Hello,
I think I have a good idea in the differences between DGPS and RTK. Like everything else I have developed my knowledge with person research, but since I am not an expert in GPS I doubt myself from time to time. With that said, I would like to post my brief interpretation of the DGPS and RTK so this group can confirm or correct my understanding. The last thing I want to do is tell some one the wrong thing and confuse them. So here is my 'Bird's Eye View.' I understand there these topics can get further in depth.
DGPS uses a static base station within a certain distance (around 6 miles) to broadcast atmospheric, clock, and ephemeris corrections to a rover via radio. The rover needs to be close in proximity of the base station in order to experience similar atmospheric conditions. This method of observations uses the L1 frequency therefore has an accuracy of 1m.
RTK uses a similar process as DGPS, but uses both L1 and L2 frequencies. The use of the additional frequencies (including L5) increases the amount computations the receiver makes to correlate the errors from the satellites. If the rover has more frequencies to measure the time delay against it can take out most of the time delay from the received message. The received message is then lined up (Delay Lock) with the Almanac stored on the rover.?ÿ
The Almanac is received by the rover from the satellites in the NAV message.?ÿ
Let me know if I'm on the right track. I have been thinking about this for a while and seem to read something new online each time I search for answers.?ÿ
Thank you,
Dan
Are you talking about DGPS as in the old Coast Guard corrections broadcast? If so you are nowhere near close to RTK precision.
Paul in PA
Both methods use an accurate position at the base to compute corrections. Both broadcast corrections over a data link (radio, internet, etc)
To simplify, DGPS broadcasts corrections to the pseudoranges. This will give an accuracy >1m. RTK broadcasts corrections to the satellite positions using phase measurements. Accuracy is cm level. These corrections are applied to the measured phases at the rover. There are other corrections as well, clock, ionosphere, etc.?ÿ
DGPS has become something of a catch-all term that encompasses both consumer-grade and commercial-grade receivers and methods. If there is a correction stream coming from a base or network of bases, whether in realtime or post-processed, it is differential GPS.
The correction streams can come from land-based sources (like the Coast Guard radio beacons that Paul mentioned, or UHF/cellular RTK or VRS for that matter) as well as corrections broadcast by geostationary satellite - such as the free services like WAAS or EGNOS on the lower end of precision, or pay-based services like Trimble's RTX that can get down to a couple of centimeters in the horizontal depending on the service and receiver you choose.
For satellite-delivered corrections the receiver doesn't necessarily have to be within 6 miles of a base station, especially for projects requiring lower precision - for pseudoranges the distances are similar enough between ground-based receivers as long as they are within a couple hundred KM. This is why a handheld consumer-grade receiver such as Garmin can get down to 1-3 meters with WAAS out in the boonies.
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Still learning ??. had a chance to try DGPS in a GIS class last year and compare it to RTK. Used Topcon's Ga/Gb receiver for RTK, and GRS1 for DGPS (only used L1, GPS, and internal antenna; GRS1 can do more, but that's the capability we used). We took some shots near a flag pole. The DGPS unit was not mounted on a tripod. It was held above the point. Orange circles are DGPS. Green triangles are RTK.
Large scale is ~3 ft across, small scale ~30 ft.
That flagpole doesn't make a great signal environment.