1、Rec. ITU-R F.1113 1RECOMMENDATION ITU-R F.1113RADIO SYSTEMS EMPLOYING METEOR-BURST PROPAGATION(Question ITU-R 157/9)(1994)Rec. ITU-R F.1113The ITU Radiocommunication Assembly,consideringa) that experiments have already shown the practicability of utilizing the frequency band between 30 and 50 MHzfor
2、 transmission by meteor-burst propagation to distances well beyond the horizon;b) that systems using this mode of propagation are already in service for burst data transmission,recommends1. that Annex 1 be used as guidance for the application of radio systems employing meteor-burst propagation.ANNEX
3、 1Radio systems employing meteor-burst propagation1. IntroductionThe phenomena of meteor-burst propagation using reflections from ionized meteor trails is described inRecommendation ITU-R PI.843. Recent advances in microprocessors and digital electronics mean that it is now possibleto construct a me
4、teor-burst communication system at a realistic commercial cost. This Annex describes two such systemsand presents results of tests carried out on these systems.2. Example meteor-burst system A2.1 System descriptionThe system is configured as a star network with a central master station and a capacit
5、y of up to 1 000 remotestations within the coverage area of 2 000 km radius (see Fig. 1). The system uses a half-duplex packet protocol withsingle segment automatic repeat transmissions in the presence of errors. Each of the remote stations is able tocommunicate with any number of other remote stati
6、ons via the central master station.Larger systems may be built using several master stations communicating with each other by full duplexmeteor-burst propagation such that all remotes can communicate with each other via combinations of master stationsover a series of 25 kHz channels. It is also poss
7、ible to construct a system without a master station by using point-to-pointtechniques when the remote stations perform the control functions. This latter system is not flexible and has to bereconfigured for any change in the number of stations.2 Rec. ITU-R F.1113FIGURE 1System block diagramRemote st
8、ationNo. 2RemotestationNo. 1RemotestationNo. 3MasterstationRemotestationNo. 999RemotestationNo. 1 000MonitorstationD01FIGURE 1/F.1113.D01 = 10 CMOne network system in Europe is designed with the maximum number of system functions concentrated in themaster station. This is to minimize the size, cost
9、and power consumption of the remote stations. The master stationperforms the functions of meteor trail illumination, system synchronization and message routing, but is transparent to theuser. It is the only component of the system which needs mains power.2.1.1 Essential system characteristicsThe sys
10、tem described operates in half-duplex mode using two frequencies around 46.9 MHz with a singlehorizontally polarized antenna at each station. In practice the master station antenna is an array of four “Yagi” antennasconnected together for omnidirectional coverage. It would be possible to use vertica
11、l polarization if required foroperational reasons such as mobile remote stations. The transmitter (see Fig. 2) is rated at 500 W. A transmit/receiveswitch allows call/listen cycles of approximately 80 ms duration. This alternate 40 ms call, 40 ms listen cycle is used toilluminate the meteor trails a
12、nd is known as probing. The probe transmissions may be addressed to individual remotestations or may be non-specific depending on system priority. The master station transmitter modulation is binary lowindex ( 30) phase shift keying (PSK) to minimize interference on adjacent channels. Synchronizatio
13、n of remotestations is achieved with Manchester encoding of the clock signal within the data.Remote stations transmit with high index ( 90) PSK modulation for maximum chance of detection by themaster station. Two protocols are supported, one for data acquisition and one for communication. In normal
14、operation,the master station sends probe words and checks received signals for valid data segments from a remote station (seeFig. 3). Immediately a valid segment is found, an acknowledgement is sent back to the remote source. At the same time acomputer memory check is made for any segments addressed
15、 to that remote station and if found, the first segment of thereturn message is appended to the acknowledgement. If a segment is returned in this way, an acknowledgement from theremote station is sought. In all cases if no acknowledgement is received, the segment is repeated until it is.Rec. ITU-R F
16、.1113 3FIGURE 2Master stationT/R switchReceiver TransmitterDiskdriveComputerPowersupplyModemStationconsoleTo monitorstationD02FIGURE 2/F.1113.D02 = 14 CMThe system is controlled by a monitor, connected to the master station by radio or landline. This connectionenables the monitor to be at the admini
17、stration offices while allowing the master station to be remote in an ideal lowambient noise location. The system monitor checks overall performance and stores all the data transmitted so that userscan collect from the monitor rather than receiving in real time.In this system a remote station for da
18、ta acquisition is optimized for low power consumption and reliableoperation in conditions typically from 30 C to +60 C. The 100 W transmitter duty cycle is limited to 1%. The receiverhas a sensitivity of 118 dBm and there is a buffer store for packets of data or short text messages (see Fig. 4).A se
19、cond type of remote station for communication is optimized for maximum information transfer rate whichis related to signal-to-noise ratio. The 300 W transmitter duty cycle is limited to 10%. The receiver has a sensitivity of121 dBm and there is a buffer store with up to 20 kbytes of memory (see Fig.
