Neutral Gas Ion Mass Spectrometer (NGIMS) NGIMS Data Throughput Document For CONTOUR NGIMS Prepared by: _________________________ Florence Tan NGIMS Digital Engineer Approved by: _________________________ Jack Richards NGIMS Instrument Manager 1.0 Scope This document describes the data throughput of the Neutral Gas Ion Mass Spectrometer (NGIMS) during the pre-Encke encounter period. 2.0 Overview The NGIMS instrument is commanded via a 1553 bus on the CONTOUR spacecraft. The 1553 bus schedule can be found on the CONTOUR website at: https://sd-forum.jhuapl.edu/CONTOUR/. 3.0 References 1. APL Document 7379-9303 CONTOUR 1553 Bus Specification Version 1.03 June 11, 2001 2. FuncEnc1A, FuncEnc2A, FuncEnc3A, FarEncA, PreEncA, PostEncA, PostEncCalA Encke encounter scripts 3. NGIMS Telemetry Format document NGIMS-FSW-06-telemetry-v2_9.doc 4.0 Types of NGIMS packets NGIMS produces 3 types of science telemetry packets: TM science, Dump packets, and Acknowledge packets. Each of these packets is 244 bytes long and is transmitted as TM packets, differentiated by the first word of each packet. TM science uses APID 480, Dump packets uses APID 482, and TM Acknowledge uses APID 481. NGIMS also produces 8 words of non-packetized data (NPTM) to the C&DH and the 8 words are repackaged as a C&DH packet (APID 0004 or 0084) to be sent to the ground. All packets should follow the CONTOUR CCSDS format for instruments. However, NGIMS packets differ from the format at several key junctures. More information about these packets and their differences from the CONTOUR CCSDS format are documented in NGIMS-FSW-06-telemetry-v2_9.doc. 4.1 NGIMS Data Collection Schedule Below is a schedule for the collection of the NGIMS science telemetry packets. Transaction Variables Values of Transaction Variables Sub-addresses T1 (NGIMS) Participating remote terminals NGIMS 1553 messages per transaction 4 16-bit words per 1553 message Message Words 1 32 2 32 3 32 4 32 Transaction frequency Up to 4 Hz Minor frame number(s) when this message can be sent Minor Frame Transactions 17 Up to 1 18 Up to 2 19 Up to 1 Number of retries 1 Summary of Collection of NGIMS Science Telemetry As the graph indicates, NGIMS can be queried for up to four packets per second. At 1952 bits per packet, NGIMS' maximum allowable data output rate is 7808 bits per second. As shown in Section 5, NGIMS' calculated maximum data output rate is 5075 rates per sec. In addition, the spacecraft has imposed (but not enforced) a data rate requirement of 4500 b/s on NGIMS for SSR data management purposes. The CONTOUR NGIMS science document (TBC) specifies that NGIMS will be turned on for TM or science data collection 14.3 (TBC) hours prior to encounter. The PI indicates that NGIMS will be turned on for 5 hours (TBC) for data collection and sensor calibration after Closest Approach. Thus, NGIMS will be producing data for approximately 19.3 hours for the Encke flyby. 5.0 NGIMS Telemetry Packet Size and Data Production As mentioned, NGIMS science data are transmitted as science telemetry or science TM packets. Each science TM packet consists of 15 Integration Periods (IPs) of data or 1 subscan every 0.495 seconds (15 * 33ms). Each subscan is packed into 80 words of science data in each NGIMS telemetry data in CCSDS packet format. 101 words were allocated as science data in each NGIMS science TM (or CCSDS) packet. To utilize every bit of transmission bandwidth, each CCSDS packet is packed with 101/80 NGIMS subscans. This paragraph calculates NGIMS science/CCSDS data rate. On average, 80/101 CCSDS packets are produced and transmitted every .495 seconds or 0.792 packet/.495 seconds or 1.6 CCSDS packets per second NGIMS TM packets are a fixed length of 244 bytes. Since 244 bytes = 1952 bits, NGIMS transmits TM packets at a rate of 1.6*1952 bits/s = 3123.2 bits/s 5.1 NGIMS Dump Telemetry (TM) Packets NGIMS dump TM packets are produced when a memory dump command is sent. When NGIMS dump TM packets are produced, NGIMS commanding philosophy mandates that NGIMS stop transmitting TM science packets. This is achieved in the command script by sending a ngi_TM_on 0 (turn off science TM packet transmission) command prior to each memory dump. Production of dump packets does not affect our science data rate because dump packets are only produced when no TM packets are produced. 