The advancement of the technology and broad availability of low-cost devices (e.g., networking technology, micro-embedded computers, and sensors, etc.) have opened new ways for the expansion of flying ad-hoc networks (FANETs). The unmanned aerial vehicles (UAVs) have the potential to change their trajectory freely in the case of FANETs. This freedom demands efficient routing to make the network connection smooth. However, designing routing protocols for FANETs is a crucial task for highly dynamic nodes like UAVs. The position-based routing demonstrated high efficiency and resilience to handle the high mobility of UAV nodes in FANETs as compared to topology-based routing protocols. Geographic routing protocols utilize the coordinates of the node to greedily proceed toward the destination. This article presents an extensive review of position-based or geographic routing protocols in FANETs. Previous survey papers have performed a comparative analysis of all the protocols on different network conditions, assumptions, tools, and settings, therefore their performance is not comparable. The novelty of this survey is that this article performs the comparative analysis of various geographic routing protocols by evaluating all the geographic routing protocols over a common network scenario with a varying number of nodes. The starting phase of this article covers the basic outline of FANETs. Thereafter, an absolute study of the geographic based routing protocol has been presented by distinguishing the routing protocols on the basis of path and packet forwarding strategy. This article further emphasizes on the strength, weaknesses, quality of service (QoS), and comparative analysis of the various characteristics of the geographic routing protocols. Finally, a performance comparison summary of all the analyzed routing protocols for FANETs has been presented. The open research issues have been discussed at the end of this article.

Flying Ad-hoc Networks (FANETs) play a major role in various civilian and non-civilian applications especially in search and rescue operations after natural disasters. Unmanned aerial vehicles (UAVs) collect critical information in the form of images and videos from disaster areas and send it to the ground station (GS) for immediate rescue operations. UAVs capture the information and send it to the GS to carry out the rescue operations without any delay. A small number of UAVs operate over a large geographic area for rescue operations when a sudden natural disaster occurs that creates a sparse or intermittent network. Therefore, routing is the most challenging issue in sparse FANETs due to the low-node density network and high mobility of UAVs Existing routing protocols for connected or dense FANETs cannot be directly adopted in extremely sparse FANETs. Only a few works exist that focus on sparse networks as most of the work is based on connected networks. Traditional routing algorithms for sparse FANETs are based on a flood-based strategy that creates unnecessary delays in the network. Some recently developed routing algorithms directly adopt greedy-based forwarding for routing in sparse networks and consider the non-realistic assumption without considering delay as a significant constraint of search and rescue operations. This paper presents a novel ferry mobility-based direction and time-aware greedy delay-tolerant routing (FM-DT-GDR) protocol for data transfer to GS during disaster relief operations in the sparse FANETs without delay. FM-DT-GDR uses a store, carry, and forward (SCF) mechanism to prevent data loss when no neighbors are present. Upon receiving the beacon from the ferry node, the search nodes calculate the nearest destination from their location between the ferry and the GS and send the data accordingly. The ferry collects data from the search UAVs and sends it to the GS. Optimized ferry trajectories significantly improve route availability between GS and search UAVs Furthermore, the proposed routing scheme efficiently selects the forwarder node to send the data to the selected destination without any delay. FM-DT-GDR provides significant improvements in packet delivery ratio, routing overhead, and end-to-end delay compared to traditional routing protocols.

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