According to BPHS (Brihat Parashara Hora Shastra, Dashaa-phala-adhyaaya, verse-34), Rahu's exaltation is in Vrish and Vrishchika respectively. But BPHS also states that Vrishchika is Sva-griha of Ketu. In same book, Ketu's antar-dashaa under Venus mahaa-dashaa states that exaltation and sva-griha of Ketu are different. Hence,different parts of Vrishchika are exaltation and svagriha of Ketu, as in the case of Mercury which has different parts of Kanya for exaltation, svagriha and trine.
This symmetric Parashari scheme excludes 3-5-9-11 signs from exaltation of any planet, which are two left and two right side signs in the picture of Parashari (North Indian) Rasi Chart with Mesha as Lagna.
Logic Behind Exaltations of Planets
The Parashari scheme is based on the following scheme in which every planet is exalted in the sign of next planet : - Sun : Mar : Sat : Ven : Jup : Moo ; Mercury is excluded in this circular ring because Mercury is exalted in its own sign but in different degree, while Rahu is exalted in sign of Venus ( Saturn is also exalted in other sign of Venus), and Ketu is exalted in sign of Mars.
In the circular ring, there are two halves. In each half, malefics are exalted in signs of other malefics (Sun : Mars : Saturn) while benefics are exalted in signs of other benefics (Ven:Jup:Moon), Mercury being neutral kept aloof. Both halves are joined together by Saturn being exalted in sign of Venus, which suggests that the otherwise benefic planet Venus has some maleficence also , which is dues to its natural affinity to Kaama.
Mercury is neither natural benefic nor natural malefic, hence it is exalted in its own sign and not in the sign of any benefic or malefic.
Moon is exalted in the sign of another natural benefic Venus, and Venus is exalted in the next benefic Jupiter. Jupiter is exalted in the sign of Moon, which completes the cycle of exaltations of benefics.
Sun is exalted in sign of another natural malefic Mars, Mars is exalted in sign of Saturn.
Hence, natural benefics become strong in signs owned by next higher natural benefic, while malefics gain strength in signs owned by next higher natural malefic.
But unlike benefics, the cycle of malefics is not completed by making Saturn exalted in sign of Sun. On the contrary, Saturn is exalted in the sign owned by a benefic Venus. Why ?
Because Venus is a benefic but it also signifies Kaama, which is God incarnate if it is dharmic as Gita states. Otherwise Kaama is the worst malefic factor and the worst killer. Vrish is the benefic sign of Venus, while Tula is the malefic sign of Venus representing Kaama. Moreover, Sun and Saturn are exalted in opposite signs.