Telemetry

For all but specific PI-driven experiments using some of Flexible Array equipment, all sensor signals digitized and time stamped at the sensor site are continuously transmitted in near real time to the Array Operations Facility.

The advantages of the digital USArray network are:

• The full bandwidth and dynamic range of the sensors can be recorded
• Data-transmission protocols provide automatic error-correction, allowing the retransmission of lost or corrupted data, thus eliminating data gaps due to transient problems with telemetry.
• Continuous data transmission amortizes the load on quality-control and archiving functions at the Array Operations Facility and Data Management Center.
• Emerging and latest communication technologies can be used, such as portable satellite systems.
• Continuous data transmission provides excellent network state-of-health monitoring, which has immediate advantages for management of maintenance logistics.
• All telemetry will uses TCP/IP as the transmission protocol.
• The type of telemetry used at each site will depend upon local environmental conditions, permit limitations, and existing (if any) communications infrastructure.

Portable VSAT Modems

The most preferable solution is portable VSAT modems. This system is a two-way satellite Internet "always-on" system consisting of a 0.5 m dish and a satellite modem that routes data from remote users through, for example, the GE-4 and Telstar 7 satellites to a downlink hub and then to the Internet. The system is targeted for home users with the expectation of low uplink and high downlink bandwidth requirements and requires an onsite computer and AC power (modem draws 30 W power). Two-way satellite-based Internet service offers download speeds close to those available via cable modem or DSL without the need for phone lines or an Internet Service Provider (ISP). The dish is connected to a satellite modem with a dedicated IP address using standard coaxial cables.

Satellite coverage is provided throughout the conterminous United States and Alaska. Cost is estimated to be $3600 for the dish and modem, and $180/month for continuous operations. As market penetration spreads into the home consumer market, these prices will likely continue to fall.

Advantages of VSAT modems include rapid deployment and demobilization time. Disadvantages include high power consumption (15-20W), and requirements for the dish to have a clear view of the southern sky.

Radio-Frequency (RF) Links

This is a well-established telemetry technology that has been in use in several PASSCAL programs for many years. RF modems are serial radios that provide a digital carrier detect signal to denote a working link. RF links will likely be used in hostile terrain, or to carry the signals to a drop point that provides access to the Internet. While the technology is quite mature, the disadvantages are potentially lengthy deployment times, and higher failure rates than the consumer-based satellite technology. Furthermore, RF modems do not communicate using IP, so the signal must be converted to IP at the drop point.

Direct Access to the Internet

Local Internet access may be available if the site is located on private property, near buildings, etc. This has the advantages of rapid deployment time, as well as providing power and possible security. However, these are often sites of considerable cultural noise.