upper() don't work with field-alias in the order by part of select statement.

I need in some select-statements case-insensetiv ordering of results,
i want to use upper()-function in order by part of statement but it
doesn't work with field-aliases.
Can I do this in another way?
I'm using PostgreSql 7.1.2 on SuSe-Linux

Example:
this works:
select p1.prs_vorname as vorname, p1.prs_nachname as nachname,
p1.lfdnr as lfdnr, pos.pos_name as position,
p1.prs_email as email, abt.abt_name as abteilung
from person p1 left join abteilung abt
on abt.lfdnr = p1.prs_abt_lfdnr,
person p2 left join positionen pos
on pos.lfdnr=p2.prs_pos_lfdnr
where p1.lfdnr=p2.lfdnr
ORDER BY nachname;

this don't work:
select p1.prs_vorname as vorname, p1.prs_nachname as nachname,
p1.lfdnr as lfdnr, pos.pos_name as position,
p1.prs_email as email, abt.abt_name as abteilung
from person p1 left join abteilung abt
on abt.lfdnr = p1.prs_abt_lfdnr,
person p2 left join positionen pos
on pos.lfdnr=p2.prs_pos_lfdnr
where p1.lfdnr=p2.lfdnr
ORDER BY upper(nachname);

ERROR: Attribute 'nachname' not found