Data telemetry via satellite

 

 

Eutelsat

A number of satellites are available for data transmission. In this part of the world, Eutelsat is one of the satellite consortiums that provide data communication facilities, they have something like 14 geostationary satellites.

SCADA and telemetry systems using Eutelsat are generally based on VSAT services - Very Small Aperture Terminal.

Or in other words, small dishes.

There are various providers offering hardware orientated towards SCADA and telemetry which use the Eutelsat VSAT services. Some of the typical features or characteristics include :-

Intelsat and Inmarsat are two other satellite service providers.

Eutelsat is widely used for the monitoring and control of remote fixed installations like pipelines, dams, and unmanned power stations. However geostationary satellites sit over the equator, and provide little or no signal coverage at the polar regions.

 

Iridium

Iridium is another satellite provider - they use a mesh of 66 low earth orbit satellites, which provide full coverage of the polar regions - this is a big selling point, as the footprints of most geostationary satellites don`t cover the polar regions. Another advantage is the size of the antenna - Iridium doesn`t need dishes, it can use small antenna just a few centimetres in length.

As well as carrying voice traffic, Iridium also provides a number of data communication services. These include :-

There are a few modems available for use with Iridium, including these ones, but there are others -

I think you can also get a version of the 9601 with built in GPS.

The Iridium data services are two way, so control information can also be sent back from the labs to the unmanned remote devices.

Iridium communicates with the user devices in a complex system that uses both TDMA and FDMA. There are 240 channels, with a channel separation of 41.666khz, and a bandwidth of 31.5khz.

Each channel is divided into 9 time slots in a 90msec time frame -

Each frame provides 2250 symbols, at a channel modulation rate of 25 ksps. User data rate is 2400 bps per link, but of course you can buy multiple links for faster speeds.

The carrier frequency for the user device communications is in the L band, in the range 1616 - 1626.25 Mhz.

Latency is around 1800msec.

Communications between satellites, and between satellites and the base stations, are in the Ka band -

There is a fortuitous similarity between the carrier frequencies of 1616 - 1626.25 Mhz for the user devices on Iridium, and the carrier frequency of 1575.42 Mhz used for the L1 civilian service of GPS - it means that whip antenna on specialised equipment can be designed so that the antenna works for both Iridium and for GPS.

However the development of the new L2C civilian GPS service that uses the existing military L2 frequency of 1227.60 Mhz may well challenge this situation, due to the much greater difference between the frequencies.

The new L2C civilian service will provide greater accuracy and less ionospheric interference, but the receiver will need to receive both the current L1 frequency of 1575.42 Mhz and the L2 frequency of 1227.60 Mhz. However I think the existing service and the new service are going to co-exist, so using the new service will be optional.

In general terms, as well as its global voice communication services, the Iridium data carrying services have been widely adopted in maritime research, with devices like unmanned buoys and unmanned autonomous underwater vehicles using Iridium data services to send data back to research labs in real time or in data bursts, for months at time - at a fraction of the cost of doing the same kind of research and data collection from ships.

However there is a certain irony in the fact that whilst Iridium has enabled a significant expansion in maritime and environmental research, it has had a detrimental effect on another area of research - namely radio astronomy. Hydroxyl is a common interstellar molecule that emits radiation at 1612 Mhz - a lot of radio astronomy research is done by listening for very faint signals on 1612 Mhz, and this is just below the Iridium user device frequency band.

It looks as if Iridium has exclusive use of the frequency band 1621.35 - 1626.25 Mhz, and shared use of the band 1616.0 - 1621.35 Mhz. There are agreements between Iridium and some countries that, for specified periods, Iridium will either restrict the service to the upper exclusive band of frequencies, or will go quiet over specified areas.

So although Iridium comes very close to providing a full global 24/7 coverage, it doesn`t quite get there.

 

Argos

Argos is a satellite system that is designed specifically for data collection and for tracking, for environmental research purposes. There are several different world wide research organisations involved in its management.

Its use is restricted to organisations who are involved in environmental research.

Argos is piggy-backed onto a constellation of weather satellites that have an orbit that cover both poles, so Argos can offer full polar coverage. As yet, I haven`t discovered how many satellites are involved, it is certainly a few. There are not enough to provide coverage of the whole earth at any one time, so coverage is characterised by an intermittent nature.

When a remote transmitter has contact with one of the satellites, it uploads its data - this data is stored on the satellite, and is downloaded to an earth station when one is in view. Argos has a network of earth stations, so this download may be almost immediate, or it may be delayed by a few or several minutes.

I can`t see from the Argos website at www.argos-system.org whether it is a one-way or a two-way system - ie, whether it only allows for the uploading of data from the remote site, or whether it allows two way data traffic. However another website at www.thalesgroup.com suggests that Argos can be used for sending control data back down to the remote site. Other websites suggest that Argos is one-way only.

Uplink bandwidth is restricted to 256 bits of data, so each uplink session can only carry 32 data bytes, and there will be minutes or hours between each uplink session.

Each remote transmitter has to have a unique identification number.

The satellites move in a sun-synchronous orbit at right angles to the equator, and across the poles. As the satellites traverse their orbit, the earth is rotating underneath them, so each satellite sweeps across a different longitudinal section of the earth in each orbit. Because the footprint is a fixed width, there are significant gaps between each footprint at the equator, but much smaller gaps at the poles. So polar regions have a higher visibility than equatorial regions. At the equator, a transmitter will see a satellite for 80 minutes in 24 hours, whilst at the poles, a transmitter will see a satellite for 384 minutes in every 24 hours.

A transmitter at the equator will see a satellite 7 times in 24 hours, a transmitter at the poles will see a satellite 28 times in 24 hours.

Because of the limits of data volume on each uplink session, uplink bandwidth is restricted to 448 bytes per 24 hours at the equator, and 1792 bytes per 24 hours at the poles.

Argos has a transmitter positional tracking ability built in, by using doppler shift on the uplink carrier frequency. In addition, this can be supplemented by GPS.

Because of the intermittent nature of the coverage, and the severe limitations of data bandwidth, research organisations would be struggling to use Argos for much more than position tracking. It would definitely be of no use for autonomous underwater vehicles, which spend most of their time underwater, just coming to the surface to contact a satellite every few hours, and uploading several kilobytes of data, and receiving downloaded control data. Especially in the artic region, where there is a risk of the AUV getting trapped in ice if it stays on the surface for more than a few minutes. An AUV would not want to sit on the surface for up to an hour waiting for a satellite to appear. For AUV`s, Iridium is king.

 

 

 

 

 

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