Estimated Position Error.....

[gearpins]

Hello every one ,
Reading my books on navigation and trying to figure out how did they (whoever they are) arrive at a figure of 0.28nm as a threshold for estimate position error ???
can anyone help?
thanks,
:confused:

[gearpins]

anyone please?:)

[Genghis the Engineer]

You aren't giving us much to work on!

In what circumstances does the author of the book give the value? Position measured how? - DR?/RNAV?/INS?, how long into flight?, etc.

G

[gearpins]

OK here is an extract from FCOM A320

CURRENT NAV MODE
EPE (RATE or THRESHOLD)
REMARK


IRS/GPS

(FOM² + 100²) in meters

FOM = Figure of Merit of GPS
If above 0.28 NM the GPS position is rejected.


IRS/DME/DME

Tends towards 0.28 NM

EPE decreases from initial value to 0.28 Nm.


IRS/VOR/DME

0.1 NM + 0.05 X DME DIST minimum : 0.28 NM

EPE increases or decreases as the distance between the a/c and the VOR/DME.


IRS ONLY

+ 8 NM/h for the first 21 min.
+ 2 NM/h after
EPE increases continuously




sorry once again for the lack of clarity before

[Genghis the Engineer]

Is that not saying that if the error is greater than 0.28nm it rejects it, it doesn't look like a statement of error calculation to me.

G

[FE Hoppy]

Looks like Airbus don't want an epe less than .28 uless GPS is valid and used.

May well have something to do with the implamentation of RNP when the system was designed. As RNP approaches at one time also specified the nav mode eg. RNAV(GNSS)

B-RNAV = RNP 5
P-RNAV = RNP 1

Default Approach value on our brazilian/honeywell equipment is RNP.3

As RNP is stll evolving I woudn't spend too much time worrying as by the time you've figured it out the game may have changed.

[gearpins]

All I am trying to do is figure why is the value 0.28?? and not say 2 or some other convenient number. I am sure its for a good reason. just trying to get to the history behind it...
Any Navigation guru out there..??:)

[FlightDetent]

I think you misinterpret the information given. On the very next FCOM page, there is some explanatory text and regulatory requirements to which the aircraft had been certified.

ENR 2 NM, Terminal 1 NM and APCH 0,5 NM (0,3 with GPS).

Aircraft itself estimates the probable positon error (based on available data sources and time from initialization / last update). This EPE value is evaluated against the certification requirements and pilot is warned whenever accuracy limit is not satisfied.

What you quote is a very in-depth knowledge on how exactly is the EPE calculated.

For instance the IRS mode with DME/DME updating the EPE may be as low as 0,28 NM. Essentially the manufacturer states that 0,28 NM is the best achievable precision in IRS/DME/DME mode. Why not 2 NM? Beacuse their equipment is much better than that. Why not 0,1 NM? I bet they wish but IRS/DME/DME installation just cannot do it.

[gearpins]

flt detent, thanks, that cleared it up a bit for me.
I was wondering if .28 nm was also a regulatory requirement?:)