This tree randomly split in two and fell over! No one was hurt, it could have been muuuch worse.
During this solar eclipse, I was lucky to be near the path of the full eclipse. Sky & Telescope have a really nice graphic showing where in North America the eclipse was visible.
I’m just north of the ideal path. Anyway, it was cloudy here in Oregon, so my pictures aren’t great. I hope you enjoy them.
The National Oceanic and Atmospheric Administration (NOAA) manages a few satellites in low earth orbit. There are three actively transmitting APT signals at the moment, NOAA15, 17, and 18. Each of these satellites passes overhead a few times a day. I’ve been interested in learning how to receive their signals for a while now, and I’ve finally succeeded!
A bit ago, I bought a “SoftRock” SDR (Software Defined Radio, read the excellent 3-part article by Bob Larkin at the ARRL site.) receiver kit from Tony Parks. (A note about his site, he puts a few kits up for sale a few times a month, so he’s almost always sold out.) I think SDR is really, really interesting. I don’t want to get too bogged down in the details of it, because it’s not the point of this post, but I’m going to briefly discuss it. Basically, the idea is that you want to have some minimum amount of electronics to deal with the antenna; letting your computer handle the rest. This can take a variety of forms, but the simplest is the QSD, or Quadrature Sampling Detector.
It sounds complex, but it’s quite like using a strobe light to look at a spinning wheel. The bright light of the strobe “samples” the world at a given interval. If it strobe rate matches the speed of the wheel, the wheel appears still. Stretching the analogy a bit further, imagine that information is written on the wheel. Using the strobe you can read it, even if it’s spinning. While that is an awful analogy, but the idea is that we can sample the radio signal at (nearly) the same frequency as the carrier of our desired signal. When we do this, the signal we want magically appears at the output. If we’re using AM (or its derivatives such as LSB or USB) we can even listen to it directly. It only gets a bit more complex when we consider the quadrature part. Quadrature just means “90 degrees out of phase.” Using another copy of the radio signal, and a sampling clock in quadrature, we can cancel out some noise and interfering signals. Sorry for the tangent, if you read this far (without falling asleep), I recommend you read the linked articles at the top of this paragraph. The math isn’t too hard, and it’s sooo powerful!
This isn’t an image of my SoftRock, it’s a slightly different version, but I don’t have an image handy. It’s a really easy kit to build, and it’s fairly inexpensive. More than that, it’s really easy to use. Once it’s all setup, you just attach it to your computer, power it, and install the antenna!
Once you’ve attached the receiver hardware to your computer, you need some software to use it. This is an image I took of a very well written SDR program on the Mac called DSP Radio. On the left side of the spectrum window, there is the live radio spectrum coming from the satellite. The green box around it represents the bandwidth of my software radio receiver. In a traditional radio, this bandwidth would be set by a filter circuit. Most communication radios only have about 15 kHz of bandwidth. This makes them unable to properly receive satellite weather images. Traditionally, you would have to build or buy a specially-built satellite receiver. With SDR, I can move a slider to scale the bandwidth way, way up! In this case, I’m using about 37 kHz of bandwidth! Notice that there’s all this empty space on the right, this is radio spectrum that I’m receiving, but there’s nothing there. Maybe you can notice the shadow of the satellite data on the right; this is an “image.” These images are the bane of all radio designers. The true test of a receiver (other than sensitivity) is how well these images are suppressed. In this case, they’re suppressed rather well, notice how bright the lines are on the left compared to the right.
The DSP Radio program takes the signals from the Softrock through the audio input of the computer. When you have something tuned in through its interface the demodulated signal appears at the audio output. I’ve been recording these signals as well as passing them to another program called WXtoIMG. This program is not great, I’ll be honest. It’s barely maintained, and you can tell it’s an ugly cross-platform mess. To even get it to work is tricky. But, what it does, it does well. The image at the head of this post was generated using it. When I made that image, I literally had to connect the audio out of one computer to the audio in of another. I’m not sure how I’m going to get around that issue. It can accept data in the form of wav files, provided that they’re linear PCM sampled at 11,025 Hz with at least 16 bit samples. The problem is that the nice political boundaries, lat/lon lines and ground image comes from the program. It does this by computing the location of the satellite and where on the earth its photographing. It has to decode the audio in real time for this to work, which means that I can’t use an audio file. For you to play around with, if you wish, I’ve included a sample wav file. It starts before the satellite pass and ends after, so if begins and ends with static.
NOAA15-baseband.wav (large file warning: 28 MB)
The included audio can be used to create the image below. I used WXtoImg to generate it, though the open source WXAPT could be used under Linux. This image was taken when the satellite was traveling south-north, so I had to flip it vertically and horizontally. On the right side of the image is the A channel, which is visible light, and the left side is the B channel, infrared. Normally, these channels are reversed left-to-right. The stripes and color bars help the decoder line up the image and adjust brightness, contrast, and gamma. (Right-click here to download full-size image: 2080 x 1260 pixels)
My next step is to write some shell scripts on a Linux box to automate this whole process. My goal is to have a page that has the latest satellite image and an archive available at all time. But first, I have to write a post about the antenna I built to receive these signals. Stay tuned!
