gps speedo/ distrubutorless tach

[josephfrantz]

has anyone used these or aware of their accuracy/ reliability. seen the pair at legendary_motorcycles.com. they look very nice just have never heard of a gps based sppedo conversion/ replacement.

[pete]

No idea on accuracy or reliability, but I wonder what would happen when you went through a tunnel or rode beside some vehicle that interrupts GPS reception?

[Killer2600]

No idea on accuracy or reliability, but I wonder what would happen when you went through a tunnel or rode beside some vehicle that interrupts GPS reception?

GPS is quite accurate, your speed will be within 1 mph of actual at a steady speed. The short coming is acceleration and deceleration where the gps indicated speed may lag actual.

As for what happens when you lose GPS reception, you fall over - everyone knows when the speedo reads 0 mph it's either your foot or the bike that has to go down

[posplayr]

GPS is quite accurate, your speed will be within 1 mph of actual at a steady speed. The short coming is acceleration and deceleration where the gps indicated speed may lag actual.

As for what happens when you lose GPS reception, you fall over - everyone knows when the speedo reads 0 mph it's either your foot or the bike that has to go down

GPS receivers directly measure inertial velocity (velocity WRT an earth centered fixed frame of reference), so the acceleration lag will be related to 1/2 of the velocity update rate. At 1 and 5 hz this would be 0.5 and 0.1 sec respectively. I suspect there would be more lag in the needle readout of a typical tach cable drive system.

[Steve]

No idea on accuracy or reliability, but I wonder what would happen when you went through a tunnel or rode beside some vehicle that interrupts GPS reception?

Accuracy is fine, reliability is fine. Going into a tunnel will interrupt satellite reception, so you will have no speed or location info until reception resumes.

What vehicles iterrupt GPS reception? I have never heard of such a thing. I have been using GPS for many years.
I have used one on many vehicles and driven through many parts of the country.
I have never lost a GPS signal due to a vehicle. Please explain.

GPS receivers directly measure inertial velocity (velocity WRT an earth centered fixed frame of reference), ...

Some, do, but not all. I used to have a Garmin StreetPilot 2610, which was a good unit for its day. Reading the manual for it, I saw that there was also a 2650 model. The difference was that the 2650 included all the intertia sensors that were to be mounted to the vehicle. In the event of loss of sufficient satellite signals, the sensors would take over, keeping track of an apparent position until satellite signals returned.

Most GPS receivers are just that: receivers. They receive a co-ordinated time signal from multiple satellites, then compute an apparent position based on the different arrival times of that signal. A fixed interval of time later (possibly one second) they receive another signal and compute a new location. The speed that is displayed is computed based on the distance between the last two computed positions and the time between them.

Until GPS receivers update your location many, many times per second (tens? hundreds?), you will see an average speed over the last second or so. For most travels, that is great, but won't work well at a drag strip.

.

[posplayr]

Some, do, but not all. I used to have a Garmin StreetPilot 2610, which was a good unit for its day. Reading the manual for it, I saw that there was also a 2650 model. The difference was that the 2650 included all the intertia sensors that were to be mounted to the vehicle. In the event of loss of sufficient satellite signals, the sensors would take over, keeping track of an apparent position until satellite signals returned.

Most GPS receivers are just that: receivers. They receive a co-ordinated time signal from multiple satellites, then compute an apparent position based on the different arrival times of that signal. A fixed interval of time later (possibly one second) they receive another signal and compute a new location. The speed that is displayed is computed based on the distance between the last two computed positions and the time between them.

Until GPS receivers update your location many, many times per second (tens? hundreds?), you will see an average speed over the last second or so. For most travels, that is great, but won't work well at a drag strip.

.

Steve,
I have integrated multiple OEM GPS receivers into electronic guidance systems starting as early as the end of 1991. For every receiver I have ever seen or evaluated (5 have been fully integrated into GN&C electronics and 10+ evaluated), all have both pseudo range and Doppler range rate measurements. That means geodetic position and velocity is computed with one set of satellite measurements (minimum 4 for 3D position) at the update rate of the multi channel receiver.

I have done extensive development creating multiple measurement sensor fusion embedded software deriving inertial state which is estimated from the various gps measurements with aiding from either pressure sensor, laser altimeters or inertial sensors.

The latency is as I previously described; you don't need even 2 position fixes to derive velocity and you certainly don't need multiple to get a filtered velocity as you describe.

Jim

[Dogma]

Steve,
I have integrated multiple OEM GPS receivers into electronic guidance systems starting as early as the end of 1991. For every receiver I have ever seen or evaluated (5 have been fully integrated into GN&C electronics and 10+ evaluated), all have both pseudo range and Doppler range rate measurements. That means geodetic position and velocity is computed with one set of satellite measurements (minimum 4 for 3D position) at the update rate of the multi channel receiver.

I have done extensive development creating multiple measurement sensor fusion embedded software deriving inertial state which is estimated from the various gps measurements with aiding from either pressure sensor, laser altimeters or inertial sensors.

The latency is as I previously described; you don't need even 2 position fixes to derive velocity and you certainly don't need multiple to get a filtered velocity as you describe.

Jim

My experience as a user has been very much like what Steve describes. Perhaps what you describe is not exposed to users in consumer hardware?

[posplayr]

My experience as a user has been very much like what Steve describes. Perhaps what you describe is not exposed to users in consumer hardware?

If you were to have a 1 hz update GPS receiver and it has a ground track speed output how would you know if you are looking at:

time difference computed velocity from position fixes
v.s.
doppler range rate derived ground track calculations?


If the display is truly very slow, and it was a GPS received developed back when Selective Availability (SA in the up to the late 90's) with the 100+ meter horizontal position degradation was active then it probably is as Steve describes.

Below I looked up the spec for the 2610 and the position accuracy without SA is now 49 feet. Velocity is to 0.05 m/s or .1 knot (it doesn't say if that is per axis or RMS or anything else).
That is well below 1 mph accuracy on a sample per sample basis. The whole reason the GPS based speedometer exists is because the Doppler velocity is so accurate. With a 12 channel receiver, you are going to seldom be out of signal strength unless is some deep forest canopy or in a tunnel.

While it is possible for 8 of 12 satellites being tracked to have an obscured LOS (e.g. as you pass directly adjacent a Semi-truck), it is going to more often that not have a GPS lock sufficient to provide velocity.
3 satellites will give horizontal position.

http://www8.garmin.com/products/sp2610/spec.html

GPS Performance



Receiver: 12 parallel channel GPS receiver continuously tracks and uses up to 12 satellites to compute and update your position Acquisition times:

• Warm: Approximately 15 seconds
• Cold: Approximately 45 seconds
• AutoLocate®: Approximately 2 minutes

Update rate: 1/second, continuous




GPS Accuracy:

• Position: < 15 meters (49 feet), 95% typical**
• Velocity: 0.05 meter/sec steady state

Dynamics: 6g’s