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en:hardware:gnss [2017/11/17 16:29]
TobiasVo
en:hardware:gnss [2022/06/09 07:28] (current)
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 ====== GNSS (GPS) ====== ====== GNSS (GPS) ======
 +original source(([[http://​en.blitzortung.org/​Compendium/​Documentations/​Documentation_2014-05-11_Red_PCB_10.4_PCB_12.3_PCB_13.1_PCB_14.1.pdf|Documentation System RED]], chapter 5.4))
  
 ===== Description ===== ===== Description =====
 +
 +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.
  
 ===== Using GALILEO ===== ===== Using GALILEO =====
 +
 +FIXME add content