GPS-determination velocity with carrier phase

Hello,

Sorry for my email, but I'm looking for help on GPS for determination 
velocity...
I'm working on GPS data, and I'm using the carrier phase for 
determination of velocity, but in static test the results have a bias  
(like offset -1m/s , 1.4m /s and 0.4 m/s) , what could be the problem? 
can anyone help-me?

I use

obs = Hv

and H is the direction cossenos
obs= derived( phase) - dtcorr*v_light + Hv_{velocity of satellite, given 
by derivation of equations of ICD  }

were dtcorr is the derive clock drift = Eph(2,k)*dt + Eph(20,k)

and solve this by means of least squares..
should I subtract something else? the relativist term should be used?

the ionosphere and troposphere could be neglected  ?

I have attached one graf witch the red is velocity derived by position 
and blue is the estimation by using carrier phase....

many thanks,
Filipe Santos

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