Re: How is speed calculated? (Nuvi 660)

for potentially a more detailed discussion....

ps56k wrote:

The basic output of each GPS measurement consists of 7 values:

(x,y,z) and (dx,dy,dz), all in the ECEF coordinate system, as well as
time. This is simply what pops out from the delta time measurements
between the arrival times from all visible sats.

The (x,y,z) is typically converted to (lat/long/altitude) by first using
the GPS geoid, then using a vendor-supplied table of altitude offsets
between the geoid and the real world.

At this point every GPS vendor has their own more or less proprietary
protocol for how to combine multiple such measurements, typically one
per second, and use it to determine maximum likelihood values for both
lat/long position and velocity.

Terje
--
"almost all programming can be viewed as an exercise in caching"

te
in a

I'd say: delta time measurements and the doppler shifts measurement

The University of Calgary and the University of New Brunswick both
post a large number of papers on-line. Here are several on velocity:

Serrano, Luis, Kim, Don, Langley, Richard B., "A Single GPS Receiver
as a Real-Time, Accurate Velocity and Acceleration Sensor,"
Proceedings of the 17th International Technical Meeting of the
Satellite Division of The Institute of Navigation (ION GNSS 2004),
Long Beach, CA, September 2004, pp. 2021-2034,
http://gauss.gge.unb.ca/papers.pdf/iongnss2004.serrano.pdf

Serrano, Luis, Kim, Donghyun, Langley, Richard B., Itani, Kenji, Ueno,
Mami, "A GPS Velocity Sensor: How Accurate Can It Be? =85 A First Look,"
Proceedings of the 2004 National Technical Meeting of The Institute of
Navigation, San Diego, CA, January 2004, pp. 875-885,
http://gauss.gge.unb.ca/papers.pdf/ionntm2004.serrano.pdf

Cannon, M. Elizabeth, Lachapelle, Gerard, Szarmes, Michael C., Hebert,
Jeffrey M., Keith, James, Jokerst, Scott, "DGPS KINEMATIC CARRIER
PHASE SIGNAL SIMULATION ANALYSIS FOR PRECISE VELOCITY AND POSITION
DETERMINATION", NAVIGATION, Vol. 44, No. 2, Summer 1997, pp. 231-246,
http://plan.geomatics.ucalgary.ca/papers/97amjh.pdf

Nadezda Sokolova, "Doppler Measurements and Velocity Estimation:
Comparison of Standard and High Sensitivity Receivers," M.S. Thesis,
Department of Geomatics Engineering, University of Calgary, December
2009,
http://www.ucalgary.ca/engo_webdocs/GL/09.20299.NSokolova.pdf

From Sokolova:

"The Doppler measurements that are used to extract velocity
information can be obtained in two different ways. The first option is
the use of raw Doppler measurements that are the direct output of the
PLL filter. The second method uses time-differenced GPS carrier phase
measurements.

In the literature, the second approach has been considered
advantageous, mainly because of the achievable accuracy and the noise
reduction obtained in the process of computing carrier phase
measurements. Differentiating the observables for velocity estimation
over a short time interval brings some advantageous effects, namely
removal of the residual effects of the tropospheric and ionospheric
delays and the provision of an ambiguity free solution.

Several methods of differentiation have been investigated for this
purpose. . . . "

An earlier paper on the subject, not freely available on-line:

May, Marvin, Nguyen, Khien, Tanju, Bereket, "On GPS Velocity,"
Proceedings of the 46th Annual Meeting of The Institute of Navigation,
Atlantic City, NJ, June 1990, pp. 23-32,
http://www.ion.org/search/view_abstract.cfm?jp=3Dp&idno=3D5147

claudegps wrote:
Sure, that's more precise.

Terje

--
"almost all programming can be viewed as an exercise in caching"