I’m back from the third (and fourth) flight, and it keeps getting better! On Friday (New Years Day) I went to the BCRCC (Benton County Radio Control Club) “Polar Bear” event. Basically, the idea is: On new years day, rain or shine, everyone comes and flies something. There is a raffle for all those that fly. I brought the Kadet, mostly because that’s the only plane I have, and the tiny Blade mSR that I got for X-mas.
The Kadet’s second flight was somewhat uneventful. Flying during an event invariably leads to crowded skies, because I didn’t have the runway to myself I just tried to stay out of people’s way. I was still pretty unfamiliar with my plane so it was nerve-wracking. Luckily I did a good job landing with all those people watching :).
Today, I went back to the field with my Kadet, and a new plane that I got at the BCRCC auction for $20 in November. It’s a “Funtana Mini” made by E-flite that has since been discontinued. Since the auction, I’ve been slowly getting the parts I needed to complete it, including a receiver and servos. I reused the motor from another plane that is no longer with us.
I piloted it on its maiden flight today. It is extremely twitchy, even on low rates. Also, something I didn’t expect was how hard it would be to land it. It requires a fairly high airspeed to maintain altitude, and there isn’t much frontal area to slow it down, so the landing was fast. I wasn’t able to stop it by the end of the runway so it spent some time in the mud. Nothing was damaged, but the mud at the airstrip is very stinky. The second flight was more fun, I actually did some aerobatics. I still had trouble bringing it down, though.
I wasn’t satisfied with the prop I was using in the first flight test, which was an 11×7 APC E-series. I bought 2 more, one with higher pitch speed (for the uninitiated, “pitch speed” is the number of inches a propeller would travel forward in one revolution in an ideal fluid) and the same thrust (which how hard it “pulls”), which I think is an 11×8.5 E-series, and one with the same pitch speed and more thrust; a 12×7 E-series, I think. I tried the one with more pitch speed first, and I’m not rushing to try the other one. I’m quite happy with the performance as it is. At full-throttle I can climb-out at 45°, and at 1/4 throttle I can maintain altitude and speed. I’m also very happy with the observed endurance. I ran the stop-watch on my radio during the second flight of the day while doing non-stop touch-and-go’s, and after 12 minutes of flying called it quits. The resting battery voltage on my 3S 2100 mAh LiPo was 11.4 volts. This roughly corresponds to 20% of the pack power remaining. With this knowledge in hand I know that I can set the count-down timer on my radio to 12 minutes and not stress-out my batteries too much.
Altogether, it was a good (long) weekend of flying!
I’ve just finished building the first radio controlled airplane kit since, i don’t know, over a decade. I tried to build one 8 yeas ago, but never finished. Anyway, It’s pretty exciting. I’ll share some of the building process with you. My intent from the beginning of the project was to build a arial platform for autopilot development, photography, and, perhaps, an attempt at FPV (first-person video).
With those goals in mind I decided to select a “trainer,” which is a very stable plane intended for teaching new pilots. There are several advantages to a trainer in this context. In a way, I’m training an autopilot to fly, so it’s a fitting. In addition, a typical trainer has a simple, boxy fuselage. I think that the simplified geometry should be easier to build around. The individual plane I settled on is the SIG Kadet LT-25, and my mom was nice enough to get it for my for christmas a few years ago.
I wanted the plane to be on the small side because I’m 100% electric for all my planes and helicopters. I don’t have any investment in nitromethane power, and I don’t want any. It’s stinky, loud, dirty and annoying. At the same time, electric power is getting better and better. The batteries are increasingly amazing in their capacity and power. Unfortunately, I don’t have any early building photos. But, I’ll talk about some of the photos I do have:
This is the only photo I have of the wing being built. It went together really well, the laser-cut parts are amazing. I’m not all that great at making the wing halves meet up right, but lots of epoxy and filler remedied the problem (hopefully). In this picture I haven’t added the fiberglass reinforcement around the seam. Finally, notice that the servo hole is a strange shape. I added a brace in the middle of the opening because 2 micro servos are slightly more narrow than one full-sized servo. Using 2 servos allows me to make the ailerons behave as flaps as well as ailerons. This way, I can adjust the lift of the wings to compensate for the additional weight of payload packages.
As I mentioned, I wanted to make it as easy as possible to add payload modules to the aircraft. To make this easier, I moved the servos for tail feathers surfaces to the tail itself. This has 2 advantages, I don’t have to waste space in the radio bay, and it should offset the weight of the comically giant motor I bought. There is some additional parasitic drag, but I hope it isn’t too much.
