The Martian ionosphere is mainly modulated by differing levels of solar EUV radiation. Since Mars lacks an intrinsic global magnetic field, the solar wind and interplanetary magnetic field (IMF) interacts directly with the planets atmosphere and upper ionosphere. The modulation of the ionosphere thus changes the effective obstacle resisting the solar wind flow. When the effective obstacle varies, the location of the formation of the bow shock boundary (where solar wind is rapidly slowed) may also vary. The ESA Mars Express (MEX) spacecraft has been sampling the Martian plasma environment for in excess of 12 years. Using superthermal electron measurements by the Analyser of Space Plasma and Energetic Atoms (ASPERA-3) electron spectrometer instrument on-board MEX, we have identified the location of the Martian bow shock boundary over this time period. Using these results, we present an updated average bow shock location model, and its use in identifying the possible modulation of the bow shock by the conditions of the Martian atmosphere and upper ionosphere.