Airborne Radar for Navigation?

Airborne Radar for Navigation?

Strange that airborne radar is so seldom used nowadays for its original purpose. Bob Buck, author of the perennially-popular book “Weather Flying” and one of the greatest aviators of all time, once told me how in olden days he used airborne radar to navigate from Rome down across the Adriatic Sea to Athens. Due to political activities in that part of the world back then, VORs were often out of service and NDBs were frequently not where their identifier indicated they should be.
Another time, after I’d conducted a radar training course for British West Indies Airlines down in Trinidad, my seatmate on the flight back to Miami was a deadheading Senior Captain. He congratulated me for having spent so much time in my course discussing use of airborne radar for navigation. He confided he still island-hopped with radar on his flights from Trinidad up to the States and back. He explained that, in the early days of his aviation career, ground-based nav facilities in South America were scarce and often inop, due to one revolution or another. He was forced to radar navigate just about everywhere he flew. It was a habit he still preferred in flying a Lockheed L1011.
Just why the use of airborne radar for navigation fell out of favor is something of a mystery and has resulted in GA, in particular, being in a very vulnerable situation. Interestingly, the original purpose of airborne radar was for navigation, not weather.
Airborne radar was created 75 years ago by the British. In those days, weather on radar was a hindrance to finding one’s way from Britain over to the Ruhr Valley in Germany for the purpose of dropping high explosives on it. But, following the war, radar navigating was never picked up in the civilian sector. We came out of the war with LORAN A, which eventually morphed over to LORAN C, then Doppler and Omega and finally GPS. Who needed radar mapping?
We Need A Backup
We in GA have settled on a dual GPS/VOR system, which is most unprofessional. Safety is always in triplets; two working, a third to keep those two honest. Most airlines provide the third, the backup, with some form of electronics. GA, for the most part, has no backup, in spite of the fact that GPS is extremely vulnerable to hacking.
Terrain mapping is a simple answer to the pressing need for a third system. Fortunately, George Lucchi of RCA at Van Nuys and George Church of Bendix at Teterboro, assisted by Skip Stevens, recognized that airborne radar can and should be used for ground mapping as well as weather avoidance. They saw to it that, from the beginning, “weather radars” had a MAP mode. In some cases, systems are optimized for enhanced ground mapping capability when MAP is selected. How? By providing a means for selecting a very short pulse.
Radar detects things by broadcasting pulses of microwave energy and listening for the “echo” from them. Engineers speak of those pulses in terms of “pulse widths” because of how they are displayed on his or her test equipment. Pilots, however, must think of pulse dimension in feet, because feet is how things are seen on a radar display. A pulse of one microsecond is 984 feet in length. The significance is that, because the pulse must travel out and back to measure distance, objects become “range smeared” on your radar by the pulse to half again its actual length. That is, an object that’s actually 984 feet front to back will be displayed on your radar as being 1,476 feet front to back when detected by a one-microsecond pulse.
Now consider, to achieve good range performance and weather penetration capability, radar engineers commonly string pulses together in groups. For instance, a string of four pulses is common. The result is a combined pulse length of 3,936 feet. Adding the smear, it becomes 5,904 feet. As a result, objects detected by that radar, say a thin flag pole, appear almost a mile in depth on your radar. Not good for mapping. A small river 3/4 miles wide will be smeared over totally. A single building will smear out to appear like a huge apartment complex.
To overcome that, on some radars, when MAP is selected on a short displayed range, the engineer causes the “pulse length” to drop back to only one microsecond or less. Detail is considerably enhanced. That’s why the all-time-great radar, in my thinking, is the Honeywell Primus 700/701. With it, in MAP mode, you can call up a pulse of only 0.1 microsecond – resulting in a range smear of only 150 feet! You can almost count fence posts with it.
With certain other radars, in order to get them to work at all, particularly at long range, the design engineer had to increase the pulse group to 6, 10, in one case 28! Think of what that does to range smearing. With a 28-microsecond string, the echoes from our thin little flag pole now becomes 41,000 feet front to back! A single house on that radar smears out to a huge blob, making it impossible to distinguish weather echoes from terrain objects when a long display range is selected.
For best ground mapping, the shorter the pulse the better. Before buying a new radar you should find out about the “pulse widths” it broadcasts. (Recently engineers have switched to “pulse compression” techniques, which improve terrain detail greatly; Garmin’s GWX 70 has it.)
Tweaking For Mapping
With an understanding of all that, the wonderful thing about using airborne radar to confirm the navigation situation is that it’s so simple and easy (assuming a pulse string of reasonable length). You select a displayed range of 80 to 120 nm when flying below FL290 and TILT to ground paint on the outer half of the display; above FL290, use a displayed range of about 150 nm and adjust TILT to ground paint from 80 nm outward.
Then. just lean back and watch the world roll by. Most of the time, you’ll find mapping in the WX mode works OK. For real precision, go to MAP, a displayed range of 50 or so nm, and tweak TILT and the CAL control (misnamed “GAIN” on most radars) up and down carefully. With many radars you’ll be amazed with the detail you can pull up.
It’s best to simultaneously tweak TILT and CAL. Unfortunately, with an MFD, through which most radars are controlled today, that’s next to impossible because the CAL control is time-shared with something else.
The neat thing about radar mapping is darkness or cloud doesn’t matter. And there’s the added benefit of being warned about weather that must be avoided. It’s simple; don’t try to fly into a radar shadow. A radar shadow will be a prominent black area just behind a red echo. If radar energy, traveling at the speed of light, isn’t going through that area, the extreme rain there will certainly stop you. Also, when topping weather, don’t allow any echo to intrude inside the 30-nm range, to avoid the CAT bubble of turbulence that builds just above some storms. Finally, note that when mapping with radar you get an instant notification of a radar failure. If the ground suddenly disappears or begins to jiggle, dance, change colors or anything weird like that, it’s off to the avionics shop you must take it.
So, ground mapping with your radar is a win, win, win situation. Once you’ve become skilled at radar navigation, you’ll wonder why you waited so long to get started. It’s eternal CAVU. It’s so simple, so soothing, so professional.•T&T

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