Network Reference - Iridium Subscriber Unit
An Iridium Subscriber Unit or ISU can either be an Iridium satellite phone or any of NAL Research's CDM and A3LA series modems. It is capable of operating from 1616.0 to 1626.5 MHz; however, the actual frequencies used are in accordance with regional spectral licenses and international frequency coordination. An ISU (as well as Iridium satellites) uses right hand circular polarization (RHCP) and provides a maximum gain of 3.5 dBic from 8.2° to 90° elevation and a maximum gain of 0 dBic at 0° elevation. The average and peak RF transmitted powers are 0.6W and 7W, respectively.
The L-band interface between an ISU and an Iridium satellite is based on hybrid FDMA/TDMA (frequency division multiple access/time division multiple access) architecture using a 90 milli-second frame TDD (time division duplex). The fundamental unit of the TDMA channel is a time-slot, which is organized into frames. A frame consists of a 20.32 milli-second downlink simplex time-slot, followed by four 8.2 milli-second uplink time-slots and four downlink time-slots with various guard times interspersed. Each frame comprises of 2250 symbols at the channel burst modulation rate of 25 kilo symbols per second (ksps).
The FDMA structure occupies 41.667 kHz bandwidth. The frequency accesses are divided into the duplex channel band and the simplex channel band. The duplex channel band is further divided into sub-bands, each occupies 333.333 kHz. In duplex operation, the Iridium network is capable of operating up to 30 sub-bands, containing a total of 240 frequency accesses. The Iridium system re-uses duplex channels from beam to beam when sufficient spatial isolation exists to avoid interference. A 12-frequency access band is reserved for the simplex channels. These channels are allocated in a globally allocated 500 kHz band between 1626.0 to 1626.5 MHz.
The L-band downlink channels use DE-QPSK (differentially encoded quaternary phase shift keying) for traffic, broadcast, synchronization, ring alert and messaging. Power Spectral Flux Density (PSFD) provided to ISU ensures adequate service link margins. The uplink traffic channels use DE-QPSK modulation. The uplink acquisition and synchronization channels both use DE-BPSK (differential encoded binary phase shift keying). BPSK is used since it provides a 3 dB link advantage, which improves the burst acquisition probability. Traffic channels operate with adaptive power control, which acts to limit power transmissions beyond what is required for a robust connection.
The L-band link between an ISU and Iridium satellite is designed for a threshold channel bit error of 0.02. This level is achieved at an Eb/(No+Io) of 6.1 dB in clear line of sight conditions. The system operates with an average link margin of 13.1 dB above this level, as required to mitigate channel shadowing. Under good channel conditions, this level is reduced by adaptive power control. Even under adaptive power control, link margin is maintained to mitigate fades that are too short in duration to be compensated for by the power control loop. Adaptive power control uses a closed loop algorithm in which the satellite and ISU receivers measure received Eb/No and command the transmitters to adjust their transmitted power to the minimum value necessary to maintain high link quality. When the entire available link margin is not required to mitigate channel conditions, adaptive power control has the effect of reducing system power consumption.
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.
The Iridium network supports GSM-based algorithms for authentication and encryption to safeguard critical data to the satellites. Furthermore, the complexity of the Iridium air interface and feeder-link interface should make developing an Iridium monitoring device extremely difficult and probably beyond the reach of even the most determined adversaries.