) Branches on the SS Tree Figure 2.28(b). 1965 picture of the MX-118 applique, a member of the ARC-50 radio family and the first to use Gold codes. (Photo courtesy of Robert Dixon.) reduced to one second and an improved ranging system yielded measure- ment in two seconds. Even though the ARC-50 possessed obvious advantages over existing radios such as the ARC-27 or ARC-34, including a hundredfold improve- ment in mean time between failures, there was Air Force opposition to installing ARC-50's in the smaller fighter aircraft. The problem revolved around the fact that pilots were accustomed to having two radios, one being a backup for the other, and size-wise a single ARC-50 would displace both of the prior sets. Certainly, the ARC-50 was a success, and Magnavox became an acknowl- edged leader in SS technology. Among the descendants of the ARC-50 are the GRC-116 troposcatter system which was designed free of a sync pre- amble, and the URC-55 and URC-61 ground-station modems for satellite channels. An applique, the MX-118, for the Army's VRC-12 family of VHF- FM radios was developed, but never was procured, partly as a result of inad- equate bandwidth in the original radios (see Figure 2.28). 2.3 BRANCHES ON THE SS TREE Many designs of the 1940s and 1950s have not yet been mentioned, but those described thus far seem in retrospect to have been exemplary pioneering efforts. It is time now to take notice of several SS systems left out of the pre- vious accounting, some of which were never even prototyped. 2.3.1 Spread-Spectrum Radar With the exception of the 1940s state-of-art descriptions of technology, we have made a distinction between the use of SS designs for communication and their use for detection and ranging on noncooperative targets, and have The Historical Origins of Spread-Spectrum Communications omitted a discussion of the latter. The signal strength advantage which the target holds over the radar receiver in looking for the radar's transmission versus its echo means that LPI is very difficult to achieve