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WAAS Receivers
| WAAS | |
|---|---|
| Title | WAAS Receivers |
| Author(s) | GMV. |
| Level | Basic |
| Year of Publication | 2011 |
The Wide Area Augmentation System (WAAS) is an GPS Augmentation system developed by the Federal Aviation Administration (FAA), with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GPS for all phases of flight, including precision approaches to any airport within its coverage area.
WAAS uses a network of ground-based reference stations, in North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received Deviation Correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve accuracy.[1]
WAAS Receivers
The WAAS SiS has been designed to minimize standard GPS receiver hardware modifications. Therefore, a WAAS-GPS receiver is like a GPS receiver but with special software inside that allows the receiver to lock onto the code used by the WAAS GEOs satellites and compute the WAAS corrections to the GPS signals. Apart from this, the receiver is just like a GPS receiver. This means that it can pick up GPS signals as well. It is also of the same size as a GPS receiver and uses the same type of antenna.
- Receiver types
WASS-GPS receivers can be designed using a chipset, hybrid component or an auxiliary card.- Chipset: consists of one or two components that must be installed on a circuit board. The routing of the RF part is sensitive. This compact solution is also the least expensive.
- Hybrid component: consists of a single component integrating the RF and signal processing parts to be installed on a circuit board. Routing is easier compared to chipsets. The price is higher than for the chipset solution.
- Auxiliary card (piggyback): all the receiver and peripheral components are integrated on a ready-to-use card connected to the final product’s main circuit board. It is an ideal solution for prototyping embedded applications. This is the most expensive solution.
- Communication protocols and manufacturer’s specifications
Manufacturers generally use proprietary protocols which give access to almost all the data (pseudoranges, satellite navigation messages, SBAS messages, etc.) associated with a standardised protocol, NMEA 0183. Some receivers also generate data in RINEX (Receiver INdependent EXchange) format. A GPS receiver with “WAAS Enabled” specification usually means that the receiver activates SBAS reception by default. It is important to have in mind that navigation services are developing swiftly, so keep in pace with international standards is mandatory.
- SBAS Services
The WAAS-enabled GPS Receivers are driven by the WAAS application market. In general, the WAAS service operator provides different services aiming at different market sectors, namely an Open Service, a Safety of Life service (SoL) and even a Commercial Service.[2]
For the Safety-of-Life (SoL) service, WAAS users include all aircraft with approved WAAS avionics using the WAAS for any approved phase of flight. The WAAS user equipment shall be compliant (certified) against several standards, i.e. RTCA MOPS DO 229 (see article SBAS Standards). The SoL civil aviation certified equipment is in the highest rank with respect its cost. There exist a large number of certified receivers manufacturers worldwide, in the US: GARMIN, Honeywell, Rockwell Collins, General Avionics, etc. The Open Service (OS) targets low cost, general purpose GPS equipment that uses the WAAS Signal-In-Space (SIS) to provide the user with an enhanced accuracy performance in comparison with the one provided by a standalone GPS device. In comparison with the certification requirements of the SoL user equipment, user equipment is not necessarily compliant with the RTCA MOPS DO 229 processing rules, but might only make use of the processing algorithms that render the accuracy corrections provided by the SBAS SIS.