Closest time

Hi,
I have two tables

Table Messages:

prim_Key           Truck           Message                  MessageTime           Country     City
1                        ABC999       'some message'       21.7.2008 16:01      ?               ?
2                        ABC123       'another message'   21.7.2008 16:25       ?               ?        

Table Positions:

prim_key          Truck          MeasuredTime       Country        City
1                      ABC999      21.7.2008 12:51    Germany       Kiel
2                      ABC999      21.7.2008 14:51    Germany       Hamburg
3                      ABC999      21.7.2008 19:20    Germany       Berlin
4                      ABC123      21.7.2008 16:20    France          Paris

We have our trucks going around whole Europe. Several times a day we exchange short electronical messages between our Dispatching and each Truck. Moreover we have a GPS system which every 2nd minute writes each Truck's position to a table. Basically I want to write an insert trigger on the Message table which updates the  Message table with the Country and City from the Positions table based on the record in the Positions table which have the closest MeasuredTime to MessageTime in the Messages table.

In this case record nr 1 in the Messages table should be updated with Germany as Country and Hamburg as City.

Thanks in advance for your help.

-Sri.