six o rbital planes. The satellites transmit navigation messages periodically. Each navigation message contains the satellite’s orbit element, clock, and status. After receiving the navigation messages, a GPS receiver can determine its position and roaming velocity. To determine the receiver’s longitude and latitude, we need at least three satellites. If we also want to determine the altitude, another satellite is needed. More satellites can increase the positioning accuracy. The positioning accuracy of GPS ranges in about a few tens of meters. GPS receivers can be used almost anywhere near the surface of the Earth. By connecting to a GPS receiver, a mobile host will be able to know its current physical location. This can greatly help the performance of a MANET, and it is for this reason that many researchers have proposed to adopt GPS in MANETs. For example, mobile hosts in a MANET can avoid using nạve flooding to find routes; neighbors’ or destinations’ locations may be used as a guideline to find routing paths efficiently. Several works have addressed location-aware routing protocols for MANETs [Jain et al., 2001; Karp and Kung, 2000; Ko and Vaidya, 1998; Lin and Stojmenovic, 1999; Mauve and Widmer, 2001; Stojmenovic and Lin, 2001]. Proposals that partition the physical area into nonoverlapping zones to facilitate routing have also been proposed [Joa-Ng and Lu, 1999; Liao et al., 2001]. One interesting feature of such zone-based protocols is that a host can easily decide which zone it belongs to, and only one representative host needs to be active to collect routing-related information. The route search cost can be reduced significantly too since nonrepresentative hosts will not flood the route request packets.