Monitor and Control
The GMRT ,an aperture synthesis array, has been set up about 80 Km. north of Pune .It consists of an array of 30 fully steerable parabolic dishes,each of 45m in diameter,located over a region of about 25 Kms.(Fig. 2).Six dishes are installed in each of the three arms of the Y-shaped array and the remaining 12 antennas are installed in and around Central Electronics Building (CEB).
A simplified block diagram of the GMRT receiver system is shown in Fig (GMRT SYSTEM).The antennas have dual polarised feeds at all the six frequencies.The feeds are mounted on the four faces of a micro-processer controlled rotating turret near the prime focus.The receiver system consists of RF,LO,IF electronics and is required to have a very low system temperature and good phase stability.The linearly polarised signals received from the sky are converted to circularly polarised signals,amplified and brought thro' two low-loss cables to the base of the antenna.Then the RF signals are converted to 130 and 175 MHz bands using the regenerated LO signals locked to a central frequency standard and transmitted to the CEB thro' the Single mode 1310 nm Optical Fibre cable and converted to the video signal of 16 MHz bandwidth by the baseband system.The back end consists of a correlator system which digitizes the analog signals at 32 MHz clock rate,introduces proper path delays to compensate for the time difference in the signal path to maximise the received signal from all the 30 antennas and helps in producing high-quality image maps.
Control and Monitor (C & M) System for GMRT
What is the purpose of C & M ?The Control and Monitor System of a Radio Telescope are required to provide the necessary co-ordination between the various building blocks of the receiver system.This system is used for Controlling the activities of the various building blocks of GMRT like FE,LO,IF,BB,SERVO and FPS etc and Monitor the healthiness of the same in each of the antenna shells and CEB.It provides the human interface to persons like Telescope Observers,Scientists and maintenance personnel for operating all the antennas from CEB.It has to monitor all parts of the telescope system for correct operation and alert the operator in case of any anomalous behaviour.And in the case of severe fault conditions,safety procedures have to be initiated locally.It has also to prevent human error from placing the telescope in a dangerous situation.This Control and Monitor System for GMRT tries to meet all the points mentioned above.
Salient features of C & M system
1)The rotation of all the thirty antennas in AZIMUTH and ELEVATION thro' SERVO CONTROL COMPUTER(SCC).
Azimuth --> -270 TO +270 deg
Elevation --> 17 TO 90 deg
--> Using ABC - SERVO RS422 communication link @ 9.6 Kbps.
2) Alignment of the required feed for observation to the focus of the dish thro' Feed Position System(FPS).
0 deg --> 233/610 MHz
90 deg --> 150 MHz
180 deg --> 1420 MHz
270 deg --> 325 MHz
--> Using ABC - FPS RS485 communication link @ 9.6 Kbps.
3)Selection of FRONT END SYSTEM parameters like observing freq. band,Noise Cal,Channel Swap etc thro' MCM '5' & MCM - FE interface card in the Common Box.
4)Sets the LO Freq. , IF bandwidth (6,16,32 MHz),IF attn.,ALC ON/OFF etc. using MCMs'2','3' & '9' located in LO/IF SYSTEM
5)Sets Baseband bandwidth (62.5 KHz to 16 MHz) using MCMs located in BASEBAND SYSTEM thro' Antcom '0' .
6)Monitors the C & M system parameters at Antenna shell using MCM '0' at NRR and at CEB using 6 MCMs thro' Antcom '31'.
7)Monitors the Temperature at various points in RFI cage in each antenna thro' Temperture Monitor system using MCM '0'.
In addition, it also provides the vital voice communication link between CEB & all the Antennas.ie.You can contact anybody in any antenna using C & M SYSTEM.
The communication medium is a single mode analog optical fibre link operating @ 1310 nm between CEB and all the antennas ie. two fibres are available for each antenna.So,the digital command & control signals are converted to analog form using non-coherent FSK techniques and sent alolng with the LO and IF signals.In the forward link,Telemetry command signals are combined with the LO carriers and transmitted to all the antennas and in the return link,Telemetry monitor signals are combined with the IF signals and brought to CEB.The FORWARD link uses 18 MHz carrier & the RETURN link uses 205.5 MHz .
