My GM862-GPS has problems with its GPS accuracy. One read out the GPS coordinates by entering AT$GPSR = 0 and then AT$GPSACP. The module responded the following:$GPSACP: 135424.999,46XX.XXXXN,006XX.XXXXE,1.4,617.3,3,215.08,0.14,0.07,120509,06But the real position (46°XX’XX.XXN and 6°XX’XX.XXE) is about 700 meters away.Is it normal? Or is there a way to make the GPS more accurate?
The lack of accuracy can have more reasons.
The accuracy of the GPS receiver is the accuracy of the Sirf chip: 700 meters away doesn’t have meaning if compared to the Sirf accuracy. But the "virtual" accuracy of GPS receiver is affected by the instant of the acquisition of the GPS position and other parameters, by the application design, and by intrinsic errors of the GPS system.Lack of accuracy related to the instant of the acquisition of the GPS position and other parameters.
When you acquire a new position (AT$GPSACP) after a reset or after a wake up (if you use power saving modes), it’s betterto wait for the 5th or 6th valid fix after turning on GPS, not just for the 1st one , and then accept the fix. This is becausein a first phase ,after reset or turn on the GPS part, the GPS chip has to calculate some parameters and a first valid fix could be done with a minimum number of parameters, but with more parameters acquired in the next fix the accuracy could change. This phenomena is not usual, but it can happen. More attention should payed when you start with a cold start and with a warm start.
Furthermore: it’s better to discard the GPS sentences with HDOP values bigger than 8, see also the following link as reference:http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS)
In GE863-GPS and GM862-GPS the fields PDOP and VDOP are not available. If full DOP info are needed, you need to use the NMEA serial port instead (read in GM862-GPS "Software User Guide").If you will use speed data (available in VTG sentences), you should pay attention also to discard sentences coming after GSAsentences with 2D fix, because the position is already affected by error. Better wait for a 3D fix.
Application design issuesThe GM862-GPS module hasn’t a LNA so to achieve good performances it is required to develop a good antenna path.This includes the antenna and the LNA section.The installation of the antenna is also affecting the performances if is in an area that could increase the multipath and noise problems.
Some intrinsic errors of the GPS system : (accuracy could vary as time goes by)Ionosphere and troposphere delay * Signal is lower as it passes through the atmosphere Signal multipath * Signal reflected (tall buildings, large rock surfaces) * This increases the travel time thereby causing errors * Fix: High position of antenna Orbital (Ephemeris) Errors * Inaccuracies of the satellite’s reported location * Fix: Ephemeris data collectionNumber of visible SV * The more SV seen, the better accuracy. * GPS units typically will not work indoors, underwater or underground, shadowing effect of mountains, buildings, etc.Satellite Geometry * Poor geometry results when the satellites are located in a line or in a tight grouping. * Ideal satellite geometry exists when the satellites are located at wide angles relative to each otherReceiver Clock Errors * Receiver’s built-in clock is not as accurate as the atomic clocks onboard the GPS satellites * Therefore, it may have very slight timing errors
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