Review Article

Locating Payloads Using Doppler Shift in the Signal of a Backup CW Beacon

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Abstract

There are a number of devices for locating high altitude payloads on their return to earth, including APRS, Iridium modems, SPOT® and custom radio transmitters. Such units can transmit the exact location, which is ideal. Having an accurate location ahead of time will often influence the direction of approach for recovery, especially if it is near obstacles such as a marsh, river or stream that is difficult to cross. Unfortunately, every device has some probability of failure due to a variety of causes, everything from component failure, loss of satellite lock or radio reception to human errors such as forgetting to charge or change batteries. Including a second independent location device as a backup can greatly increase the odds of locating the payloads, but it can also double the required weight. A simple, lightweight, nearly foolproof, continuous wave (CW) beacon can serve as an attractive backup. These units transmit a simple beep heard on the radio. The transmitter is typically located using a directional antenna by comparing signal strength in different directions from different locations. This technique can be tricky in practice, as real-world conditions create reflections making it difficult to identify the actual source direction. Furthermore, the signal works line-of-sight, so a payload on the ground might only be detected up to a half mile away in some cases, requiring a good rough guess of location to even get started. If one has access to an aircraft, the signal can usually be received up to 20 or more miles away from the air, but narrowing the location from an airplane can still be tricky using a directional antenna and signal strength. This paper describes a technique we have developed for quickly and accurately locating a CW beacon using the Doppler shift heard in the received signal. Using a continuous wave (CW) beacon of 433 MHz, an aircraft traveling directly toward the source at 100 knots will experience a shift in the signal to 433,000,074 Hz. This 74 Hz change in the radio frequency will result in a 74 Hz shift in the audio frequency heard in a CW receiver. The audio signal will therefore vary between +/- 74Hz of a center frequency depending on the direction of travel relative to the beacon. Our experience has shown that we can narrow down the location of the transmitter much more quickly and accurately by listening to the Doppler shift in the received signal than we can using a directional antenna and signal strength. To aid in this process, we have developed a system based on a Raspberry Pi® that will listen to the Doppler-shifted beacon combined with the GPS track to automatically compute the location fairly precisely. While we have currently experimented in the use of this system in an aircraft, the technique also has some feasibility using an automobile should the beacon land in proximity of suitable roads, and we are also developing it for deployment on a quad-copter.

Keywords: Payload recover, CW beacon, Direction Finding, Doppler shift

How to Cite:

Eason, R. & Patton, J., (2021) “Locating Payloads Using Doppler Shift in the Signal of a Backup CW Beacon”, Journal of High Altitude Ballooning 1(1).