### Determining if a Callsign Certificate is Valid

When a Callsign Certificate is issued for a Callsign, any previous Callsign Certificate for that Callsign is rendered invalid. LoTW will reject QSOs digitally signed by an invalid Callsign Certificate.

Each Callsign Certificate has a unique serial number. LoTW maintains a list of your valid Callsign Certificates that includes the serial number for each. To verify that a Callsign Certificate in TQSL is valid, you can compare it's serial number with the serial number displayed by LoTW.

TQSL displays a Callisign Certificate's serial number when you direct it to display that Callsign Certificate's properties:

To display LoTW's list of valid Callsign Certificates,

1. | Log in to your LoTW Account
2. | Click the Your Account tab
3. | In the Logbook Web Account Menu on the left, click the Your Certificates button; LotW will display your Callsign Certificates, with the serial number for each:

If the serial number displayed in TQSL matches the serial number displayed by LoTW, the Callsign Certificate in TQSL is valid.