# General QuestionNeed some distance definitions.

#### primarch1

##### New member
Hello everyone. I just thought I'd ask for some help from those who are a bit more mathematically inclined than I am.

I've been a orbiter user for many years now. My favorite craft are the XR Fleet, the XR2 being my favourite, and of course the arrow freighter, which is my interplanetary craft of choice. I think its autoland ability is extremely cool.

What I'd like to ask about is what some of the definitions for altitude and speed in orbiter actually relate to. Please be gentle here. Math has never been my strong point.

So, first of all, the indicator that measures forward velocity. What is that exactly. Being also a flight sim buff, I'm used to speed being measured in knots, but orbiter seems to use something else.

And the altitude measurements. The default reading is K. I assume that is kilometers is it?

And also, when you get further out from planets, the distance switches to M. 1.5M from the earth puts you in a medium orbit for example. What is M defined as?

I apologise if any of these questions sound stupid, this is why I haven't asked this in so long but now I'd like to know. If I knew what the definitions for the readings were it would help me use them better.

#### DaveS

Donator
Beta Tester
Orbiter is scientific in its nature, so it uses the SI system, rather than the US Imperial System. So any distances are measured in meters, with K (kilo) being the scientific notation for thousand. G (giga) is is million. Masses are in kilograms (kg). /s means per second, so m/s is meters/second. km/s is kilometer per second. AU is Astronomical Unit which is the average distance between the sun and Earth (149 597 871 kilometers). And pc is parsec which is a unit of distance and not time and one pc equates to 3.26 light years.

#### francisdrake

Orbiter is metric. Distances are measured in meters [m] and in multiples of it, like kilometers [km] here often abbreviated as 'k', or Megameters 'M' (which is 1000 km).
Time is in seconds [s ]. Speed is distance per time [m/s], [km/s], etc.
The prefixes are explained here, but you will only encounter a few of them, like kilo, Mega, Giga.

The measured speeds depends on your reference system.
The true airspeed is measured in relation to the ground, as for an aircraft.

But in a greater context, the ground is moving, because the Earth is rotating.
So in the Orbit-HUD the speed is a bit different from the Surface HUD, and also the direction of the flight vector is a little different, as this is the speed relative to the Earth center.

The easiest way to get used to it is to memorize a few typical speeds:
• Earth orbit speed is around 7.8 km/s.
• Speed of sound is around 340 m/s.
• Space Shuttle landing speed is around 100 m/s.

After some time you will get the hang on typical speeds and distances.
The metric system has advantages over:

Asteroid the size of 112 camels to pass Earth Wednesday.

#### primarch1

##### New member
Orbiter is metric. Distances are measured in meters [m] and in multiples of it, like kilometers [km] here often abbreviated as 'k', or Megameters 'M' (which is 1000 km).
Time is in seconds [s ]. Speed is distance per time [m/s], [km/s], etc.
The prefixes are explained here, but you will only encounter a few of them, like kilo, Mega, Giga.

The measured speeds depends on your reference system.
The true airspeed is measured in relation to the ground, as for an aircraft.

But in a greater context, the ground is moving, because the Earth is rotating.
So in the Orbit-HUD the speed is a bit different from the Surface HUD, and also the direction of the flight vector is a little different, as this is the speed relative to the Earth center.

The easiest way to get used to it is to memorize a few typical speeds:
• Earth orbit speed is around 7.8 km/s.
• Speed of sound is around 340 m/s.
• Space Shuttle landing speed is around 100 m/s.

After some time you will get the hang on typical speeds and distances.
The metric system has advantages over:
Thank you! that gives me a basis to look into. and answers my questions. I can use that to learn more. This is why I love orbiter so much. it challenges one to learn. it took me months in the early days for example to work out how to launch a craft from earth, intercept an orbiting target and dock with it. but this at least gives me a basis to learn from.

I like to think, I'm certainly no mathematician, I don't know a lot, but at least I'm willing to find out!

thank you for the help!

#### Urwumpe

##### Not funny anymore
Donator
There are add-ons that measure in other units, which are more comfortable for US users.

But generally, Orbiter is metric internally. EXCEPT one thing. AU. Astronomical units are no SI unit, but the astronomic basic unit for measuring distances in the solar system. one AU is the average distance of Earth from the Sun.

About 145 Gm (Gigameters) are one AU, One Gm is 1000 Mm (Megameters), One Mm in turn are 1000 km (kilometers). Its pretty strange for Europeans as well (who would rather say 145 million km than 145 Gm), but its SI engineering units.

#### Krishnan

##### I believe, my friends, caravans of rockets
I know others have provided answers, but I'll think this is a great introduction to some of the stuff without extremely complicated math. I remember it was on the old orbitersim page and it helped me a lot.

Basics of Spaceflight
Basics of Spaceflight PDF

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