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Figure 39.9 Beacon slot structure – preamble, pilot, and data sections.
Figure 39.10 Relationship between GPS and MBS system timing.
The MBS system time is synchronized with GPS system time at an absolute level. Figure 39.10 shows the MBS system time relationship with GPS system time, the relationship between MBS system time and MBS beacon transmissions, as well as the time of arrival (TOA) at the MBS receiver of the MBS beacon signals. As shown in Figure 39.10, MBS time equals GPS time + Fixed Offset. Note that all beacons are synchronized to transmit as per MBS system time. MBS system time is synchronized to the phase center of the transmit antenna. In the MBS signal transmission from the beacon, the peak of the first chip of the first preamble PRN sequence within a slot is aligned to MBS system time. The MBS transmission for beacons in the N‐th slot starts at an offset of (N × 100) ms from the MBS system time second boundaries. Note that any fixed offset between MBS and GPS system time is transmitted through the data packets.
Figure 39.11 shows a block diagram describing signal generation in an MBS transmitter. Note the similarity of the MBS signal generation section to the GPS C/A code transmit section [1]. The logic shown in Figure 39.11 controls the selection of the PRN sequence (system‐wide preamble versus beacon‐specific pilot/data spreading sequence) for different parts of the slot as well as the optional beacon‐specific frequency offset for the pilot/data sections. The data packet content can consist of trilateration information, atmospheric information, timing information, and/or system control messages. The data content may also be encrypted to prevent unauthorized usage of the data stream. A forward error correction/detection scheme is added on the data frames to protect against channel errors. Once the spreading is done, pulse shaping is applied to generate the signal, which is then up‐converted to the MBS frequency and amplified before transmission over the air.
Figure 39.11 Transmitter block diagram.
39.1.3 Comparison of MBS Signal with GNSS Signals
Table 39.1 shows a comparison between the GPS L1 C/A code signal and the MBS 2 MHz signal. The MBS system is a broadcast system similar to the GPS, the only difference being that the MBS system is terrestrially broadcast. The MBS is a spread‐spectrum system which has pretty much all its key characteristics identical to GPS C/A except that the spreading codes selected may be optimized for multipath. With regard to multiple access, the MBS system uses a combination of CDMA/TDMA/Frequency Offset Multiple Access, whereas the GPS system uses CDMA. The MBS system provides a cross‐correlation rejection of >40 dB as against the worst‐case CDMA code rejection of 23 dB in GPS systems. The MBS system’s beacon transmissions are synchronized to one another, just as for GPS satellite transmissions. The MBS data bit duration and coding in MBS are similar to the Wide Area Augmentation System (WAAS) or European Geostationary Navigation Overlay Service (EGNOS) systems. The MBS data content is, in general, analogous to the GPS data content with the MBS beacon coordinates corresponding to the GPS ephemeris/almanac and the MBS timing correction corresponding to the GPS satellite clock correction. Note also that MBS data may be encrypted using a conditional access scheme to control receiver access to the data stream. Location estimation using the MBS system is similar to the GPS system in that trilateration can be done using pseudorange measurements. The key difference is that the MBS trilateration process may estimate altitude independently using the receiver barometric pressure reading. The MBS receiver can operate in both stand‐alone and assisted modes, just as a GPS receiver. Also, the position may be computed at the receiver or at the server.
Table 39.1 Comparison between GPS L1 C/A code and MBS 2 MHz signal
| System Feature | GPS L1 C/A Code | MBS 2MHz Signal |
|---|---|---|
| System type | BroadcastSatellite system | BroadcastTerrestrial system |
| Signal properties | Spread spectrumFull bandwidth 20 MHzFirst null‐to‐null bandwidth 2.046 MHzSinc‐shaped spectrum with slow sinc roll‐off of spectrum sidelobes | Spread spectrumFull bandwidth 2.046 MHzFirst null‐to‐null bandwidth2.046 MHzSinc‐like spectrum shape similar to GPS in‐band and very sharp spectrum roll‐off beyond null‐to‐null bandwidth |
| Spreading codes (chip rate, code length, duration) | Chip rate: 1.023 McpsCode length: 1023 chipsSpreading: BPSKCode duration: 1 msSpreading code type: Selected Gold codes | Chip Rate:1.023 McpsCode length: 1023 chipsSpreading: BPSKCode duration: 1 msSpreading code: GPS family of Gold codes, optimized for multipath |
| Multiple access | CDMASatellite transmissions cross‐correlation > 23 dB | CDMA/TDMA/frequency offset (optional)Beacon transmissions cross‐correlation > 40 dB TDMA slots 100 ms over 1 s transmission periodSlots can contain preamble, pilot, and data |
| Synchronization | Relative and absolute synchronizationSV transmissions at antenna aligned to each other and to common GPS system time | Relative and absolute synchronizationBeacon transmissions at antenna aligned to each other and to common MBS system timeMBS system time aligned to GPS system time |
| Data (rate, modulation, and coding) | Bit duration: 20 msBPSK modulation dataNo forward error correction | Bit duration: 1 msBPSK modulation dataForward error correction scheme similar to WAAS/EGNOS |
| Data (content) | Satellite orbit information, clock corrections, and atmospheric corrections through ephemeris and almanacData on the C/A code is not encrypted | Beacon locations, beacon clock corrections, and atmospheric informationData may be optionally encrypted using a conditional access scheme to control receiver access |
| Receiver trilateration method | Pseudorange‐based 3D trilaterationGPS system timing (pps) available as by‐product | Pseudorange‐based 2D or 3D trilaterationMBS system timing available as by‐product |
| Receiver operation mode | Stand‐alone and assisted modesAssisted mode provides greater sensitivity and quicker TTFFPosition computed on receiver or on the network | Stand‐alone and assisted modesAssisted mode provides greater sensitivity and quicker TTFFPosition computed on receiver or on the network |
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