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Related Party Transactions (Details) - USD ($)
12 Months Ended
Apr. 30, 2019
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2020
May 10, 2019
Feb. 09, 2019
Feb. 08, 2019
Dec. 31, 2017
Long-Term Incentive Awards                
Revenue $ 55,407,000 $ 29,594,000            
Unbilled related party receivables 9,028,000 3,145,000            
Design and Development Agreement                
Long-Term Incentive Awards                
Maximum net value 133,360,000              
Revenue 55,407,000 29,594,000            
Board member | Consulting agreement                
Long-Term Incentive Awards                
Amount paid to related party $ 55,000 48,000 $ 80,000          
HAPSMobile                
Long-Term Incentive Awards                
Ownership percentage 10.00%     5.00% 10.00% 10.00% 5.00% 10.00%
Unbilled receivables and retention | HAPSMobile                
Long-Term Incentive Awards                
Unbilled related party receivables $ 9,028,000 $ 3,145,000