We'll see the system details later.
System operation (ONLINE SYSTEM)
What is ONLINE System?
The ONLINE SOFTWARE running on the UNIX workstation provides the user interface for sending all the USER COMMANDS like antenna postioning,setting various receiver parameters etc to different systems in MRR & all the ANTENNAS and displays the status of the same.
Let us see how the user commands are processed by the ONLINE system.
The user commands are formatted by UNIX workstation and sent to all the antennas thro' COMH at MRR & Antcom located in each of the antenna shell.
More details will be given by Prof. A. P. Rao in his talk on "ONLINE SYSTEM FOR GMRT"
Communication Handler ( COMH )
COMH is the main communication handler located at MRR C & M rack, handles all the packet communication between UNIX workstation and ANTCOM. COMH is operated in TDM mode ie. it sends the formatted user commnads to the first antenna in the subgroup & waits for the acknowledgement.If it receives the error free reply before the timeout period, then it accepts the data & selects the next antenna in sequence & the operation continues.But in case if it doen't get any reply before the timeout period or if the reception is erroneous then it tries three times and logs the error as Timeout or Checksum error and pass on this information to the ONLINE system for further action.
Antenna Computer ( ANTCOM )
ANTCOM is the vital Antenna base computer located in each of the antenna shells.It receives various parameters sent by COMH and performs all the tasks and passes the vital information to various systems like Servo Control Computer,MCMs and FPS thro' the following three communication links.
- Main FSK communication link between COMH & ANTCOM at 250 Kbits/sec using OF system.
- Asynchronous data communication link between ANTCOM & SERVO thro' RS 422 link.
- Asynchronous data communication link between ANTCOM & MCMs thro' RS 485 link.
Monitor and Control Module ( MCM )
MCM is a general purpose Micro-controller based card which provides 16 TTL Control O/Ps and monitors 64 analog signals.Antcom communicates with MCMs thro' RS485 communication link @ 9.6 Kbaud rate and sets various FE,LO and IF system parameters.
Servo Control Computer
The Servo Control Computer located in each antenna shell accepts the movement command,position information etc. thro' Antcom-SCC link @ 9.6 Kbits/sec rate,validates the data and obeys the command.It returns the antenna status information periodically thro' the same link and will be displayed by the ONLINE software on the monitor.
We'll see the specifications & the circuit details of COMH, ANTCOM & MCM later. Next we'll see the details of System Specifications and the various communication links used in the C & M system.
System Specification of C & M System
- Non-coherent FSK techniques are used for data transmission over OPTICAL FIBER links.
- Data Baud Rate is 250 Kbits/sec.
- Bit interleaved techniques are used for multiplexing Telemetry,Voice,Sync.,Dial and Aux.Channels.
- Polynomial and checksum ERROR DETECTION with ARQ capability.
- The Bit Error Probability is 10 ¯10
Bit rates available for various services:
| DATA (COMH - ANTCOM COMM) | ---> | 125 Kbits/sec |
| VOICE (TELEPHONY VOICE) | ---> | 62.5 Kbits/sec |
| DIAL (TELEPHONY SIGNALLING) | ---> | 15.625 Kbits/sec |
| SYNC (SYNCHRONISATION PATTERN) | ---> | 15.625 Kbits/sec |
| AUX1 (AUXILIARY CHANNEL1) | ---> | 15.625 Kbits/sec |
| AUX2 (AUXILIARY CHANNEL2) | ---> | 15.625 Kbits/sec |
| TOTAL BIT RATE | 250 Kbits/sec | |
Details of Signal Flow in the C & M System
So far, You have been exposed to the functions of C & M system,various data rates used by the system and the type of modulation used etc.But how all these things are achieved ? This system block diagram will give you a clear idea of the inner workings of the C & M system and the signal flow thro' the various subsystems.
