Originally Posted by lordgeoff
Sorry guys. You have it wrong.
Satellites only send simple signals which basically say, this is my position in space, this is my position in space. The satellites repeat this information very quickly. If you have ever watched the strings arriving at your GPS receiver. You would see the strings are very simple and repetitive.
Don't get too hung up on the Doppler effect. It merely means moving toward or moving away from an object.
The string from the satellites is received by the GPS and through calculations from CONSECUTIVE strings from the Satellite can calculate how far it has moved toward or away from the satellite. Hence the GPS RECEIVER can then make further calculations such as the average speed between these two points.. I.e. Average speed which in effect is instantaneous speed as it is calculated each second or less.
There is no communication from the GPS receiver on your boat to the Satellite as it is only a receiver. Imagine the billions of GPS receivers, walking Apps, hand heads, motor
vehicles, trucks, having a continuous two way chat with the satellites. It would be Comms meltdown.
From your link...
"GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed."
The calculations or algorithms of both the satellite and GPS receiver are complex. The calculations such as HEADING, SOG, etc are simply calculated using the distance and time between the two CONSECUTIVE positions. VMG, ETA, TTG etc are again simple calculations once a waypoint is introduced into the calculations.
Your link confirms what I have been saying the SPEED is the average between two positions. To all intents and purposes an average speed calculated each second or less is as good as an instantaneous.
Your link also confirms the range of speeds the GPS receiver is capable of from zip to hundreds of mph/kph.
is simple the Comms standard for most GPS receivers. I think the new version is NMEA2000 or something like that. Raymarine
still has it proprietary code SEATALK
Lordgeoff. I cannot understand why you continue peddling this nonsense. The fact is people that understand this technology at an engineering level have explained to you how this works. Equally the internet
is full of technical explanations that explain this technology. The trouble is GPS is a conceptually simple system that's actually very very complex in reality , hence the incorrect explanations that abound,
GPS systems derive instantaneous speed from the Doppler effect which appears in the carrier wave from the sat. This speed for example is fed into the kalman filter to achieve better position accuracy.
All modern chipsets compute Doppler speed
Indeed as you say heading ( on single
antenna ) systems are derived from position on RTK systems with two antennas heading is derived using inferometry and not position computation
Note . What appears on the NMEA output is often different. Garmin
for example had traditionally not stored instantaneous speed in its track log for example in some products
Also. GPS satellites do not repeatedly transmit position. The pseudo code that is continuously broadcast merely serves to indentify the particular satellite , its primary purpose is to allow clock recovery / sync in the receiver.
Remember the position of the satellite is a far smaller contribution to positional error then clock error. The complexity in GPS is to regenerate the accuracy of the onboard atomic clock in a cheap
receiver powered by a simple crystal oscillator. In fact if you just used a crystal oscillator uncorrected , after a week the position error would put you on the far side of the moon.
Hence the complexity in clock recovery. The position of the satellites is much less of a concern and can be transmitted much slower them anything else.
Overlaid on the c/A pseudo code is a data transmission
, that contains the satellites almanac and more importantly ephemeris. This data is transmitted at a rate of 60 bits per second , very slow.
This is because the satellites orbit is tightly constrained and known in advance. The online data ( navigation message ) is primarily ephemeris corrections.
The navigation. Message information can actually come by lots of comms methods like in the case of assisted GPS.
PS. No one hear has said the receiver transmits to the sat
Note that VMG have nothing to do with GPS per se
It's all here http://www.gps.gov/technical/ps/1995...cification.pdf
Sent from my iPhone using Cruisers Sailing Forum