1. Introduction

Modern 3D position-sensitive detectors with advanced (nanosecond level) readout technology will ensure unprecedented background rejection capability, which in conjunction with good energy and spatial resolution will achieve sensitivity at the required level.  
Two secondary instruments are envisaged:

• a few-degree field-of-view (FOV) imaging X-ray telescope,
• an optical/NIR telescope.

Both should be gimble-mounted, and rapidly repointable along one coordinate axis, thus providing GRB localizations to better than 1 arcmin.

The total weight of the instruments is of order 1.5 tons, and the total satellite weight is approx. 2.5 tons.

2. Gamma-ray Monitor (GRM)

GRM is a combination of two main parts:

• electron tracker, based on Silicon detectors,

• calorimeter, based on LaBr3 scintillators.

Gamma-rays are detected in GRM in three ways:

• may Compton scatter in Si-tracker, where track of the electron is recognized, while photon energy is fully stored in calorimeter,
• may split into electron-positron pairs. Both particles's paths are tracked simultaneously in tracker,
• may deposed their energy into the calorimeter directly

To minimize charged particle background (mainly energetic electrons) GRM is surrounded by anticoincidence shield (ACS) made of plastic scintillator.

Scientific parameters:

FOV: cone, full opening angle 160 degrees,
energy range: 0.2 - 50 MeV,
spectral resolution: 2% @ 662 keV (LaBr3),
angular resolution: 3deg @ 500 keV, 1deg @ 2MeV,
effective area: 400 cm2 @ 500 keV, 100 cm2 @ 2MeV

Technical parameters:

Calorimeter:
LaBr3 mass: ~750 kg,
Channels: ~150,000,
Tracker:
Si mass: ~50 kg,
Channels: ~400,000
ACS:
plastic mass: 155 kg

3. X-ray Monitor XRM

X-ray Monitor. XRM

Due to high required effective area (1000 cm^2) XRM is a 7 Wolter-I X-telescopes sensitive in 0.2-10 keV energy range. The field of view requires to cover GRM error circle. With extra 50% margin it gives 3 deg diameter.

XRM parameters:

• Focal length: 1600 mm
• Largest mirror diameter: 365 mm
• Smallest mirror diameter: 76 mm
• Angular resolution 20'' (FOV-averaged)
• Effective area: 2,450 cm2 @ 1.5 keV
• 140 cm2 @ 8 keV
• Image area: 384 x 384 pixels