Tracking Satellites: The Nitty Gritty Details

December 16, 2020 by Chris Lott 18 Comments

If you want to listen to satellites, you have to be able to track them as they pass over the sky. When I first started tracking amateur satellites, computing the satellite’s location in the sky was a part of the challenge. Nowadays, that’s trivial. What’s left over are all the extremely important real-world details. Let’s take a look at a typical ham satellite tracking setup and see how it all ties together.

Rotators for Steering

The popularity of robotics, 3D printing, and CNC machines has resulted in a deluge of affordable electric motors and drivers. It’s hard to imagine that an electric motor for rotating an antenna would be anything special, but in fact, antenna rotators are non-trivial engineering designs. Most of the challenges are mechanical, not electrical — the antennas that they drive can be huge, have significant wind loading and rotational inertial, and just downright weigh a lot. A rotator design has to consider bearings, weather exposure, all kinds of loads, not just rotational. And usually a brake is required to keep the antenna pointed in windy conditions.

There’s been a 70-some year history of these mechanisms from back in the 1950s when Cornell Dubilier Electronics, the company you know as a capcacitor manufacturer, began making these rotators for television antennas in the 1950s. I was a little surprised to see that the rotator systems you can buy today are not very different from the ones we used in the 1980s, other than improved electronic controls. Continue reading “Tracking Satellites: The Nitty Gritty Details” →

Building A GPS With Bug Eyes And Ancient Wisdom

September 6, 2019 by Tom Nardi 12 Comments

The Global Positioning System (GPS) is so ingrained into our modern life that it’s easy to forget the system was created for, and is still operated by, the United States military. While there are competing technologies, such as GLONASS and Galileo, they are still operated by the governments of their respective countries. So what do you do if you want to know your position on the globe without relying on any government-operated infrastructure?

According to the team behind [Aweigh], all you have to do is take a cue from ancient mariners and insects and look up. Using two light polarization sensors, a compass, and a bit of math, their device can calculate your latitude and longitude by looking at the daytime sky. With their custom Raspberry Pi shield and open source Python 3 software, the team envisions a future where fully-independent global positioning can be tacked onto all sorts of projects.

The concept relies on the Rayleigh model, which is essentially a polarization map of the sky. As light from the sun is scattered in the Earth’s atmosphere, it creates bands of polarization which can be identified from the ground. Essentially it’s the same principle that makes the sky appear blue when viewed with human eyes, but if you have two light sensors looking at the proper wavelengths, you can use the effect to figure out where the sun is; which the team says is precisely how some insects navigate. Once the position of the sun is known, [Aweigh] operates like a modernized, automatic, sextant.

Naturally, this is not an ideal solution in all possible situations. In an urban environment, a clear view of the sky isn’t always possible, and of course the system won’t work at all once the sun goes down. In theory you could switch over to navigating by stars at night, but then you run into the same problems in urban areas. Still, it’s a fascinating project and one that we’re eager to see develop further.

Incidentally, we’ve seen automated sextants before, if you’re looking for a similar solution that still retains that Horatio Hornblower vibe.

Hacked LCD Shutter Glasses See The Light

July 12, 2018 by Tom Nardi 14 Comments

It’s always a little sad to see a big consumer technology fail. But of course, the upside for us hacker types is that the resulting fire sale is often an excellent source for hardware that might otherwise be difficult to come by. The most recent arrival to the Island of Unwanted Consumer Tech is 3D TV. There was a brief period of time when the TV manufacturers had nearly convinced people that sitting in their living room wearing big dorky electronic glasses was a workable solution, but in the end we know how it really turned out.

Those same dorky glasses are now available for a fraction of their original price, and are ripe for hacking. [Kevin Koster] has been playing around with them, and he’s recently came up with a circuit that offers the wearer a unique view of the world. Any reflective surface will look as though it is radiating rainbows, which he admits doesn’t show up as well in still images, but looks cool enough that he thought it was worth putting the board into production in case anyone else wants in on the refraction action.

To explain how it works, we need to take a couple of steps back and look at the mechanics of the LCD panels used in these type of glasses. At the risk of oversimplification, one could say that LCDs are sort of like capacitors: when charged the crystals align themselves in such a way that the polarization of the light passing through is changed. Combined with an external polarization filter, this has the end result of turning the panel opaque. To put the crystals back in their original arrangement, and let the light pass through again, the LCD panel is shorted out in the same way you might discharge a capacitor.

What [Kevin] found was that if he slowly discharged the LCD panel rather than shorting it out completely, it would gradually fade out instead of immediately becoming transparent. His theory is that this partial polarization is what causes the rainbow effect, as the light that’s passing through to the wearers eyes is in a “twisted” state.

[Kevin] has provided all of the information necessary to build your own “Rainbow Adapter”, but you can also purchase a kit or assembled board from Tindie. If you’re looking for other projects to make use of those 3D glasses collecting dust, how about turning them into automatic sunglasses or having a go at curing your lazy eye.

Real-Time Polarimetric Imager From 1980s Tech

April 20, 2018 by Tom Nardi 14 Comments

It’s easy to dismiss decades old electronics as effectively e-waste. With the rapid advancements and plummeting prices of modern technology, most old hardware is little more than a historical curiosity at this point. For example, why would anyone purchase something as esoteric as 1980-era video production equipment in 2018? A cheap burner phone could take better images, and if you’re looking to get video in your projects you’d be better off getting a webcam or a Raspberry Pi camera module.

