The GSAT4 Application Project (GAP4) is being developed at SAC, Ahmedabad and NESAC, Shillong is one of the identified sites for this Project. The GSAT-4 satellite is designed to cater to the requirements of high bandwidth and Ultra Small Aperture Terminal (USAT) by incorporation of Ka‑band multiple spot beams with frequency reuse and onboard signal regeneration and switching. It is primarily designed as a Ka-band mission with 8 spot beams having both regenerative and bent-pipe transponder systems. The satellite also carries a payload for geo-satellite based navigation augmentation and two beacon sources.
The most important aspect of the GSAT-4 is that it houses on‑board router (provided on the regenerative transponder) to take care of hub functionality. It provides twin benefits. On the one hand, on-board hub operation obviates the need of double-hopping thereby reducing signal latency effects which is a common problem of any ordinary VSAT-to-VSAT communication system. On the other hand, it reduces the EIRP (Effective Isotropic Radiated Power) requirements on the transmission side by about 5-6 dB thus lowering the size and operating costs of ground stations. In fact, this technology of ISRO was driven by the need to do away with big VSAT structures replacing them with USATs (Ultra Small Aperture Terminal).
Three types of Ka-band ground terminals are developed under GAP-4 project:
Bent-pipe terminals: These terminals communicate through GSAT-4 bent‑pipe transponders and cater for the data-rate of upto 40 Mbps.
Wideband (WB) terminals: These 2.048 Mbps terminals communicate through GSAT-4 regenerative payload’s WB connectivity.
Narrowband (NB) terminals: These 64 Kbps terminals communicate through GSAT‑4 regenerative payload’s NB connectivity.
The following applications are planned under GAP-4 programme:
Portable GIS application
Interconnectivity Among the Remote Sensing Centers
Installations of equipments are in progress at NESAC. After launching the GSAT-4 satellite, program will start immediately.