BACKLOG AND ESTIMATED POTENTIAL CONTRACT VALUE Our total backlog, including funded and unfunded portions, was $89.5 billion on December 31, 2020, up 2.9% from $86.9 billion at the end of 2019. Our total backlog is equal to our remaining performance obligations under contracts with customers as discussed in Note B to the Consolidated Financial Statements in Item 8. Our total estimated contract value, which combines total backlog with estimated potential contract value, was $134.7 billion on December 31, 2020, up 6.7% from $126.2 billion at the end of 2019. The following table details the backlog and estimated potential contract value of each segment at the end of 2020 and 2019: Funded Unfunded Total Backlog Estimated Potential Contract Value Total  Estimated Contract Value December 31, 2020 Aerospace $ 11,308  $ 318  $ 11,626  $ 2,800  $ 14,426  Marine Systems 23,646  26,336  49,982  4,876  54,858  Combat Systems 14,341  226  14,567  9,774  24,341  Technologies 9,488  3,826  13,314  27,727  41,041  Total $ 58,783  $ 30,706  $ 89,489  $ 45,177  $ 134,666  December 31, 2019 Aerospace $ 13,168  $ 181  $ 13,349  $ 2,989  $ 16,338  Marine Systems 20,012  24,175  44,187  5,453  49,640  Combat Systems 14,474  439  14,913  4,322  19,235  Technologies 9,876  4,620  14,496  26,485  40,981  Total $ 57,530  $ 29,415  $ 86,945  $ 39,249  $ 126,194  For additional information about our major products and services in backlog see the Business discussion contained in Item 1. AEROSPACE Aerospace funded backlog represents new aircraft and custom completion orders for which we have definitive purchase contracts and deposits from customers. Unfunded backlog consists of agreements to provide future aircraft maintenance and support services. Beyond total backlog, estimated potential contract value represents primarily options and other agreements with existing customers to purchase new aircraft and long-term aircraft services agreements. Following reduced order activity in the first half of 2020 due to the COVID-19 pandemic, orders in the second half of 2020 reflected improved demand for Gulfstream aircraft. We received orders for all models of Gulfstream aircraft, including strong order activity for the new G700 aircraft, which is scheduled to enter service in the fourth quarter of 2022. Despite the impact of the COVID-19 pandemic, the segment achieved a book-to-bill ratio (orders divided by revenue) of 0.9-to-1 in 2020. Demand for Gulfstream aircraft remains strong across customer types and geographic regions, generating orders from public and privately held companies, individuals, and governments around the world. Geographically, U.S. customers represented more than 55% of the segment’s orders in 2020 and 38