This short note is to acquaint Arecibo users with a potential RFI threat for certain L-band observations. This involves a new broadband civil GPS system, GPS L5, planned for the band 1176.45 +\- 12 MHz (1164 - 1188 MHz). Recent information slates this system for a first launch in 2004, with a full satellite constellation in 2011. It is proposed that the signals of GPS L5 be 6 dB stronger than those of the current GPS L1. The "HI redshift space" that may well be lost on an essentially permanent basis will be at least 0.195 < z < 0.220. Assuming this system is implemented as described above, there is also likely to be a severe effect on high-redshift searches for galaxies containing OH megamasers (OHMs) at z ~ 0.42.
Galaxies have been detected from Arecibo in "the GPS L5 band" both in HI emission and absorption. However, detection of HI (and OHM) emission requires particularly long integrations at these redshifts, even with the 305-m telescope. In addition, 1125 - 1225 MHz is a band that Arecibo pulsar astronomers now use regularly, and find to be relatively clean.
In view of the above, NAIC now appeals to potential users of the above frequency band to consider at a high priority the submission of Arecibo proposals that absolutely need to observe within the proposed GPS L5 frequency band. This appeal is seen as the astronomical equivalent of an archeological "rescue dig" of a site about to be destroyed by land developers!
For completeness, we mention that a new coded GPS signal is planned for 1227.6 MHz, this being co-frequency with the existing GPS L2 transmissions.