Validity of RAIM Prediction ION ITM 2011 San Diego, CA Jan , 2011 Takeyasu Sakai Electronic Navigation Research Institute, Japan Takeyasu Sakai Electronic Navigation Research Institute, Japan

ION ITM Jan Slide 1 Introduction Background of RAIM:Background of RAIM: –GNSS is an essential tool for implementation of PBN; –GPS needs to be augmented in terms of integrity for aviation applications; GPS itself does not provide enough integrity as GNSS; –The ICAO (international civil aviation organization) GNSS standard defines several augmentation systems: SBAS, GBAS, and ABAS; –RAIM (receiver autonomous integrity monitoring) is a kind of ABAS which is a mechanism that the receiver ensures a certain level of integrity; –FAA TSO-C129 and RTCA DO-208 require RAIM function. Use of GPS/RAIM (GPS augmented by RAIM):Use of GPS/RAIM (GPS augmented by RAIM): –In many countries, GPS/RAIM is usable for IFR flights; –Mostly RAIM prediction is required before flight to check the availability of GPS/RAIM for the intended operation; –Recently Japan Civil Aviation Bureau (JCAB) ordered an investigation on how reliable RAIM prediction is; This presentation is summary of the investigation.

ION ITM Jan Slide 2 Augmentation Systems SBAS Satellite-Based Augmentation System GPS-compatible signal from GEO ABAS Aircraft-Based Augmentation System RAIM, Baro aiding GBAS Ground-Based Augmentation System VHF signal from the ground MSAS GNSS: GPS + augmentation system (SBAS, GBAS, or ABAS);GNSS: GPS + augmentation system (SBAS, GBAS, or ABAS); RAIM (receiver autonomous integrity monitoring): a mechanism of ABAS;RAIM (receiver autonomous integrity monitoring): a mechanism of ABAS; TSO-C129 certified receiver must implement RAIM function.TSO-C129 certified receiver must implement RAIM function. GPS must be augmented in terms of integrity GPS constellation

ION ITM Jan Slide 3 Fundamental of RAIM Measures range between GPS and receiver Detects failure of satellite by range anomaly Requires 5 or more GPS satellites;Requires 5 or more GPS satellites; The receiver warns pilot (or FMS) when it detects failure of GPS satellite;The receiver warns pilot (or FMS) when it detects failure of GPS satellite; RAIM FD: Detects failure and outputs warning;RAIM FD: Detects failure and outputs warning; RAIM FDE: Detects failure and exclude the anomalous satellite, then continues operation (For FDE, requires 6 or more satellites).RAIM FDE: Detects failure and exclude the anomalous satellite, then continues operation (For FDE, requires 6 or more satellites).

ION ITM Jan Slide 4 Part 1 Background and Motivation

ION ITM Jan Slide 5 Current Situation in Japan Regulation:Regulation: –In Japan, use of GPS/RAIM is currently limited to supplemental navigation because availability of GPS/RAIM is not enough; –Supplemental navigation system: Aircraft must equip other primary navigation system and the route is planned based on the primary navigation system; –Many small business planes equip TSO-C129 GPS receivers but they cannot fly RNAV route directly going to the destination by GPS/RAIM only; –Additionally, RAIM prediction is mandatory; If GPS/RAIM is predicted unavailable for the intended operation, they should change the flight plan. Recent situation:Recent situation: –Business operators have strongly requested JCAB to allow using GPS/RAIM as a primary navigation source which enables to fly RNAV route without additional equipment; –JCAB is now considering to relax the regulation and allow GPS/RAIM as a primary navigation source.

