What is GISS?
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In accordance with the radio regulations and the international division of the electromagnetic spectrum, operating space-based radar systems with SAR operating principles can use spectrum areas in the L-band 1215-1300 MHz (signal bandwidth up to 85 MHz) and 8.025—8650.0 MHz (bandwidth 625 MHz) in the first X (3.5 cm) band, or 9.3—9.9 GHz (600 MHz bandwidth) in the second X (3 cm) band.
In practice, in the L-band SAR systems used a frequency of 1270 or 1275 MHz with a LFM signal bandwidth from 10 MHz to 30 MHz. In the X-band, the second (3 cm) band is mainly used, work is carried out at central frequencies of 9600, 9615 and 9650 MHz with a LFM signal bandwidth up to 100 MHz.
Since simultaneously operating spacecraft for radar sensing of the Earth are in orbits of no more than one to two dozen, and they, according to a given observation program, probe the required area for several minutes, record no more than 30 minutes on one orbit in these ranges, then, naturally, noticeable problems of electromagnetic they do not cause compatibility with other systems.
It is another matter when the GISS uses the same range of the electromagnetic spectrum permitted on a primary basis, but is used continuously and around the clock, it is required to consider the possibility of such functioning from the point of view of mutual interference and electromagnetic compatibility with other systems.
2.2.1 In the L-band (1215—1300) MHz, in addition to radar facilities, satellite radio navigation systems (SRNS) operate. The frequency plans of the signals of the operating SRNS are shown on the slide in Fig. 2.9.
In the so-called L2 range of functioning of the SRNS, their signals are continuously emitted:
- GPS — 1227.6 MHz, PM PRS signal bandwidth 10.5 Mbps;
- GLONASS — 1242.933 — 1249.5 signals with FD, PM PSP rate on each carrier is 0.511 Mbps;
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GALILEO — 1278.75 MHz, composite PM PRS signal bandwidth with a speed of 10.5 Mbps, total bandwidth 25.6 MHz
- (1268.4,01294.0 MHz); COMPASS — 1268.52 MHz, PM PRS signal bandwidth 10 Mbps;
- QZSS — 1227.6 MHz, BPSK PRS signal, rate 1.022 Mbps.
To ensure the operation of the GISS, compatible with all SRNS, there remains one section 1254.64 - 1263.4 MHz with an OFDM signal bandwidth up to 8.8 MHz, which, in principle, allows providing radar resolution up to 2:—10 meters in refinement mode and up to 50 m in survey mode.
The main part of the spectrum in the range allocated on a primary basis for the operation of radar systems is occupied by the E6 signals of the European SRNS (see Fig. 2.10).
There are no particular problems with navigation and terrain maps in Europe, just as there are no problems with providing cellular communications in sparsely populated areas. The E6 signal makes it possible to ensure automatic operation of graders during the construction of highways with specified slope angles and an accuracy of creating a band up to 2 cm. Occasional operation in this range of radar satellites with high power at frequencies of 1270-1275 MHz with a band up to 30 MHz practically does not interfere with their joint use this section of the bandwidth. Since GISS is cost-effective to create as an international system, and the issues of harmonizing the frequency spectrum used are under the jurisdiction of the administrations of each of the states of the world community, the decision on the frequency used and the width of the radiation spectrum over their territory will be determined by each of the participating countries. For most countries (Russia, China, India, African countries, Brazil, etc.), the problem of providing the population with personal satellite communications and broadband Internet access is important, with a significant interest in continuous radar control over the adjacent territory and airspace, then over their territory the E6 signal range of the European SRNS can be widely used.
Onboard repeaters of the GISS SC operate with a programmable mode of operation. And they can use the agreed frequencies and bands for the formation of OFDM signals used for communication and radar surveillance over the corresponding territories. For most countries, the mode of operation of the GISS in two L-band sections, shown in Fig. 2.11. In sparsely populated areas and in ocean theaters, signals from all SRNS in the L5 and L1 bands can be used to determine the coordinates. The countries of Europe will decide for themselves whether to use a limited section of 5 —8 MHz for radar surveillance or, as part of navigation equipment for road operations, introduce an additional channel for subtracting from the spectrum of the E6 signal, the spectrum of the probing (communication) GISS signal received via a separate directional reception path at the frequency 1275 MHz.
