Visualizing RC Radio Range

Signal Hill, Hilltop Park, CA

Two words I love are "useful" and "free."  When I think of things that are useful and free, Google Maps comes to mind.  And yet, there are areas where Google maps aren't so useful.    What if you simply want to map your transmitter's range for your next flight?   In this hypothetical example, we'll look at Hilltop Park in Signal Hill, CA.   What better place to illustrate your transmitter's signal than Signal Hill? If you visit it on the right day in Los Angeles, it's lovely.  You can see all the way to Downtown Los Angeles and out to Long Beach.  The strange thing about the park is that it's only  110m(365') above sea level.  But, it's in the basin of Los Angele which is fairly flat, thus you can get a wonderful panoramic view.   Of course, the first thought that comes to mind is not "This would be an excellent makeout spot" but instead "Hey I want to fly my plane from up here!"  It's a bad idea for a number of reasons, aside from the legality of it the fact that it's densely populated once you leave the park and a crashed plane very easily could create property damage.  But the point remains, what radio system could potentially get you to the waters of Long Beach?

I am going to bring up four popular receivers

Spektrum AS3X-  300m range?




FrSky XM+-1K  range




FrSky L9R->3K Range



TBSCrossfire-6K+ range



The FrSky is fairly well published.   Spektrum says the AS3X is "Full Range" and doesn't really disclose hard numbers.  Thus, I will trust it for the first 1000 feet.  The TBS Crossfire runs in the 900mHz range and under the right conditions and right tools can go 30K if not more.  6K is perhaps an understatement, or maybe not?  More about radio range later.

One incredible mapping overlay

Here's a link to the map I am about to share.  In it, you'll see four circles.

AS3X

The first circle is the Spektrum receiver's range.  300 meters is around 1000 feet.  The good news is that it will cover the entire area of the park.  The bad news is water is nowhere in site. The AS3X is a Spektrum receiver with flight stabilization built-in. It's really more of a  line-of-sight receiver than one built for FPV.   I'm sure range is further than 1000 feet, but 5 times further?  Unlikely.  There is a Home Depot and Costco just below the park and chances are good that you can still see your plane after it goes out of range. That means that the receiver is fine for park flying but for long range FPV we have to look elsewhere.

AS3X Ramge

XM+

FrSky XM+


The XM+ is an SBus receiver with just one set of pins.  On that one set of pins it puts out  16 channels.  It's tiny and very light.  The advertised range is 1km though others have gone much further under certain conditions.   Low to the ground in busy areas the range can be below 900 meters.





The small circle in the center is the Spektrum AS3X, the larger circle is the XM+.  That's what five times the range looks like.   Impressive but still nowhere near water.  It's great for FPV where you're staying fairly close to where you're standing.  1km is a bit too far to see with most planes thus it's ideal for FPV.  Still, we need more range if we're going to make it Long Beach.

FrSky L9R

It's time to pull out the big guns.  The "LR" in "L9R" stands for "Long Range."  The specs say that the L9R has a range that is greater than 3km.   Cool, but, what is that?   Since they came up with the number 3km that's what we'll use.

L9R

And now, water is on the map for the first time. As you can see the smaller circle is the XM+ and the tiny circle is AS3X.  The water is within sight, and the L9R can do at least 3K, so why not just go for it? Of course, if you lose signal and don't have INAV correctly programmed someone will find a free plane.



TBS Crossfire

One does not merely buy an FrSky radio system.  When you buy the Taranis you're investing in FPV.  Still, if you want to go further there are more options. When you get into OpenTX and INAV what you're essentially doing is using a radio to have one computer negotiate with another computer.  You will not be able to see your plane 1K out and you cannot trust that your video transmitter is going to work flawlessly.  You need a failsafe, which is a computer that will fly your plane back to you should you lose signal in either your radio receiver or video transmitter.  And yet, if you want more range you'll have to move on to newer technology.

TBS Crossfire

The Crossfire by Team Black Sheep is in the 900mhz band.  You don't need a Ham Radio license to fly on this band.  Some people are claiming flights over 30K with the TBS Crossfire.  And now, for the first time, we see that this system can cover a range from Signal Hill out to the Queen Mary and back.  You can see all four circles.  If your plane can achieve a speed of 48K/Hr the total flight would take 15 minutes.

