ISS motion control & navigation

Orientation
Links

This can be described as figuring out which way the ISS is orientated, and where it is. This is a horribly complicated subject which I don’t quite understand (it involves a lot of fearsome-looking algebra), but I’ll summarize what I know as best I can. For reference I am mostly using Motion Control and Navigation of the Russian Segment, a 1998 Star City training manual, and the NASA Guidance, Navigation and Control manual, and have quoted verbatim from these for some of the descriptions. (Like most of these types of manuals, these could be marketed as a cure for insomnia.)

Russian Guidance, Navigation and Control (СУДН, SUDN) equipment: Zvezda СУДН equipment comprises 4 sun sensors, 2 rate gyros, 3 horizon sensors, 2 GLONASS receivers, 2 magnetometers. There are 32 attitude thrusters and 2 reboost engines.

To provide reference points for navigation, the ISS uses a three-axis co-ordinate system on both segments. The illustration shows both U.S. and Russian segment co-ordinates; the latter are shown with a dashed line. The U.S. positive axes (X, Y, Z) point forwards, starboard and downwards towards the Earth, respectively. (Negative axes point in the opposite direction to each of these.) The Russian segment’s positive X-axis points aftwards, Y to the zenith (top) and Z to port (in the right-handed co-ordinate system). That’s a very simplified description, I might add!

The Russian system of design and construction for the modules involves the concept of planes, (more precisely, half-planes). These are located at angle of 90° to each other along the longitudinal axis (X-axis). Planes are designated by Roman numerals. As a rule, they are orientated:

into the zenith (upwards);
to the right (starboard), if we look in the forward direction of the module;
into the nadir (downwards);
to the left (port) in the forward direction of flight.

Attitude control keeps the ISS pointed in the right direction (so that it isn’t floating, say, upside-down) relative to the Earth. Control is maintained either by the U.S. or Russian segments. Attitude control for the U.S. segment is provided by four Control Movement Gyroscopes mounted on the Z-1 Mast. These weigh 300 kg each and are numbered (clockwise from bottom left) 1, 2, 3, 4. The wheel at the center of each gimbal assembly spins up to 6600 rpm, providing the gyroscopic effects used to generate control torques. They use non-propulsive attitude control, and thus are the main method of attitude control.

The ISS is generally orientated in one of three attitudes:

LVLH/ОСК (OSK): Local Vertical/Local Horizontal (Russian equivalent: ОСК). Z-axis points towards Earth (nadir), Y-axis points perpendicular to orbit plane, X-axis points to the velocity vector (forward directon of orbit). This is the attitude the ISS will fly permanently after STS-115 installed the P3/P4 Truss (ISS assembly mission 12A).
X-POP/РСО (RSO): X-axis Perpendicular to Orbit Plane (Russian equivalent: РСО). The X-axis always points at orbital noon, so at some parts of the orbit the ISS is pointing up out of the orbital plane (i.e. perpendicular).
X-TEA/РОСК (ROSK): X-axis Torque Equilibrium Attitude. Used when the Space Shuttle Orbiter is docked with the ISS. Similar to LVLH.

Below, Russian SUDN equipment is briefly described.

GIVUS
«ГИВУС»

The Russian gyroscope is the GIVUS, «ГИВУС» (usually referred to by its acronym as the full meaning is too much of a mouthful in either language). It “is meant for measuring increment integral from the angular momentum vector projection of the ISS RS to the device’s measuring axes (i.e. it measures attitude rates).” There are 4 measurement channels that operate independently of each other. (There were to be 6 Kentavr gyroscopes installed on the yet-to-be-built Universal Docking Module (УСМ, USM), but this has yet to eventuate.)

DO
ДО

DO, attitude control thrusters on the Service Module and Progress (16 on the latter) provide maneuvering and reboost capabilities for the ISS. The Progress is preferably used for this as Zvezda’s thrusters have a limited burn lifetime. The ISS needs to be reboosted every 3 months or so due to orbital decay – there is still some atmosphere even at its average height of 400 km, and this produces drag; the ISS would eventually fall out of orbit if not reboosted (and if it landed on an occupied area of Earth, it would ruin quite a few people’s days).

