LODESTAR-SCN-1 · Rev A
Stellar Cartography Navigation Interface Control Document
Lodestar is a standalone star tracker. It reduces a zenith plate to camera attitude in ICRF and, with UTC, a geographic fix. The same words an IRS already understands go out on ARINC 429, with extra stellar labels for a cartography computer.
ARINC 429 low-speed bus
Twelve 32-bit words, 12.5 Hz (80 ms). Odd parity, SSM = NO on lock, NCD when searching. Labels 310/311/314/324/325 match IRS present-position and attitude so a stellar cartography computer can merge this source with an INS without a new label map.
ARINC 653 partition
Publish sampling port STELLAR_NAV_OUT, 64-byte payload (16 words, last four reserved). Refresh 80 ms. Destination: navigation fusion partition. No queuing — latest plate always wins.
ARINC 664 / AFDX
VL to the ship nav domain carries the ICD JSON (this packet) plus the raw 429 image. Com port: lodestar.nav.stc. UDP 4290 on the development host.
Camera frame
+X right on the plate, +Y down, +Z boresight. Geographic fix assumes +Z is the local vertical (mast-mounted zenith camera). For a hull-aligned tracker, apply the installation DCM before feeding IRS labels.
Time
UTC is mandatory. Position is LST − GMST; a 1 s time error is ~0.25' of longitude (~460 m at the equator). Use the same time source as the ship chronometer.
Label map
| Label | Name | Std |
|---|---|---|
| 150 | UTC BCD hours/minutes/seconds. Lodestar packing: HH MM SS in bits 11–29. | ARINC 429-15 |
| 270 | Boresight RA Right ascension of camera +Z in ICRF, stored as 0–360 wrapped to ±180. | LODESTAR ICD |
| 271 | Boresight Dec Declination of camera +Z in ICRF. | LODESTAR ICD |
| 272 | Star FOM Figure of merit 1 (best) through 7 (no nav), IRS-style. | LODESTAR ICD |
| 273 | Stars Used Count of catalog identifications in the current QUEST solution. | LODESTAR ICD |
| 274 | Residual RMS Plate residual after QUEST, arcminutes. | LODESTAR ICD |
| 310 | Present Position Latitude Geographic latitude from zenith RA/Dec. Camera +Z treated as local vertical. | ARINC 429-15 / 704A |
| 311 | Present Position Longitude Geographic longitude from LST − GMST. | ARINC 429-15 / 704A |
| 314 | True Heading Azimuth of image-up (camera −Y) on the local horizontal, degrees true. | ARINC 429-15 / 704A |
| 324 | Pitch Angle Boresight elevation above the local horizon (90° = zenith). | ARINC 429-15 / 704A |
| 325 | Roll Angle Rotation about boresight relative to a north-up plate. | ARINC 429-15 / 704A |
Catalog
Hipparcos J2000, 515 stars to visual magnitude 4. Identification uses a pyramid (triangle + fourth star) then QUEST. Typical zenith plate: 14–27 IDs, residual under 0.5 arcmin, geographic error under a kilometre when UTC is good.