In aviation, satellite navigation is generally only used to determine the absolute position of aircraft. We show that the signals can also be used for safe relative navigation provided that a data link exists between the two aircraft. The link can be used to form a double difference combination of code phase measurements and determine a three dimensional baseline vector. The baseline vector is protected by protection levels which determine the 7 3 10 − × error bound of the baseline estimation. Thus, the distance vector can be used to perform safe approximation maneuvers in instrument weather conditions. We derive the protection level expression and test the baseline vector estimation using data from two real satellite navigation receivers on the ground. Moreover, we simulate an intercept mission using a Spirent GNSS7790 simulator and show that with the derived protection bounds an approximation up to 10 m is possible.