Satellite Visibility - Iridium
The Iridium Satellite system is global in coverage. If a the Field Tracker 2000 has a clear view of the sky, genearlly a satelite is visible. The Field Tracker 2000 provides the strength of the received signal on a scale of 0 to 5, where 5 is the strongest signal. Visibility of satellites may be temporarily blocked by mountains, buildings, items in a backpack, etc. However, since Iridium satellites pass from horizon to horizon to an observer on the ground within 15 minutes, if no satellites are in view due to mountains or buildings, generally a satellite becomes visibile within a few minutes.
When an Iridium satellite is visible, transmissions to and from Field Tracker 2000 units are normally transferred in less that 1 minute, and often within 30 seconds. While the Field Tracker 2000 is in sleep mode (indicated by the green light flashing every 3 seconds), messages are received and/or transmitted when the Field Tracker wakes up at the programmed interval and a satellite is in view.
Satellite Visibility - Orbcomm
One of the reasons Solara offers communication equipment that operates with the Orbcomm system is that Orbcomm provides complete, global coverage at an low price. It is generally suited to situations with a requirement for a low price, and longer duations in message transmission than provided by the Iridium Satellite system is not an issue.
Information sent through the Orbcomm satellites in the mid-latitudes (up to about 65°) are directly relayed to a ground station. Delay of up to 15 minutes may be experienced when satellites are not directly visible. The Solara Tracker messages contain location reports or notification messages.
Orbcomm Satellite Visibility in the Arctic
Messages at high latitudes sent through satellites not within direct view of a ground stations are stored on a satellite until it passes over a ground station.
Twenty-eight Orbcomm satellites are readily visible up to about 65° north latitude, with one available about every 15 minutes. These satellites are in direct view of a ground station at all times over North America and Europe.
Two Orbcomm satellites orbit close to the north pole, with visibility times between 45 minutes and 4 hours, depending on the time of day. These satellites are not in view of a ground station when crossing over the north pole.
Orbcomm has launched replacement satellites in 2008, so the overall time delay for times between satellites will decrease as each satellite is commissioned and brought into service.
Time for Message Delivery
Generally, based on field use observations, it has been observed that users between 55° north latitude and 65° north latitude see their messages transmitted within one hour about 85% of the time, two hours about 10% of the time, and up to a maximum of 4 hours 5% of the time.
For users south of approximately 55° north latitude, message thoughput times are generally faster, with the presence of thick tree canopy and buildings in urban areas also affecting satellite visibility.
The fastest time for a message to be recieved by the monitoring computer is about 15 seconds. This occurs when a satellite is visible in the sky from the Tracker and the satellite can see a ground station.
Factors Affecting Message Delivery Times
The message delivery time depends on how well it sees satellites and how quickly the Orbcomm system can receive the message. For example, if a satellite is obscured by a hill, it will not be able to receive a message. If the Orbcomm system is very busy, it may take a few minutes extra to get the message through. If the monitoring user is not logged into the Solara server to see the message as it is sent, the message will be stored on the Solara server for them to retrieve later.