Data communications: From Pathfinder to public safety
>From the Mars mission to applications for public safety and business, data communications technology is experiencing improvements in error reduction, applicability, security and ubiquity.
Wireless data communications technology is steadily improving for the mobile environment. Better use of available spectrum, software support, secure transmissions and extended applications all contribute to the adoption of data communication. Some of the applications this year have literally been “out-of-this-world.”
Roving around a red planet It may be worthy of some historical note that this is the first time MRT has covered mobile radio technology deployed on another planet. As shown on this month’s cover and in the photo below, part of the hardware contributing to the success of NASA’s Mars Pathfinder mission is a set of radio modems allowing the six-wheeled Sojourner Rover survey vehicle to communicate with the Lander base station.
For the Mars mission, the Jet Propulsion Laboratory (JPL) elected to use a Motorola RNet 9600 SLM modem, which was designed by Dataradio, Atlanta, and which also uses that company’s components. One interesting point about this space hardware is that it is essentially an “off-the-shelf” product designed for terrestrial use.
The “transparent” radio modem is a compact (1″32.5″33.3″) unit with a built-in 2W UHF transceiver. “Transparent” in this case refers to the modem’s transmission of characters exactly as they are presented to the RS-232 port, without adding packetization, addressing or error checking. This minimizes delays for use with other protocols that handle those additional functions for Pathfinder and reduces the earth-bound data throughput by other systems to about 2,400bps.
Handling binary data at speeds as fast as 9,600bps, the modem incorporates logic and modem circuitry, based on Dataradio’s proprietary modem chip, which was then coupled with a Motorola transceiver to make the final product. The low idle-state current consumption of the unit, 35mA, is one of the reasons the modem was selected for the Mars mission.
Two modem units are actually being used on the Mars surface, one in the Rover, the other in the Lander. They transmit telemetry, control and status information between the two mission vehicles over a range of about 500m. The 2W terrestrial version of the modem covers a range of about 50km. Two UHF whip antennas carry the signals. The center frequency for the transmissions is 459.7MHz, with a 25kHz bandwidth.
The only modifications for the Mars application were the addition of heating units to withstand temperatures as low as 21108C, and replacement of some plastic connectors with hardwired connections to withstand the unique “bump and run” balloon-assisted landing that was executed by the lander.
Meanwhile, back on Earth… For more “down-to-earth” applications, Dataradio has produced units for public safety mobile computing and for systems control and data acquisition (SCADA).
Two formats for vehicular information systems, designated PS2000 and PS2001, include NLETS/NCIC database access, status indication, free-form messaging and email. The PS2000 version provides a set of pre-formatted inquiry forms allowing field officers to check license tag, driver ID and stolen vehicle information. An alert feature provides a warning notice for officer attention. The terminal software enables communications with both the base and other mobile units via private digital communications. The PS2001 version is based on a scalable message switch and provides terminal emulation that allows portable computers to be used in place of mobile data terminals (MDTs), with either keyboard or pen-based operation.
Forms can be selected by function keys or a drop-down menu. The messaging features include individual, group or all-call capability. The systems are network-ready for “mug shot” or fingerprint transmission and also provide messaging capability for the Internet. There is also an optional field-reporting system.
The public safety systems include frequency coordination and licensing serv-ices, as well as traffic analysis and a radio coverage study to ensure performance. All hardware, including mobile and base radios, mobile computer and mounts, antennas and other components are provided with a detailed design and implementation plan.
Linking mobile uses to SCADA Dataradio’s Integra modem system allows both fixed SCADA and integration of SCADA into mobile data networks. Four configurations include fixed point-to-point SCADA/telemetry, a wide-area network for SCADA/telemetry, combined mobile data and SCADA, or wide-area mobile data and SCADA. In a full channel, transparent mode, the modem is capable of passing data at speeds as fast as 9,600bps (8,000bps on a half-channel system). For data collection platforms incapable of RTS/CTS handshaking, the modem has a data-detect feature that initiates the transmission.
The modem avoids packet collisions by dynamically allocating channel resources. “Listening” before transmitting allows the modem to use 70% of a channel.
Extending networks with ISDN Intraplex, Westford, MA, has produced a series of multiplexers, designated Intralink, that allow mobile radio operators to use integrated services digital network (ISDN) circuits to extend their networks. It has also created the Securelink multiplexer line designed specifically for agencies using the federal government’s Securenet for transmission of encrypted voice traffic. Both products provide for transmission of voice, data, audio, graphics and video information.
The use of digital ISDN services provides a supplement to the combination of analog microwave and high-capacity T1 lines carrying traffic between dispatch and transmission towers for EMS, police and fire operations. As many as four clear or encrypted full-duplex channels can be supported on two B-ISDN circuits, which can be routed independently to separate locations.
Can you keep a secret? Encryption technology formerly restricted to military applications, is now available for safeguarding public safety, commercial and industrial data. Harris, Rochester, NY, provides a communications security terminal (CST) designed to secure voice, fax and data traffic across a range of communications media, including radio and cellular networks. The CST automatically generates session keys and can be operated without any user configuration. User-specific encryption keys can be inserted and altered using a smart card that can be programmed on a PC using Harris-supplied software. A single terminal can be used in several secure networks. The terminal can serve in a stand-alone secure mode, or it can be connected to an installed PABX to provide security for an entire local network.
Using data networks for messaging Ardis, Lincolnshire, IL, has announced the availability of email and two-way messaging applications for the Microsoft Windows CE operating system on its nationwide data network. Designed for hand-held PCs offered by several manufacturers, the Windows CE system can be linked by a Motorola PM100D wireless modem card to create a mobile messaging system with links to Windows-based desktops, the Internet or intranets. The Ardis system currently is deployed in more than 400 metropolitan areas in the United States, with more than 80% population coverage.