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New Post-Processing Option for Sub-Meter Accuracy
Magellan has recently released updates of MobileMapper receiver and office software that provide many enhancements. Among these new features is the ability to differentially correct MobileMapper positions in post-processed mode for submeter accuracy. The new version of receiver software includes an option to log the GPS measurements that are required for post-processing. The new version of MobileMapper Office includes the actual differential correction module that merges the receiver’s data with measurements recorded by a reference receiver.

The MobileMapper receiver is set to record GPS measurements for post-processing when the customer inputs an activation code specific to his receiver's serial number. This optional receiver firmware is available for purchase from Magellan’s MobileMapper dealer network or, in the United States and Canada, from the Thales Navigation website at http://professional.magellangps.com/en/solutions/mobilemap/.

Accuracy Test Parameters
Accuracy tests were conducted over several days under different constellations of GPS satellites. Various configurations were also tested. The "rover" receivers were all MobileMappers but some of the tests were conducted using the MobileMapper’s internal antenna and some were conducted using the external L1 antenna from a Thales Navigation ProMark 2 survey system. Some data was collected with the receiver held horizontal to the ground and some with the receiver held vertically. (The MobileMapper’s antenna functions best when vertical.)

Various reference station receiver were used: MobileMapper with internal and external antenna, a Thales Navigation iCGRS geodetic-grade receiver and two survey grade receivers used at US National Geodetic Survey CORS sites.

Accuracy Test Results
The table below summarizes the post-processing accuracies attainable with MobileMapper. In general PDOPs were in the 3 to 5 range and the number of satellites tracked was between 5 and 9.

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In most cases, WAAS corrections were used in real-time. The post-processing engine, however, does not use these corrections. MobileMapper with the post-processing option is able to deliver 70-cm accuracy on baselines up to 48 km using the receiver’s internal antenna. Using an external antenna it delivered 50-cm accuracy on the same 48 km baseline. On shorter baselines MobileMapper is capable of 40-cm accuracy. Across all the tests, accuracy with the MobileMapper receiver is 90 cm with internal antenna and 60 cm with a ProMark2 external antenna.

In addition to achieving submeter accuracy for point features, the new MobileMapper post-processing option also allows differential correction to submeter accuracy of all points making up line and area features. The result is not only accuracy in an absolute sense, i.e. each point is within one meter of where the MobileMapper receiver actually was, but the map representation of entire line or area features more closely match their actual shape.

The illustration below is a MobileMapper Office map screen showing a series of line features recorded while walking out the word "submeter" in a parking lot. Each letter is about 4.5 meters tall and 2.5 meters wide. Painted lines marking out parking spaces were used as a guideline. The receiver, with internal antenna, was hand-held while the person recording the "map" estimated where to hold the receiver to best spell out the letters.

Not only is each point making up the letters within one meter of truth, the shapes of each individual line feature are true to form. This allows users to map line and area features with a high degree of detail.

This high degree of precision provided by MobileMapper’s post-processed differential correction also supports the calculation of the area of polygonal features. The example below is of an architectural feature 6.64 m in diameter that comprises an area of 34.63 square meters. After differential correction in MobileMapper Office and export to MIF format, MapInfo calculated the recorded area to be 34.61 square meters (0.5% error).



Requirements for Submeter Accuracy
The MobileMapper system records and processes continuous streams of GPS carrier phase information to deliver submeter accuracy. The longer this stream of data is unbroken, the better the accuracy will be. Because carrier phase signals are sensitive to environmental factors such as tree canopy, it is important to have an open view of the sky as much as possible.

In addition, submeter accuracy can only be achieved with relatively low PDOP (Positional Dilution of Precision) and a sufficient number of satellites tracked by the receiver. If the PDOP goes much higher than 4 or 5 and if the number of tracked satellites is less than 5, the probability of submeter accuracy will be less than the specified 95%.

How to Optimize MobileMapper Accuracy

1. Set the logging interval for lines and areas to 1 second.
2. Hold the receiver at eye-level so your head doesn't block the antenna.
3. Hold the receiver so it is vertical.
4. Hold the receiver stationary for a few seconds before logging any feature.
5. Try to position the receiver directly over the feature and walk smoothly when logging lines and areas. MobileMapper Office will reveal deviations in a path as small as 20 or 30 centimeters. You can avoid such deviations by setting the logging interval to a higher time or distance value.