An estimator for determining the range of a space vehicle relative to some origin by using astrophysical signals of opportunity is developed. The signals used are x-ray photons emitted by pulsars. The estimator is an extended Kalman filter, which estimates the time difference of arrival of x-ray photons at the space vehicle and the origin. The performance of the estimator is validated by Monte Carlo simulations and postprocessing x-ray data from the NASA/Japan Aerospace Exploration Agency Suzaku mission. It is found that the proposed algorithm performance is comparable (as measured by estimation error covariance) to the maximum-likelihood estimator used on this problem, which has been successfully used on a similar x-ray pulsar navigation problem in the past.