Satgen 516 Iridium Signals by GM4IHJ (BID SGEN516) 1999-02-13 The trials and tribulations of the Iridium Low Earth Orbit Mobile Phone satellite network, have prompted several queries from Satgen readers. Published frequency data shows an allocation of 1610.6 to 1626.5 MHz, with 1618.55 to 1626.5 specifically for the downlink ( ie as far away as possible from the 1610 to 1613.8 radio astronomy hydroxyl band ). The original design of 66 satellites , 11 in each of 6 orbit planes has been supplemented by a further 20 satellites so far, in order to replace 12 defective sats and provide some spares. Reception of the downlinks, for identification but not interpretation of their message content, is possible using a simple vertical dipole in a corner reflector. Looking south. Feeding an ICOM R7000 Rx or similar, set to receive SSB or AM. Half hemisphere overhead passes doppler 1626.332 MHz to 1626.297 in the local morning. Afternoon passes doppler approx 1626.270 to 1626.230. You do not hear a clear tone or a continuous signal, you hear clicks. These clicks then doppler down about 20 to 40 KHz before the simple antenna loses them . A new sequence of clicks starts at 1626.332 or 1626.270 approx as satellites follow at 9 minute intervals. On passes far from overhead you hear only the central portion of the doppler shift. There will clearly be other frequencies in use at other times ( eg perhaps a curious sequence 1626.470 to 1626.420 but these are not presently confirmed as Iridium). If you hear a signal one day at time XXXX, you will hear it again the next day at XXXX + 3 minutes. At times you hear more than one satellite. This is best sorted out by staying on a fixed frequency for 9 minutes during which you may hear up to 3 signals, spaced in time about 1 minute between the end of one and the start of the next. The reason for these curious clicking sequences may be that the modulation being used is Time Division Multiplex . Whereby each individual voice call is compressed in time, then other user voice calls are time sequence stacked - like ready sliced bread. On reception there is a complex expansion of the signal to reproduce the original voice characteristics. This expansion is of course only available if you have the $3000 ? handset. The greater the number of voice channels in use, the greater the number of clicks. But presently the system appears to be very lightly used and hence the reception of only sharp single clicks or, the occasional double click. As observers will note the actual signal bandwidth of these "clicks" is quite large , hence the very strong resistance to Iridium from some members of the radio astronomy community. Please note that the system described above is by no means the complete answer to Iridium reception. Observers using more sophisticated search equipment will get better results. The actual handset itself seems to use a vertically stacked pair of quadrifilar helices for reception. Which should produce much more consistent omni directional results over a broader range of frequencies/ full hemisphere, than the narrow fixed reception cone system described here. Good Hunting .