama.
I hope we don’t create another agency, but that’s what they usually do, create another agency to watch over the existing agencies.
I think there has to be open and honest communication between the agencies. That seems to have gone away.
I believe that all private and public/private groups should have a solid voice to their prospective agencies. They certainly tried in this scenario, but it didn’t seem to work. Did they petition the wrong agencies? I don’t know.
The FCC reaches out to the telecom carriers all the time, sometimes maybe too much, but with the right intentions. They also have meetings and conferences to understand the needs of the industries.
The FAA also talks to its constituents with great success. They have to put safety above everything else. They seem to be doing a pretty good job.
All agencies have failures and dropped balls, it happens. We’re only human. But I believe they do what they can for the greater good.
My hope is that they learn a lesson and find a way to work better together going forward.
So I think the FAA had the right idea, asking for more money to upgrade the RALTs, but they went about it in a terrible way. Poor planning if that was the end goal. They could have asked back in 2018 and the FCC, or the Department of Treasury more likely, probably would have set something aside.
Will the Airlines keep the RALTs they have today?
Personally, I hope they upgrade to something better and more sophisticated. However, sophistication scares airlines because it generally means that they could fail more often.
I talked to so many people that wonder if the RALTs could be upgraded. For example, cars are using video, LIDAR,
I wonder if it can work? I wonder if they need to make some upgrades.
I know what you’re thinking of the airline industry, why do we have to change? Fair question.
When was the last time the RALTs did a major upgrade or a new technology?
There has been a lot of controversy around 5G, this makes things worse. The fact the press said 5G could cause planes to crash didn’t help anyone. They didn’t specify C Band, which is pretty much what the FAA wanted.
|3GPP||3rd Generation Partner Project, basically a governing body over wireless communication standards|
|5G||Fifth Generation Mobile Networks|
|BTS||Base transceiver station|
|C Band||A portion of spectrum that was once allocated for satellite transmission but is now being repurposed for 5G communications|
|CTIA||Cellular Telecommunications Industry Association which is now called the Wireless Association.|
|dBm||Decibel Milliwatts used to measure power radiation in decibel|
|DOT||Department of Transportation|
|EIRP||Equivalent Isotropically. Radiated Power|
|ERP||Effective Radiated Power|
|FAA||Federal Aviation Administration|
|FCC||Federal Communications Commission|
|FDD||Frequency Division Duplex|
|Mbps||Megabits per second|
|PSD||power spectral density of a signal|
|RALT||Radio Altimeter, which is the same as Radar Altimeter|
|RTCA||Radio Technical Commission for Aeronautics|
|TDD||Time Division Duplex|
|W||Watts, a power measurement|