But occasionally the old ways of doing things offer possibilities that modern methods don’t. This fascinating white paper from [David Prutchi] describes in intricate detail how a 1982 JVC KY-1900 professional video camera purchased for $50 on eBay was turned into a polarimetric imager. The end result isn’t perfect, but considering such a device would normally carry a ~$20,000 price tag, it’s good enough that anyone looking to explore the concept of polarized video should probably get ready to open eBay in a new tab.

Likely many readers are not familiar with polarimetric imagers, it’s not exactly the kind of thing they carry at Best Buy. Put simply, it’s a device that allows the user to visualize the polarization of light in a given scene. [David] is interested in the technology as, among other things, it can be used to detect man-made materials against a natural backdrop; offering a potential method for detecting mines and other hidden explosives. He presented a fascinating talk on the subject at the 2015 Hackaday SuperConference, and DOLpi, his attempt at building a low-cost polarimetric imager with the Raspberry Pi, got him a fifth place win in that year’s Hackaday Prize.

While he got good results with his Raspberry Pi solution, it took several seconds to generate a single frame of the image. To be practical, it needed to be much faster. [David] found his solution in an unlikely place, the design of 1980’s portable video cameras. These cameras made use of a dichroic beamsplitter to separate incoming light into red, blue, and green images; and in turn, each color image was fed into a dedicated sensor by way of mirrors. By replacing the beamsplitter assembly with a new 3D printed version that integrates polarization filters, each sensor now receives an image that corresponds to 0, 45, and 90 degrees polarization.

With the modification complete, the camera now generates real-time video that shows the angle of polarization as false color. [David] notes that the color reproduction and resolution is quite poor due to the nature of 30+ year old video technology, but that overall it’s a fair trade-off for running at 30 frames per second.

In another recent project, [David] found a way to hack optics onto a consumer-level thermal imaging camera. It’s becoming abundantly clear that he’s not a big fan of leaving hardware in an unmodified state.

IPhone Polarizing Camera Solves Filter Orientation Problem Using Flash

July 4, 2016 by Jenny List 6 Comments

One of last year’s Hackaday Prize finalists was the DOLPi, [Dave Prutchi]’s polarimetric camera which used an LCD sheet from a welder’s mask placed in front of a Raspberry Pi camera. Multiple images were taken by the DOLPi at different polarizations and used to compute images designed to show the polarization of the light in each pixel and convey it to the viewer through color.

The polarizer and phototransistor taped to the iPhone.

[Dave] wrote to tip us off about [Paul Wallace]’s take on the same idea, a DOLPi-inspired polarimetric camera using an iPhone with an ingenious solution to the problem of calibrating the device to the correct polarization angle for each image that does not require any electrical connection between phone and camera hardware. [Paul]’s camera is calibrated using the iPhone’s flash. The light coming from the flash through the LCD is measured by a phototransistor and Arduino Mini which sets the LCD to the correct polarization. The whole setup is taped to the back of the iPhone, though we suspect a 3D-printed holder could be made without too many problems. He provides full details as well as code for the iPhone app that controls the camera and computes the images on his blog post.

We covered the DOLPi in detail last year as part of our 2015 Hackaday Prize finalist coverage. You can also find its page on Hackaday.io, and [Dave]’s own write-up on his blog.

Polarizing 3D Scanner Gives Amazing Results

December 23, 2015 by Al Williams 16 Comments

What if you could take a cheap 3D sensor like a Kinect and increase its effectiveness by three orders of magnitude? The Kinect is great, of course, but it does have a limited resolution. To augment this, MIT researchers are using polarized measurements to deduce 3D forms.

The Fresnel equations describe how the shape of an object changes reflected light polarization, and the researchers use the received polarization to infer the shape. The polarizing sensor is nothing more than a DSLR camera and a polarizing filter, and scanning resolution is down to 300 microns.

The problem with the Fresnel equations is that there is an ambiguity so that a single measurement of polarization doesn’t uniquely identify the shape, and the novel work here is to use information from depth sensors like Kinect to select from the alternatives.

Continue reading “Polarizing 3D Scanner Gives Amazing Results” →

Augmenting Human Vision With Polarimetric Cameras

December 15, 2015 by Brian Benchoff 30 Comments

Light is just a wave, and the wavelength of light determines its color and determines if it can cook food like microwaves, or if it can see through skin like x-rays. There’s another property of waves human’s don’t experience much: polarization, or if the light wave is going up and down, side to side, or anywhere in between.

[David Prutchi]’s project for the Hackaday Prize was like many projects – a simple, novel idea that’s easy and relatively cheap to implement. It’s a polarimetric camera meant to see what humans can’t. By seeing the world in polarized light, the DOLPi can see landmines, cancerous tissue, and air pollution using only a Raspberry Pi and a few Python scripts He gave a talk at this year’s Hackaday SuperConference about polarization cameras and the DOLPi project. After enjoying the video, join us after the break for more details.

Continue reading “Augmenting Human Vision With Polarimetric Cameras” →