RSTA-MEP and the Linux Crewstation
We recently completed a prototype Linux crewstation for the Reconnaissance, Surveillance and Target Acquisition Mission Equipment Package (RSTA-MEP). This article briefly describes the whole system, then focuses on the crewstation portion. The Raytheon RSTA-MEP program provides the capability to assess the battlefield quickly through real-time information from the fusion of onboard and offboard sensors. Advances in sensors and software provides for wide-area-search (WAS) imaging, automatic target detection (ATD) and aided target recognition (AiTR) capabilities. These capabilities provide the crew with real-time data, including target position, classification and priority. Combining this with the US Army's Tactical Internet allows the crew to formulate and contribute to a common operating picture of friendly and enemy forces. This vehicle is a technology demonstrator to show what emerging capabilities can be added into existing and future reconnaissance vehicles.
In its current incarnation, the vehicle has thermal sights to allow the user to see in the daytime or at night. The primary sensors on the mast are a long-range Forward Looking Infrared (FLIR) sensor, an Inertial Navigation System (INS) and a Global Positioning System (GPS) receiver. In addition to what's on the mast, there are several Raytheon NightSight infrared sensors attached to the vehicle so that the rest of the crew can look at the immediate area around the vehicle.
The mast is four meters high, and including the height of the vehicle, the sight is over five meters high. The vehicle has a three-member crew: driver, commander and scout/operator. The driver can also use the NightSight sensors to drive in the dark and look around for security purposes. The commander also has controls for the NightSight sensors, operates the connection to the Tactical Internet and directs the other two. The scout/operator uses the Linux crewstation to operate the mast-mounted sensors and their associated embedded systems.
The RSTA-MEP system is mounted onto an H1 Hummer and consists of a mast-mounted sight, embedded computers and a crewstation PC running Linux (Figure 1). These parts connect together as shown in Figure 2.
The embedded computers are digital signal processors that control the mechanics and electronics of the sensor (for example, pointing the sensor or cooling its detector) and some of the image processing. PowerPC boards running VxWorks and single-board computers running Microsoft Windows NT and Sun Solaris also are used. The applications running on these computers include the Force XXI Battle Command Brigade and Below (FBCB2, a US military digital command and control system), target detection and recognition, real-time image processing and communications. The package also includes a GPS receiver, inertial navigation system and digital map functionality. The embedded systems communicate with each other using Ethernet and Virtual Interface protocol (VI) on Fibre Channel.
The prototype Linux crewstation is the successor to an earlier system. The ideal case would have been for our new crewstation to fit in exactly the way the predecessor crewstation had. Our initial attempts to use VI on Fibre Channel failed. Our embedded systems group had considerable experience with vendor compatibility issues, so in selecting Fibre Channel hardware we were limited to vendors who supplied cards and drivers for both VxWorks and Linux. We couldn't find any of those who supported VI protocol. Our second attempt was to try to use disk emulation and imitate a hard drive connected to Fibre Channel, so we could at least stay on the same media.
The results there also were unsatisfactory, so we went to gigabit Ethernet. Ethernet would carry both the video from the sensor and the command and status data between the crewstation and the embedded systems. When looking at gigabit Ethernet, the home audience must consider four things: packet size, media, interconnecting and network interface card. Regular Ethernet has a maximum packet size of 1,500 bytes. An emerging standard for gigabit Ethernet is to allow a 9,000 byte maximum, called jumbo packets. For this project, our concerns about vendor compatibility between the embedded side and the Linux side pushed us to regular packet size.
The second consideration is media. Gigabit Ethernet cards come in two varieties, copper and optical fiber. Although copper is susceptible to electromagnetic interference (EMI), fiber is mechanically delicate. We chose copper because it's cheaper. If EMI became a problem, we always could get an optical card later without changing software. The choice of copper also meant we were compatible with our lab's infrastructure, courtesy of autonegotiation. Our existing network was 10/100 copper.
The third consideration is interconnect. The situation doesn't change too much from 10/100 Ethernet; you have switches, hubs and crossover cables. Switches route traffic so it's seen only by the intended recipient. They handle connections with differing speeds and duplexes, and they have the all-important blinky lights (the status lights on the switch that blink to show activity) to help with debugging. The disadvantages of switches are the cost and that you need a managed switch if you want to use a packet sniffer.
Hubs are the second choice. On the plus side, they are cheaper than switches and have the status lights. On the minus side, we know of no hubs for gigabit Ethernet (only switches), so if you use a 10/100 hub, you sacrifice speed. Hubs also send all packets everywhere, which is good if you're trying to sniff packets but bad if you're trying to limit the amount of traffic on an interface.
Crossover cables are the simplest option. They're the cheapest choice; they require no additional equipment, and you can be sure that no packets are coming from an outside source. On the other hand, there are no blinky lights, no way to connect an outside packet sniffer, and if one interface goes down (common for restarting embedded hardware), so does the other.
We chose switches, although the choice between switches and crossover cables is still a subject of religious debate. We also can pass on a caution about gigabit cabling. Professionally made category 5e or 6 cables are preferable to home-brew cables.
The fourth consideration is your network interface card; they generally come in 32- and 64-bit flavors. The 64-bit cards typically perform better with less draw on your PCI bus' resources. Although we didn't perform a trade study on available products, we chose the Intel Pro/1000 Server Adapter.
We chose to use TCP/IP on Ethernet. Although TCP is slower than UDP, it is a reliable protocol that compensates for any dropped, duplicated or reordered packets. We wanted to get the best-quality video in the face of possible EMI on the vehicle, so we deemed that built-in error correction was essential. Also, because no information is lost, duplicated or delivered out of order, command and status information would be reliable. When coding the socket layer for this, we had to tune the sizes of the socket send and receive buffers (using setsockopt with the SOL_SOCKET SO_RCVBUF and SO_SNDBUF options) to get enough throughput for the video. We also turned off Nagle's algorithm (setsockopt with IPPROTO_TCP and TCP_NODELAY) to reduce the latency between the crewstation and the embedded system, making it more responsive to sensor-pointing commands from the grips attached to the crewstation.