No, Chauvihar is not absolutely compulsory for doing Navkarshi, but:

1. Ideal / traditional practice

- The best (uttam) way is: - In the evening: take Chauvihar pachchakkhan (no food/water after sunset). - Next morning: do Navkarshi (first food/water 48 minutes after sunrise, after Navkar Mantra). - This way, your vrat runs continuously from sunset to Navkarshi time and has more spiritual benefit.

1. If Chauvihar is not possible

- Due to health, age, travel, work, etc., if you cannot do Chauvihar, you can still do Navkarshi alone. - Doing only Navkarshi is still punya and better than not observing any discipline.

1. Pachchakkhan aspect

- If you specifically take “Chauvihar + Navkarshi” pachchakkhan, then yes, both are included. - If you take pachchakkhan only for Navkarshi, then you are bound only from early morning till Navkarshi time (whatever you state in the vow).

1. Digambar / Shvetambar view

- Both traditions respect Chauvihar and Navkarshi as separate but very valuable daily aachars. - In both, combining Chauvihar with Navkarshi is considered better, but Navkarshi by itself is allowed and meritorious when Chauvihar cannot be done.

You can read more detailed, simple explanations here and here