A method in a first network node for estimating a first timing offset difference between a first timing offset of a first base station, RBS1, and a second timing offset of a second base station, RBS2, which first network node, first base station, RBS1, and second base station, RBS2, are comprised in a cellular network, the method comprising: - receiving (301, 701) Uplink Time Difference Of Arrival, UTDOA, measurements from a second network node, which UTDOA measurements relate to a first UTDOA time difference between a first measured receiving time when an uplink signal from a user equipment is received at the first base station, RBS1, and a second measured receiving time when the uplink signal from the user equipment is received at the second base station, RBS2, - receiving (303, 702) Observed Time Difference Of Arrival, OTDOA measurements from a third network node, which OTDOA measurements relate to a first measured OTDOA time difference between a third receiving time when a downlink signal from the first base station, RBS1, is received at the user equipment and a fourth receiving time when a downlink signal from the second base station, RBS2, is received at the user equipment, and - estimating (304, 703) a first timing offset difference between a first timing offset of the first base station, RBS1, and a second timing offset of the second base station, RBS2, based on the first UTDOA time difference and the first measured OTDOA time difference, characterized in that the UTDOA measurements further relate to a second UTDOA time difference between a fifth measured receiving time when the uplink signal from the user equipment is received at a third base station, RBS3, and the first measured receiving time when the uplink signal from the user equipment is received at the first base station, RBS1, wherein the OTDOA measurements further relate to a second measured OTDOA time difference between a sixth receiving time when a downlink signal from the third base station, RBS3, is received at the user equipment, and the third receiving time when the downlink signal from the first base station, RBS1, is received at the user equipment, and wherein the estimating (304, 703) further comprises estimating a second timing offset difference between the first timing offset of the first base station, RBS1, and a third timing offset of the third base station, RBS3, and wherein the estimation of the second timing offset difference is based on the second UTDOA time difference and the second measured OTDOA time difference. - receiving (301, 701) Uplink Time Difference Of Arrival, UTDOA, measurements from a second network node, which UTDOA measurements relate to a first UTDOA time difference between a first measured receiving time when an uplink signal from a user equipment is received at the first base station, RBS1, and a second measured receiving time when the uplink signal from the user equipment is received at the second base station, RBS2, - receiving (303, 702) Observed Time Difference Of Arrival, OTDOA measurements from a third network node, which OTDOA measurements relate to a first measured OTDOA time difference between a third receiving time when a downlink signal from the first base station, RBS1, is received at the user equipment and a fourth receiving time when a downlink signal from the second base station, RBS2, is received at the user equipment, and - estimating (304, 703) a first timing offset difference between a first timing offset of the first base station, RBS1, and a second timing offset of the second base station, RBS2, based on the first UTDOA time difference and the first measured OTDOA time difference,