ION ITM Jan Slide 6 Supplemental Navigation Source Supplemental Navigation Equipage:Equipage: –Aircraft must equip other primary navigation system; –The route is planned based on the primary navigation system. Operation:Operation: –Pilots must monitor primary navigation source to check correctness of supplemental navigation; –If pilots find any doubt in supplemental navigation, they must change navigation source to the primary; –RAIM prediction is mandatory. Benefit:Benefit: –The route is limited by the coverage of primary navigation system; –Cannot fly RNAV route by GPS/RAIM only. PrimaryNavigation SupplementalNavigation

ION ITM Jan Slide 7 Primary Navigation Source Primary Navigation Equipage:Equipage: –Aircraft must equip other backup navigation system to fly the alternate route; –For approach, the backup navigation system must be available at the alternate airport; –The route is planned based on the primary navigation system. Operation:Operation: –RAIM prediction is mandatory; If predicted unavailable, they should change the flight plan; –In case that the primary navigation source fails, pilots change navigation source to the backup and fly the alternate route (airport). Benefit:Benefit: –Possible to plan the route based on the primary navigation source; Enabling RNAV by GPS/RAIM. BackupNavigation PrimaryNavigation Alternate Route PlannedRoute

ION ITM Jan Slide 8 Sole Mean Navigation Equipage:Equipage: –Aircraft do not need to equip any other navigation system; –RAIM FD cannot serve as a sole mean navigation; At least RAIM FDE is necessary because continuous operation against failure of GPS satellite is necessary. Operation:Operation: –RNP operation with enough integrity of navigation; –RAIM prediction is still required. Benefit:Benefit: –Small planes with less equipage can fly to remote islands. Sole Mean Navigation

ION ITM Jan Slide 9 RAIM Prediction Is GPS approach possible on the planned arrival time? Is GPS/RAIM available along with the planned route? DestinationAirport OriginAirport RAIM prediction: It is required to check availability of GPS/RAIMRAIM prediction: It is required to check availability of GPS/RAIM for the intended operation before flight; Because GPS/RAIM does not have enough availability and often unavailable;Because GPS/RAIM does not have enough availability and often unavailable; If GPS/RAIM is predicted to be unavailable, flight plan should be changed;If GPS/RAIM is predicted to be unavailable, flight plan should be changed; In Japan, ATM Center performs RAIM prediction everyday for Enroute, Terminal, and Approach, respectively, and issues NOTAM for unavailable operations.In Japan, ATM Center performs RAIM prediction everyday for Enroute, Terminal, and Approach, respectively, and issues NOTAM for unavailable operations.

ION ITM Jan Slide 10 Expected Investigation Possible problem: Missed Alarm EventPossible problem: Missed Alarm Event –There might be ‘Missed Alarm Event’ that RAIM prediction says ‘GPS will be available’ but actually GPS/RAIM is not available; Then the pilot uses backup navigation system or changes the destination to the alternative airport; –Relaxing the regulation to allow use of GPS/RAIM as a primary navigation source, ATC controllers have a concern and wish to know how often such an event occurs and how wide the affected region is; –JCAB decided to observe the situation by a two-year-trial with limited users and to conduct an investigation to estimate the effects. Investigation ordered by JCAB:Investigation ordered by JCAB: –Problem is due to unscheduled outage (failure) of GPS satellite; –Reliability of GPS satellites: the number of satellites, availability, and failure rate; –Lists unscheduled outage of GPS satellite; –Detailed investigation of ‘Missed Alarm Event’ that RAIM prediction said OK but actually GPS/RAIM was unavailable: Illustrates affected region and temporal range.

ION ITM Jan Slide 11 Concern of ATC Controllers GPS fails, change route Sudden outage due to failure Wow !!! ‘Missed Alarm Event’: RAIM prediction says ‘GPS will be available’ but actually GPS becomes unavailable;‘Missed Alarm Event’: RAIM prediction says ‘GPS will be available’ but actually GPS becomes unavailable; Many aircraft flying by GPS/RAIM suddenly lose primary navigation simultaneously;Many aircraft flying by GPS/RAIM suddenly lose primary navigation simultaneously; How much does it affect to ATC operation?How much does it affect to ATC operation? GPS fails, change route GPS fails, change route GPS fails, change route ATC Controllers

