Rasivalaya are instruments for measuring celestial latitude and longitude. There are twelve instruments, each for one measurement to be done when the corresponding sign of the Zodiac transits the Meridian.
The Rasi Valaya are instruments that become active when the Zodiacal constellation corresponding to the given Rasi Valaya, transits the Meridian.
Meridian is an arc in the sky connecting the North and the South, and passing through the Zenith. With the rotation of the Earth, when a celestial body crosses this arc, it is said to transit the Meridian.
Indian Astronomy emphasised the measurement of celestial positions with respect to the Ecliptic(the apparent path of the Sun in the sky).
Celestial Latitude is the position of an object to the North or South of the Ecliptic. Celestial Longitude is its position as measured along the Ecliptic. The point of reference for measuring the Celestial Longitude is the Vernal Equinox, one of the intersection points between the Ecliptic and the Celestial Equator.
The Celestial Pole is a stationary point in the sky, while the position of the Ecliptic Pole changes with time. The Gnomon of a Rasi Valaya instrument points to the Ecliptic Pole, when the Zodiacal Constellation corresponding to that Rasi Valaya transits the Meridian.
At that instant, the Celestial Longitude of an object can be measured using the quadrant of the Rasi Valaya, while the markings along the length of the Gnomon can be used to determine the Celestial Latitude. The instant when the instrument is to be used, can be determined by another observer, from observations with the Jayprakash, in the day time. A knowledge of the time when the Meridian Transit takes place, and noting of the time with the Samrat Yantra, could also be used. Note the strategic location of the Rasi Valaya in between the Samrat Yantra and the Jaiprakash.