5.2 NGIMS Acknowledge Telemetry (TM) packets In addition, NGIMS will produce an acknowledge TM packet in response to commands sent to the instrument. The acknowledge TM packet, which is also 244 bytes long, will acknowledge 1 to 8 commands that are received per one-second timeframe. In other words, if 8 commands are sent within a 1-second window, the system will send one acknowledge packet back. Similarly, if 1 command is send in a one-second window, the system will also send one acknowledge packet back. Given that the NGIMS spacecraft allocated data rate is 4500 bits per second, and NGIMS produces 3123.2 bits per second, there is a surplus 1376.8 bits to be allocated for the transmittal of the TM Acknowledge packet. As mentioned earlier, the acknowledge TM packet consists of 244 bytes. This means that the system can transmit a acknowledge TM packet every 1952/1376.8 seconds or 1.4177 seconds or 0.705 packets per second. Looking at this from the commanding point of view, the ground support equipment can transmit up to 8 commands within a one-second period every 1.4177 seconds, and still keep below our data production promises. 5.2 NGIMS Maximum data rate NGIMS has a commanding philosophy that caps commanding rate to 1 per second. If NGIMS were freed from the spacecraft allocated rate of 4500 bits per second, and commands are send every second, then the maximum number of NGIMS packets per second is 1.6 data packets + 1 ack packet = 2.6 packets per second or 5075.2 bits/s 6.0 NGIMS Maximum Data Volume NGIMS will be on for 19.3 hours during the Encke flyby - 14.3 hours pre- Closest Approach (CA) and 5 hours post-CA. If data production is at a pre-set constant of 4500 bits per second; during the 19.3 hour period, NGIMS will produce a total of 19.3*3600*4500 bits = 312.66E6 bits If NGIMS sent 1 command per second during the entire data period, NGIMS will produce a maximum of 2.6 packets per second or a maximum data volume of 19.3*3600*2.6*1952 bits = 352.624896E6 bits However, we know that we will not be producing data at the maximum rate ie. producing 1.6 TM packets per second and sending a command every second, causing NGIMS to produce 2.6 packets/sec. The more likely scenario is NGIMS produces TM packets at 1.6 packets per sec and the spacecraft sends ~1800 commands to NGIMS over the entire 19.3 hour pre and post encounter period. We used 1800 commands because that is the total number of commands in the FuncEnc1A, FuncEnc2A, FuncEnc3A, FarEncA, PreEncA, PostEncA, PostEncCalA Encke encounter scripts. This translates to 1 command every 38.6 seconds. This rate hides the bursty nature NGIMS' commanding; each 'cluster' of commands are often followed by long no-command periods for data acquisition. Thus, the total data produced by NGIMS in this type of scenario will be 1.6 TM packets per sec for 19.3 hours + 1800 ack packets total = 1.6*1952*3600*19.3 + 1800*1952 = 220.513536E6 We can then calculate the typical bit rate for NGIMS to be Total data (bits)/Total Time (sec) = 220.513536 E 6/(19.3*3600) = 3174 bits/sec 7.0 Summary Maximum NGIMS output data rate 2.6 packets per second or 5075.2 bits/s Typical NGIMS output data rate 3174 bits per second Maximum NGIMS Data Volume (19.3 hours) at maximum command rate of 1/s 352.624896E6 bits Typical NGIMS Data Volume (19.3 hours) at typical command rate of 1 command/38.6 sec 220.513536 E 6 NGIMS/GSFC-FT01 CONTOUR NGIMS NGIMS Data Throughput Document Rev. : 0.1 Date: 24 June 2001 Page: 1