The ARINC708 Protocol
Weather radar data is transmitted using the ARINC708 protocol. The ARINC708 word consists of a 64-bit header and a 512 data values of 3 bits each. The full word is thus 1600 bits long.
Note
ARINC data are presented LSB first, so the first bit transmitted is the least significant bit of the first byte.
The Header
The 64 bit header starts with a label of 8 bits, which is always 0o055 (or equivalently 0b10110100). The header contains the following fields:
Bit(s) |
Name |
Description |
|---|---|---|
0-7 |
Label |
Always 0o055 |
8-9 |
Control Accept |
See |
10 |
Slave |
0=Master, 1=Slave |
11-12 |
Unused |
|
13-17 |
Mode Annunciation |
See |
18-24 |
Faults |
See |
25 |
Stabilization |
0=Off, 1=On |
26-28 |
Operating Mode |
See |
29-35 |
Tilt |
See |
36-41 |
Gain |
See |
42-47 |
Range |
See |
48 |
Unused |
|
49-50 |
Data Accept |
See |
51-62 |
Scan angle |
See Scan Angle |
63 |
Unused |
Control Accept
Bit 9 |
Bit 8 |
Description |
|---|---|---|
0 |
0 |
Do not accept control |
0 |
1 |
IND1 accept control |
1 |
0 |
IND2 accept control |
1 |
1 |
IND1 and IND2 accept control |
Mode Annunciation
Bit |
Mode annunciuation |
|---|---|
13 |
Automatic sensing of a turbulence alert has occurred |
14 |
Automatic sensing of a reflectivity weather alert has occurred |
15 |
Clutter elimination is active |
16 |
Reduced sector scan is in operation |
17 |
Aircraft attitude and/or tilt exceeds the system’s design limits |
Faults
Bit |
Fault |
|---|---|
18 |
Cooling fault |
19 |
Display fault |
20 |
Calibration fault |
21 |
Altitude input fault |
22 |
Control fault |
23 |
Antenna fault |
24 |
Transmitter/receiver fault |
Operating Mode
Bit 28 |
Bit 27 |
Bit 26 |
Operating mode |
|---|---|---|---|
0 |
0 |
0 |
Standby |
0 |
0 |
1 |
Weather (only) |
0 |
1 |
0 |
Map |
0 |
1 |
1 |
Contour |
1 |
0 |
0 |
Test |
1 |
0 |
1 |
Turbulence (only) |
1 |
1 |
0 |
Weather and turbulence |
1 |
1 |
1 |
Reserved (Calibration annunciuation) |
Tilt
Bit |
Tilt (degrees) |
|---|---|
35 |
-16 |
34 |
+8 |
33 |
+4 |
32 |
+2 |
31 |
+1 |
30 |
+0.5 |
29 |
+0.25 |
Gain
Bit 41 |
Bit 40 |
Bit 39 |
Bit 38 |
Bit 37 |
Bit 36 |
Gain (dB) |
|---|---|---|---|---|---|---|
1 |
1 |
1 |
1 |
1 |
1 |
Calibration |
0 |
0 |
0 |
0 |
0 |
0 |
Max |
0 |
0 |
0 |
0 |
0 |
1 |
-5 |
0 |
0 |
0 |
0 |
1 |
1 |
-11 |
1 |
1 |
1 |
1 |
1 |
0 |
-62 |
Range
Bit 47 |
Bit 46 |
Bit 45 |
Bit 44 |
Bit 43 |
Bit 42 |
Range (nm) |
|---|---|---|---|---|---|---|
0 |
0 |
0 |
0 |
0 |
1 |
5 |
0 |
0 |
0 |
0 |
1 |
0 |
10 |
0 |
0 |
0 |
1 |
0 |
0 |
20 |
0 |
0 |
1 |
0 |
0 |
0 |
40 |
0 |
1 |
0 |
0 |
0 |
0 |
80 |
1 |
0 |
0 |
0 |
0 |
0 |
160 |
1 |
1 |
1 |
1 |
1 |
1 |
315 |
0 |
0 |
0 |
0 |
0 |
0 |
320 |
Data Accept
Bit 50 |
Bit 49 |
Data Accept |
|---|---|---|
0 |
0 |
Do not accept data |
0 |
1 |
Accept data 1 |
1 |
0 |
Accept data 2 |
1 |
1 |
Accept any data |
Scan Angle
The scan angle is given by bits 51-62. Bit 62 is the most significant bit, and takes a value of 180 degrees. Each less significant bit takes the value of half the next more significant bit. For example, bit 61 takes a value of 90 degrees, and bit 60 takes a value of 45 degrees. The least significant bit, bit 51, takes a value of 0.087890625 degrees.
The Data
The data payload is 1536 bits long, divided into 512 3-bit fields. Each
field gives a categorized radar return, summarized in arinc708_data_bins.
Bit 2 |
Bit 1 |
Bit 0 |
Category |
Representative Color |
|---|---|---|---|---|
0 |
0 |
0 |
No precipitation (<Z2) |
Black |
0 |
0 |
1 |
Light precipitation (Z2 to Z3) |
Green |
0 |
1 |
0 |
Moderate precipitation (Z3 to Z4) |
Yellow |
0 |
1 |
1 |
Heavy precipitation (Z4 to Z5) |
Red |
1 |
0 |
0 |
Very heavy precipitation (>Z5) |
Magenta |
1 |
0 |
1 |
Reserved (out of cal indication) |
|
1 |
1 |
0 |
Medium turbulence |
|
1 |
1 |
1 |
Heavy turbulence |