# Positioning Using IRIDIUM Satellite Signals of Opportunity in Weak Signal Environment
## 2 Iridium Satellite Orbit and Signal Characteristics  
The number of visible satellites for receiver and positioning measurements are related to the constellation distribution (Iridium satellite orbit) and Iridium signal characteristics, respectively. This section mainly analyzes the Iridium constellation structure and the signal characteristics. It can provide the basis for the Iridium satellite SOPs positioning in weak signal environment. The concept of Iridium satellite system is first proposed by [PERSON] in 1987. The Iridium system, with its constellation of 66 low Earth orbit (LEO) satellites, is a satellite-based, wireless personal communications system, providing voice and data services to virtually any destination on earth [9]. The generation Iridium satellites are almost out of use, expect two satellites are on standby. Iridium Satellite LLC in 2007 announced to launch IRIDIUM NEXT, a second-generation satellite, consisting of 66 active satellites which are used to take the place of the generation Iridium satellites, with another nine in-orbit spares and six on-ground spares. Today, 75 IRIDIUM NEXT satellites are in-orbit operational in six orbital planes, each with an inclination angle of 86.6 degrees to the equatorial plane. The orbit altitude of 70 satellites is 625 km, while the other five satellite orbit altitudes is 720 km. All the satellites have an orbital period of 97 min. At least one satellite can be seen in low latitudes anytime and anywhere. The Iridium system uses a combination of space division multiple access (SDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA) [10]. The adjacent 12 of the 48 spot beams on each satellite are grouped into a set, to share the total user downlink frequency band based on SDMA and FDMA. For every spot beam, channels are implemented using a TDMA architecture based on time division duplex (TDD) using a time frame. The user uplink and downlink are in different time slots of the same frame of the same TDMA carrier. The frequency accesses are divided into the duplex channel band and the simplex channel band which are used as the traffic channel and signaling channel, respectively. Every time the frame also consists of a simplex part and duplex part. As a result, the simplex signal and duplex signal are in different time slots and different frequency bands. The Iridium frequency allocation and frame structure are shown in Figure 1.  
The system uses L-band frequencies of 1616 to 1626.5 MHz for the user link. The frequency band shared by duplex channels is from 1616.0-1626.0 MHz. The 10 MHz band is organized into sub-bands, each of which contains eight frequency accesses. Each sub-band occupies 333.333 kHz (8 * 41.667 kHz, and the guard band is 10.17 kHz). The Iridium is capable of operating with up to 30 sub-bands, containing a total of 240 frequency access. A 12-frequency (12 * 41.667 kHz, and the work band is 31.50 kHz) access band is reserved for the simplex (ring alert and messaging) channels. These channels are located in a globally allocated 500 kHz band between 1626.0 and 1626.5 MHz. The simplex channels of every beam have 12 frequency accesses and 240 frequency accesses of duplex channels that are divided into 12 spot beams equally [11]. As a result, every spot beam has 32 frequency accesses. Every spot beam has 80 duplex channels since every frequency access is with four duplex frequency accesses by TDD. Every satellite can at most get 3840 duplex frequency accesses (4.8 Kbps) since every satellite has 48 beams. The TDMA frame is in total 90 ms long. It consists of a 20.32 ms downlink timeslot which shares simplex frequency band, followed by four 8.28 ms duplex uplink timeslots and four duplex downlink timeslots that share duplex frequency band. There are 2250 symbols per TDMA frame at a channel burst modulation rate of 25 ksps. A 2400 bps traffic channel uses one uplink and one downlink timeslot per frame [12].