Units and conventions
Every quantity on this site states its unit. This page is the reference for what those units are, and for the conventions that are easy to get wrong because nothing errors when you do.
Units
| Quantity | Unit | Notes |
|---|---|---|
| Distance, altitude, range | km | Meters only inside physical formulae, where stated |
| Angle | degrees | Radians only inside formulae, where stated |
| Time, duration | s, or h where a flag says -hours | Epochs are RFC 3339 UTC |
| Frequency | GHz | Flags say -ghz |
| Power | dBW | dBm appears only where a source quotes it |
| Antenna gain | dBi | Relative to isotropic |
| Figure of merit | dB/K | Gain over system noise temperature |
| Carrier to noise density | dBHz | |
| Noise temperature | K | |
| Mass | kg | |
| Ballistic term | m^2/kg | |
| Radiation term | m^2/kg | |
| Availability | percent | For example 99.9 |
| Data rate | bit/s and multiples |
Flags name their unit: --altitude-km, --inclination-deg, --frequency-ghz,
--duration-hours, --step-seconds. When a flag does not name a unit, the table
above applies. If neither answers it, that is a documentation defect worth reporting.
Angles and their ranges
| Angle | Range | Zero means |
|---|---|---|
| Latitude | -90 to +90 degrees | Equator; positive north |
| Longitude | -180 to +180 degrees | Prime meridian; positive east |
| Inclination | 0 to 180 degrees | Equatorial prograde; above 90 is retrograde |
| RAAN | 0 to 360 degrees | Vernal equinox direction |
| True anomaly | 0 to 360 degrees | Perigee |
| Elevation | -90 to +90 degrees | Local horizon; negative is below it |
| Azimuth | 0 to 360 degrees | North, increasing clockwise through east |
Retrograde orbits are inclinations above 90 degrees. A sun-synchronous orbit is retrograde, typically near 98 degrees, which surprises people who expect “polar” to mean exactly 90.
Sign conventions
| Quantity | Positive means |
|---|---|
| Longitude | East of Greenwich |
| Latitude | North of the equator |
| Doppler shift | Closing range, so the received frequency is higher |
| Link margin | The link closes with headroom to spare |
| Elevation | Above the local horizon |
Western longitudes are negative. A station specification such as
"Madrid,40.43,-3.7,0.6" is latitude 40.43 degrees north, longitude 3.7 degrees
west, altitude 0.6 km. Entering it as positive puts your station in Turkey, and
nothing in the output will look obviously wrong.
Station specification
Ground stations are given as a comma-separated string:
Name,latitude_deg,longitude_deg,altitude_km--station "Madrid,40.43,-3.7,0.6"Altitude is above the reference ellipsoid, in kilometers. A station at sea level
is 0, not 0.0006.
Naming
| Surface | Convention | Example |
|---|---|---|
| CLI flags | lower-kebab-case, unit suffixed | --altitude-km |
| CLI enum values | lower-kebab-case | two-body |
| API fields | lower_snake_case | duration_hours |
| API enum values | lower_snake_case | two_body |
The CLI and the API do not share spelling. The default propagation model is
two-body with a hyphen on the command line and two_body with an underscore over
HTTP. Passing the CLI spelling to the API returns invalid_request. Treat the two as
separate contracts.
Earth model
| Constant | Value used |
|---|---|
| Equatorial radius | WGS-84 semi-major axis |
| Flattening | WGS-84 |
| Gravitational parameter | Standard Earth GM |
Altitudes are heights above the ellipsoid, not above terrain and not above mean sea level. For a ground station in mountainous terrain the difference is real and you should supply the ellipsoidal height.
Numbers in output
Printed results are rounded for reading. Exported files carry full precision, so quote from the export rather than from the console when precision matters.
Percentages are printed with one decimal place. Margins are printed in dB with one decimal place. Neither rounding implies that level of accuracy in the underlying model; see accuracy and limitations.
Next steps
- Accuracy and limitations for what the numbers are worth.
- Glossary for the terms themselves.
main (pre-release)