20、 5).The communication protocol makes use of the natural diversity in space and time of the meteor trails toprovide automatic polling to a maximum of 1 000 remote stations. When a path is present, the data is sent andacknowledged during the same event. Long data strings require several events and rec
21、ombination at the receiver. Text istransmitted in a series of segments which are all acknowledged at the end of the message. It is possible to continuouslymonitor the signal-to-noise ratio and adaptively change the data rate between 2 kbit/s and 64 kbit/s to maximize thethroughput.4 Rec. ITU-R F.111
22、3FIGURE 3Typical meteor-burst data exchange protocol1.2A.2B.2C.Master station alternately transmits and listens2. Master station receives message(s) from remoteRemote station has message to send and its receiver listens for probe signalRemote station message length 24 bytesListen Probe Listen Probe
23、Listen Probe Listen Probe ListenListen Probe Listen Listen Ack Listen Probe Listen AckListen Data segment 24 bytes ListenListen Additional segment ListenIf all messages cannot be sent on one meteor event, the remaining messages/parts of message are stored untilthe next available event Listen Probe L
24、isten Data segment 24 bytes Listen (Additional segment)3A.4A.4. Master station hears remote but there is no data queued at either stationRemote station receives message(s) from master3. Master station sends message(s) to remoteListen Probe Listen Ack Listen Probe Listen ProbeListen No text Listen Li
25、sten Ack ListenRemote station hears master but has no data to sendListen No text ListenD03FIGURE 3/F.1113.D03 = 21 CM PLEINE PAGERec. ITU-R F.1113 5FIGURE 4Typical data acquisition stationT/R switchReceiver TransmitterCommunicationsmicroprocessorDataacquisition microprocessorOperatorterminalSignalco
26、nditioningSignalsfrom sensorsControlto plantD04FIGURE 4/F.1113.D04 = 12 CMFIGURE 5Typical text and data communication stationT/R switchReceiver TransmitterCommunicationsmicroprocessorText handling microprocessorOperatorconsoleD05Auxiliaryserialdata portFIGURE 5/F.1113.D05 = 11.5 CM6 Rec. ITU-R F.111
27、32.1.2 Noise sensitivity of meteor-burst systemsMeteor-burst links have been shown to be relatively unaffected by atmospheric disturbances such as theAurora Borealis. In the ideal case the performance of a system using meteor-burst propagation would be limited only bygalactic noise. The receiver the
28、refore has to be designed to decode signals whose magnitude is comparable with galacticnoise. In practice there is also man-made noise which is often transient and very large compared with galactic noise. Forthis reason, the receiver has to have high dynamic range and some form of protection to obvi
29、ate the saturating effects ofhigh energy spikes.2.1.3 Design and use, variation with frequencyThe system could work at any frequency from 20-120 MHz provided that regular ionization was not present.The range would be up to 1 600 km using ionized trails from meteors at 80-100 km above the Earth. The
30、preferredfrequency range is 40-50 MHz to maximize the information duty cycle.2.2 Test results2.2.1 Results from operationsPerformance achieved within the European network for all ranges has been better than predicted by most of thepublished computer models. The greatest differences are at short rang
31、es where achieved data rates have been up to fivetimes the predicted values. Experience in the British Isles has proved that reliable communication is provided at allranges between 0 and 1 600 km. The system is used to take measurements at remote locations and regularly send theresulting data back t
32、o offices. Typically 8 12-bit samples are sent from each remote location during each 15 mininterval.In tests during December and January of years from 1986 to 1989, eight low power data acquisition remotestations were set up to send data at 5 min intervals over meteor-burst links having ranges of: 1
33、 520, 1 040, 816, 608, 464,256, 141 and 24 km. All data were successfully received throughout the test period within the 5 min interval before thenext data were released.2.2.2 Observed interference and noise sensitivity effectsExperience in Europe shows that the single most significant factor in det
34、ermining the performance of a meteor-burst link is the level of in-band interference at the site location. Other services in that frequency range use line of sightpropagation. Interference effects can often be reduced by rotating or changing the elevation angle of the antenna awayfrom its normal pos
