(a) Initial model. (b) Result of the first iteration with 1300 parameters and 1600 iterations. (c) Result of the second iteration with 5300 parameters and 800 iterations. (d) Result of the third iteration with 21 500 parameters and 400 iterations. (e) Result of the fourth iteration with 85 000 parameters and 200 iterations. (f) Result of the fifth, and last, iteration with 340 000 parameters and 100 iterations. The L² norm of errors is 3.5 m for a 400 m difference in the data.