Radio Range-Having a healthy sense of scepticism

We'll be flying at a dry lake in the coming months.  There is nothing around.  No hills, no trees, no buildings, no radio interference to speak of. Just flat, wide open land.  The radio range will be off the charts because there isn't much to interfere with a radio signal.  This map is the polar opposite. Trees, buildings, some small hills, and a solid ton of radio interference, even on 900mhz.

I'm certain that people are getting a solid 1K on the AS3X, while others have advised me that they get 3K from the XM+ and there is a video of the L9R going 14K while people are taking the TBS Crossfire out 30K+. If you can monitor RSSI values while flying it certainly helps.

Here's a video from the park in question. At one point you can see the Home Depot and Costco where he probably would have lost range with his drone.

A more practical application.

One of my buddies has a timeshare in Cab San Lucas.  I've gone with him before and flew planes on the beach, it's a  ton of fun.  At the end of the harbour is Los Arcos, which is famous for its beauty.  The question is, what radio can he use to make it to Los Arcos?

Los Arcos, Cabos San Lucas Mexico

Cabo San Lucas

This is a 1K map and this is about as close as we can walk without having to get a cab ride to the other side of the Marina. This is a 1K radius which means that the circle is 2K  Los Arcos is just outside the 1K radius, but not by much.   Be sure to update your firmware to get RSSI readings.  Be certain that your failsafe settings are legit because you may be using them.

The Golden Gate Bridge

Golden Gate Bridge, San Francisco, CA

Golden Gate Bridge

Drones are banned from flying on or near the Golden Gate Bridge.    But let's imagine they were fine, or you have a wonderful lawyer on retainer.   You can see that the L9R would cover the length of the bridge.  The assumption would be that you would fly from the San Francisco side near the Golden Gate Bridge visitors center.

Lake Minnetonka

In the final example, I am imagining that I will make another trip to the city my wife is from, Minnetonka, MN. And I can talk someone to dropping me off on some desolate part of the lake. Before I even step foot on the ground I can figure out how far I can fly with an XM+ and where to turn around.

Lake Minnetonka

Flight planning

You can use this tool (again, here's the link)  so that you can quickly look at the range of locations you plan to fly.  The tool works in metric and imperial units and you can save a link and  keep it handy so that you can think of flight paths and landmarks to help you keep orientation.   You can do "What if" scenarios and decide if you need to upgrade your receiver.  As you can see from the examples you can view a map or satellite map.

Radio range

It's impossible to say exactly how far your radio will work.   Instead of one factor there are as many as seven factors that have influence over your range.  If you fly low to the ground in an area with a lot of radio traffic the range may be less than what's listed.   If you're flying high in low radio traffic areas it may be considerably more range, perhaps twice to three times the suggested range.     If you have a receiver with RSSI that will help alert you when your receiver reception is critical.  Of course, all of this is based on the notion that your VTX will work further than your RX which is often not the case.  With this tool, you can also start mapping the range where your VTX has fallout.

One huge benefit

It's easy to lose orientation easily when flying.  With a GPS and INAV configured you'll always know where home is.   But, I wouldn't rely only on that.   Looking at the coverage circle on a map you can get a sense of where you're likely to fall out of range. You can look for landmarks in the area so that you know how far you can comfortably venture out to and where you should start turning back.  You can even make a second circle that is 80% the size of the main circle,. You can visualize where you can make your turns.  Look for recognizable landmarks from the air to help you know where to turn and how to get back.

Why your plane loses altitude in angle mode

The mystery

I wanted to know some simple truths in life, such as who built the pyramids,  why hasn't the United States converted to the metric system, will the Smiths ever make another studio album and why did Nikki Miller turn me down for the prom?

Some things in life are too complex to grasp.  But, looking at my broken C1 Chaser on the ground perhaps there were a few things I could still learn?  In the angle mode it kept dropping, and  I had it too close to the ground,  directly overhead and lost perspective.  Not the smooth INAV experience I was hoping for.