35、ition. A rotatable low gain wide beam antenna allows the use of a wide area of sky: variations ofantenna angle of up to 25 in any plane can be accommodated with little effect on the link performance. It should benoted that the meteor-burst technique is able to work with reduced signal-to-noise ratio
36、, thus maintaining the linkintegrity, albeit at a lower data rate.2.2.3 Observed variation with frequencyThe system described uses only a single pair of frequencies around 46.9 MHz; hence there is as yet nooperational data on variation with frequency.3. Example meteor-burst system B3.1 System descri
37、ptionTests on a data transmission system using a meteor-burst channel were carried out in France during the firsthalf of 1988, at various times of day and night, on a point-to-point link between two fixed stations 350 km apart.Rec. ITU-R F.1113 7DTE DCEVHFT/RAAVHFT/RDCE DTET/R switching T/R switchin
38、gFIGURE 6Basic block diagram of the linkD06FIGURE 6/F.1113.D06 = 9.5 CM The DCE sub-assembly performs FSK modulation and synchronization functions (RF modem). The DTE sub-assembly performs communication, coding and data processing functions.The transmitted message is formatted in blocks. There are t
39、wo types of blocks: header blocks (one block per message), data blocks (number of blocks dependent on message length).Encoding is based on blocks and involves a Reed-Solomon error correcting code (23.13) which maycompensate error rates of up to 5%.The bit rate used is 16 kbit/s; characters are made
40、up of 6 bits.The procedure is of the stop-and-wait “half-duplex” type with repeat transmission (repetition of only theblocks not received).Tests were carried out: At several transmitter power levels: 1 kW 200 W 100 W, a few tests at 50 W At different frequencies: around 40 MHz With different message
41、 sizes: 900 500 250 100 50 10 characters.Horizontally polarized 5-element Yagi antennas were used, with a radiation pattern which illuminates theentire coverage area with a high concentration of meteors.These antennas are therefore particularly suitable for this type of transmission; they have a gai
42、n of 9.5 dBi.3.2 Test resultsThree types of tests were carried out.8 Rec. ITU-R F.11133.2.1 Channel testMeasurement of the following characteristics for a given observation time T:m : mean duration of a meteor trailm= trails durationnumber of trailsm : mean duration between two meteor trailsm= durat
43、ion between two trailsnumber of trailsHence, do: channel opening duration, i.e. average percentage of time during which the channel is open.dommm=+The mean duration of a meteor trail, m, is calculated by determining continuity of operation of the procedure.For a 1 kW power level, meteor-burst channe
44、l activity gave a mean trail duration of 400 ms and a meanduration between two trails of 700 ms.For a 200 W power level, there was a slight drop in activity (mean trail duration: 330 ms mean durationbetween two trails: 800 ms).However, at a power level of 100 W, there was very little activity (mean
45、trail duration: 350 ms meanduration between two trails: 43 s).It may be deduced that the channel opening duration was 36% at 1 kW, 29% at 200 W and 0.8% at 100 W.The tests also showed that mean effective meteor activity is practically stable over 24 h; increased activityat 0600 h and reduced activit
46、y at 1800 h, as indicated in Recommendation ITU-R PI.843, was not apparent.Note 1 These test results for channel activity at high transmission power levels (1 kW 200 W) are appreciably higherthan the theoretical results obtained using the formula derived from the COMET experiments.It would seem that
47、 they can be explained only by the simultaneous presence of another propagationmechanism. The antenna height above ground (17 m), chosen to provide links of up to 1 000 km, gives appreciable gainsat low angles of elevation. This might tend to favour tropospheric scatter links, theoretically possible
48、 over the distanceconcerned and operationally feasible with the procedures used for the meteor-burst channel. The existence of thisphenomenon has been confirmed by estimating, opening by opening, propagation time of the signal (troposphericdiffusion at low altitude corresponds with a much shorter pr
49、opagation time than reflections from meteor trails at highaltitude).3.2.2 Bit error ratio testThe test allowed measurement of bit error ratio on the trails due only to channel characteristics, without the useof error correcting codes.A received message was compared with a 900 character reference message. All errored bits (per character)were stored in a file: a window of 60 characters was slipped into this file in one character steps. When the bit error ratiowas 5% in this window (this figure corresponds to the maximum capacity
