The "RH" in Indian sounding rockets stands for "Rohini Herald." Rohini was a satellite that was launched by India's national space agency, the Indian Space Research Organization (ISRO), in 1967. It was the first satellite to be put into orbit by an Indian-made launch vehicle, the Rohini Satellite Launch Vehicle (SLV-3). The success of the Rohini mission marked a major milestone in India's space program and established the country as a player in the global space arena.

Since then, ISRO has continued to develop and launch a wide range of satellites and spacecraft, including communication satellites, Earth observation satellites, and lunar and interplanetary missions. The organization has also developed a family of sounding rockets, which are suborbital vehicles used to carry scientific payloads to high altitudes for the purpose of conducting experiments and collecting data.

The Rohini Herald (RH) series of sounding rockets is one of the latest developments in ISRO's sounding rocket program. These rockets are designed to carry a range of payloads, including atmospheric and ionospheric sounding experiments, upper atmospheric research, and remote sensing missions. The RH series is characterized by its compact size and lightweight, which makes it ideal for launching from mobile platforms.

The RH series has already been used for a number of successful missions, including studies of the Earth's upper atmosphere and ionosphere and the measurement of air density and wind patterns. In addition, the RH series has been used to launch a number of remote sensing payloads, including payloads for the measurement of atmospheric pollutants, as well as payloads for the study of the behaviour of the ionosphere and its effects on communication and navigation systems.

In conclusion, the "RH" in Indian sounding rockets refers to the Rohini Herald series of suborbital vehicles developed by ISRO. These rockets have been designed to carry a range of scientific payloads for atmospheric and ionospheric research, remote sensing, and other applications. With their compact size and lightweight design, the RH series has proven to be a versatile and reliable platform for conducting various missions in the upper atmosphere.