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How does the radio altimeter reading change with the bank angle?

Following example: Let's say, we're flying at 1000' AGL and a bank angle of 0° above a endless flat surface. The radio altimeter shows 1000' as well.
Now if we roll to either side to a bank angle of 10° and maintain 1000' AGL, what does the radio altimeter read now?

Does does the radio alt increase accordingly or is the radio altimeter adjusted to always face down?

I'm especially interrested in the way how Airbus handles this situation.

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    $\begingroup$ The RA antenna emits and receives signals in a cone approximately 60° wide (I think). The RA shows the distance to the closest object in that cone. Up to 30° bank (and pitch), RA should still work and show 1000ft in your scenario. Comment instead of answer, as I can’t back it up at the moment. $\endgroup$
    – Cpt Reynolds
    Commented Jul 26, 2018 at 22:14

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The Rad Alt will show the same value of 1000'.

The radio altimeter antennas provide a radiation pattern that covers at least +/- 15 deg in pitch and +/- 20 deg in roll, though most cover more - typically +/- 20 deg in pitch and +/- 30 deg in roll.

The transmitted signal fans out equally over the entire pattern area from the transmit antenna and is reflected back where it is received by the receive antenna. The returned signal is spread over a short period of time due to the varying distances the signal must cover. The closest point will be the first received. As the aircraft changes pitch or roll, the location of the closest point will move within the pattern, but the shortest distance will still be the same.

The DSP software also processes the signal to filter out smaller objects such as the approach lighting towers.

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