WHAT IS EPE?
Quote follows from Garmin Engineering
Joe,
The EPE is an estimation based upon the information the receiver
can determine. SA consists of artificial clock errors and
artificial ephemeral errors. Both of these effects, as well as
atmospheric effects, can result in a positional area of uncer-
tainty, which can be measured and will add to the receivers EPE.
Bias errors cannot be measured and will typically not be detected
in the EPE calculation.
The 12XL will typically have a better EPE than other units due to
the 12 channel correlator and the use of all tracked satellites
in the positional computation.
EPE is an estimation, rather than a measurement, but all
measurable factors are used in the estimation algorithm. We
consider the details of our EPE and FOM calculations proprietary.
We calculate EPE our own way. URE and HDOP are definitely
significant factors in the calculation. We calculate an over-
determined solution, and fully understand the characteristics of
SA, and are able (in our opinion) to provide for a better
estimate of current position error than the simplistic calcula-
tions will indicate.
Many folks have and will demand to know our specific calcula-
tions, but we consider these to be proprietary and we do not
release the specific formulas. This is similar in our FOM
calculation, we use a lot of finesse in our software which other
manufacturers have not been able to duplicate. This is further
evidenced by Dr. Wilson's reports on our accuracy compared to
other receivers. If the tests were performed, I believe you would
see closer correlation between our EPE values and actual errors,
as compared to other manufacturers units.
Garmin International
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One experimenter has found that about 2/3 of the time, the
ACTUAL error is less than the EPE readout on a Garmin 45 GPS unit
tested. Your figures may vary. So.. EPE really is an estimate
not a guarantee or a measurement.
Joe Mehaffey
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Sam Wormley offers the following additional information on EPE.
Garmin does not give an explanation for the EPE calculation
displayed on a number of their handheld receivers. EPE is an
acronym for "Estimated Position Error". See page 18 of the GPS
12XL Owner's Manual and Reference. The behavior of EPE is not
identical on all of Garmin's receivers... some reflect the dgps
mode and others do not.
A number of sources indicate the error in position can be
estimated by the simple calculation DOP * URA with the former
calculated in the GPS receiver from the geometry of the satel-
lites used in the position solution and the later derived and
computed from the GPS message transmitted by each satellite.
DOP - Dilution of Precision - TDOP, PDOP, HDOP, VDOP, etc. are,
in effect, multipliers that, when used with some measure of
positioning statistic, can give you a realistic expectation of
probability of error.
URA - User Range Accuracy - this is a quantity that is transmit-
ted in the navigation message that is the predicted (not
measured) statistical ranging accuracy. Since it is defined for
SPS (Standard Positioning Service), it includes SA.
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Bill Stone wrote [March 1997]:
What does Garmin mean by EPE?
EPE = HDOP * URA (1-sigma) would give a measure of the 68%
confidence circle, i.e., 68% of your position fixes would fall
within and 32% would fall outside.
EPE = HDOP * URA (1-sigma) * 2 gives a measure of the 95%
confidence circle, i.e., 95% of your position fixes would fall
within and 5% would fall outside.
EPE = HDOP * URA (1-sigma) * 0.73 gives a measure of the 50%
confidence circle, i.e., 50% of your position fixes would fall
within and 50% would fall outside. This is also known in the
literature as CEP (Circular Error Probable).
This latter expression is what several of you have indicated that
you think Garmin's EPE calculation is for the Garmin 12XL. There
may also be an "EPE filter" to minimize the EPE figure from
jumping about to much.
There should be a "reasonable correlation" between HDOP and outer
radial position from a known geodetic point (could be determined
over a long period of time by statisical averaging). I use the
word "reasonable" because the comparison involves the points
wandering around mostly within a circle of confidence, whereas
HDOP will relate to the radius of that circle of confidence.
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To further complicate the analysis, witout an absolutely clear
view of the sky, you can expect the HDOP to take jumps as
satellites are obstructed from view. This might not be a bad
idea to collect data under conditions less than desireable. John
Franke reports:
"It is interesting to see what happens under certain
alinements. I recently had a case where the receiver (6
channel, parallel) was tracking 4 satellites but was doing
2D NAV and the HDOP was 32. A quick look at the satellite
position screen showed the 4 tracked satellites to be in a
straight line. There were other satellites available but
the view to them was blocked by buildings. So, knowing the
orbits repeat but are several minutes earlier each day, I
watched the next day to see the alinement develop and the
PDOP change. In less than twenty minutes the receiver went
from 3D NAV with a PDOP of 2.3 to 2D NAV and a HDOP of 30
back to 3D NAV and PDOP of 2.4! Quite impressive and
educational to observe".
The NMEA sentence $GPGGA recorded periodically over at least a
twenty-four-hour period may provide enough data to draw a
conclusion. This is a good experiment for all to try who have
the capability to capture data.... you will learn much.
Many of you may have better approaches than mine to gather and
analyse data to resolve the EPE issue.... and, of course it goes
without saying, we would hold Garmin in high[er] esteem if Garmin
would give a complete technical discription of the calculation.
And... if that technical language is over the typical readership
heads, I'm sure there are many qualified to translate.
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Sam Wormley - http://www.cnde.iastate.edu/gps.html