Visit our Popular Forums

**JPA Cate** · 21-03-2015, 17:49

Quote:

Originally Posted by lordgeoff

Last time then I am off to the football....
GPS= Global Positioning System.
Satellite SIGNAL to GPS RECEIVER= POSITION ON THE GLOBE.
Clever GPS RECEIVER = FROM CONSECUTIVE POSITIONS CALCULATES SPEED, VMG, TTG, HEADING, XTRAK ERROR, SOG etc etc etc etc.
GPS = BEST PIECE OF NAVIGATION EQUIPMENT ON THE BOAT.....INCLUDING PAPER CHARTS
Cheers I'm off to the footy.

Are you really this obtuse, or are you just pulling our legs? Perhaps football is a better sport for you than sailing and navigation...

And to be sure, GPS is indeed wonderful. Its wonderfulness is especially obvious to those who started out voyaging with celestial. But it doesn't work in the manner you keep trying to explain. Please pay attention to those folks who actually do know how it works, who cite technical references to back their statements and who show conclusively that modern GPS systems rely upon Doppler effects to calculate velocity.

Jim, using Ann's computer

Sailor Doug · 21-03-2015, 19:31

I can't find a manufacture that publishes that gps speed is calculated from Doppler effect. Lots of papers published that are interesting but no manufactures. All gps units seem to use Doppler to improve satellites location using complex algrthames.

Sent from my iPad using Cruisers Sailing Forum

transmitterdan · 21-03-2015, 22:16

They use Doppler to get velocity. Knowing velocity they can get a more accurate location.

lordgeoff · 22-03-2015, 02:06

Quote:

Originally Posted by transmitterdan

They use Doppler to get velocity. Knowing velocity they can get a more accurate location.

Once again.......transmitterdan
From the guys who have made a few GPSs...GARMIN
"GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more."

lordgeoff · 22-03-2015, 02:15

Quote:

Originally Posted by Sailor Doug

I can't find a manufacture that publishes that gps speed is calculated from Doppler effect. Lots of papers published that are interesting but no manufactures. All gps units seem to use Doppler to improve satellites location using complex algrthames.

Sent from my iPad using Cruisers Sailing Forum

DOPPLER EFFECT is simply the term describing one object moving towards or away from another object. The movement creates an apparent change in frequency. The best example is the sound the police siren makes as it moves toward you compared to the sound the siren makes when moving away. Hence attaching the term DOPPLER EFFECT to the transmissions from the GPS satellite network. As I have shown Transmitterdan below. The satellite signals are used by the GPS receiver to establish the position of the GPS receiver. All other calculations are derived from consecutive GPS positions of the GPS receiver.
Garmin has a good lay explanation here.

Garmin | What is GPS?

**Wotname** · 22-03-2015, 03:44

@ lordgeoff, a short extract from Wikipedia - Global Positioning System: "GPS units can measurements of the Doppler Shift of the signals received to compute velocity accurately".

chala · 22-03-2015, 06:41

To quote Wikipedia accurately.

“GPS units can use measurements of the doppler shift”

Sailor Doug · 22-03-2015, 07:41

From Doppler they know if a satellite is moving away or towards them that allows calculated correction of satellite position only.
Gps is quite amazing, at lest three satellites rotating very very high, very fast elliptical orbit. Spinning semi sphere and a boat siting on it in a fluctuating fluid. Device on the boat (gps) is calculating relative direction, speed and position on this rotating sphere. Only a very weak common message is given to all devices trying to solve this problem. It is amazing how accurate and quickly you processor does this. Did I also mention from memory the gps needs to know orbit of satellites, surface of earth at boats position and correct it for surface undulation.

If you give up your knot log you might as well give up your watch and compass cause gps does all three of them better.

Sent from my iPad using Cruisers Sailing Forum

transmitterdan · 22-03-2015, 09:26

Quote:

Originally Posted by lordgeoff

Once again.......transmitterdan
From the guys who have made a few GPSs...GARMIN
"GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more."

[emoji14]eace:

And once again you are wrong. GPS receivers need velocity to derive an accurate position quickly. Velocity is much easier to compute than position and comes first in the sequence of events within a receiver. Garmin uses doppler to get velocity just like every other receiver.

lordgeoff · 22-03-2015, 09:30

Quote:

Originally Posted by Sailor Doug

From Doppler they know if a satellite is moving away or towards them that allows calculated correction of satellite position only.
Gps is quite amazing, at lest three satellites rotating very very high, very fast elliptical orbit. Spinning semi sphere and a boat siting on it in a fluctuating fluid. Device on the boat (gps) is calculating relative direction, speed and position on this rotating sphere. Only a very weak common message is given to all devices trying to solve this problem. It is amazing how accurate and quickly you processor does this. Did I also mention from memory the gps needs to know orbit of satellites, surface of earth at boats position and correct it for surface undulation.

If you give up your knot log you might as well give up your watch and compass cause gps does all three of them better.

Sent from my iPad using Cruisers Sailing Forum

Yes absolutely correct Doug. However a nice watch always looks cool when you go into a bar. Knot log and compass looks a bit out of place and does not really make you look cool in the same bar.
The speed of calculations and accuracy of the GPS system and associated plotter never ceases to astound me. There are some really smart guys out there that create these systems.
Makes all of us a lot safer on the water.
Cheers

lordgeoff · 22-03-2015, 09:40

Quote:

Originally Posted by transmitterdan

And once again you are wrong. GPS receivers need velocity to derive an accurate position quickly. Velocity is much easier to compute than position and comes first in the sequence of events within a receiver. Garmin uses doppler to get velocity just like every other receiver.

transmitterdan. mate you have nearly got it. Well done. Just slip in " GPS receivers firstly calculate position which then enables calculation of speed and many other pertinent navigation pieces of data i.e SOG, VMG, TTG, ETA, XTRACK ERROR etc etc etc.
Welcome to the club. Took a while but you have now nailed it.

lordgeoff · 22-03-2015, 09:43

Quote:

Originally Posted by transmitterdan

And once again you are wrong. GPS receivers need velocity to derive an accurate position quickly. Velocity is much easier to compute than position and comes first in the sequence of events within a receiver. Garmin uses dcoppler to get velocity just like every other receiver.

Sorry Transmitterdan you are not disagreeing with me you are disagreeing with Garmin and the GPS engineers. I am just quoting GARMIN in the excerpt above.
Feel free to continue the debate with Garmin. Maybe Garmin has got it all wrong and you are right........

**Dockhead** · 22-03-2015, 10:22

In another thread, there was a serious debate about how GPS works. I'm not sure why there's such a big debate -- it's not a matter of opinion. But in any case -- here's a place to discuss it. Enjoy.

I hope Goboatingnow will come by and straighten us out -- he's a pro in the field and forgot more yesterday, than the rest of us will ever know about how GPS works.