Inmarsat satphone operation
The following explanations give an idea of the differences in operation between the Inmarsat GAN and Mini-M satphones, and demonstrate how units of each type are typically assembled and operated. These are currently the two most common Inmarsat systems used by newsgatherers.
Inmarsat GAN
There is a limited selection of brands of Inmarsat GAN satphones, and the Inmarsat GAN satphone typically consists of two parts – the antenna and the transceiver unit – integrated together.
The unit has a fold out panel antenna, and the whole unit is pointed approximately towards the satellite. The unit typically has both an audible and visual display of signal strength, and as the unit is rotated slowly towards the satellite, the signal strength indication will increase. The elevation angle of the antenna also has to be adjusted to obtain optimum signal strength.
Why is the antenna-pointing not as critical as for the larger SNG uplinks? The answer lies in the operating frequency of Inmarsat satphones. They all operate in L-band (around 1600 MHz), and therefore because the beamwidth is considerably wider, less pointing accuracy is required.
Because the pointing is not so critical, it is possible to even transmit videophone pictures from the decks of large warships. Incidentally this can be done with Ku-band SNG uplinks as well, but the antenna needs to be only around 1 m, and will need manual tracking as the ship moves through a relatively calm sea.
Siting the satphone
To operate successfully, the satphone needs to be able to ‘see’ one of the four Inmarsat satellites. When siting the antenna, a compass is used to check that there is a clear view towards the satellite – to the south if in the Northern hemisphere, and to the north if in the Southern Hemisphere. The satphone can be used inside a building if the antenna is pointing out through a window – the window does not need to be open, but beware of metallized coatings on the glass used to prevent glare or offer privacy. The window obviously needs to offer a view of the sky in the right direction.
• Do not use the compass near the satphone while it is switched on, as the electromagnetic field from the satphone antenna can cause a false reading.
• Do not try and take a reading from the compass near vehicles (metal affects the reading), nor inside buildings, as many modern structures have an integral steel frame that will affect a compass reading. Tall buildings or trees close-by must not obstruct the view towards the satellite. For safety reasons, the antenna should not be pointed towards areas where people may pass or gather, and ensure that no cables create a trip hazard.
A few seconds after the satphone is switched on, the handset display should give various status indications, telling the user which LES is selected (usually the one last used). To make a reliable HSD call, the antenna must be pointing very accurately at the chosen Ocean Region satellite. This is not the case for simple voice calls, which with a much lower data rate do not need such critical alignment. There is usually a chart supplied with the terminal that will give an estimate of the azimuth and the elevation of the antenna. The whole satphone may be rotated until the antenna points in the direction indicated by the compass. The terminal display gives an indication of signal strength.
Safety
The antenna produces significant levels of non-ionizing radiation and the manufacturers’ recommendations for minimum clear distances in front of the antenna must be followed: 2–3 m is a typical ‘safe zone’. Standing behind the antenna is perfectly safe, and it is advisable to mark out the safe zone with visual warning tape. However, in HSD mode, satphones have an automatic transmission cut-out which operates within 15 s of an obstruction appearing in front of the antenna. This is not the case when it is operating in voice-only mode, when care must be taken that no one stands in front of the antenna. Such is the resilience of the data link for voice transmissions, the satphone may be able to maintain a connection even with a person standing in front of the antenna.
Operating
All commands are keyed into the DECT handset, which displays all information needed. Operational controls are accessed from the handset with error messages and the progress of the call displayed on the handset. Once aligned and set up, the satphone is ready for operation. Each manufacturer’s unit will vary in the keystrokes required to set the mode of operation and dialling sequence. Different codes are used in the dialling sequence to establish the type of call (voice, HSD, etc.) as indicated earlier in this chapter. Any additional units such as videophone, digital audio encoder, or Store and Forward units will also need to be connected before an HSD call can be made.
Inmarsat Mini-M
The operation of the Inmarsat Mini-M satphone is very straightforward. The satphone comes in a small laptop-sized case, and the antenna is usually integral to the case. It is simply a matter of flipping the antenna up, pointing it approximately in the direction of the satellite, and optimizing the position in azimuth and elevation to get the strongest signal strength. Once this has been done, the handset can be used to dial the call directly. Because the RF power from the Mini-M satphones is much lower, there need only be a one metre area kept clear in front of the satphone for safety reasons.