FMC POS REF update

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Pressing R6 POS REF on the Progress page displays the POS REF page.

On my current flight, the FMC (GPS) position is shown with the R1 ARM key displayed. The Inertial position is shown as 4.01 nm different (from what?), and the GPS and RADIO positions different by 0.07nm and 0.03 nm respectively. The last line shows the Required Navigational Performance as 2.00 nm and the Actual Navigational Performance as 0.06 nm. The VOR DME at R6 shows PLR.

Can someone explain the meanings of the items on this page and what is the meaning of 'update', particularly as applied to inertial reference position?

Any help is much appreciated.

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1 hour ago, Richard McDonald Woods said:

Can someone explain the meanings of the items on this page and what is the meaning of 'update', particularly as applied to inertial reference position?

You'd do well to do a little research into the old CIVA INS to understand the basic concepts of INS, but I'll give a high level overview:

We have three sources here:

1. VOR/VOR, VOR/DME, DME/DME - "Radio"
2. INS
3. GPS

Viewed independently:

1. I can determine my position by finding a VOR radial, and another VOR radial (VOR/VOR), crossing them, and using the Lat/Lon of both VORs, triangulate my position as a Lat/Lon (DME/DME is similar). I can also find a VOR radial, and my DME distance out on it and form a Lat/Lon out of it. This is reasonably accurate, provided you stay in range of radio-based navaids.
2. INS starts out accurate and then degrades over time, because for every position after the initial known position, you're at an assumed position based on the INS picking up subtle movements. In other words, I start from known point 0,0, and move east for 1 mile. I am now at 0,1, but the sensor's accuracy is slightly off, so I'm actually at 0.000001,0.9 (not as far east, and ever so slightly north of my INS/assumed position. For this reason, you can update the INS with a new "known" position determined by either radio or GPS.

Part of the reason we still have INS in the realm of GPS (the latter not being subject to this "drift") is that INS information is immediate, however, because we're using instruments that pick up movement, rather than calculate movement. In other words, INS "sees" the various accelerated forces on all axes - rate of turn, speed-by-accumulated-accel ("I accelerated at 1m/s/s for 30s, so I'm at 30m/s"), and so on. It's what drives the PFDs (and is why there's the reversionary ATT mode on the old INS selectors). GPS data can do the same if you have enough sats in view, but there would be lag, because it needs the current position and at least one "past" data point to calculate that. In other words, INS is immediate data based on accelerations right now, and GPS is delayed data based on current plus n-1 data points.
3. In order to get a Lat/Lon, we use a satellite constellation to calculate (triangulate) a position. This is more accurate than the basic radio method because there are more points to check against, with much better coverage. There's more to it than that, but that's the high level version.

Why all of them? Redundancy, but also accuracy. Radio alone is kind of weak. INS + Radio gives you better accuracy. Adding in GPS makes it more accurate. The mix of all of that is the FMC Position, and the difference between that mixed position and the other sources (individual or mixed, as shown on the page - it's the mixed position unless the line says Left/Center/Right or L/C/R on the line itself) is that referenced difference. So a difference of 4.1 on INS means the INS thinks you're in a spot 4.1nm away from the mixed spot.

Edited by scandinavian13
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Hi Kyle,

Many thanks for your usual concise explanation.

I shall look up more on INS.

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