Three papers are presented that develop new estimation algorithms for the relative navigation of high-altitude spacecraft and for systems that have implicitly defined measurement times. The first two papers focus on the use of carrierphase differential GPS techniques with integer ambiguities to estimate the relative states of spacecraft formations operating at altitudes above the GPS constellation. Those papers' contributions include an optimally integrated integer ambiguity factorization method for a square-root information estimator, a robust cycle-slip detection and recovery algorithm, and an extended Kalman filter with an adaptive ionosphere model that allows to make multimodal use of dual-frequency GPS data. The third paper develops a new process noise model and extended Kalman filter that sensibly processes implicitly constrained radio navigation measurements. The filter's models combine dense output RungeKutta numerical integration with a piecewise polynomial approximation to continuous-time white process noise.