Short answer
BeiDou's unique two-way satellite messaging service. Users transmit a short message + position request on L-band uplink (1610–1626.5 MHz), and receive a response + correction on S-band downlink (2483.5–2500 MHz). The only GNSS with built-in global SMS-grade communication.
Detailed explanation
RDSS is the active-positioning + short-messaging service built into BeiDou from the start (BDS-1 in 2000). Unlike passive GNSS where the receiver only listens, RDSS user terminals actively transmit a short burst up to the BDS GEO satellites, which relay it to a ground control centre, which computes position and sends a response back over the S-band downlink.
The transmit-receive nature of RDSS makes it slower (typical fix interval 1–10 seconds vs. continuous for passive GNSS) and lower-capacity (limited message throughput, must share with other users), but it gives China-region users a unique capability: no-cellular satellite messaging. Maritime, expedition, remote-area, and emergency-response users can exchange ~78 Chinese characters per message globally over BDS-3.
BDS-3 expanded RDSS to a worldwide service in 2020 and added the related Global Short Message Communication (GSMC) — handling up to 14 Chinese characters per message globally for emergency / IoT use.
RDSS drives a separate product category in any GNSS antenna catalogue: integrated L-band Tx + S-band Rx antennas, often with RHCP transmit and LHCP receive elements in one housing. The BDS short-message antenna family is a GNSource specialty — see the TDXL-CZBLS4003 series for typical Tx/Rx combo products.
Where you'll see this
BDS Short Message Communication
Browse product lineRelated terms
BeiDou
Chinese GNSS, fully operational with BDS-3 since 2020. Broadcasts global civilian signals on B1I/B1C (1561 / 1575.42 MHz), B2a/B2b (1176.45 / 1207.14 MHz), and B3I (1268.52 MHz), plus the unique BDS RDSS short-message service.
L-band
The 1–2 GHz radio frequency band used by virtually every GNSS civilian signal. Most public GNSS signals cluster around 1176 MHz (L5/E5a/B2a), 1227 MHz (L2), 1561–1602 MHz (L1/E1/B1/GLONASS L1), and 1268–1278 MHz (B3I/E6).