This article first appeared in Mountain Rescue Magazine. Winter 2015 issue
With increasing use being made of web-based applications such as SARLOC, SARCALL and ViewRanger (BuddyBeacon) together with the clear advantages of being able to connect MRMap to its servers, the availability of an Internet connection to Search Managers has become increasingly important. This presents relatively little difficulty when teams are managed from a fixed location, such as a Rescue Centre of Police Station, but is more challenging when mobile Control Vehicles are used. This short article describes how the Dartmoor Rescue Group (DRG) is attempting to solve the problem and is written in the hope that our approaches may be of interest to other teams.
Search and Rescue on Dartmoor and the surrounding areas is undertaken by the four Dartmoor Search and Rescue Team (DSRT) charities that comprise DRG. Almost all DRG’s Callouts involve deploying a Control Vehicle to a rendezvous (RV) point, usually somewhere on Dartmoor, where there is sufficient parking for team members and from where a misper search or casualty recover can be undertaken most conveniently. Under these circumstances, the only options for Internet connection appear to be satellite or the cellular ‘phone network unless an opportunity exists to piggyback on a third party’s Wi-Fi. Within the Dartmoor National Park boundary, however, only one mobile ‘phone mast has been allowed and all service providers have their aerials at various heights on the VHF radio mast at North Hessary Tor. As a consequence, only a G2 (GPRS) data signal is available over most of the Moor and there are many areas where no signal can be received at all on a mobile ‘phone. Better reception is available, of course, on the edges of the Moor from masts outside the National Park boundary.
DSRTs Ashburton, Okehampton and Tavistock have opted to use the cellular ’phone networks to connect their Control Vehicles while Plymouth is currently planning to use the more expensive but potentially more reliable satellite approach. Ashburton and Tavistock are using the same equipment and this is described in more detail.
The heart of the Ashburton/Tavistock systems is a 434T WiFi 3G Modem Router (Photo 1) which, in addition to having 4 Ethernet ports has a PSTN port which allows a standard telephone to be connected. Ashburton connect the router to their Search Management PC using one of the Ethernet ports while Tavistock makes use of the router’s Wi-Fi signal to connect to its Search Management laptops.
The router is connected to an omnidirectional A0121 external aerial offering a claimed 5dB of gain. The equipment was provided by Solwise and details can be found on their comprehensive website at solwise.co.uk. To access the mobile ‘phone networks at a cost acceptable to the two teams a network roaming SIM, provided by Gould Two Way Radio Ltd is used, called “g2sim”. Details of this are at www.g2sim.co.uk. To date, Tavistock’s experience with the A0121 external aerial, mounted as it is on a hinged 2m aluminum mast, (Photo 2) and the g2sim is that a mobile telephone signal is received and MRMap connects to its servers at RVs where an ordinary mobile ‘phone detects no service.
One of the attractions of the Ashburton/Tavistock approach is that the roaming SIM allows Internet connection while the Control Vehicle is being deployed. Having initiated a Callout using SARCALL, a Search Manager currently loses visibility of the team members’ responses until Internet connection is re-established at the RV. Tavistock’s experience is that internet connection using the roaming SIM is adequate to keep a laptop or tablet updated with SARCALL responses as the Control Vehicle deploys, even with the cellular aerial clipped down in its travelling configuration. See Photo 3. Thus the Search Manager has an updated statement of team member’s responses on arrival at the RV.
Trials at different RVs continue but the feasibility of building a cellular/Wi-Fi repeater has been discussed with Paul Stephenson, Solwise‘s Managing Director. If needed at an RV with no cellular signal, the repeater would be deployed to an elevated location (on a hillside, for example) offering line-of-sight to both a mobile ‘phone mast and the ControlVehicle. The link between the ‘phone mast and the repeater would be cellular while the repeater to Control Vehicle link would use Wi-Fi technology. The schematic at Figure 1 embodies the principle using components available from Solwise. A relatively small capacity lead-acid battery would satisfy the modest RF powers involved so such a repeater would be highly portable compared with the portable VHF repeaters currently used.
To receive the Wi-Fi signal at the Control Vehicle, another Access Point/Client Bridge would be used. This would be mounted on the roof of the Control Vehicle and would be trainable in the direction of the repeater. It is envisaged that the 8dB of built-in antenna gain provided by the ENS200 would be sufficient for this application. The connection components between Access Point/Client Bridge and the existing Control Vehicle’s 434T Modem Router would be as shown in Figure 1.
As yet, the need to build the repeater has yet to be justified operationally … which is a pity as it would be good fun getting it up and running!
Dartmoor Rescue Group
Solwise Products mentioned