Speaking of the beast, this is the comically huge motor. It’s a scorpion, which claims to be better than the others because the magnets are rated to 200 degrees celsius while also having the strongest magnetic field (which is apparently rare). It’s rated to 80 Amps and 1200 Watts continuously. That is WAAY more than what I need for this model. I’m not sure what I was thinking when I bought it, but it is what it is. I spent some time on the motor calculator to figure out a prop/battery combination that seems appropriate to the weight of the model. Most people have theirs coming in at a finished weight of 4 pounds, so I assumed 5 pounds as a starting point and went from there.
A common rule-of-thumb for electric flight is that for “reasonable” sport flying 40 to 50 Watts/Pound are necessary for brushed motors and 35 to 45 for brushless motors. In the performance results notice that the current (24 Amps) for a 3-Cell LiPo (11.1 Volts) ends up with about 266 Watts and 53 Watts/Lb, which is just about right. My speed controller is rated for a 4-Cell LiPo, which would enable about 400 Watts of power. I doubt I’ll need that much, but I guess it’s nice to have the option.
Several companies, such as APC, developed special props for electric motors. They are much more efficient than the older models. There is renewed interest in propeller design, even in the general aviation circles. One of the leaders of the movement is Paul Lipps. He has an interesting article on EAA’s (experimental aircraft association) Experimenter magazine. Some qualities of his designs are also present on this propeller. As demonstrated in the photo above, the angle of attack at the root of the propeller is much greater than other model props. This caused a problem for the spinner I planned on using. I had to carve the opening larger to accommodate the prop and give some room for comfort.
I skip ahead here a little, but here is a photo of the motor and speed control installed in the fuselage. At this point, the motor is just sitting there but it doesn’t look any different bolted in. I later moved the speed control slightly. In the photo it’s sitting above a balsa panel, I was able to move it underneath. This left plenty of room on top of the panel for the battery pack.
I wanted to build a place for the GPS and autopilot circuitboards. I knew that I could install them in the radio compartment, but it would have been covered by the wing. This photo is looking through a lightening hole on the top of the fuselage, just behind the wing. I found some plastic ‘U’ channel and thought it would allow me to slide electronics trays into this bay. It’s necessary because once the covering film is installed, I won’t have access to the bay from this angle.
Here is another view of the GPS tray. I had to take this picture from the firewall to get the right angle. You can see the bracing I made underneath and alongside the plastic channels to add rigidity. On the bottom of the fuselage, you can see 2 balsa sticks. Those are the mount for the tailwheel steering servo.
Here, the tailwheel steering servo is mounted. I decided that I could sacrifice this amount of space in the bay. I didn’t want the servo to be mounted upside-down on the bottom of the plane because it’s rainy in Oregon, and I didn’t want water to splash up from the main gear onto the servo. I also thought that if I had a landing onto tall grass there was an increased risk of damaging the servo because there would be more places for things to get caught. The funny-looking bulge on the right is there so I can remove and install the servo. The sticks are just wide enough to fit the body of the servo, but there is a wire coming out that makes it hard to mount otherwise. You can see that the control wires rub against the side of the former. I later carved them a little to allow for clearance.
As things move along, here is a photo of the plane partially covered. The entire fuselage is covered, and the top of the wing is, too. You can’t really see, but the tail surfaces are covered and installed. I haven’t bought the wheels at this point, either.
Now, things are just about finished. I installed an eye-bolt in the top of the wing at the Center of Gravity (CG) to check the plane for lateral and longitudinal balance. Notice that there is a metal tube sticking out of the leading-edge of the right wing. I decided to install a pitot tube for use later. It’s made out of aluminum and goes into a silicone tube down the length of the wing to another aluminum tube that exits out the bottom of the wing. I used Bernoulli’s formula for determining air speed using a pitot tube and determined that I need to get a 1500 Pa (1.5 kPa) differential pressure sensor, assuming 50M/sec as a maximum speed (which is about 100Mph).
Anyway, back to the plane, I’ve finished covering it. I added a yellow stripe along the whole bottom of the left wing that wraps around the front of the right. This is a high-contrast indicator of the orientation of the plane. These aid in the pilot’s ability to determine the orientation of the plane.
The idea is to make the 2 possible orientations of the plane look very different through the use of color. In this example, my plane would be different because the image on the bottom-left would have a stripe along the length of the wing and on the one in the bottom-right image, there would be a single diagonal stripe on the right wing
Well, long story-short, I’m done building the airframe. I need to work on the electronics now. Luckily (I’m trying to be up-beat about this) the weather is absolute shit, so I’ll have plenty of time to work on it. Hopefully there will be a break that I can use to take it on it’s maiden flight.
There it is all finished. Note that the deck isn’t wet because I sprayed it off. We’re supposed to have “torrential” rain this weekend. 🙁