At the same time, the wide base of the E6 signal is sufficient to ensure the required accuracy of determining the coordinates at the work site, especially considering that a narrow beam (=12°) of a powerful sounding signal will scan over the entire 5000 km visibility zone of the European part with the eastern part of the Atlantic with a period not more often 10-30 seconds.
The entire frequency range of 1215—1300 MHz can be used for the operation of subscriber terminal communication equipment on uplinks. The modes and value of the radiation frequency of the receiving-transmitting terminal equipment will be determined by the control channel in the total signal of the spacecraft radiation generated under the influence of the control ground complexes and on-board communication service control modules. The receiving and high-power transmitting paths of the Land X-band are spaced across different spacecraft, and in terminal equipment, the separation of several decades of MHz between the receiving and transmitting paths can be compensated by digital methods of subtracting the effect of its emitted signal from the signal in the receiving path.
For a number of special applications and IoT communication services, the corresponding terminal equipment can and should work to transmit directly in the band of a powerful sounding signal, but this is a separate story.
The successful operation of the on-board receiving complex of communication services in the L-band will also be hampered by pulsed radiation from primary airfield radar stations (PARS) of air traffic control (ATC). These stations operate at a relatively low power, pulse duration is about 10 ps, pulse repetition rate is about 1.2—1.5 kHz, and they are located in the airport area, where, as a rule, there are no problems with the provision of cellular services. In sparsely populated regions on the borders of states, military radars for detecting air targets (DATs) radar can also operate in this range. Taking into account the interests of using the GISS spacecraft for the same purposes, for an even greater detection and tracking range of air targets, the relevant administrations can coordinate their work and transfer the radiation range of the military radar of the ATD (as well as the civilian ATC radar) in the band above the frequency of 1300 MHz.
The influence of residual emissions and a limited number of emissions from the ATC radar and ATD radar on the quality of signal reception from the user terminal equipment on board L/X (receiver /transmitter) of the GISS spacecraft can be eliminated by an appropriate choice of the used interleaving interval of the transmitted information and methods resistant to these radar emissions coding. The information decoded without errors from the output of the onboard receiving submodules without errors will be sent to the onboard router.
2.2.2 From the point of view of a systems electromagnetic compatibility, it is more expedient to use the X (3.5 cm) range 8025.0—8650.0 MHz for the operation of the GISS. According to the radio regulations, the 7.9—8.4 GHz band is allocated for the operation of military satellite communication system stations on uplinks. Therefore, directed narrow beams (-10°) radiations of a high power OFDM probing/communication downlinks signals of the GISS will not interfere with the functioning of the military DAT radar. Figure 2.12 shows two of the many conceptually possible use cases for a frequency plan in a GISS.
It is possible to allocate up to five 80 MHz bands for use on various X / L spacecraft in the GISS orbital constellation with 288—576 spacecraft, providing traffic transmission in overlapping service areas. In each of the 80 MHz bands, it is possible to continuously transmit traffic through 3, 7 or 19 directional beams with a relatively low radiation power (about 20 W) to the terminal equipment of users with tracking high directivity antenna systems in the OFDM signal band up to 30 MHz. In the second sub-band 30—40 MHz, it is possible to transmit the OFDM signal through the scanned space in a narrow beam (=10°) with high power for high-speed IP traffic and radar sensing.
It is possible to allocate on each of the X / L spacecraft up to seven bands of 40 MHz for transmission through seven beams, covering the entire visibility zone for transmitting traffic to the terminal equipment of users with tracking PAA with high directivity. An eighth bandwidth of 120 MHz can be dedicated to the scanning narrow beam signal throughout the entire area. It will be able to provide almost continuous radar sensing and transmission of interrogation and control (ringing) signals for terminals with omnidirectional antennas and high-speed downstream traffic information flow from territorial networks to their address. The final decisions will be made when the GISS is created.
In the receiving paths of the GISS spacecraft, using digital filtering and reprogrammable SDR modems, it is possible to implement the required frequency plans and modes of operation to provide a wide range of communication services. Thus, in the X-band, there are many options for implementing frequency plans for using this range, both for the provision of communication services and for almost continuous radar sounding.
In the X (3.5 cm) band, there are practically no radars using this frequency band. Even if they appear in some region, then the methods of parrying their influence stated above for the L-band will be applicable in the X-band.