The ARTEMIS spacecraft is the primary space segment element of the SBAS AFRICA infrastructure. It is a geo-stationary telecommunications satellite located at 21.5°E longitude, launched in 2001 by the European Space Agency (ESA) and currently owned since January 2014 by Avanti Communications. On board is hosted an L1 (1575.42 MHz) navigation transponder which was formerly operated as part of the European Geostationary Navigation Overlay Service (EGNOS). In the SBAS AFRICA infrastructure the navigation transponder broadcasts the SBAS navigation data service, i.e. the signal-in-space (SIS), via GPS/SBAS PRN130. The SBAS AFRICA signal-in-space is generated on the ground at the ARTEMIS Ground Uplink Station (GUS) hosted at Avanti Communications Gateway Earth Station in Makarios, Cyprus. The GUS comprises a navigation baseband subsystem that generates the SBAS L1 signal in compliance with RTCA DO229D Minimum Operational Performance Standards for Global Positioning System/Satellite-Based Augmentation System Airborne Equipment and an RF subsystem that uplinks the signal to the navigation transponder via a 4.9 metre Ku-band antenna.
HYLAS 2 VSAT Network
The SBAS AFRICA VSAT network is a dedicated communications network providing Ka-band VSAT data connectivity for the deployed SBAS AFRICA Ground Monitoring Stations (GMS) via the Avanti Communications HYLAS 2 spacecraft located at 31°E longitude. The VSAT network enables the GMS to transfer GPS tracking and measurement data to a centrally-hosted (at the Avanti Communications Gateway Earth Station in Goonhilly, UK) data server, otherwise known as an NTRIP Caster, which consolidates and archives all SBAS AFRICA navigation measurement data according to formats standardised under the Networked Transport of RTCM via Internet Protocol (NTRIP) as used for streaming differential GPS (DGPS) over the Internet.
HYLAS 2 was launched into geostationary orbit in August 2012. Deploying 24 fixed beams and one steerable beam, HYLAS 2 provides customers and service providers with the flexibility to address current and future broadband communications markets across Europe, the Middle East, the Caucasus and Africa.
Ground Monitoring Stations
Eighteen (18) SBAS AFRICA Ground Monitoring Stations (GMS) are deployed throughout the projected SBAS service coverage area, initially in South Africa but extending into Southern African Development Community (SADC) member states in future where up to eight (8) additional stations will be deployed. The GMS, provided by NSL of Nottingham, UK, track all-in-view GPS spacecraft on L1 and L2 frequencies and obtain raw measurement data for each GPS spacecraft. The raw GPS measurement data are transmitted in real-time over the HYLAS 2 VSAT network via the NTRIP protocol to the SBAS AFRICA Navigation Processing Facility. Each GMS comprises a GPS/SBAS L1/L2 receiver/antenna subsystem and associated data management and communications elements that emulate an NTRIP server to enable data transfer to the central NTRIP caster. Data connectivity is provided by an integral VSAT modem. The SBAS AFRICA infrastructure also has the capability to ingest TRIGNET-sourced GPS measurement data in real-time to assure the required level of service coverage in terms of accuracy, availability and continuity of service.
Navigation Processing Facility
Hosted at the Avanti Communications Gateway Earth Station in Goonhilly, UK, the SBAS AFRICA Navigation Processing Facility (NPF) ingests via the SBAS AFRICA NTRIP caster the GPS tracking data from the deployed Ground Monitoring Stations in Africa and computes in real-time on a GPS epoch-by-epoch basis the navigation data messages necessary to provide the SBAS AFRICA service in accordance with RTCA DO229D Minimum Operational Performance Standards for Global Positioning System/Satellite-Based Augmentation System Airborne Equipment. The NPF is implemented using the magicSBAS platform, a demonstrator for Space-Based Augmentation Systems, provided by GMV of Madrid, Spain. magicSBAS collects GPS tracking data (measurements and ephemeris) from the GMS network of reference stations (South Africa in this implementation), computes satellite orbit and clock errors, ionospheric delays and integrity information, and cross-links the navigation message data in real-time to the SBAS AFRICA Ground Uplink Station in Makarios, Cyprus. magicSBAS supports various formats for station data and correction messages, including NTRIP, RINEX, and RTCM, and runs in real-time or in post-processing fast-replay mode.
SBAS User Receivers
The SBAS AFRICA messages broadcast via ARTEMIS will be accessible to users with a standard, off-the-shelf SBAS-enabled GPS receiver compliant with RTCA DO229D Minimum Operational Performance Standards for Global Positioning System/Satellite-Based Augmentation System Airborne Equipment . The SBAS AFRICA Signal-in-Space will be broadcast on GPS L1 (1575.42 MHz) via PRN 130 throughout the ARTEMIS global beam area (GBA) centred on 21.5°E longitude.