<< G-SAT-SAR can help in reducing sensor-to-shooter dissemination time thereby ensuring an almost continuous realtime imaging >>
Latest trends in the field of remote sensing and GIS-based information have changed the dimensions of future warfare. Real-time remote sensing and GIS-based intelligence are going to play a crucial role in the battlefield and are going to decide winning strategies. Possession of this realtime capability will demonstrate the potential of the nation to sustain the war and ultimately will play a defining role in deciding the winner.
The battlefield scenario and mission applications are constantly changing. It is crucial that combat commanders and warfighters have a real-time application of GIS information for matching weapons with the relevant targets so that requirement can be rapidly configured to meet the ever changing information needs. For an effective military combat, the sensor-to-shooter loop is the most significant factor. The response time required from the time the hostile targets are launched till they are engaged is rapidly plummeting due to the advancements in remote sensing technologies. Therefore it becomes imperative to cut down on the time in the sensor-to-shooter loop with suitable real-time GIS information. This will be most critical for enhancing combat effectiveness in modern network centric warfare.
Generally, quality and clarity of microwave (radar) images are a function of the length of the transmitting antenna from which EM waves are transmitted. Antenna size and shape are a major concern when radar is mounted on an aircraft or a spacecraft. To overcome this limitation, a series of pulses are transmitted out as the platform travels and the returns obtained from each pulse are combined to form a picture simulating the effect of using a long antenna, hence the name Synthetic Aperture Radar (SAR).The concept of geosynchronous orbit based synthetic aperture radar (G-SATSAR) will be a suitable and realistic idea for application of continuous real-time GIS information for future applications.
In the present day scenario, most of the operational SAR based GIS systems are low-earth orbit systems with relatively narrow swaths and long repeativity. For continuous real-time GIS information such as war field or dynamic terrain monitoring, a large number of satellites are required. If the LEO based SAR sensor system is replicated to GEO based platform, the advantages could be many in terms of area covered, real-time information and reduction in the number of satellites for a particular application. Also, synthetic aperture radar attracts significant interest because of its capability to perform in all weather conditions including night. It is not possible to meet the growing demands of wide swath and continuous monitoring applications with LEO orbit based SAR sensors. Hence, the concept of geosynchronous satellite based SAR is certainly expected to overcome these limitations.
Some of the types of G-Sat-SAR system configurations are:
- Mono static radar principle based G-Sat-SAR imaging methodology
- Bi-Static radar principle based G-Sat-SAR imaging methodology.
Mono static radar principle based G-Sat SAR imaging methodology
In this methodology, as the name suggests, a single SAR antenna placed on geosynchronous orbit performs the functions of transmission and reception (Figure 2). A single G-Sat-SAR can cover 42 per cent of earth for continuous imaging in all weather conditions. Achievable antenna size, radiated peak power, range and azimuth resolution along with challenges in the design of antenna, data processing solution and corrections for atmospheric perturbations have to be accounted for the benefits of geosynchronous satellite based SAR.
Figure 1: Existing operational imaging methodology
Figure 2: Proposed G-Sat SAR imaging methodology
Figure 3: Proposed Geo-Leo SAR imaging methodology
Bi-Static radar principle based G-Sat SAR imaging methodology
As the name suggests, in this methodology the transmitting antenna is located at G-sat and the SAR receiving antenna is located on LEO satellites. Depending on the imaging requirement, the number of LEO satellites can be determined. This is a simple modular solution where small satellites can be accepted in LEO orbits and failure of one or two satellites will not affect the imaging operations. This system can be further complimented with AWACS at some locations for specific defence and security applications as shown in figure 4. For this, AWACS has to be fitted with SAR receiving antenna system in combination with LEO satellites. These two combinations, with and without AWACS, offer a great flexibility of imaging operating modes. Large transmitting power and less return power are ideally suitable for microwave (radar) imaging operations in these two configurations.
The two G-Sat- SAR imaging methodologies will certainly enable powerful continuous observational capabilities which provide large instantaneous reach and very high mapping rates, while taking space based imaging technology to higher elevation. This realisation will have profound impact on the military effectiveness for future wars and will ensure sensor-toshooter time in near real-time.
A few simulations have been found in the literature showing antenna diameter size of 30m, peak powers in the range of 15 to 60 KW, azimuth resolution of 100km to 2m depending on the mode of operation, orbital inclination, frequency of operation, antenna and beam steering capability and finally signal processing schemes. However, due to advancement in technology, the data processing time is continuously reducing. Also miniaturisation of processors in the near future may allow onboard data processing. Even the size of satellites is decreasing. Hence G-Sat based SAR imaging technologies are achievable and estimated to be operational by 2030 which will be most economical.
The G-Sat-SAR configurations proposed in combination with LEO satellites will have certain advantages for smaller size satellites, antennas, power ranges and will offer flexible deployment opportunities for defence and security applications. Continuous imaging operations are possible even with failure of some LEO satellites. The main disadvantage will be synchronisation of positioning of LEO satellites for particular imaging operations. However technology advancement is always available for rescue. Cost considerations will be certainly higher for this LEO and AWACS configuration method.
Figure 4: Proposed Geo-Leo AWACS imaging methodology
The basic reason for using microwaves for remote sensing is that they are very different from other sources. The measurements can be carried out at any time and is not dependent on the background sources such as sun which penetrates clouds. One of the disadvantages is that the long wavelengths mean large antenna is required even for achieving usable spatial resolution. Moreover, active microwave systems such as synthetic aperture radars tend to have more weight, need more power and are larger in size. At present, all SAR sensors are operating in low orbits with narrow swath widths and long repeat periods. Repeat period is the key parameter for remote sensing hence there is a need to progress to G-Sat-SAR to overcome these limitations. The above mentioned concepts of G-Sat-SAR system configurations are a step in the direction for achieving frequent or near realtime continuous imaging. The monostatic radar method is the most efficient method in terms of resources and equipment required with less cost. But the technology has to be experimented with suitable trials for realisation. For this, electronic beam steering, spot imaging and signal processing need to be designed specifically for implementation of G-Sat-SAR imaging. This will cater to the needs of defence and security in the following manner.
- It will cut down reaction time for the military commanders thus aiding in decision making
- Large area of interest as deemed fit by respective military commanders can be imaged continuously and in near real-time
In the bi-static system, even though the number of satellites will be more, failure of some satellites will not affect the imaging operations. Also, it is possible to have smaller satellites with lesser power in this system. However, synchronisation of receiving satellites and AWACS will be a difficult proposition. But if usable imaging G-Sat-SAR technologies are realised, they will redefine continuous imaging in near realtime with reduced costs.
- Gp Capt KP Gowd “New Techniques for Dynamic Radar Target Recognition” Published in the Proceedings of International Radar Symposium India (IRSI- 2005), Bangalore, India. 17-19 Dec 2005 pp 725-728.
- “Target Recognition Technologies for Enhancing Combat effectiveness in Real Time”, Published in the proceedings of GeoIntelligence Asia 2011 under the theme “Combat Identification- Increasing Combat Effectiveness” by Geospatial Media and Communications at New Delhi, India 14-15 Jun 2011. (GeoIntelligence, Vol 1, Issue 4, Jul-Aug 2011, pp29-31).