Astrographic Navigation

## Astrography

Earth is understood as home to most humans, and this has shaped our perception of the cosmos. Earth, specifically the centerpoint of Earth’s sun, Sol, is the “Middle of the Universe”. Universal Galactic Coordinates (UGC) use the meridian that runs through Sol as the origin axis of the polar coordinate system. This system uses 3 values to determine one’s position anywhere in the galaxy:

* Radial - Measured in Radly - the angle between the Earth Meridian and the current location.
* Range - Measured in Lightyears - the linear distance to the galactic center.
* Inclination - Measured in Radly - the angle between the Earth Plane (the circle formed by the movement of the Earth meridian) and the current position.

Sol is the point in the galaxy which marks the starting point for polar measurements, a line called the Earth Meridian. This line is a straight line from Sol’s Center of Mass to the calculated Galactic Center. From here, radial values increase counterclockwise. Additionally, the Earth Meridian is the reference angle for inclination. Anything “Above” the Earth Meridian has a positive angle value, from 0(inclusive) to 180(exclusive); anything “below” the Earth Meridian has a negative value in the same range. Some notes: values between 90 and 180 are on the opposite side of the galactic disk from earth; a value of 180 is neither positive nor negative, it is on the opposite side of the galaxy in-plane with earth; and as a result of these facts, radial values between 270 degrees and 90 degrees cannot be paired with inclination values between 90 and 180. All of these situations are, of course, edge cases; since the width and breadth of humanity are neither one greater than 3 degrees from the earth meridian.

### Units of Astrography

* Radly (lyrad)- *contr. of Radial Lightyear* - the value of an angle whose arclength is 1 lightyear and whose radius is roughly equal to the distance from Earth to the Galactic Center. Defined as 40 microradians, slightly smaller than an arcsecond. Used to measure Radial and Inclination values.
* Lightyear (ly) - the distance light travels in one Earth Year. For navigational purposes, this distance is defined as exactly 63241.1 Astronomical Units.
* Astronomical Units (AU) - the mean distance from the center of Earth to the center of Sol. Defined as 149.6 million kilometers.

#### Subunits

The base units above are often sub-divided by their time variable, as described below.

##### Linear:

* Light Month (lmo) - the distance light travels in a month. 1/12 of a lightyear, or 5194.34 AU.
* Light Day (ld) - the distance light travels in a day, 173.145 AU.
* Light Hour (lh) - the distance light travels in an hour, 7.21436 AU.
* Light Minute (lm) - the distance light travels in a minute, 0.120239 AU or 17990000 km.
* Light Second (ls) - the distance light travels in a second, 299792 km.

##### Polar:

* Radial Light Month (lmorad) - 3.285415 microradians
* Radial Light Day (ldrad) - 109.514 nanoradians
* Radial Light Hour (lhrad) - 4.56308 nanoradians
* Radial Light Minute (lmrad) - 76.051 picoradians
* Radial Light Second (lsrad) - 1.267522 picoradians

### Coordinate Display

Galactic Coordinates are calculated based upon a ship’s relative position to the Galactic Center and to Earth, and are often accurate to the tenth of a lightsecond, about 30,000km. Once calculated, they are displayed as three clock values, one for each axis. In general, units are understood, linear values are measured in light-time distances, radial values are measured in light-time angles.

Common display formats are:

Range; YYY:MM:DD:HH:MM:SS.S

Radial; YYY:MM:DD:HH:MM:SS.S

Inclination; YYY:MM:DD:HH:MM:SS.S

Or

R/YYY:MM:DD:HH:MM:SS.S D/YYY:MM:DD:HH:MM:SS.S I/YYY:MM:DD:HH:MM:SS.S

While the first format is common in many human interface applications, the machine readability of the second format, called the RDI format, has made it most popular. This popularity has led to the position clock, regardless of format, being referred to as the RDI or “Ridi” Clock.

## Navigation

Finding one's location in Space requires some use of a combination of one of the following methods.

1. Arithmetic - derives current position from last known position, with input factors for speed, acceleration, and solar wind. Becomes less accurate the further a ship travels from its last known position. Cannot be used in a new star system without first calibrating to a known position.
2. Parallax - uses the relative position of distant stars to calculate a current position. Needs an estimate of current location to know which stars to look for. Has two modes: *static* mode which uses some reference coordinate (such as a nearby planet, star, or other known object) to determine parallax location. *Dynamic* mode requires a ship to be moving, the faster the more accurate, but does not need a reference coordinate.
3. Gravimetric - uses gravimetric sensors to determine location. Heavier ships have more accurate, cheaper gravimetric instruments since the hull mass distorts spacetime in a predictable way. Has two different functions: *Reference* mode can detect the position (relative to the ship) of a known stellar mass such as a planet or star. If this stellar mass has a known coordinate, position can be calculated based on this reading. *Exact* mode requires either a significant hull mass or highly advanced gravimetric sensor, but will give the position of a ship relative to the galactic center with moderate accuracy.
4. Rangefinding - an infrared range finder can find distance to stellar bodies with relative accuracy. Only one dimensional, this method cannot be used on its own to calculate position.
5. Radio - locations are broadcasted from reference points in a star system. Much like GPS, the communication time and known location of the signal sources can be used to triangulate position.

These positioning systems are often used in conjunction with one another to gain accurate positioning. The result is that a ship's position can often be known incredibly precisely in most star systems, with a drop in accuracy the further a ship goes from a reference point such as a planet or other source. The main failure of these systems is in uncharted star systems where the position of stars and planets has not been precisely calculated relative to the galactic center. In these cases, ships may still navigate using in-system coordinates (like galactic coordinates, but using the star as the center and the first planet as a meridian line) but will require computers to be reconfigured. Many exploration and survey vessels come with special navigation computers for this exact purpose.