1. A method comprising: measuring a plurality of pseudoranges using an antenna; performing a first position fix iteration using the plurality of measured pseudoranges to form a first measurement error vector; performing a second position fix iteration using the plurality of measured pseudoranges to form a second measurement error vector; computing a first mean of the first measurement error vector; subtracting the first mean from the first measurement error vector to form a first zero-mean error vector; computing a second mean of the second measurement error vector; subtracting the second mean from the second measurement error vector to form a second zero-mean error vector; computing a sum vector of the first and the second zero-mean error vectors; and determining whether an integrity failure has occurred based on the sum vector.