So, as the glue was drying, yet again for this version of the plane I asked the smartest people I know, who happened to be on the INAV Fixed Wing Group what happened. Someone wrote in a nice reply that answered the question.



Level vs altitude

A perfectly level plane won't lose altitude, right?   Or perhaps a better question is, what do you want?  Do you want your plane to fly perfectly level or do you not want to lose altitude? Because a  level flight usually is a flight that drops.  When we first set up our flight controllers what we're asking is for the plane to fly perfectly level.    And my flight controller was performing perfectly.

But, planes aren't exactly perfect, and they don't fly exactly efficiently. It has to do with the way that the air travels across the wing.  The tendency is that at level flight many planes will sink.  But, it really depends on the airfoil.  Some planes are designed with a bit of reflex built in.   But, most just sink when level.

2.5 is my magic number

To combat the loss of altitude we have to pitch the nose up ever so slightly to compensate.  One of the first things I do when I get a new flight controller before I even begin soldering is to flash INAV.  When it's flashed one of the very first steps is a calibration.  I will teach the flight controller what the six axes are in my world.   I find it best to do this on a desk and hold the flight controller perfectly still for each reading.  I find that these are the six truths that the flight controller must learn.   But, shouldn't you do this in a plane instead?  It's like having a compass (not the same, but similar) that you mount in your car.  If you mount it on your windshield at an angle, do you need to recalibrate your compass?  No, because it will always know where north is, no matter how it's mounted. 

My view is that straight up and straight down really aren't open for interpretation. Neither are the other 4 axes.  The position of the flight controller may change, however.  That's what we have to account for. When your flight controller has learned it's six truths it can make accommodations for its environment.  You can mount it upside down, 90 degrees left or right yaw, or even 180 degrees.  But we have to tell the flight controller that it's not mounted exactly as expected. 

In the INAV Configurator, in the configuration tab on the top left, there are boxes where we tell the flight controller how it's mounted. This is where you can account for it being 90 or 180 degrees off yaw or upside down (180 degrees pitch).  If I am not making any other pitch adjustments I change the number to  2.5 (positive). That doesn't automatically fix everything, but it's a very good place to start.  If the plane still sinks in normal flight I can take it up a half a degree at a time.  If the nose is up too high it will gain altitude too easily and may seem like the control surfaces have even less authority. 

To answer a question people will have, yes, most of the time it works fine, or well enough to allow me to perform an autotune.

 I don't want people reading this and thinking "I don't want to lose altitude and crash, why not give it 5, 10 or 15 instead?"   If you want a real-world example while this isn't such a good idea, get on your bicycle and ride an 8% grade.   8% is a small number, right?   You'll be huffing and puffing and grunting all the way up the hill.  I have one near my home that's over 2km long  By the time I'm at the top I can see the valley below,. The valley is where I start out at before the 8% climb. Little numbers make a big difference.

Play with it.

If the plane is still sinking, a bit, take the number up to 3.   If it's not sinking but you're concerned about lift or authority over the control surfaces, drop it down to 2.   If it's too off for your tastes, land the plane and don't fly it again until adjustments are made.

There is no right number for every plane. So, experiment, and be choosy about what works for you.   I find that 2.5 helps get me to the point where I can autotune the plane.  Every time you change this pitch number in the configurator you will need to do a new autotune and possibly an auto trim of the plane.

We'll be working on a master-tune document, but I wanted to get this out there right away.

I will give you some other clues as well.

Your plane really should fly well in manual. If it doesn't and you're not up to snuff on tuning skills you need to ask for help.    You have to fight to get the CG right and the control surfaces working to keep the plane in the air.   And if you can get a gr a good eat flying experience with your plane in manual the rest will fall into place. 

If your plane flies well in manual but not well in stabilized modes you should perform an autotune.  Hold onto your hat because for a short period of time the plane will fly even worse.  Then it will fly much better over time and you will really like the results.  Be sure to save.

If you are in the stabilized modes and you have no authority to speak of you are most likely  tail heavy.   A quick way to tell is to put the plane in manual, get it up high and point the nose at the ground.  If  the nose balloons back up with speed you are  tail heavy. 

Fine tuning is an art in INAV.  Be very picky about what you like and work on improving your plane for a good experience.