ION ITM Jan Slide 12 Part 2 Summary of Investigation

ION ITM Jan Slide 13 GPS Satellite Failures Event OccurredNANU IDNANU IssuedSVNRepairedCase # 07/08/24 17: /08/24 17:554007/09/18 20:28 07/09/15 12: /09/15 13:075307/09/15 16:13 07/10/08 04: /10/08 04:395907/10/08 16:36 07/10/08 06: /10/08 06:375807/10/08 16:49 07/10/08 21: /10/08 21:344107/10/09 01:43 07/10/09 08: /10/09 08:566007/10/09 11:16 07/10/10 08: /10/10 08:285107/10/10 11:04 08/03/25 02: /03/25 02:364008/03/27 23:49 08/04/02 09: /04/02 12:275808/04/02 15:08 08/04/26 02: /04/26 03:136108/04/29 12:14 08/08/05 23: /08/05 23:493508/10/07 20:49 09/07/25 01: /07/25 02:245409/07/26 16:00 There were 30 cases of sudden failure of GPS satellite for years 2007 to 2009;There were 30 cases of sudden failure of GPS satellite for years 2007 to 2009; Among them, 12 cases listed here affected RAIM prediction in Japan;Among them, 12 cases listed here affected RAIM prediction in Japan; 5 cases on 07/10/08 to 07/10/10 are due to trouble in the GPS MCS; There were 8 independent cases for 3 years; Frequency of the Event: 8 / 3 = events/year.5 cases on 07/10/08 to 07/10/10 are due to trouble in the GPS MCS; There were 8 independent cases for 3 years; Frequency of the Event: 8 / 3 = events/year. MCSTrouble

ION ITM Jan Slide 14 Investigation of Each Case Detailed investigation of each case:Detailed investigation of each case: –Comparison between output of RAIM prediction and actual receiver behavior for each case of GPS satellite failure; –Illustrates affected region and temporal range of ‘Missed Alarm Event’. ATM Center RAIM prediction system:ATM Center RAIM prediction system: –Assumes GPS/RAIM receivers meet TSO-C129 requirements (Classic RAIM); –Computes ARP (approximate radial-error protected) index and compares with the threshold predefined for each phase of flight; ARP is rough estimation of the largest horizontal position error similar with HPL (horizontal protection level); –If ARP exceeds the threshold, GPS/RAIM is unavailable for the associate operation. Phase of Flight Enroute (RNAV5) Terminal (RNAV1) Non Precision Approach Maximum ARP Allowed 2262 m1135 m352 m Threshold of ARP index for each phase of flight

ION ITM Jan Slide 15 Assumptions Estimation of RAIM prediction behavior:Estimation of RAIM prediction behavior: –Implemented ARP computation algorithm and parameters same as RAIM prediction system in the ATM Center; –Assume RAIM prediction is performed on 21:00 local time (12:00 UTC) everyday based on ‘FORECAST’ NANU info. (scheduled maintenance of GPS satellites); –GPS satellite position: computed with archive of almanac information; Mask angle is 7.5 degrees; –NOTAM is issued if GPS/RAIM is unavailable for 5 minutes or more; –The estimation result is compared with the operational NOTAM (Available since 9/27/2007); They completely matched. Estimation of GPS/RAIM receiver behavior:Estimation of GPS/RAIM receiver behavior: –Assumes onboard GPS/RAIM receivers determine if GPS/RAIM is available or not by the computation identical with ATM Center; –Computes ARP index based on ‘ACTUAL’ NANU information (failure and maintenance report of GPS satellites) and compares with the threshold.

ION ITM Jan Slide 16 Case : Temporal Variation Case 07/08/24 17:51 SVN40 Computed ARP variation for each case;Computed ARP variation for each case; Target location is Osaka Int’l Airport; At the center of Japan and GPS approach is available there;Target location is Osaka Int’l Airport; At the center of Japan and GPS approach is available there; Case : ARP increased at the same time as SVN40 failure, but less than threshold.Case : ARP increased at the same time as SVN40 failure, but less than threshold. Threshold for Approach (352m) Threshold for Terminal (1135m) Threshold for Enroute (2262m) ARP by RAIM prediction Actual ARP Difference Actual > Prediction Prediction SatelliteFailure

ION ITM Jan Slide 17 Cases to Cases to Case 07/10/08 04:37 SVN59 Case 07/10/08 06:33 SVN58 Case 07/09/15 12:50 SVN53 Cases and : RAIM prediction said GPS/RAIM is unavailable only for Approach; Actually it became unavailable for Enroute and Terminal, and for Approach for longer than the prediction;Cases and : RAIM prediction said GPS/RAIM is unavailable only for Approach; Actually it became unavailable for Enroute and Terminal, and for Approach for longer than the prediction; Case : No NOTAM because it was not operated yet;Case : No NOTAM because it was not operated yet;

