UDP Packets Sent by the TCC

Contents

• Introduction
• Packet Format
• Version History

Introduction

The TCC broadcasts UDP packets containing position information. These packets are broadcast 1/second (even if the axes are halted). Remember that UDP is not a reliable protocol, so integrity of the packets cannot be guaranteed.

The broadcast port depends on the telescope. As of 2007-09-04 these ports are:

• SDSS 2.5m telescope (tcc25m): port 1200
• 3.5m telescope (tcc35m): port 1235

Packet Format

The format of version 2.3 (major.minor) packets is as follows:

Each packet contains two components: a header, followed immediately by data.

Packet Header
Variable	Type	Description
Size	int4	Number of bytes in this packet (including this header).
Type	int4	A code identifying the type of packet; codes are defined where?
MajorVers	int4	Major version number; incremented if packet is changed in a way that is more than simply an extension. Users should check that the major version number is exactly as expected.
MinorVers	int4	Minor version number; incremented if packet data is extended. Users should check that the minor version number is no smaller than expected.

Packet Data
Variable	Type	Description
TAIDate	double	Date at which data was or will be current (TAI, MJD seconds)
SlewEndtime	double	Date at which current/last slew ends/ended (TAI, MJD seconds); NaN if the telescope is not slewing (or, possibly, tracking).
CoordSys(8)	byte	Coordinate system (as a string)
Epoch	double	Epoch (decimal year; Besselian for FK5, Julian otherwise).
ObjNetPos(2)	pos, vel double	Net target position in user coordinates; axis 1,2 (e.g. RA,Dec or Az,Alt).
Boresight(2)	pos, vel double	Boresight position (instrument x,y).
RotType	padded int4	User-specified type of rotation; see rotation type codes.
RotPos	pos, vel double	User-specified rotation angle; the exact meaning depends on RotType.
ObjInstAng	pos, vel double	Orientation of the object with respect to the instrument.
SpiderInstAng	pos, vel double	Orientation of the secondary spider with respect to the instrument.
TCCPos(3)	pos, vel double	Mount position of (azimuth, altitude, instrument rotator) axes, as requested by the TCC.
SecFocus	double	User-set secondary mirror focus offset.
AxisCmdState(3)	padded int4	Commanded state of (azimuth, altitude, instrument rotator) axes; see axis commanded state codes.
AxisErrCode(3)	padded int4	Error code of each axis; if the TCC has commanded a halt, this explains why (azimuth, altitude, instrument rotator) axes; see axis error codes.
ActMount(3)	pos, vel, time double	Actual mount position, velocity and time as reported by the azimuth, altitude and rotator axis controllers.
AxisStatusWord(3)	padded int4	Status word reported by azimuth, altitude and rotator axis controller. Invalid if the corresponding axis does not exist.

"padded int4" means the data is contained in the first 4 bytes and the last 4 bytes are padding for data alignment. The padding bytes will probably be 0 but please do not rely on that.

"pos, vel double" is position, velocity as a pair of double precision numbers. Position is in degrees, velocity in deg/sec, and the time is given by the TAIDate field.

"pos, vel, time double" is position, velocity and time as three double precision numbers. Position is in degrees, velocity in deg/sec and time is TAI (MJD, seconds).

Rotation type codes are as follows (as defined in tinc:axedef.for). The corresponding names are described in TCC Commands: Rotate.

RotType	Description
0	None
1	Object
2	Horizon
3	Physical
4	Mount

Axis commanded state codes are as follows. This corresponds to TCC Message Keyword AxisCmdState, but with integer values (as defined in tcc include file basics.h) instead of strings.

AxisCmdState	Description
-1	NotAvailable: axis is not available
0	Halted
1	Drifting
2	Slewing
3	Halting
4	Tracking
5	BadCode: unknown command state

Axis commanded state codes are as follows. This corresponds to TCC Message Keyword AxisErrCode, but with integer values (as defined in tcc include file basics.h) instead of strings.

AxisErrCode	Description
-3	HaltRequested: halt requested
-2	NoRestart: axis left halted via /NoRestart
-1	NotAvailable: axis not available
0	OK (no error)
1	MinPos: position too small
2	MaxPos: position too large
3	MaxVel: velocity to large
4	MaxAccel: acceleration too large
5	MaxJerk: jerk too large
6	CannotCompute: could not compute the position
7	ControllerErr: could not communicate with the axis controller
8	TCCBug: a TCC bug
9	BadCode: unknown error code

Notes:

• See the TCC Message Keywords Dictionary for full definitions of most items in the "data" section of the packet.
• pos, vel double is a structure containing two doubles: "pos" and "vel"; it contains the same information as the TCC's "coordinates" (p,v,t triplets), but with time given by the "TAIDate" entry in the packet.
• Comparing SlewEndtime and TAIDate will tell you if the telescope is still slewing (as of when the packet data was generated).
• CoordSys and Epoch are guaranteed not to change unless SlewEndtime also changes.

Version History

2.4 2014-09

Added AxisErrCode, ActMount and AxisStatusWord fields.

2.3.1 2015-05

Added reference to tcc35m-1-p and made minor tweaks to the text.

2.3 2009-01

Added AxisCmdState field.

2.2 2000-10

Added SecFocus field.

2.1 1995-10

New packet format.