PPP delivers accuracy up to 3 centimetres. A typical PPP solution requires a period of time to converge to decimetre accuracy in order to resolve any local biases such as the atmospheric conditions, multipath environment and satellite geometry. The actual accuracy achieved and the convergence time required is dependent on the quality of the corrections and how they are applied in the receiver. However, PPP systems typically provide a greater level of accuracy and charge a fee to access the corrections.
A typical PPP system is shown in Figure Most people do not consider the altitude as the height above the ellipsoid, but as the height above Mean Sea Level. MSL is the same as pressure altitude when corrected for temperature and barometric pressure , but different from HAE. Mean Sea Level has had nothing to with the level of the sea, it relates to some stakes driven into the ground hundred years ago that seemed at the time to be roughly mean sea level at that point.
Pressure altitude is not some sort of absolute geometric altitude, it is related to the gravity under the position being measured. If the measured position is over a high density rock, like iron ore, then the gravity is higher, and the pressure is increased over the simple HAE.
Conversely ocean water is less dense and has the opposite effect. This is very noticeable in the Hawaiian Islands. Many different datums can be used to calculate height. These datums are based on different ellipsoids used to approximate sea level. GCVD is the standard datum in Canada since Clearly there is no point knowing your precise position to a few cm if you are not certain of your datum and vertical datum ellipsoid , with epoch.
This will be important later as you are asked to input your choice of horizontal and vertical datums to your PPP service. For best results a minimum of 6 hours, and preferably 24 hours, of continuous observations are required.
The plot file is fed into gnuplot to turn it into a png image file. The image will contain a scatter plot of all the positions reported by your GPS, as well as summary statistics. The statistics include the mean latitude, mean longitude, mean altitude and other computed values. Verify your GPS is communicating with gpsd by running cgps and confirming that you have a stable 3D fix. Display the png with your favorite image viewer. To use display from Imagemagick : display scatter. Maybe you want to collect just 10 minutes of data 20 epochs at 30 second interval to verify that your tool-chain is working before doing a 24 hour run.
Simple, just change gpsprof -n to gpsprof -n 20 and then proceed as above. Maybe your gpsd host does not have Python installed. Just run gpsprof remotely. On the host you will need to run gpsd with the -g parameter so that it can be accessed over the network. Plain GPS determine their position by measuring the distance to several GPS satellites and calculating a position solution. The main limitation is that the position of any GPS satellite is not known to better than a meter or two in real time.
PPP uses the raw GPS measurements from a worldwide network of precisely fixed ground receivers to precisely calculate the actual orbits of all the satellites. The most accurate orbits "Final" take around 14 days to determine. To use these orbits you need to collect the raw measurements from your GPS, then upload them to a service to compute a more precise fix.
To relay on an external orbit and clock provider, this may limit accuracy, real-time capabilities and multisystem approaches. To overcome this problem, the PPP service must be able to compute their own orbit and clock products using a regional network of stations e. In this case, the accuracy of the orbits and clocks is slightly degraded but this degradation occurs mainly outside the area where the stations are deployed.
Inside this region, the combination of orbit and clock products is such that the positioning performances are good. Indeed, it is possible to achieve positioning performances at the same level as with a global network. This opens very interesting possibilities to regions already operating networks of GNSS receivers e. Such service could complement RTK for areas far away from any of the base stations requiring much less stations than a classical RTK approach for the same level of precision.
It has been observed that PPP solutions based on regional networks, even for small regions, are as accurate as PPP solutions using global networks. This opens new ways for providing precise positioning services at regional basis.
Book Google Scholar. Astrodynamics — In: Proceedings of the first international symposium on precise positioning with the global positioning system, Rockville, US, pp — GPS Solut 22 1 Navigation 64 4 — Download references. We would like to thank them all for their contributions to this software. You can also search for this author in PubMed Google Scholar. Correspondence to Jianghui Geng. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
If you have an interesting program or software package you would like to share with our readers, please pass it along; e-mail it to us at gpstoolbox ngs. Reprints and Permissions. Geng, J. GPS Solut 23, 91 Download citation. Received : 04 May Accepted : 01 July
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