Gora Mohanty wrote: > are largely due to atmospheric fluctuations. Corrections for these are > derived from the difference between the well-known actual position of the > base station, and the received realtime position. These can be broadcast > through various means, e.g., an Internet DGPS server, and should be good for > a few hundred km around the base station. The cooperative aspect of this > comes in because of the benefits of multiple base stations.
Proper differential corrections for GPS are about measuring and correcting pseudoranges, the measured distances between GPS satellites and the receiver. SBAS systems (WAAS/EGNOS/MSAS/etc) and RTCM SC-104 (DGPS) do the corrections differently, but the basic idea is the same, to correct the pseudorange measured by the GPS receiver. There are also other mechanisms to doing differential corrections, but they are mostly used in the surveying sector, either real-time or post-processed. Anyway SBAS/DGPS corrections can't be applied to a computed position (lat/lon/alt). The corrections also can't be generated at a reference station by just calculating the difference between a position returned by uncorrected GPS and the real/surveyed position. This is because you can't guarantee that two receivers, no matter how close together, use the same satellites and identical weighting and filtering in the position solution. So you need to go down to individual satellite pseudoranges. SBAS systems actually go down even further than that and break the corrections into error components like satellite clock error, orbit error and ionospheric delay. This means that both at the correction generating end and at the correction use end the GPS receivers have to have support for differential GPS built-in. Or as an alternative, you have to have access to the pseudorange measurements and have software that applies/generates the corrections and calculates the positions. You just can't post-correct a typical NMEA position ($GPRMC/GGA) and expect to get good and reproducible results. The Internet DGPS systems based on relaying RTCM SC-104 corrections work ok, if you are within a couple of hundred kilometers from the reference station. Multiple correction streams can't easily be combined, you have to somehow select one (typically the one geographically closest to you). SBAS systems supply corrections to a large geographical area at once, up to a continent (or two). But the trouble there is how to get the SBAS corrections into your GPS receiver, if you aren't receiving the corrections directly from the geostationary satellites. For example the EGNOS correction stream is available via Internet from SISNeT, but a typical GPS receiver can't handle the corrections input via it's serial or USB port. At least in theory you could convert the SBAS data into virtual RTCM SC-104 reference stations and feed that into receivers that support it. But then again nowadays RTCM SC-104 support often just isn't there in the GPS receiver firmware.. Tapio _______________________________________________ talk mailing list [email protected] http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/talk
