A control unit 10 detects the presence/absence of receiving position
information based on the position data provided from a GPS receiver 14.
While the GPS receiver 14 is supplying no position data, the control unit
10 computes the travel distance of a vehicle according to a vehicle speed
pulse signal input from a vehicle speed pulse input unit 18. When the
obtained travel distance exceeds a predetermined distance, the control
unit 10 determines abnormal reception due to faulty equipment. The
predetermined distance is set in advance based on a reception disability
area, etc. When the control unit 10 determines abnormal reception due to
faulty equipment, it turns on an orange indicator. Thus, the person in
the vehicle can recognize the abnormal reception due to faulty equipment.