ION ITM Jan Slide 18 Cases and Cases and Case 07/10/09 08:55 SVN60 Case 07/10/08 21:04 SVN41 Case : ARP increased at the same time as SVN41 failure; GPS Approach became unavailable later at 21:38 and continued unavailable for 31 minutes until 22:09;Case : ARP increased at the same time as SVN41 failure; GPS Approach became unavailable later at 21:38 and continued unavailable for 31 minutes until 22:09; RAIM prediction could not work and NOTAM was not issued for Case ;RAIM prediction could not work and NOTAM was not issued for Case ; Case : RAIM prediction said GPS Approach is unavailable; Actually it became unavailable for Enroute and Terminal, and for Approach for longer than the prediction.Case : RAIM prediction said GPS Approach is unavailable; Actually it became unavailable for Enroute and Terminal, and for Approach for longer than the prediction.

ION ITM Jan Slide 19 Cases and Cases and Case 08/03/25 02:30 SVN40 Case 07/10/10 08:24 SVN51 Case : RAIM prediction said GPS Approach is unavailable; Actually it became unavailable for Enroute and Terminal, and for Approach for longer than the prediction;Case : RAIM prediction said GPS Approach is unavailable; Actually it became unavailable for Enroute and Terminal, and for Approach for longer than the prediction; Case : ARP increased due to failure and GPS/RAIM could not be used for Approach; RAIM prediction could not work and NOTAM was not issued.Case : ARP increased due to failure and GPS/RAIM could not be used for Approach; RAIM prediction could not work and NOTAM was not issued.

ION ITM Jan Slide 20 Cases and Cases and Case 08/04/26 02:46 SVN61 Case 08/04/02 09:46 SVN58 Case : ARP increased due to failure, but less than threshold;Case : ARP increased due to failure, but less than threshold; Case : ARP increased due to failure and GPS/RAIM could not be used for Approach; RAIM prediction could not work and NOTAM was not issued.Case : ARP increased due to failure and GPS/RAIM could not be used for Approach; RAIM prediction could not work and NOTAM was not issued.

ION ITM Jan Slide 21 Cases and Cases and Case 09/07/25 01:28 SVN54 Case 08/08/05 23:38 SVN35 Case : NOTAM was issued by RAIM prediction but actually GPS/RAIM was available;Case : NOTAM was issued by RAIM prediction but actually GPS/RAIM was available; Regular RAIM prediction does not predict the effect on 04:30 UTC, but extra prediction was performed as response to NANU reporting failure of SVN35 and the extra NOTAM was issued;Regular RAIM prediction does not predict the effect on 04:30 UTC, but extra prediction was performed as response to NANU reporting failure of SVN35 and the extra NOTAM was issued; Case : ARP increased due to failure and GPS/RAIM could not be used for Approach; RAIM prediction could not work and NOTAM was not issued.Case : ARP increased due to failure and GPS/RAIM could not be used for Approach; RAIM prediction could not work and NOTAM was not issued. False Alarm

ION ITM Jan Slide 22 Spatial Impact Observes affected region for some cases that RAIM prediction could not work properly:Observes affected region for some cases that RAIM prediction could not work properly: –For consideration of effects on ATC operation; –‘Missed Alarm Event’: RAIM prediction said OK but actually unavailable; –Illustrated as colored plots on a map as follows: RAIM prediction says GPA/RAIM is available and actually it is available (No Alarm); There is no RAIM NOTAM and GPS/RAIM is available normally. RAIM prediction says GPS/RAIM is unavailable but actually it is available (False Alarm); RAIM NOTAM is issued and GPS/RAIM is not used for operation; There is no safety problem but availability of GPS/RAIM navigation is lowered. RAIM prediction says GPS/RAIM is unavailable and actually it is unavailable (True Alarm); RAIM NOTAM is issued and GPS/RAIM is not used for operation; This case means RAIM prediction works properly. RAIM prediction says GPS/RAIM is available but actually it is unavailable (Missed Alarm); RAIM prediction fails to issue NOTAM reporting GPS/RAIM is unavailable; GPS/RAIM suddenly becomes unavailable during operation. Predict: Available Actual: Available Predict: Unavailable Actual: Available Predict: Unavailable Actual: Unavailable Predict: Available Actual: Unavailable

