Cockpit of a Hawker Siddeley Trident with the moving map display

A Live Map Display In A 1960s Airliner

We tend take GPS navigation for granted these days, so it’s easy to forget that it became only available in the last few decades. Aviation navigation used to be significantly more challenging, so how was the Hawker Siddeley Trident, a 1960s airliner, fitted with a live updating map display? In a fascinating dive into aviation history the British Airliner Collection has spun up an insightful article on the magic behind these moving map displays.

Without access to satellite navigation or advanced electronics, engineers had to get creative. Enter the Trident’s moving map display, a marvel of ingenuity that predated the GPS systems. Using a combination of Doppler radar and some clever mechanics, pilots could accurately determine their position without relying on any external signals.

The system makes use of four Doppler radar beams, arranged in what was known as the Janus array. This configuration corrected for errors caused by changes in altitude or wind drift, ensuring accurate ground speed readings. The movable antennas mounted under the cabin floor could adjust its orientation to maintain alignment with the actual direction of travel, calculating drift angle precisely. Combined with compass information and flight time from a known start point to to indicate the current position with a pointer on a rolled paper map. The system was well ahead of it’s time, and significantly easier to use and more accurate than the Decca radio navigation system in use at the time.

It’s mind boggling to see the solutions engineers came up with without much of the digital technology we take for granted today. Gyroscopes for inertial navigation, the cavity magnetron for radar and radial engines were all building blocks for modern aviation.

Thanks for the tip [poiuyt]!

A Doppler Radar Module From First Principles

If you’ve ever cast your eyes towards experimenting with microwave frequencies it’s likely that one of your first ports of call was a cheaply-available Doppler radar module. These devices usually operate in the 10 GHz band, and the older ones used a pair of die-cast waveguide cavities while the newer ones use a dielectric resonator and oscillator on a PCB. If you have made your own then you are part of a very select group indeed, as is [Reed Foster] and his two friends who made a Doppler radar module their final project for MIT’s 6.013 Applications of Electromagnetics course.

Their module runs at 2.4 GHz and makes extensive use of the notoriously dark art of PCB striplines, and their write-up offers a fascinating glimpse into the world of this type of design. We see their coupler and mixer prototypes before they combined all parts of the system into a single PCB, and we follow their minor disasters as their original aim of a frequency modulated CW radar is downgraded to a Doppler design. If you’ve never worked with this type of circuitry before than it makes for an interesting read.

We’ve shown you a variety of commercial Doppler modules over the years, of which this teardown is a representative example.

Hot Wheels Toy Turned Radar Gun


[Ken] was strolling through a department store one day looking for a gift for his daughter when he stumbled across a Mattel’s Hot Wheels Radar Gun for $30. He purchased it, took it home, and tested it out. Surprisingly, the device had the ability to not only scan toy cars, but also regular size vehicles, spinning bicycle wheels, and joggers as well. As his mind began to churn coming up with new ideas, he purchased another toy and repackaged it creating a more professional grade DIY radar speed detector.

The process was pretty simple. First, he disassembled the device getting to the Doppler radar system inside, which was similar to the professional radar guns that police officers used. This toy was able to transmit a continuous wave at 10.525GHz, measuring the returning frequency of returning waves that bounced off of moving objects. However, the detection range of this toy was severely limited. [Ken] then upgraded the antenna housing unit with a 3″ diameter acrylic document tube, making the quality look a lot better. After that, the system was attached to a tripod allowing for the device to be easily transported and setup near a busy traffic road, quietly watching the speed of cars driving by.

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