Is it possible to find an airliner nowadays with Doppler radar used for navigation purposes?
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asked Dec 27, 2017 at 9:05
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Only in military applications nowadays, primarily helicopters as the system doesn't require the alignment process of the INS system, is not subject to GPS jamming, is very accurate when flying low (as opposed to high and fast over oceans), and most helicopters don't do the occasional barrel roll (the system is belly mounted to track the ground below).
(Calm seas messed with the Doppler early-on due to the double reflection – surface and sub-surface.)
However for the airliners, as you ask, it was short-lived, but it helped usher in a new era.
TWA in c. 1962 fitted its Boeing 707 fleet with dual redundant Bendix DRA-12 Doppler systems and CNA-24A computers (pictured above). After demonstration flights to the FAA for technology proving, TWA was the first to let go of the navigator (the fourth crew member back then). It was the first time long-haul over-water flights were operated without a dedicated navigator.
The inertial navigation system in that decade was still a top military secret. But the technology and computing power were making their way into the civilian world.
First was the sub-inertial navigation system (part INS; part Doppler), such as the Sperry SGP 500 gyro platform coupled with a Decca Type 72 Doppler. It was cheaper than a pure-INS system and had improved accuracy over a pure-Doppler system.
Initially operators favored the sub-INS system, because of the much quicker alignment time. In that era, any disturbance to the aircraft – while being refueled or boarded – would add to the alignment time. That was c. 1969.
1971 – just under a decade from TWA's initiative – was when the INS first made it onto an aircraft as default equipment straight from the factory. The plane was the Boeing 747, the INS system was the Carousel IV, and the rest is history.
(Doppler navigation was also used on general aviation aircraft, and much like the VOR-based RNAV equipment, such as the King KNS 80, both yielded to the GPS.)
References:
- TWA Relies on Doppler, Flight International, 11 October 1962
- New sub-inertial navigation system, Flight International, 6 March 1969
- Carousel IV in the 747, Flight International, 1 July 1971
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2$\begingroup$ Though just for what (little?) it's worth, it's not RADAR, but most GPS receivers determine your current speed using the Doppler effect. $\endgroup$ Commented Dec 28, 2017 at 6:59
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$\begingroup$ According to Wikipedia it's calm seas that provided insufficient returns to accurately determine velocity. $\endgroup$ Commented Mar 20, 2018 at 16:58
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$\begingroup$ @JerryCoffin GPS receivers do not use the doppler effect to compute the speed of the receiver as the satellites are (usually) going to be moving many orders of magnitude faster than the receiver. The receiver does has to compensate for the doppler effect as it induces a frequency shift; the receiver has to independently track the carrier frequency and code offset of each satellite, but only the code offset is used in determining the position. Ultimately the speed of the receiver is computed by observing the change in the computed position over time. $\endgroup$ Commented Oct 3, 2019 at 4:01
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$\begingroup$ @alex.forencich: That was (at least partly) true a decade ago or so (but even then, mostly of low-end receivers, not ones likely to be used for aircraft navigation). I doubt it's true of even low-end receivers any more though. Doppler data is faster to obtain and more accurate. Here are a few links giving more details: insidegnss.com/wp-content/uploads/2018/01/…, insidegnss.com/how-does-a-gnss-receiver-estimate-velocity, pdfs.semanticscholar.org/fda5/…, $\endgroup$ Commented Oct 3, 2019 at 5:24
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$\begingroup$ @JerryCoffin Interesting! Never knew that was a thing. I guess that shouldn't be too surprising; with the amount of processing power available these days, just pull in every possible piece of information and throw it into a kalman filter to improve the positioning accuracy. $\endgroup$ Commented Oct 3, 2019 at 5:37