AS 680/682
Subsystem Description Description, Operation and Maintenance
ADS−B
2−2 Ed. 10.07
For remote operation (i.e. not integrated within an existing ATC network) the Ground Station can be
optionally equipped with a modem to bridge its external Ethernet interface over a telecommunication
line (e.g. ISDN, Fibre Optical Network, etc.). Standard is the Ethernet LAN communication.
The Ground Station equipment can be supplied with DC voltage, e.g. generated by Solar power, or
optional with AC from mains, using the AC/DC converter option.
2.1.2 Physical Breakdown
See Fig. 2−2.
The Ground Station hardware architecture is shown in Fig. 2−2. The configuration delivered for one
ADS−B Ground Station AS 680 includes the following main components:
− 1 omnidirectional antenna
− 1 GPS antenna
− 1 Antenna Amplifier Unit (AAU) which comprises a low−noise pre−amplifier (LNA) and an RF filter
and lightning protection to ensure a good signal−to−noise ratio at the input of the reception chan-
nel and to avoid losses due to the attenuation by antenna cables. It is powered via the RF line from
the Ground Station receiver. The AAU can be mounted with a support on a mast.
− a low−loss cable (e.g. 1/2" cellflex), with a required total maximum attenuation of 4 dB, to connect
the antenna and the AAU, as well as the AAU and the equipment rack
− a Signal Processing Unit (SPU), core module of AS 680, integrated into a 3HU, 19" subrack
− a separate Site Monitor (SM) as external test tool, integrated into a 1HU, 19" subrack
The cables are listed in more detail in section 3.1.4, Figs. 3−1 to 3−4.
The signal processing unit (SPU) is based around a VME−bus system. It includes a receiver unit
(RXU), a signal processing board (SPB), a single board computer (SBC) and a GPS timing system
(GTS). Received RF signals are converted into video signals by the RXU’s logarithmic receiver, and
analyzed by the signal processing board in order to reliably detect ADS−B signals. The decoded data
are collected and further processed by the application software of the SBC. The GPS Timing system
provides a positive system time reference to support the SBC’s real time clock. It also provides addi-
tional information about GPS status, like position, dilution of precision, number and identity of satel-
lites visible and – optionally – also GPS integrity information in the same way as an ADS−B target
(RAIM / HPL). The Ground Station constantly verifies GPS health by checking the deviation of the
measured GPS position versus the configured Ground Station position.
The Ground Station application software runs on an embedded version of the operating system Linux
providing high robustness and flexibility for network communication and remote operation. Changes
in, or updates of the application can be performed while the Ground Station is operational and do not
require a reboot. Thus the system downtime is minimize. Remote upload of new software releases
as well as configuration files is handled in a secured and fail−safe way which is essential for true re-
mote operation.
2.1.3 External Interfaces
The interfaces are identified in Fig. 2−2, as well as in Fig. 2−6. The Ground Station has the following
RF and analogue signal interfaces:
− an ADS−B 1090 MHz interface (antenna/AAU/receiver), Fig. 2−2/3
− a monitor output of the received ADS−B video signal (base band), Fig. 2−2/11
− a GPS L1−band 1575 MHz interface (active antenna/receiver), Fig. 2−2/4
− 2 monitor probes on the antenna to inject test signals, e.g. coming from a site monitor, Fig. 2−2/9
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