lustyd
Well-Known Member
How it's used is implementation specific, so although it can achieve that goal, it's not always the mechanism used.
Plotter\Chart anomoly
- Thread starter Caladh
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srm
Well-Known Member
Perhaps a little, very much simplified, history might help clarify the arguments.
When GPS first became operational civil receivers were only allowed one (of two) frequency. The receiver had an accurate clock that was regulated by the satellites. By calculating the travel time of the signal a pseudo-range was obtained. A position was calculated from the intersection of pseudo-ranges from a group of satellites. The early marine receivers could only output time and a 2 or 3 D position depending on the number of satellites used. Other information was calculated from this basic data.
Position accuracy of the civilian service was better that anticipated. It appears that the US DoD was a bit embarrassed that they had given the world the means to guide a missile in to the Oval Office so the signal was deliberately degraded by Selective Availability. This led to various workaround by different organisations such as US Ports transmitting corrections that eventually gave local, regional, and national Differential GPS services.
The Clinton Administration eventually accepted that SA was holding back the further development and use of GPS and set it to zero. This opened up GPS to sectors that required greater accuracy than obtained by travel time distances such as land survey (which is where I came across the use of GPS carrier wave measurement).
Obviously, road vehicle navigation also requires far greater accuracy than maritime use. Chip design will have developed along with these increased requirements for greater accuracy so that carrier wave measurement will have become available to marine chart plotter manufacturers, should the designers wish to make use of it.
When GPS first became operational civil receivers were only allowed one (of two) frequency. The receiver had an accurate clock that was regulated by the satellites. By calculating the travel time of the signal a pseudo-range was obtained. A position was calculated from the intersection of pseudo-ranges from a group of satellites. The early marine receivers could only output time and a 2 or 3 D position depending on the number of satellites used. Other information was calculated from this basic data.
Position accuracy of the civilian service was better that anticipated. It appears that the US DoD was a bit embarrassed that they had given the world the means to guide a missile in to the Oval Office so the signal was deliberately degraded by Selective Availability. This led to various workaround by different organisations such as US Ports transmitting corrections that eventually gave local, regional, and national Differential GPS services.
The Clinton Administration eventually accepted that SA was holding back the further development and use of GPS and set it to zero. This opened up GPS to sectors that required greater accuracy than obtained by travel time distances such as land survey (which is where I came across the use of GPS carrier wave measurement).
Obviously, road vehicle navigation also requires far greater accuracy than maritime use. Chip design will have developed along with these increased requirements for greater accuracy so that carrier wave measurement will have become available to marine chart plotter manufacturers, should the designers wish to make use of it.
requiem
Well-Known Member
I'm sure someone will let me know where I go wrong, but I believe the quartz clock in most receivers is nowhere near accurate enough for this purpose beyond a very rough ranging, and thus the phase differences in the received signal are used to obtain a more precise distance. This creates a system of equations that can then be solved for the x, y, z, and t values. You can likely guess the first three, and the final value is the clock error.
As to the plotter; the developers have a choice: if the GPS hardware API is able to output a COG value based on Doppler measurements, they could use that directly. Or, they could simply take a chain of successive (x,y) position values and estimate COG based on those.
Buck Turgidson
Well-Known Member
Yes. The receiver will mathematically correct its internal time when it has 4 valid signals because they can only intersect at one point in space at one time. The receiver continuously calculates the time offset whenever it has 4 signals.
At this point the system is still effected by ionospheric interference which can be minimised by either D-GPS using SBAS or GBAS corrections or internally if the receiver is dual or triple band where it can calculate for individual satellites using the different Doppler shift experienced by the two or three frequencies being received.
Refueler
Well-Known Member
Lets see what the 'experts say' ...
Refueler
Well-Known Member
Your first paragraph is why it requires minimum 4 sats to provide a 3D fix .... 3 sats or less and the less accurate RCVR clock error can create serious differences.
Refueler
Well-Known Member
You imply that Differential was only a US based factor - in fact it was an international item .....
At the time SA was 'switched off' was at a time when satelite maintenance was on a slippery downward slope ... sat replacements were falling behind schedule etc. Funding was under pressure.
Road Vehicle nav - as already posted has a trick it uses : Jump to Road. You can see this clearly when travelling along a highway and then you take a parallel slip road ... quite often - the nav will still follow the main road till you clearly separate on the slip road. Also when going off-road - the unit asks you if you accept Off road ...
The use of Doppler shift is used along with Time to ascertain far more than just simple position - agreed - the common ground is positional difference for most.
You can check out Garmin and various other actual GPS manufacturers instead of reading some Car related / off-roaders link ...
lustyd
Well-Known Member
It could, although the UK appears to have switched off DGPS stations last March, probably because with modern equipment it's not really needed.
KompetentKrew
Well-Known Member
Your link is to a company that sells a dedicated GPS speedometer using this principle.
It seems like a developer who uses U-Blox GPS modules can use them to calculate doppler speed, but it's not a built-in function of the device itself. [thread]
Meanwhile another supplier of GPS speedometers, specifically for boats, states that their product uses relative position change to calculate speed.
It doesn't make sense to me that chartplotter manufacturers would implement the more complicated doppler speed calculations, when they're receiving position constantly from the GPS (once a second, if not several times per second) and considering it's trivial to calculate speed from the boat's relative position change and that this accuracy is good enough for our purposes. The chartplotter is recording position constantly for the track so the developer of the chartplotter software can use this to calculate speed, applying smoothing / averaging as desired.
Finally, this thread is about OP's observations that the chartplotter shows the boat's heading flickering when it's stationary in its berth or on the hard - I guess we've all noticed this ourselves. I've just switched on my B&G Vulcan and it immediately shows a SOG varying between 0.1 and 0.2,, as well as varying COG, so I guess that proves it?
Buck Turgidson
Well-Known Member
The link was to show the formulas commonly used by modern GPS chips. Hence my statement to that effect in the post. Take a look at the spec of a typical newer GPS receiver such as the GARMIN GPS24xd. They achieve the quoted level of stability at low speed using the techniques discussed. Older receivers still show the dithering especially on startup when horizontal dilution of precision is high but this is becoming a thing of the past with more capable receivers such as the one quoted. Note it is dual band meaning it receives L1 and L5 (E1,E5) signals from each satellite. Those two signals transmitted at different frequencies propagate at different speeds through the ionosphere and the difference between the two signals is used to calculate the time correction required. I'm sure you can google multi-band GPS receiver for a more complete description.
Buck Turgidson
Well-Known Member
Differential GPS refers to any form of external correction so although the maritime DGPS service I think you are referring to(that would be a GBAS Ground Based Augmentation System) may be gone, there is still SBAS available throughout many parts of the world including Europe where it is implemented through EGNOS.