July 8, 2016

MADRAS instrument

The MADRAS instrument is a nince-channel self-calibrating microwave imager. It is the main instrument of the MEGHA-TROPIQUES mission, jointly managed by the CNES and ISRO agencies.

MADRAS is a conical scanning radiometer, it is a total power type. The rotation speed is around 25 rpm.

MP4 Format ~29.3 Mb (or WMV format ~13.5 Mb)

MADRAS has operating frequencies in the range 18.7GHz-157GHz. The table below gives the main mission objective for each channel.

Frequencies	Main mission objective
18.7 GHz	Rain above oceans
23.8 GHz	Integrated water vapour
36.5 GHz	Liquid water in clouds, rain above sea
89 GHz	Convective rain areas over land and sea
157 GHz	Ice detection in clouds

The microwave radiation from the scene is collected by the antenna reflector and focused to the respective horns. The scene is scanned by rotation of the complete antenna i.e. the reflector and the horns. During every rotation, the angular sector where the antenna beam faces at the platform is used to calibrate the receivers. A calibration mirror is used to collect the energy coming from the cold sky. An on board hot target provides another calibration point. The radiometric sensitivity requirements imply that the receivers must be close to the horns so they are implemented in the mobile part. The purpose of the receivers is to deliver signals, the magnitude of which is proportional to the brightness temperature of the scene. A rotating joint allows the transfer of the electrical signals (video, TM/TC, power supplies, heater lines) between the fixed part and the mobile part.

Channel definition for each channel

Channel nb.	Frequencies	Polarization	Spatial resolution (km)
M1	18.7 GHz	H + V	40
M2	23.8 GHz	V	40
M3	36.5 GHz	H + V	40
M4	89 GHz	H + V	10
M5	157 GHz	H + V	6