ost 10 ms. Eventually it was demonstrated [148] that the tone burst was not necessary and the four 7- stage clocks were approximately aligned by time of day at 5 min intervals in initial search situations. Transcontinental field trials of the F9C system commenced in August, 1954 [142]. The transmittere was located in Davis, CA, and the receiver in Deal, NJ, to provide an eastbound HF link for F9C tests. A conventional teletype link was supplied for westbound communication (see Figure 2.22). Initial tests verified what many suspected, namely, that multipath could severely reduce the effectiveness of SS systems. While at low data rates an ordinary FSK receiver would operate based on the energy received over all propa- gation paths, the high time resolution inherent in an SS receiver would force Early Spread-Spectrum Systems the receiver to select a single path for communication, resulting in a con- siderable loss in signal level. Based on these early trials, several of the pre- viously mentioned modifications were made, and in addition, it was decided to add diversity to the system to combat multipath. Two receivers with anten- nas displaced by 550 feet were used for space diversity tests, and two corre- lators were employed to select signals propagated by different paths, in tau-diversity (time delay) tests. A second set of field trials began in February 1955 to determine the effects of these changes on performance of the transcontinental link. Results showed that an ordinary FSK system with space diversity and integrate-and- dump reception still significantly outperformed the F9C, with tau-diversity showing some hope of improving F9C performance. Both local and remote jamming tests were conducted in this second series, the interfering signal being an in-band FSK signal with MARK and SPACE frequency spacing identical to that of the F9C data modulation. The remote jammers were located at Army Communication Station ABA in Honolulu, HI, and at the Collins Radio Company in Cedar Rapids, IA. With tau-diversity, the F9C