Already you have seen the operation and command flow of the ONLINE system.But this block diagram will give you an idea of the data flow and how the various data rates are used.User commands for various antennas are processed by UNIX WS and sent to the COMH thro' the PC router.The PC router acts as a buffer and accepts the TCP/IP data @ 10 Mbits/sec(Link A),strips the TCP/IP headers and sent it as asynchronous data with the headers to COMH @ 38.4 Kbits/sec.This RS232-C link (Link B) is converted to RS 422 signals (differential TTL signals) and vice-versa.
Details of various communication links used in the C & M system of GMRT
| 1 | ETHERNET LINK | UNIX WS | <=====> | PC ROUTER | 10 Mbits/sec |
| 2 | ASYNCHRONOUS | PC ROUTER | <=====> | COMH | 38.4 Kbits/sec |
| 3 | ETHERNET LINK | COMH | <=====> | ANTCOM | 125 Kbits/sec |
| 4 | ETHERNET LINK | ANTCOM | <=====> | MCM | 9.6 Kbits/sec |
| 5 | ETHERNET LINK | ANTCOM | <=====> | SERVO | 9.6 Kbits/sec |
| 6 | ETHERNET LINK | COMH | <=====> | ANTCOM | 250 Kbits/sec |
| 7 | ETHERNET LINK | CEB | <=====> | ANTCOM | 62.5 Kbits/sec |
COMH ----- COMMUNICATION HANDLER
ANTCOM ----- ANTENNA COMPUTER
MCM ----- MONITOR AND CONTROL MODULE
SDLC ----- SYNCHRONOUS DATA LINK CONTROL
HDLC ----- HIGH-LEVEL DATA LINK CONTROL
FSK ----- FREQUENCY SHIFT KEYING
CEB ----- CENTRAL ELECTRONICS BUILDING.
Communication Handler ( COMH )
COMH is the main communication handler located at MRR C & M rack, handles all the packet communication between UNIX workstation and ANTCOM.Let us see the details of COMH
Antenna Computer ( ANTCOM )
The ANTCOM is the brain behind the operations in each and every antenna shell and services all the relevant peripherals in a cycle time of 1 second.
ANTCOM has got the same circuitry as COMH and handles one more serial communication - ANTCOM - MCM communication using INTEL 82510 Communication Controller @ 9.6 Kbits/sec in addition to other two serial communications(125 Kbits/sec SDLC/HDLC data and 9.6 Kbits/sec SERVO data).
Now, we'll see how ( ANTCOM ) reassembles the data sent by UNIX WS/COMH.
Next,we'll see the block diagram of MCM and discuss.
So for , I have mentioned about VOICE COMMUNICATION . Let us see how the analog voice is digitized and combined with the rest of the data and the functions of Telephone signalling etc.
We have been talking about various subsystems of C & M and discussing the salient features etc It was mentioned somewhere in the talk that the C & M system can take of any errors occurring in the transmission /reception path and detect it.
How ERROR DETECTION is achieved ?
Let us explore !
Finally,we must know the STATUS OF THE C & M system and its performance.
Let us see how the analog voice is digitized and combined with the rest of the data and the functions of Telephone signalling etc.
INTEL 29C17 CODEC IC has been used for digitizing voice using 7.8 KHz clock and it produces 62.5 Kbits/sec rate.The CODEC uses "A law " for data companding\expanding.
Cyclic Redundancy Check (CRC) and checksum methods are used for error detection.
SDLC/HDLC supports 16 bit CRC error detection. CRC can detect all the single errors,double errors and burst errors up to 16 bits in length and can detect 95 to 99 % of burst errors of >16 bits.
For example,you can treat the binary data as message polynomial of certain degree as G(X) .A generator polynomial is a type of code used in CRC-12, CRC-16 and CRC-CCITT - P(X).
G(X) * X ^ n-k / P(X).
The remainder is sent along with the data as CRC.
The data sent = (G(X) * X ^n-k) + C(X).
C & M systems for CEB and all the thirty antennas have been produced and installed in the respective antennas.All the systems are working satisfactorily and being used extensively by the ASTRONOMERS for observations.
These systems are being monitored regularly for stability,reliability.Also all the communication parameters like Forward and Return link Checksum and Timeout errors,ANTCOM-MCM link errors and ANTCOM-SERVO link errors are being monitored and logged.