TWINS Telemetry Description 5/2/2011 See TWINSTelemetry_T.xls spreadsheet. There are 14 pages to the telemetry spreadsheet. Each page represents a subsection of the overall telemetry scheme used for TWINS and its spacecraft. The fourteen pages are as follows: Configuration, Data Products, Data Summary, Modes, Headers, Sync Bytes & Checksum, Image, Test, Memory Dump, Housekeeping, Neutral Data Conv., DE Data Conv., Singles Conv., and Status Conv. Each page will be discussed below. The Configuration page is simply a header to the overall documentation. It relays the revision date of the spreadsheet as written by Aerospace Corporation. The Data Products page contains the Telemetry Data Contents Summary. It discusses the CCSDS packets and their various contents depending upon instrument mode. The Data Prodcuts are Neutral-atom pixels, Lyman-alpha pixels, Direct Events, Singles, Memory, Housekeeping, Instrument Status, Headers, and Checksum. The Data Summary page contains the number of bytes used by the various data products during the 4 different TWINS science modes. The four scientific modes are Static Test, Static Image, Dynamic Test and Dynamic Image. The two Test modes do not transmit Neutral Atom pixels. The Modes page contains a description of which message type is transmitted during the seven telemetry modes as listed. Each telemetry mode will generate one or more telemetry message types. TWINS has four message types: memory dump, housekeeping, image, and test. Each message contains one or more packets output at intervals of either a) time or b) actuator scan position. Synchronization (i.e., to time or position) of the output data depends on the telemetry mode. The Headers page discusses the primary and secondary headers included in each message. It also discusses the wrapper which is also added to every message. Primary and secondary headers contain important information for ground data processing such as time tags, telemetry mode, packet type, packet ordering, etc. Headers are output once for each packet. Primary and secondary headers are each 6 bytes wide. The Sync Bytes & Checksum page relates how to calculate the checksum and verify correct transmission and storage of the raw data. Checksums are used to validate the contents of each packet. They are 1 byte wide and their location is fixed at the end of each packet. The Image page discusses the Image Packet Contents for either Dynamic Image or Static Image mode. Dynamic image has 19 packets per message; static image has 16 packets per message. This page contains a description of each byte. The main component of these messages is the Direct Events data. Direct events are 5-byte data elements containing raw data for each validated particle event. They are sampled every sector and their number will vary depending on the particle event rate. They will be output every sector crossing or every 1.333 seconds depending on the telemetry mode. The Test page is similar to the Image page, except it relates the package contents for Dynamic Test and Static Test modes. The main component of these messages is the Direct Events data. Direct events are 5-byte data elements containing raw data for each validated particle event. They are sampled every sector and their number will vary depending on the particle event rate. They will be output every sector crossing or every 1.333 seconds depending on the telemetry mode. The Memory Dump page describes the memory dump packet contents by byte. Memory data for any 32 kbyte block of instrument memory will be downlinked when requested by a ground command. The Housekeeping page lists the contents of the housekeeping packet. It includes several variables that are contained in a single byte which utilize masking to read the actual value. Housekeeping data contains both analog monitors and instrument configuration data. This data will be sampled and output at fixed time intervals depending on the telemetry mode. Housekeeping data is 1 byte wide. The Neutral Data Convention page lists the 3072 bytes included in each neutral atom image. Neutral-atom pixels are sampled every azimuthal bin (sector) and compressed from 2 bytes to 1 byte. Each pixel is classified according to sensor head (2), species (3), polar angle (32), and energy (16). Each 4-deg. sector will generate 3,072 neutral-atom pixels. They will be output every sector crossing or every 1.333 seconds depending on the telemetry mode. The Direct Event page defines the byte ordering of the 5-byte direct event data falling within every azimuthal sector. Each five bytes is a unique raw data set for a single particle event. The format of this data is defined in the User's Guide for the TWINS FEE-CTL Brd dated July 2001, Draft B. The maximum number of bytes is dependent on the particle event rate and the telemetry mode. Empty slots in the direct event data are called spares. Spares will be filled with all zeroes. The Singles page outlines the byte ordering of the 26-word singles data within every azimuthal sector. Singles data are 26 counters each 2 bytes wide and sampled every sector. They will be output every sector crossing or every 1.333 seconds depending on the telemetry mode. The Instrument Status Data is defined in the Status Convention page. The byte ordering of the 32-element instrument status data within every azimuthal sector is described on the page. Instrument status data are those parameters which need to be tightly correlated to science data (i.e. science data are neutral-atom pixels, Lyman-alpha pixels, direct events, and statistics) and will therefore be included in the image message and test message.