Why Can’t You Rely on Accuracy Readings from Some GPS/GNSS Apps for Advanced GNSS Receivers Like the Columbus P9 Race?

Many GPS apps on the market estimates accuracy use horizontal dilution of precision (HDOP) and vertical dilution of precision (VDOP). For instance, estimated horizontal accuracy is calculated by multiplying HDOP by 4.7, and estimated vertical accuracy by multiplying VDOP by 4.7.

However, advanced GNSS receivers like the Columbus P9 Race operate beyond this method. Their accuracy cannot be reliably derived from HDOP and VDOP values obtained through GSA (GNSS DOP and Active Satellites) NMEA sentences. Using the HDOP/VDOP conversion for these devices often results in substantial errors. Instead, advanced GNSS receivers output accuracy directly using proprietary message formats. For example, some u-blox GNSS receivers (like the Columbus P-9 Race) use UBX sentences to provide a more accurate, direct accuracy reading.

Correct Method to Obtain Accuracy of the P-9 Race

To get an accurate reading from the P-9 Race, use the u-center software, available from u-blox’s website, and follow these steps with the device completely stationary:

1. Go to Receiver -> Connection, and select the port where the P-9 Pro is connected.

2. Go to View -> Messages View.

3. Find UBX – NAV – POSLLH, right-click on it, and select Enable Message.

4. The Accuracy Estimate on the right will display the expected positioning accuracy.

5. Note: It’s recommended to wait a few minutes during this test. The accuracy value will initially appear higher and gradually stabilize at a lower, final value, representing the device’s actual accuracy.

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