Broadband radar frequency question

To the editor: In Chuck Husick’s recent article on broadband radar, he had a sidebar (“Using FM continuous wave to measure range,” May/June 2009) that gives this example of frequency-shift ranging:

“(If) the modulating frequency varied from 1 Hz to 100 Hz during the time it takes the transmitter’s signal to travel 1,000 feet and the measured frequency of the signal reflected from the target was 50 Hz, we would know that the target was located 500 feet from the transmitter.”

Shouldn’t that be 250 feet from the transmitter?

The signal can only travel half of the total distance out, then it has to be reflected back and travel the other half of the total distance as a return signal, right? So if the radar system measures that the signal traveled a total distance of 500 feet, it has to have been reflected from a surface 250 feet away?

Maybe I’m being too fussy, but after a career in cartography I tend not to merely split hairs when it seems important — but to slice them and make dandruff sandwiches. And as a person who is planning to sail beyond the sunset one day, I’m really impressed with FMCW radar. Thanks for an excellent article and an excellent magazine.

—Richard Westlake lives in Rockville, Maryland.

Contributing editor Chuck Husick responds: Thanks for your thoughtful question about the frequency of the signal reflected from a target and processed by the BR24 radar.

The modulation frequency detected when the returning energy reaches the receiving antenna will be equal to the frequency that was present when the signal reached the target. In the example, the modulating frequency was 50 Hz when the transmitted energy reached the target. That frequency will be present in the returning signal and will not change as a function of distance. It would be possible to locate a receiving antenna at some distance from the transmitting antenna, for example a mile beyond the transmitter, and still detect the returning signal and determine the distance from the transmitter to the target. This characteristic is one of the reasons the BR24 can detect targets with great accuracy while using very low power. In short, the only concern is the modulating frequency that existed when the target was illuminated; the distance back to the receiver does not alter the information being used to determine range.

By Ocean Navigator