Iridium Geo-Location - Iridium SHOUT ns General Description Manual

based on a 7-bit alphabet, which is encoded and transmitted as 8-bit data, hence the 140
octet (byte) maximum message size.
SMS is a store and forward method of transmitting messages to and from an Iridium
modem. The short message from the modem is stored in a central Short Message Service
Center (SMSC) which then forwards it to the destination. In the case that the recipient is not
available, the SMSC will attempt to deliver the SMS message until it is delivered or the validity
period expires. SMS supports a limited confirmation of message delivery. The sender of the
short message can request to receive a return message notifying them whether the short
message has been delivered or not. With this option, the originator gets a confirmation that
the message was delivered to the SMSC. Unlike standard GSM, the Iridium SMS can only
acknowledge that the message was delivered to the SMSC and not the end-destination.
SMS messages can be sent and received simultaneously while a voice call is in progress.
This is possible because SMS messages travel over and above the radio channel using the
signaling path, whereas the voice call uses a dedicated "traffic" radio channel for the duration
of the call.
C.3 Iridium Geo-Location
The Iridium network makes calculations of the geographical location (geo-location) of an
ISU each time a call is placed. The technique employed to determine the geo-location of an
ISU is based on measurements of the ISU and satellite propagation delay and Doppler
frequency shift. These measurements are used to estimate cosines of spherical angles that
identify the ISU's location relative to the satellite by the gateway.
The Iridium network can locate an ISU to within 10 km only about 78% of the time. The
so-called error ellipse can have a large eccentricity with the major axis oriented in the azimuth
dimension and the minor axis oriented in the radial dimension. The position of the ISU in the
radial dimension relative to the satellite can almost always be determined to within 10 km
with just one measurement. Errors in the azimuth dimension relative to the satellite are
largest along the satellite's ground path and tend to increase with distance from the satellite.
Geo-location errors in the east-west dimension, therefore, are sometimes more than 100
times greater than in the north-south dimension.
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