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GPS Accuracy Considerations
Before we address the accuracy of Ashtech/Spectra
receivers, lets get some definitions and basics out of the way.
If your in a hurry to compare GPS receivers:
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the Ashtech/Spectra Precision GPS Accuracy page.
Let's be honest, GPS
Salespeople are kind of like the stereotypical used car salesperson.
And we kind of pull a fast one when we talk about GPS accuracy:
GPS receivers are specified under good/great
conditions. Open sky, 5 or more SV's (Satellite Vehicles), low
PDOP, no reflections. But lets face it, you won't always be
working in great conditions. You will be in urban canyons, under trees, trying
to work through periods of very low SV counts.
To make matters worse, we use confusing terms,
change the terms and play the one-up game so we can better
compete with our competitors.
Read on to learn about GPS accuracy, common
pitfalls and GPS techniques.
When we quote GPS accuracy for GIS real time
measurements, we usually quote CEP (Circular Error Probability). CEP is
defined as the radius of a circle centered on the true value that
contains 50% of the actual GPS measurements. So a receiver with 1
meter CEP accuracy will be within one meter of the true measurement
50% of the time. The other 50% of the time the measurement will be
in error by more than one meter.
Note that it is probable that CEP
measurements of the same point on the ground will differ by twice
the probability. For example if a receiver has CEP of 1 meter, then
it is probable that measurements of the same point will differ by 2
When you look at the accuracy specifications for a
consumer GPS receiver, or the MobileMapper CX in real-time, WAAS
corrected mode you will get an accuracy statement like “Real-Time
Accuracy <1 Meter CEP”.
This means that the MMCX with WAAS corrections,
will be within 1 meter of the true ITRF 2000 (reference frame)
coordinate, 50% (half) of the time. Assuming (of course) that the
receiver has a clear view of the sky, there are 5 or more satellites
visible and the PDOP is reasonable.
Remember that we are quoting horizontal accuracy;
the vertical accuracy will be 2-3 times worse.
DRMS (also called RMS, 1Sigma)
The square root of the average of the squared
horizontal position errors with 65% probability.
Twice the DRMS accuracy, with a 95%
For example, the ProMark 3 receiver in static
operation has a horizontal accuracy rating of 0.005 meters 2DRMS
95%. That means we expect the receiver to deliver 5 millimeter
accurate measurements 95% of the time (when used within the
The radius of a circle centered at the true
position, containing the position estimate with probability of 95%.
To a first order, you can approximate one error
estimate from another.
1 meter CEP = 1.2 meters RMS =
2.1 meters R95 = 2.4
= 1 and that VDOP / HDOP = 1.9 and PDOP / HDOP = 2.1)
A great example of accuracy slight of hand can be
found when we compare the MobileMapper Pro to the MobileMapper CX in
real-time WAAS corrected operation. The MobileMapper Pro accuracy is
specified as 3.0 meters 2DRMS. The MMCX accuracy is specified as 1
It sounds like the CX is a lot more accurate than
the MMPro, right?
Well not a lot more: the MMCX is 2.4 meters 2DRMS,
nearly the same as the MobileMapper Pro.
Baseline and PPM
Survey receivers include static and RTK
specifications. The datasheet for the PM3 Survey Static mode looks
So what is up with the “1 ppm” (1
part-per-million) and "2 pm" on the Horizontal and Vertical specs?
Baseline is the distance from the rover to the
reference station. If you set your reference base up on the
southeast corner of a section and occupy the northwest corner with
your rover the baseline is 1.4 miles or 2,250 meters.
Assuming that our static occupation is long enough
to fix (typically 5-15 minutes) and meets the conditions of footnote
3 (5 or more SV’s, SBAS, reasonable multi-path) we would expect our
measurement to have an rms (that is root-mean-square 1-sigma)
0.005 meters + 1E-6 * 2,250 = 0.00725
meters (7.2 millimeters)
1 part-per-million of baseline is pretty standard
for most GPS receivers so just remember:
1 KM -> 0.001 meters (1
10 KM -> 0.010 meters (1 centimeter)
100KM -> 0.100 meters (10 centimeters)
Typically GPS receivers are always specified with
vertical specifications that are twice as bad as the horizontal.
Depending upon satellite geometry (where the satellite vehicles are
in the sky) the vertical error may actually be less than the
horizontal. But in general just remember that the Vertical accuracy
is going to be half as good as the horizontal.
Multiple Baseline Solutions
It bears mentioning that single frequency (L1) GPS
measurements start to fall apart for baselines longer than 15 KM
(about 9 miles). This rule holds true for ‘Single Baseline
Solutions’, which are solutions where there is one reference base
and the rover.
It is possible to post process L1 static data
against multiple reference stations (like CORS stations) in a
network solution (sometimes called a multiple baseline solution). A
multiple baseline solution benefits from software adjustment based
on known vector lengths between the CORS stations. In a well
designed network, with long occupations, one might expect to get 3
cm solutions, even with 400 mile vectors between the reference
Ashtech's GNSS Solutions program has just
introduced (in version 3.10) a new VRS function that constructs a
'Virtual Reference Station' from three nearby physical reference
stations. This technique allows static and Stop & Go occupations to
fix very quickly because the baseline is very short.
