Satellite Telemetry Options For Hydrographers

Matt Saunders - Unidata , 01 July, 2012

Unidata has continued its work on integrating several other satellite services into its range for remote monitoring. This article provides a summary of common satellite services available.

Unidata has tested / completed testing / integration of Iridium, Thuraya, Orbcom and Inmarsat as well as Globalstar, and we have done modelling on the data latency and equally importantly, the costs of these services. We have also modelled power consumption of these services. It is now clear to us that while these services are all satellite technology there are subtle aspects of each service which need to be considered. There are three distinct applications, where some service providers are more suitable than others.

If you have the need for what we call “alert” function, services such as Iridium short burst data, Inmarsat ISAT Data pro or Orbcom are probably best, especially Iridium and Inmarsat ISAT Data pro which are premium services. However, the data transmission costs escalate greatly for these services when the data rate is anything more than an occasional transmission of a short packet of data.

If you have the need for a traditional logging and reporting function, such as regular hydrographic data from a measurement site, perhaps with water level, flow and quality, then services such as Globalstar and Inmarsat are probably best as the data transmission costs are more reasonable for larger volumes of data.

In early 2012, Inmarsat released a new service, the Inmarsat BGAN M2M, and this appears to be the current leader in airtime pricing.

As technology improves, and as users choose to have a low resolution or high resolution image or perhaps a video capture capability, we expect the data load from traditional logging and reporting sites will grow. This will make the cost of sending larger amounts of data more regularly more important. For example, the routine transmission of an image, albeit a compressed image, from a measurement station camera is a new facility which Unidata, and other suppliers are now supporting.

Satellite services are also used for only the most remote locations. Another important consideration for such very remote locations is power consumption. The ability to turn off the satellite modem most of the time to conserve power is critical. Some services allow for this easily. However, some services need to establish a “session” each time the modem is powered on. The communication overheads in establishing a session are high, increasing the overall monthly cost to sometimes prohibitive levels.

The physical location of the measurement station is another consideration, and there is a clear distinction between Low Earth Orbit Systems and Geostationary Systems. If a Geostationary System is chosen, the satellite antenna must be located such that there is a clear view towards the geostationary satellite. Due to the satellite’s location in the sky, this may sometimes not be possible, especially in deep valleys. If a Low Earth Orbit System is chosen, these systems transit the sky and satellite antennae, and regardless
of location, should be able to “see” a satellite most of the time.

The commercial aspects and ongoing viability of all satellite providers needs to be considered. The primary purpose of the Inmarsat satellites used to be to provide for safety of ships at sea. Inmarsat is short for the “International Marine Satellite Organisation” and is well funded for this purpose and has been operating for many years.

Low earth Orbit systems are different, with Iridium providing the highest satellite numbers today, although the Iridium satellites are now closer to the end of their design life.