GNSS (GPS)

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The main purpose of the GPS is the 1PPS signal which gives all worldwide station the same absolute time base. The 1PPS-line is directly connected to an I/O pin of the STM32F4 MCU. It is also connected to an ADC input pin to check the quality of the 1PPS-pulse in case of an error.

Position data is sent by the GPS module every second over an serial bi-directional connection to the MCU. The MCU itself can send commands to the GPS, but does that only if needed (i.e. during initialization). The baud rate for that connection is variable and can be set in the web-interface. However, you should avoid slow baud rates as it can take too long for the GPS to send the whole data before the next 1PPS-pulse. We recommend baudrates of 38400 and higher.

In the GPS settings you have the option to enable SBAS. This is a service which can improve position accuracy and is provided for different regions of the world (i.e. EGNOS, WAAS). The accuracy will be improved due to additional information about current status of the ionosphere. The needed data is sent by satellites on geostationary positions. To receive their signals, your GPS antenna needs a good view to them. For the northern hemisphere that means there should be no buildings, trees etc. on the south side of the antenna.

In general, the GPS will receive more satellites when having good view in direction of the equator. That means, if you have only the possibility to place your GPS in-house, you should choose a place on the south side of the building (or north side on the southern hemisphere). This fact is more important, the higher your latitude is.

The GPS module not only transmits position and time, but also several status information about its reception. You can see some of the values on the LCD and on the web-interface. Please note that those values will change very often, because the GPS satellites are moving fast, weather and condition of the ionosphere can change quickly. You should see similar values every 24 hours, as the satellite constellation will be almost the same. The most important values are listed here:

• The signals of tracked satellites are used to calculate position and time data. The count of tracked satellites shouldn’t fall below 6 too often. A value of four is the absolutely minimum! Higher values are always better.

• Satellites in view gives the number of satellites, which could be tracked under perfect conditions (i.e. on a airplane). The number of tracked satellites should be close to the number of satellites in view. »>For example, when you have 15 satellites view and only 4 tracked, then you should search a better place for the antenna.

• PDOP stands for position dilution of precision. There are also values for vertical and horizontal position (VDOP, HDOP). The DOPs give the best estimation about the “GPS-quality”. The lower the values, the »>better the position accuracy. Values below 1.0 are almost perfect and those below 3.0 are still far good enough for our purposes. The DOPs give a idea whether the constellation of tracked satellites is good. For example, even if you receive 8 satellites your PDOP could be bigger that 7 (very bad), because the GPS sees only satellites from a part of the sky and cannot calculate a good position.

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