There is reference of Sayan. Here I am giving my own observation.

In the pictures,there is an interesting data regarding the distance between the Sun(here addressing as Varuna)and the Usha/ Dawn.

Though the Usha is term related with a particular Time ,the measurement here is in distance

30 Yoganas.

1 Yogana is 12km.

Why this type of inter relationship is given?

The Rishi is viewing the Dawn in a particular place. Followed by the rising Sun .THE TIME GAP between the ‘Civil Dawn’/’Civil Twilight’ and ‘the Sun rising’ is ALWAYS constant. This is given in the first two words of this REEK NUMBER 8.

So observing any fixed object illuminated by the dawn followed by the Sun THAT CONSTANT TIME can be deduced. if the length of the Horizon (That the Sun traverses by a day (12 hours or asper the Veda the Unit was ‘DANDO’),then from this the distance between the Dawn time and Sun rising can be deduced.

Here is given 30 Yogans.This confirm s they knew the Length of the Horizon also. I will try to show you that latter on.

https://www.ga.gov.au/scientific-topics/astronomical/astronomical-definitions

Civil twilight

Beginning of morning civil twilight.

Defined as the instant in the morning, when the centre of the Sun is at a depression angle of six degrees (6°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, the illumination is such that large objects may be seen but no detail is discernible. The brightest stars and planets can be seen and for navigation purposes at sea, the sea horizon is clearly defined.

HORIZON

The curvature is the reciprocal of the curvature angular radius in radians. A curvature of 1.0 appears as a circle of an angular radius of 57.3° corresponding to an altitude of approximately 2,640 km (1,640 mi) above Earth’s surface.

https://en.m.wikipedia.org/wiki/Horizon

So 1°means 46km.

So 6°means

276km

So the Sun remain 276km away from the Usha or Dawn.

1 yogan means 12km.

30 yogan means 360km.

Difference is 360-276=84km.

This may come for the Hymns or Suktas are Visioned at the spot which is higher from the Sea Surface.So a differences between the data I have cited at Sea Level and this Hymn is expected. And from that difference the exact location wherefrom this Hymn was Visioned ,can be deduced.