Optical devices

Infrared horizon sensors (ИКВС, IKVS) are used to maneuver in the orbital co-ordinate system and to perform alignment of the inertial co-ordinal system. They use the infrared emittance of the Earth and its near atmospheric layer.

Sun sensors (СД, SD) are used to determine the Station’s inertial attitude and to perform one-axis alignment and inertial co-ordinate system correction. The СД is a scanning device that determines the position of the sun in reference to the device’s co-ordinate system.

The БОКЗ, BOKZ star mapper is used to determine the Station’s attitude by finding a field of stars in its field of view.

There are various hand-held devices for crew members’ use, including the zoom sight Пума, Puma that monitors and determines co-ordinates of objects.

The Russian «ГЛОНАСС», GLONASS satellite system is the Russian equivalent of the American Global Positioning System; these radionavigation satellites provide positioning data.

ISS orientation

NASA also have a Macromedia Flash animation on their site explaining ISS orientation. Below is a transcription.

The ISS incorporates different flight attitudes in order to maximize power and minimize negative thermal effects. In order to understand the three attitudes employed by the ISS, a basic understanding of the ISS body coordinates is needed.

ISS will fly in three different flight attitudes until the main solar arrays are in position. The attitudes are:

X-axis in the Velocity Vector (XVV)

Nominal flight attitude. Currently this attitude is flown in low Beta angles, but will be the only attitude flown after the primary solar arrays are in place (after assembly mission 12A).

XVV is defined in the Local Vertical, Local Horizontal (LVLH) rotational reference frame. The positive Z-axis (extending from the base of the Station) points towards the Earth’s center (nadir). The Y-axis points opposite the angular momentum vector (i.e. the forward direction of travel). In essence, the ISS is traveling with the Destiny lab foremost.

X-axis Parallel to H (XPH)

Inertial flight attitude during assembly designed to maximize power generation during mid- to high solar beta angles.

XPH is defined using a quasi-inertial reference frame, XPOP, or “X is Perpendicular Out of Plane”. X is aligned with the angular momentum vector “h”. Z is positive pointing nadir at orbital noon. Y completes the right-hand rule. This attitude was developed for the assembly stage in which only a single axis of solar array sun-tracking is available (i.e. the two solar panels mounted on the “Mast”), hence maximizing power production.

Y-axis in the Velocity Vector (YVV)

Rotational flight attitude similar to XVV, but rotated 90 degrees in yaw (i.e. the ISS is flying sideways, with one end of the Truss pointed in the direction of travel). This attitude is flown in mid- to high beta ranges during assembly stages.

YVV is the flight attitude required to meet payload Earth viewing requirements. This attitude also serves to keep vehicle components from violating high temperature limits while maintaining a lower drag porfile.

Each attitude is designed to be flown during different Beta ranges. The overlap allows the Flight Control Team to decide which attitude is best for that day’s activities. Only XVV will be flown when the main solar arrays are in position (assuming the Shuttles get flying again, etc.).

YVV is also adopted when a Soyuz undocks from the nadir port of Pirs.

XVV = 0° to 60°
XPH = 10° to 75°
YVV = 40° to 75°

The Solar Beta angle, ß-angle, is the angle measured between the sun vector and the orbital plane of any Earth-orbiting object; in this case, the ISS.

The plane of the Earth’s orbit around the sun is known as the sun vector.
The Earth’s equator is inclined 23.4° to the sun vector.
The ISS’s orbit is inclined 51.6° to the Earth’s equator.

High solar beta angle

Most sun exposure to one site of vehicle;
Short or no eclipse times/majority of time spent in sunlight;
Angle between the sun vector and orbital plane is large.

Low solar beta angle

Longer eclipse times/shorter amount of time in sunlight;
Angle between the sun vector and orbital plane is small.