Taking the Raspberry Pi over hill and dale

Using the Raspberry Pi as a Navigation Device

When equipped with a battery power pack, cable mouse, a foldable Bluetooth keyboard, and a composite display screen, the Raspberry Pi notebook is almost perfect. Figure 5 shows the prototype of an autarkic navigation device. The Raspberry Pi at the bottom on the right serves up the monitor, a mouse, and also a GPS receiver with its USB connections. The median power consumption with this setup stays under 800mA.

Figure 5: Using the Raspberry Pi as a simple navigation device.

Ordinarily, FoxtrotGPS loads maps for streets and roadways from online sources. If desired, the program saves excerpts of maps as grid images, so they can later be called offline, as is done here. The program Navit [1] offers a more elegant solution for the map issue. It works with vector maps. These require much less memory than pixel-based graphics.

Using the Raspberry Pi as a Flight Tracker

Airplanes send continuous status signals over the 1090MHz frequency band. A DVB-T USB stick can receive these ADS-B signals with software that has been modified for this purpose. I introduced the relevant software program, Dump1090, in some detail in the previous issue [2]. The map displayed on the screen retrieves the application from the Internet. If WiFi is not available, then a mobile phone can be used as a hotspot. Alternatively, you can make do with the status information shown in the terminal window even in the absence of an Internet connection.

Dump1090 does not optimize the output for small screens. Modifications can be made with HTML or JavaScript source code. The same goes for distance input, since it disregards altitude. When an airport is nearby, it does not matter whether an approaching airplane is just a few kilometers away or whether the plane is flying at an altitude of 10km.

In this project, I confined myself to the public_html/config.js settings in the configuration file. The following variables:

SiteLat = 51.0
SiteLon = 7.0

specify the observation point for the map and the calculation for the distance. Here this is 51 degrees latitude and 7 degrees longitude. The variable:

SiteCirclesDistances = new Array(100,150,200,300)

displays circles at a distance of 100km, 150km, 200km, and 300km from this point (SiteLat/SiteLon). However, these circles are barely visible in Figure 6.

Figure 6: The ADS-B control screen for the Dump1090 application shows air traffic within the radius.

The external antenna that comes with the DVB-T stick does work, but you will usually get much better results with an antenna that has better reception. A coaxial collinear (CoCo) antenna offers a good solution, and you can assemble one out of some simple hardware. In about 30 minutes, you can build one using an antenna cable and a suitable plug.

In order for the antenna cable to work as an antenna, you will need to slit it open at intervals of a half-wavelength. The focus of the antenna in a level setting will become stronger as the number of slits increases. During testing, I used a five-element antenna with an increase in strength from 3dB to 6dB. However, when more than 10 elements are present, the loss in performance becomes excessive.

Instead of going to the trouble of cutting away the shielding, it is better to cut individual antenna parts as shown in Figure 7. Then you can pull the center wire to the shielding of the adjacent piece and stabilize the construction with a small thin rod (Figure 8). The resulting wiring places the contact bridge at a distance of half of a wavelength (11cm) via the center lead and another half-wavelength through the shielding. The same goes for the other contact, but in reverse order.

Figure 7: An 11cm element of a CoCo antenna, which I cut from an antenna cable.
Figure 8: Here I have combined five elements into a CoCo antenna and connected them to the DVB-T stick.

You should be careful when putting the antenna together that you do not place the elements too close to one another. If you do, then the shieldings may touch, and the antenna shorts out. The best approach is to use an ohmmeter to measure the resistance between the center lead and the shielding so that there is bypass between the two leads. Your little DIY antenna can be plugged in with an F plug, which is a mass-produced component normally used for building a TV antenna. Attaching a suitable adapter (Figure 9) between the F plug and the MCX adapter lets you secure the antenna to the DVB-T stick.

Figure 9: An adapter lets you connect the antenna with the DVB-T receiver.

My DIY antenna was a success during testing. I was able to achieve a five-fold increase in the number of airplanes that appeared on the screen. Additionally, I was able to track the planes up to a distance of 240km (Figure 10). Depending on the location, however, the antenna only had clear visibility towards the south.

Figure 10: The five-element CoCo antenna receives signal from airplanes that fly at a distance of up to 250km.

From a ground location, it is possible to observe planes flying at an altitude of 10km in a distance of 360km. Once further away, the planes disappear over the horizon. However, you are already only two degrees over the horizon at a distance of 200km. Therefore, if you want to see planes that fly beyond this point, you need an undisturbed view to the horizon.

Thanks to the mobile solution with the Raspberry Pi, there are no longer any problems in finding an optimal observation point. The DIY antenna can be put together and connected to the TV dongle quickly. The antenna's lead cable should be as short as possible and the DVB-T receiver should be close to the antenna. You have more leeway with the USB cable between the receiver and the Raspberry Pi. It is perfectly okay for this cable to run for a few meters between the connected devices.

The transmission direction is dependent on the level where the antenna sits perpendicular. Therefore, it is theoretically possible for the antenna to not receive any signals from objects flying directly overhead. Practically speaking, because reflections and other disturbances, this does not occur.

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