15. REVENUE

On December 31, 2018, the Company had $2.4 billion of remaining performance obligations, which represents the transaction price of firm orders less inception to date sales recognized. Remaining performance obligations exclude unexercised contract options and indefinite delivery/indefinite quantity contracts. The Company expects to recognize sales relating to existing performance obligations of approximately $1.3 billion, $0.4 billion, and $0.7 billion in the fiscal years 2019, 2020 and thereafter, respectfully.

Contract liabilities by segment are as follows:


	Space
As of December 31, 2018	Systems		Imagery1		Services		Total
Contract liabilities	$	172	$	247	$	2	$	421

	Space
As of December 31, 2017	Systems			Imagery1	Services		Total
Contract liabilities	$	266	$	330	$	4	$	600

1 The contract liability balance associated with the Company’s EnhancedView Contract was $184 million and $278 million as of December 31, 2018 and 2017, respectively. During the year ended December 31, 2018, imputed interest on advanced payments increased the contract liability balance by $26 million, and $120 million in revenue was recognized, decreasing the contract liability balance. The contract liability balance associated with the Company’s EnhancedView Contract is expected to be recognized as revenue through August 31, 2020. There were no deferred contract costs on the Consolidated Balance Sheets associated with this contract as of December 31, 2018 or December 31, 2017.

The decrease in total contract liabilities was primarily due to revenue recognized.

The Company’s primary sources of revenue are as follows:



Year ended December 31, 2018	Space Systems		Imagery		Services		Eliminations		Total
Product revenues	$	851	$	—	$	—	$	—	$	851
Service revenues		191		841		258		—		1,290
Intersegment eliminations		87		4		8		(99)		—
	$	1,129	$	845	$	266	$	(99)	$	2,141

Year ended December 31, 2017	Space Systems		Imagery		Services		Eliminations		Total
Product revenues	$	1,119	$	—	$	—	$	—	$	1,119
Service revenues		141		228		143		—		512
Intersegment eliminations		10		2		1		(13)		—
	$	1,270	$	230	$	144	$	(13)	$	1,631

Year ended December 31, 2016	Space Systems		Imagery		Services		Eliminations		Total
Product revenues	$	1,285	$	—	$	—	$	—	$	1,285
Service revenues		132		41		100		—		273
Intersegment eliminations		4		1		—		(5)		—
	$	1,421	$	42	$	100	$	(5)	$	1,558

Certain of the Company’s contracts with customers in the Space Systems segment include a significant financing component since payments are received from the customer more than one year after delivery of the promised goods or services. The Company recognized orbital interest revenue of $32 million, $35 million and $31 million for the year ended December 31, 2018, 2017 and 2016, respectively related to these contracts, which is included in revenue from construction contracts.

The approximate revenue based on geographic location of customers is as follows:


	Year ended December 31,
		2018		2017		2016
United States	$	1,371	$	744	$	447
Canada		152		330		424
Asia		334		295		312
Europe		118		207		285
Australia		18		29		30
South America		134		17		58
Other		14		9		2
	$	2,141	$	1,631	$	1,558

Revenue from significant customers is as follows:


	Year ended December 31,
	2018		2017		2016
Government:
U.S. Federal Government and agencies	$	890	$	294	$	98
Canadian Federal Government and agencies		96		194		212
Commercial and other		1,155		1,143		1,248
Total revenue	$	2,141	$	1,631	$	1,558