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Abstract
PACRIM 96 AirSAR flight illustrated the advantage of using SAR data for various applications such as coastal study and oceanography, geology, forestry, and also archaeology. From the characteristics of the AirSAR P L and C bands, AirSAR data can be used for the exploration of archaeology sites as demonstrated in this paper. By analyzing the combination of P, L, and C bands data, we can identify human activities at 800 years ago.

This study illustrate the integration of technology in the field of remote sensing and image processing to discover the world as never been before in Thailand.

Introduction
Remote sensing has a wide range of applications as has been realized for quite sometime. As one of the applications, archaeology has been an application that remote sensing can assist in identification in many area. For example, when NASA launched an imaging system called SIR-A on the Space Shuttle. Archeologist Farouk El-Baz had asked NASA to fly SIR-A over the eastern Sahara desert, hoping it could make sense of the anomalous rock formations he had been studying there. No one was quite prepared for the images that came back. The Sahara is the driest place on earth right now, but SIR-A was able to penetrate the sand and reveal an ancient landscape below that, amazingly, had been carved by running water. (NOVA, 1996)

Background Information
Phimai town had been built during the Khmer empire. The town itself had been established as a region center of the empire. There are many evident exist like stone inscription describing the live during the period. In addition, there was a communication routing from Angkor to Phimai as described in many stone inscriptions.

Location
The location of Phimai is aligned with the location of Angkor as shown in the map (Figure 1).

Figure 1 The map of showing the relationship of Phimai and Ankor

Airsar Data Specification
The AirSAR data has the following specification. According the wave length, P band has the longest wave length of 67 cm.

Bands:	P (0.45 Ghz, 67 cm) L (1.26 Ghz, 23 cm) C (5.31 Ghz, 5.7 cm)
Polarization	Full pol
Bandwidth	20, 40, 80 (Available after May 1998) MHz
Resolution, range	7.5, 3.75, 1.875 (Available after May 1998) m (without considering the tapering effect.)
Resolution, azimuth	1 m single look (JPL, 1999)

Dataset and Information Usage

• AirSAR
  The AirSAR data had been taken on Dec 6, 1996 as a part of the PACRIM 96 project. The flight had been organized and supported by JPL and the Royal Thai government.
• Air Photo
  The air photo taken in 1954 had been used for integration of analysis. The reason to use old air photograph was because during 1954, there was still not have a lot of new infrastructures constructed in the area (Figure 3A).
• Map
  The map of the scale 1:50,000 had been obtained to study the current transportation network, and rough estimation of the terrain (Figure 3B).
• Historical Information
  The historical information had been gather from various source that we can find, mostly from archaeological books about Angkor, and the report from the Royal Thai Archaeology Department about the excavation at Phimai and the Northeast region of Thailand.

Methodology

1. Analysis of AirSAR

Initially, the work had been started by the analysis of the combination of different polarization from P, L, and C bands to identify the most suitable combination of bands for the work. First the combination had been compared with the map of the area to compare the object that could be detected from the current infrastructures. After the combination had been identified, we continued to identify the infrastructures that were not modern infrastructures (Figure 2).

Figure 2 Analysis of AirSAR data from different polarization


2. Integration with Air Photo

After combination of different polarization had been analyzed, the air photo (1954) had been used to compared with the image for further study with undisturbed environment as shown in the air photo and also with 1:50,000 map (Figure 3A, 3B).


(A) Fig 3 A The air photo of Phimai area (1954)	(B) Fig 3 B 1 : 50,000 map of Phimai area



3. Pattern recognition with Khmer culture

In order to identify the ancient infrastructure, we need to compare identified object with the classified ancient infrastructure. In this case, we had compared the infrastructure in question with the most well known Khmer's infrastructure, infrastructure in Angkor area (FREEMAN and JACQUES, 1999).


(A) Fig 4 A The RGB composite from P,L,C bands combination	(B) Fig 4 B PLC image and identified discovery



4. Identification of the water reservoir

After the comparison, we could identify the structure of water reservoir in Phimai. The water reservoir, in Khmer was called "baray", was located on the south of Phimai temple with the distance of 2 kilometer (Figure 4A, 4B). From the identification, the direction of the baray was in corespondent with the direction of the town itself. When we compared the pattern of the baray at Phimai and Angkor, they matched perfectly. The additional information that we got from the pattern in Angkor was that in the middle of the baray, there was a temple on the island. This pattern was appeared on both east baray and west baray in Angkor.


5. Identification of unknown temple on the island in the middle of baray

As the result of the comparison between Angkor and Phimai, we had tried to further analyze the AirSAR data. From the DEM generated from C band, we found a high elevation area in the middle of the baray (Figure 5A). We expected that it is the temple in the same pattern as in Angkor. After the field survey and contacting an archaeologist in the area, we found the evident on the ground that supported the assumption. The most interesting finding about this temple was that the temple align itself directly to the main temple of Phimai in the same direction of the alignment of the Phimai temple and the baray (Figure 5B).

(A) Fig 5 A Hill Shade image showing the high elevation at the center of the baray	(B) Fig 5 B PLC image showing the position and alignment of main temple and discovered temple

Finding Result
As explained in the previous section, our analysis had found ancient infrastructures in Phimai area. The finding from this research are : baray, temple in the middle of baray, and ancient roads to Phimai temple (Figure 6). From these findings, there will be followed up by the reconstruction of the finding in virtual world.

Figure 6 PLC image overlaid with the ancient infrastructure

Conclusion
The baray was probably the largest one in Thailand. From the study, we would like to extend the study to identify historic roadway, communication links and human settlement of Khmer empire using remote sensing / GIS and then develop historical and archaeological multi-media GIS for the area of study, which is the area covered between Angkor Wat and Phimai.

References

• Freeman and Jacques, Ancient Angkor, River Books, 1999
• JPL, AirSAR Integrated Processor Documentation Version 0.14, Oct 1999
• NOVA, The Sky's Eye: Remote Sensing in Archaeology, 1996