One question I ask myself is if the accuracy improves if it is a long transmission
time. A short data burst like one from an SMS text may be much worse in accuracy than as written in the following. The two positions 700 miles apart yet taken only 7 minutes apart seem to support this theory.
Iridium accuracy references
Tracking transceiver units
Without an extra global navigation
satellite system, receiver tracking is difficult, but not impossible, as the position of a mobile unit can be determined using a Doppler shift calculation from the satellite. These readings however can be inaccurate with errors in the tens of kilometers. Even without using Doppler shifts, a rough indication of a unit's position can be found by checking the location of the spot-beam being used and the mobile unit's timing advance.
The position readings can be extracted from some transceiver units and the 9505A handset using the -MSGEO AT command. In the past, Iridium has used this method of tracking to block service
to U.S. embargoed countries, such as North Korea
and other politically unpopular regions, such as Northern Sri Lanka. It is also used to stop geographically bounded plans from being used outside the designated area.
The Iridium Extreme phone introduced in 2011 has a GPS location service
embedded in the device, which the user can use to locate themselves or include in SMS messages. It can also be used to provide advanced services like Geo-fencing. A red emergency
button on the top of the unit can be pressed to send the unit's position to emergency rescue
agencies or other number pre-programmed by the user or distributor.
NAL - Network Reference - Iridium Subscriber Unit
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 geo-location process proceeds as follows:
The ISU sends the satellite an uplink geo-location burst, saving the delay and Doppler corrections needed to send the message.
When the satellite receives the uplink geo-location burst from the ISU, it measures the time and frequency offsets of the burst relative to its time and frequency standards.
The satellite then responds with a downlink burst, which the ISU uses as an acknowledgement that the satellite has received the previous uplink geo-location burst.
When the downlink burst arrives, the ISU checks to see if it is satisfied with its estimates for the timing and Doppler. If so, it then transmits an uplink ACCHL message to the satellite that includes the propagation time and Doppler frequency offsets that were used by the ISU during the last geo-location uplink burst. If the ISU did not receive a response, or if the ISU is not satisfied with the accuracy of the exchange, the ISU will repeat the process again at step 1.
The Iridium network can locate an ISU to within 10 km only about 78% of the time. The location accuracy can be much higher; however, the information is not available to commercial
users. 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.