Aerospace was part of a team established to analyze the various alternatives for a new military GPS frequency. The researchers faced no restrictions in terms of the candidate frequencies or bandwidth, but their initial examination quickly ruled out all frequencies except for a narrow band around the current two GPS frequencies. For ranging to a satellite the way a GPS receiver does, narrow bandwidth modulation (below approximately 1 megahertz) does not provide usable positioning. Lower frequencies do not penetrate the ionosphere but are reflected, resulting in the phenomenon called "skip," which makes the atmosphere opaque to radio transmissions. Higher frequencies are heavily attenuated by moisture in the atmosphere, especially at low grazing angles. On the other hand, components of transmitters and receivers can reasonably accommodate percentage bandwidths of below a few percent without introducing serious distortion or undue expense. Therefore, the candidate choices for this military frequency were right around the L or S bands. Not surprisingly, these are exactly where most satellite systems are vying for spectrum. At a loss for additional frequencies, Aerospace suggested a different approach. Applying the concept of spectrum reuse, whereby niche frequency bands are sometimes filled within allocations of different services, the unused portion of the GPS registered frequencies were identified. While the C/A and P(Y) codes are centered on the L1 and L2 carriers, the outer edges of the band are virtually unused. An Aerospace-patented application of biphase modulation was suggested to "split" the new military signal, allowing it to straddle the center of the band. Thus, the M code was born.