This paper presents a method for extending the launch window duration of the Global Positioning System Replenishment satellites to be launched into the existing 24-satellite GPS constellation. The method was developed by Lockheed Martin Missiles and Space to extend the current launch window of the GPS II Replenishment satellites under procurement by the United Slates Air Force. For any given day of the year, a GPS IIR space vehicle must be capable of being launched into any one of six orbital planes, each inclined 55 degrees, and each spaced 60 degrees apart in right ascension. Thus, any given day has six identical launch windows spaced four hours apart which permit access to any of the six different planes. A direct injection into any one of the GPS orbit planes will permit a launch window no greater than sixteen minutes in order to achieve the correct right ascension within the required accuracy limits of +/-2 degrees. However, GPS satellites are first injected into a highly elliptical, 39 degrees inclination transfer orbit possessing an apogee greater than the final GPS orbit radius. Since the apogee of this transfer orbit is greater than the final orbit, it is possible to perform the transfer on either side of the transfer orbit apogee and extend the launch window by a factor of three. The final GPS orbit is achieved by a single burn of a solid rocket motor either at pre-apogee or post-apogee from the transfer orbit A mission plan allowing only a pre-apogee burn would Limit the launch window to only eight minutes accounting for system errors. The transfer burn simultaneously circularizes the orbit increases inclination to 55 degrees, and achieves the proper right ascension for the selected GPS plane. Orientation of the space vehicle solid rocket motor to achieve final GPS orbit is based on a technique presented in this paper called 'Drift Targeting'. Assuming no errors, the solid rocket burn will produce the desired right ascension and inclination with the ascending node possessing a pre-selected drift toward its final location. The effects of uncertainties in the elements of the transfer orbit space vehicle motion and pointing, and burn impulse and timing have been combined in a Monte Carlo analysis to determine their constraining effect on the extended launch window duration. The impact of a burn postponement of one day is also factored into the launch window determination as an added contingency.
