s can provide coverage more or less anywhere, but the cost can become prohibitive relative to income. Cellular networks are generally speaking optimised for capacity and increasingly now high per user peak data rates. The wider channel spacing and wider operational bandwidths needed generally reduce the coverage available in any particular band. This chapter looks at low-earth orbit and geostationary satellite systems but we start with two-way radio systems and specialist user requirements. As with terrestrial cellular networks, satellite networks and two-way radio networks are increasingly doing a lot more than just supporting voice traffic and are becoming increasingly information centric and, in the jargon of the sector, 'mission critical'. At the application layer substantial standards commonality is beginning to emerge across all delivery systems, for instance the use of MPEG and JPEG encoding and compression schemes and MPEG-based content descriptors. At the transport layer IP protocols are becoming more pervasive and potentially allow much greater transparency across multiple radio bearers. However, the benefits of this transparency can only be realised if a closer commonality between physical radio bearers can be developed over time. Traditional Specialist User Expectations Specialist users include public safety agencies, fire, police, ambulance, defence users and nonemergency users in transport and distribution, energy and small to medium business users. Traditional expectations in terms of user functionality include open channel working between multiple users based on push to talk functionality, back-to-back direct mode op- eration where users can talk to each other directly without a base station in between and ground-to-air/air-to-ground communication, particularly important for military users or event management where aerial surveillance is required. Making Telecoms Work: From Technical Innovation to Commercial Success, First Edition. Geoff Varrall. © 2012 John Wiley & Sons, Ltd. Published 2012 by Jo