The reporter experienced erroneous altitude information from the ARTCC equipment by 400 ft.

Attributes
ACN	286810
Time
Date	199410
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : vxv
State Reference	GA
Altitude	msl bound lower : 37000 msl bound upper : 37000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl tower : see
Operator	common carrier : air carrier
Make Model Name	B737-400
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 11400 flight time type : 3000
ASRS Report	286810
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified
Resolutory Action	none taken : unable none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Navigational Facility
Air Traffic Incident	other

Narrative:

While in level cruise at FL370, ZTL called with traffic information 'at 12 O'clock, 10 NM at 350, verify altitude at 370.' our cruise altitude of FL370 was verified. Center responded that his scope showed FL366 and an alarm went off. When queried further, he said that for 2 updates, which is approximately 20 seconds, his system showed our altitude at FL36.6. No traffic conflict, and stated that it was due to the radar site location. He indicated to us that this type of situation occurs frequency and his call to verify our altitude was to make sure, even though 'he knew we were at FL370.' this exchange lasted approximately 2-3 mins and left me somewhat perplexed. I wonder how we can work within a system that has frequency reports in error, that affects both ATC and flcs. Something around this area needs to get fixed.

Original NASA ASRS Text

Title: THE RPTR EXPERIENCED ERRONEOUS ALT INFO FROM THE ARTCC EQUIP BY 400 FT.

Narrative: WHILE IN LEVEL CRUISE AT FL370, ZTL CALLED WITH TFC INFO 'AT 12 O'CLOCK, 10 NM AT 350, VERIFY ALT AT 370.' OUR CRUISE ALT OF FL370 WAS VERIFIED. CTR RESPONDED THAT HIS SCOPE SHOWED FL366 AND AN ALARM WENT OFF. WHEN QUERIED FURTHER, HE SAID THAT FOR 2 UPDATES, WHICH IS APPROX 20 SECONDS, HIS SYS SHOWED OUR ALT AT FL36.6. NO TFC CONFLICT, AND STATED THAT IT WAS DUE TO THE RADAR SITE LOCATION. HE INDICATED TO US THAT THIS TYPE OF SIT OCCURS FREQ AND HIS CALL TO VERIFY OUR ALT WAS TO MAKE SURE, EVEN THOUGH 'HE KNEW WE WERE AT FL370.' THIS EXCHANGE LASTED APPROX 2-3 MINS AND LEFT ME SOMEWHAT PERPLEXED. I WONDER HOW WE CAN WORK WITHIN A SYS THAT HAS FREQ RPTS IN ERROR, THAT AFFECTS BOTH ATC AND FLCS. SOMETHING AROUND THIS AREA NEEDS TO GET FIXED.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.