Method and apparatus are disclosed for accurately determining position from GPS satellites and received on a ship using C/A code group delay observable TAU, 308 f.sub.o implicit carrier phase observable PHI, L1 band implicit f.sub.o carrier residual phase observable PSI.sub.L1, and L2 band implicit f.sub.o carrier residual phase observable PSI.sub.L2. A precise measurement of the group-phase delay range to each satellite is made based upon the L1 center frequency carrier phase. A correction for ionospheric effects is determined by simultaneous observation of the group delays of the wide bandwidth P code modulations in both the L1 and L2 bands. These group delays are determined by measuring the phases of carrier waves implicit in the spread-spectrum signals received in both bands. These carriers are reconstructed from both the L1 and L2 band signals from each satellite without using knowledge of the P code. The unknown biases in the L1 center frequency carrier phase range measurements are determined from simultaneous, pseudorange measurements, with time averaging. The instantaneous position of the ship may then be determined from the ranges so determined, with both the bias and the ion-spheric effects having been eliminated.