They are complementary sensors. It's a much easier engineering feat to combine two (cheap) sensors that are good at different things and fusing this information than creating one perfect sensor that does everything.
Private moon landers (the Japanese being most recent one) keep crashing because they rely on a single high-quality altimeter and expect it to work perfectly, all the time. If they had a complementary low-quality backup altimeter that operated independently, they would have had a less failure prone distance estimation system.
Private moon landers (the Japanese being most recent one) keep crashing because they rely on a single high-quality altimeter and expect it to work perfectly, all the time. If they had a complementary low-quality backup altimeter that operated independently, they would have had a less failure prone distance estimation system.