First Images from Japan’s Daichi

On 30th November 2005 the Japan Aerospace Exploration Agency (JAXA) announced Space Activities Commission (SAC) approval for launch of ‘Daichi’, Japan’s Advanced Land Observing Satellite (ALOS) within a time window of 20th January to 28th February 2006. The satellite was eventually rocketed beyond Earth’s atmosphere on 24th January from Tanegashima Space Centre. We present its features and first images from this novel new earth-observation instrument.

The Japanese Earth observing programme consists on the one hand of satellites developed primarily for atmospheric and marine applications and on the other of satellites primarily developed for land applications. Japan’s Advanced Land Observing Satellite, Daichi, belongs to the latter category. Daichi aims at providing Japan and other countries in the Asian-Pacific region and elsewhere with basic input data for determining land-coverage and land-use, for extracting (three-dimensional) topographic and other features and for the generation of Digital Elevation Models (DEM). Daichi also carries out earth observation of regions for sustainable development, global disaster monitoring and surveying of natural resources.

Characteristics
With a lift-off weight of 3,850kg, Daichi has since 24th January 2006 been orbiting Earth at an equatorial altitude of 692km in sun-synchronous orbit and at an inclination of 98 degrees. The satellite completes an orbit in 99 minutes and the nadir revisit frequency is 46 days. However, the available pointing facilities enable the satellite to re-observe areas every two days. Daichi has on board three different sensor systems:

• Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM)
  
• Advanced Visible and Near Infrared Radiometer
• Array type L-band Synthetic Aperture Radar (PALSAR).

In the sequel the characteristics of these three instruments are de-scribed in greater detail.

PRISM
The PRISM consists of three independent optical sensors which all record the panchromatic band (0.52 - 0.77µm). The spatial resolution, also called pixel size or Ground Sample Distance (GSD), is 2.5m and the reflectance values are captured with a bit length of 8 bits. The instrument looks simultaneously forward, nadir and backward. The configuration of sensors allows for stereo mapping and creation of DEM. Because there is virtually no time difference between the recordings of the overlapping images three-dimensional feature extraction and DEM generation can be carried out in a reliable way. The in-track distance between the two images of the same terrain point acquired by the forward-looking sensor and the backward-looking sensor is around 700km, result-ing in a base-to-height ratio of 1.0. The forward and backward telescopes are inclined +24 and -24 degrees from nadir to accomplish this base-to-height ratio. The PRISM is a push-broom scanner. The linear array CCD of the nadir-viewing sensor consists of 28,000 elements, resulting in a swath width of 70km. The forward-viewing sensor CCD and the back-ward-viewing sensor CCD each consist of 14,000 elements, resulting in a swath width of 35km.

AVNIR-2
AVNIR-2 is the successor to AVNIR, a sensor onboard the Advanced Earth Observing Satellite (ADEOS) launched in August 1996. Like the AVNIR, the AVNIR-2 is also a bush-broom scanner, but instead of 5,000 pixels per linear array the AVNIR-2 CCD consists of 7,000 pixels, whilst the spatial resolution has improved from 16m to 10m. The AVNIR-2 records three bands in the visible part of the spectrum (blue: 0.42 - 0.50µm, green: 0.52 - 0.60µm and red 0.61 - 0.69µm) and one in the near-infrared band (0.76 - 0.89µm). The reflectance values of each of the four bands are captured with a bit length of 8 bits. The swath width at nadir is 70km. The instrument has an across-track pointing facility for disaster monitoring purposes: the sensor can be pointed +44 to - 44 degree off nadir. The pointing facility enables the sensor, within an across-track range of 1,500km, during one pass to cover a swath 70km to 135km in width, depending on the pointing angle. The instrument is particularly suited to mapping coastal zones, determination of land coverage and for providing basic input data for land-use classification.

PALSAR
The PALSAR instrument, developed as a joint project by JAXA and the Japan Resources Observation System Organization (JAROS), is a synthetic aperture radar using microwaves (L-band), which allows for day-and-night and all-weather land observation. The centre frequency of the L-band used is 1,270MHz. In fine-resolution observation mode the incident angle is 8 to 60deg, the observation swath is 40 to 70km, and range resolution is 7m to 44m for Chirp Bandwidth 28MHz and 14m to 88m for Chirp Bandwidth 14MHz. In addition to the conventional fine-resolution mode there is the ScanSAR observation mode, which enables acquisition of SAR images within a strip of 250 to 350km in width, depending on the number of scans, by carrying out three to five scans at the expense of spatial resolution. The incident angle is 18 to 43 degrees and range resolution is 100m. The bit length is 8 bits for both modes

First Images
Initial, functional post-launch testing of the instruments produced several interesting pictures. One is a perspective view of Mount Haruna, observed by the PRISM on 14th February 2006 at 10:30am. Using the backward-viewing, nadir-viewing and forward-viewing images, a DEM was generated. The nadir-viewing image was then draped over the DEM, enabling viewing of Mount Haruna in perspective from all possible positions and angles. On the same day the PRISM took images of Shimizu Port, near
Shizuoka city in Japan. The spatial resolution of 2.5m enables identifying ships in the port, houses, roads, and even cars on the roads. Combined with AVNIR-2, pan-sharpened imagery may be obtained which would form precise and detailed basic material for feature mapping purposes. On 15th February 2006, as the satellite passed over the Bunngo Suidou area from south to north, PALSAR acquired an image of the famous Mount Fuji and the Izu Peninsular. This night-time image recorded at about 10:16pm was taken from an angle of 41.5 degrees to the right of the PALSAR, at a distance of 1,000km.

Concluding Remarks
The strong points of the PRISM are its high geometric precision, even when no Ground Control Points are used, and the in-track stereo-image acquisition facilities. The pointing facility of the AVNIR-2, enabling the sensor to capture any strip within a range of 750km on either side of the nadir line, provides beneficial opportunities. The favourable characteristic of the PALSAR is the ScanSAR observation mode.