Not only can a good GPS salesperson put a spin on
Accuracy, but we can pull some real slight-of-hand during a demo. I
can take two receivers that are exactly the same and collect data
with both of them, at the exact same time and get results that are
30 times worse on one vs. the other!
And you won’t be able to tell how I did it.
(Unless you read the next few paragraphs.)
How to make a receiver inaccurate:
1. Block the receiver’s view of the sky:
a. Good =
hold receiver away from your chest, with your back to the
North at arms length. The GPS antenna can see every
b. Bad = hold
receiver under my belly, with my back to the South. The GPS
antenna is blocked by my body for most satellites. Actually,
all you may need to do is casually pass your hand over the
top of the antenna once every five minutes and the receiver
should fail to ever fix ambiguities.
2. Let the receiver fix:
a. Good =
Keep the GPS antenna continuously oriented so that the
antenna ALWAYS has a clear view of four or more SV’s.
b. Bad = Once
every five minutes, make sure the antenna looses sight of
all SV’s to force dump the GPS’s lock. All you need to do is
point the GPS towards your stomach and lean over it, then
take a step and pull it out.
3. Hold the receiver still for 3 epochs before
logging a feature:
a. Good =
wait five seconds after the receiver stops moving before
logging a new point. Log at least 15 seconds over the point.
Continue to hold the receiver at the point for another
second after ending the shot.
b. Bad = move
to a new position and immediately log a point. Or even
worse, don’t stop while the point is taken. Just keep
4. Use the correct reference frame:
a. Good =
when using WAAS corrections, you are getting ITRF2000.
Convert your NAD83 coordinates to ITRF and compare ITRF
positions against ITRF positions.
b. Bad =
compare WAAS corrections against NAD83 published
coordinates. There is an excellent 7-point FAQ on Optimizing
Single Frequency accuracy at:
Don’t’ Ignore Reference Frame When using WAAS
corrected GPS to achieve sub-meter real-time, the corrections are
framed to ITRF00, not NAD83. While ITRF00 is a better datum in all
respects, NAD83 is preferred by most all applications in the United
States. There typically is about 1 – 2 meters difference between
these frames. (See
http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml for the
translation program) Reference Frame mismatch will appear as a
consistent offset to users and is often misconstrued as receiver
If you don't understand Reference Frames, try an
internet search on "reference frame geodetic" for plenty of great
Some Common Issues that Ruin GPS Measurements and Projects
GPS receivers don't work under trees. GPS antenna needs to be
above your head/body. GPS is difficult in urban canyons. If you
want to work in difficult conditions, get PM500's with GLONASS.
More satellites = more easy = faster = better.
Holding receiver with antenna pointed towards the ground (or
some direction other than up.) Consider using external antenna.
Holding the receiver directly next to a pipe or pole (or under a
tree) when measuring position. Consider measuring with offset.
Not allowing enough time for position to fix or init while
running static or Stop and Go shots.
Wrong Reference Frame. Mismatched epoch dates. Wrong datum
(NAD83 vs. NAD27). (See above.)
Feet or Survey Feet. What is the difference? (1 foot = 0.3048
meters; 1 survey foot = 0.3048006096 meters)
Ellipsoid elevation or Orthometric elevation. NAVD88 or NGVD29?
Grid or ground distances?
Wrong equipment for the job. You can't measure sewer inverts to
1 cm and measure rim shots to 1 meter if you are running a flow
study. You need a 1 cm rim shot too.
Ignoring productivity: If you are going to shoot 30 shots per
day, a static PM3 pair or PM3.RTK or PM500 will all do a great
job. But if you want to store 1,000's of points per day or stake
hundreds of shots per day, then dual-frequency RTK with GLONASS
will make you much more productive.
Mission Planning: what's mission planning? There are times
during every day when it is probable that you will have a low
satellite count. Static shots will take twice as long, it will
take 4 minutes to fix a PM3.RTK pair instead of 10 seconds. If
you don't want to mission plan, purchase PM500's with GLONASS so
you always have plenty of birds in the sky. If you are willing
to take a break for 30 minutes and wait for some more satellites
to rise, then consider PM3 receivers.
Figuring out what you are doing wrong after you have spent an
entire summer systematically collecting poor quality data.
Don't fall in to the trap of saving $50 when your
purchasing $7,000 worth of equipment and purchase from a dealer who
does not know the difference between:
- ITRF2000 and NAD83 CORS 96
- Feet and Survey Feet
- Ellipsoid and Orthometric Elevation
- Grid and Ground distances
Let your dealer know what your intend to do, and
listen to their advice. A local dealer is usually going to be a much
better resource because they can provide on-site demos and equipment
comparisons. But if you don't have a good local dealer, try to find
a great national dealer who has experience with your application.
Don't hesitate to rent equipment to find out if a particular model
will meet your needs. Don't hesitate to take the dealer's training
Before you make a huge equipment investment, make
sure that the equipment will meet your expectations before you
commit to an expensive purchase.
Often you will save money by paying an experienced
professional to help with your project design and planning! You
won't typically get that kind of help looking for the lowest price
on the web!
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