GPS jamming becoming a real problem
#21
If your ANP rapidly rises, the IRU’s might have been initialized with a bad position. I don’t know about the other Boeings, but on the 737 it displays a “last position”. Some pilots were using that to initialize the ADIRUs (and some instructors were teaching that). The problem with doing it that way is the last position shown isn’t where you are at the gate, it’s where the airplane transitioned to ground mode on the previous flight. If I had to guess, I would say it is where the jet slowed to about 80 knots or so on the previous landing. You can pull out the 10-9 and use the lat/long tick marks in the margin to see it for yourself. For most airports it isn’t a big deal, but in LAS or ORD you have just told the IRU’s that they are over a mile from where they actually are. Now you lose GPS and suddenly you have an ANP problem. Use GPS position to initialize the IRU’s or if that isn’t working, use the gate coordinates.
#22
Gets Weekends Off
Joined APC: Mar 2005
Posts: 1,888
I had one years ago where the variation would be shifted dramatically. Say from E3 to W30. Your location would be accurate over CPs and destination, but not enroute. Because of this, we would not get a GPS caution as it didn't know there was a problem. Took us a few flights to figure out what was going on.
#23
If your ANP rapidly rises, the IRU’s might have been initialized with a bad position. I don’t know about the other Boeings, but on the 737 it displays a “last position”. Some pilots were using that to initialize the ADIRUs (and some instructors were teaching that). The problem with doing it that way is the last position shown isn’t where you are at the gate, it’s where the airplane transitioned to ground mode on the previous flight. If I had to guess, I would say it is where the jet slowed to about 80 knots or so on the previous landing. You can pull out the 10-9 and use the lat/long tick marks in the margin to see it for yourself. For most airports it isn’t a big deal, but in LAS or ORD you have just told the IRU’s that they are over a mile from where they actually are. Now you lose GPS and suddenly you have an ANP problem. Use GPS position to initialize the IRU’s or if that isn’t working, use the gate coordinates.
#26
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,089
If your ANP rapidly rises, the IRU’s might have been initialized with a bad position. I don’t know about the other Boeings, but on the 737 it displays a “last position”. Some pilots were using that to initialize the ADIRUs (and some instructors were teaching that). The problem with doing it that way is the last position shown isn’t where you are at the gate, it’s where the airplane transitioned to ground mode on the previous flight. If I had to guess, I would say it is where the jet slowed to about 80 knots or so on the previous landing. You can pull out the 10-9 and use the lat/long tick marks in the margin to see it for yourself. For most airports it isn’t a big deal, but in LAS or ORD you have just told the IRU’s that they are over a mile from where they actually are. Now you lose GPS and suddenly you have an ANP problem. Use GPS position to initialize the IRU’s or if that isn’t working, use the gate coordinates.
One should verify what the former residual IRU performance was, if available, before bringing the system up, however, to get an idea of which unit might be the most efficient in the event of a fail down and a need to rely on one without other nav input. Again, just good airmanship to now which unit shows the strongest and most accurate, before starting the flight. I very seldom see anybody do this.
It's a bit like not verifying the fuel upload. I see most look at the fuel indication and if it matches what's wanted, the crew is happy. It seems to be a lost basic that one should look at the previous ending fuel, fuel upload, add them and note any discrepancy that isn't explained by APU burn, etc. Simply things, but important, and should be done every time.
As for jamming issues: while there may come a time when ground based navaids or gone, that time is not today. I've never been an advocate of an all-eggs-in-one-basket approach, and am not today. There are locations, of course, that rely only on RNAV and GPS for approaches, arrivals, departures, etc, but when operating IFR to such locations, good airmanship dictates having options that do use ground based navaids, including alternates in mind. It's been mentioned that these concerns are chiefly for general aviation aircraft. Again, good airmanship dictates having a plan B.
It's possible to paint one's self into a box. Don't do that.
I also fly general aviation, sometimes in aircraft with no nav systems; it's either charts or a handheld GPS (and charts); with loss of GPS, then it's pilotage and dead reckoning.
The latter two still work. If one insists on going IFR, and has only RNAV capability, then we're back to good airmanship, which means having a plan to get to VMC and to fly visually.
#27
I believe the FMS automatically updates the IRS with the runway position when you hit the TOGA button on a Boeing. Only concern with putting in exact coordinates for alignment is if your thrust management computer or auto throttles are inop. Otherwise last position or just the airport coordinates works just fine.
#28
Banned
Joined APC: Oct 2008
Position: Window Seat
Posts: 1,430
#29
If you're oceanic, on INS only, then ANP should very gradually increase over time without GPS. Same for a hypothetical total jamming of all sat and ground based nav over land.
Most of the airplanes we fly used to do RNAV all day long without GPS, and many of them did oceanic with INS only a few decades ago. It shouldn't all just go to poop if you lose GPS.
#30
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,089
Fly better equipment, or learn to properly initialize before you leave the gate.
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