The FireBIRD mission is made up of two satellites, TET-1 (Technology Experiment Carrier) and BIROS (Bi-spectral InfraRed Optical System). They circle the Earth in a polar orbit, passing over the poles at an altitude of 500 kilometres.
These orbits are also called sun-synchronous orbits because the satellites always cross the equator at the same time. For TET-1, this is at 11:30, and for BIROS, 09:30. The satellites have been placed in a half orbit of each other, so that one passes the zeroth latitude from North to South at 9:30, whilst the other changes from the southern to the northern hemisphere at 11:30. It takes them 90 minutes to complete one orbit, totalling 16 longitudinal orbits per day. In 20 days, they have swept the entire Earth's surface area, along all latitudes, using their highly sensitive optical systems. They move together in a free constellation, meaning they are not in fixed formation, as was the case for the satellites in the TanDEM-X mission.
The core of both FireBIRD orbiters
Both satellites are equipped with the same camera systems. These consist of one optical and two infrared cameras forming the Hot Spot Recognition System (HSRS). The bi-spectral infrared optical system (left and right opening) is made of two identical infrared sensors. They only differ in the spectral range for which they have been calibrated. One sensor detects electromagnetic waves in the mid-infrared, the other is calibrated within the thermal infrared range.
The optical sensor also covers the visible spectral range. This camera produces higher resolution than the infrared sensors and can be used for a more accurate identification of the geographical location of thermal images. In addition, the image taken by the optical camera must first be approximated by applying a diminutive algorithm to the infrared image.
This does not mean that the HSRS resolution is low, however. To compare: If the infrared sensor of MODIS, a moderate resolution imaging spectroradiometer on a NASA satellite, has a range of 1000 metres, then the infrared cameras on the FireBIRD satellites have a range on the ground of just 178 metres.
In addition to spatial resolution, the sensitivity of the sensors is also greater than previous systems: For example, the MODIS system and VIIRS (NASA), are already detecting hot spots from 10 megawatts onwards; FireBIRD satellites are gathering data from high-temperature events from one megawatt onwards. This means that small high-temperature events can also be detected, including those with low heat generation. The use of two cameras in two spectral ranges also enables the detection of fires that are considerably smaller than the ranges. Even if a pixel is still the size of six football pitches, fires the size of the centre circle are being detected. Chemical heat generation and smouldering fires can also be located.
Overview of the central parameters of the optical and IR cameras on TET-1 and BIROS
|Optical camera||Infrared camera|
|Wavelengths||380 to 750 nanometres||3.4 to 4.2 micrometres and|
8.5 to 9.3 micrometres
|Focal distance||90.9 millimetres||46.39 millimetres|
|Visual field||19.6 degrees||19 degrees|
|Pixel size||7 x 7 micrometres||30 x 30 micrometres|
|Number of pixels||3 x 5,164||2 x 512|
|Ground sample distance||42.4||356 metres|
|Scan range||42.4||178 metres|
Both orbiters are based on the same bus concept. The satellite bus is the supply unit on board that ensures operation and also the payload: Solar panels are responsible for the energy supply. The tri-axial position provides a stable basis for this. The mobility of the three axles also provides them with a higher level of flexibility when selecting a target.