ION ITM Jan Slide 23 Case Enroute Computed for Case for Enroute operation; Case looks affected widely;Computed for Case for Enroute operation; Case looks affected widely; Interval: 10 minutes per frame;Interval: 10 minutes per frame; No effect immediately after the failure; The region where GPS/RAIM is unavailable without NOTAM appears 110 minutes after the failure.No effect immediately after the failure; The region where GPS/RAIM is unavailable without NOTAM appears 110 minutes after the failure. Predict: Available Actual: Available Predict: Unavailable Actual: Available Predict: Unavailable Actual: Unavailable Predict: Available Actual: Unavailable Interval: 10 min

ION ITM Jan Slide 24 Case Enroute: 130 min Case : 130 minutes after the failure of SVN 53;Case : 130 minutes after the failure of SVN 53; The region where GPS/RAIM is unavailable without NOTAM grew up to a half size of Japan.The region where GPS/RAIM is unavailable without NOTAM grew up to a half size of Japan. Predict: Available Actual: Available Predict: Unavailable Actual: Available Predict: Unavailable Actual: Unavailable Predict: Available Actual: Unavailable Snapshot: 130 min after failure

ION ITM Jan Slide 25 Case Terminal Computed for Case for terminal operation;Computed for Case for terminal operation; No effect immediately after the failure; The region where GPS/RAIM is unavailable without NOTAM appears 30 minutes after the failure and disappears;No effect immediately after the failure; The region where GPS/RAIM is unavailable without NOTAM appears 30 minutes after the failure and disappears; The red region appears again 100 minutes after and covers whole of Japan.The red region appears again 100 minutes after and covers whole of Japan. Predict: Available Actual: Available Predict: Unavailable Actual: Available Predict: Unavailable Actual: Unavailable Predict: Available Actual: Unavailable Interval: 10 min

ION ITM Jan Slide 26 Case Terminal: 110 min Case : 110 minutes after the failure of SVN 53;Case : 110 minutes after the failure of SVN 53; The red region covers whole of Japan; Terminal area operation is affected nationwide;The red region covers whole of Japan; Terminal area operation is affected nationwide; Such a situation continued for 40 minutes.Such a situation continued for 40 minutes. Predict: Available Actual: Available Predict: Unavailable Actual: Available Predict: Unavailable Actual: Unavailable Predict: Available Actual: Unavailable Snapshot: 110 min after failure

ION ITM Jan Slide 27 Case Approach Computed for Case for non precision approach operation;Computed for Case for non precision approach operation; Prediction and actual are identical immediately after the failure;Prediction and actual are identical immediately after the failure; The region where GPS/RAIM is unavailable without NOTAM appears 20 minutes after the failure and disappears;The region where GPS/RAIM is unavailable without NOTAM appears 20 minutes after the failure and disappears; The red region appears again 80 minutes after and covers whole of Japan.The red region appears again 80 minutes after and covers whole of Japan. Predict: Available Actual: Available Predict: Unavailable Actual: Available Predict: Unavailable Actual: Unavailable Predict: Available Actual: Unavailable Interval: 10 min

ION ITM Jan Slide 28 Case Approach: 110 min Case : 110 minutes after the failure of SVN 53;Case : 110 minutes after the failure of SVN 53; The red region covers whole of Japan; Non precision approach is suddenly stopped nationwide;The red region covers whole of Japan; Non precision approach is suddenly stopped nationwide; Such a situation continued for 120 minutes.Such a situation continued for 120 minutes. Predict: Available Actual: Available Predict: Unavailable Actual: Available Predict: Unavailable Actual: Unavailable Predict: Available Actual: Unavailable Snapshot: 110 min after failure

ION ITM Jan Slide 29 Case Approach: 10 min Predict: Available Actual: Available Predict: Unavailable Actual: Available Predict: Unavailable Actual: Unavailable Predict: Available Actual: Unavailable Case ; 10 minutes after the failure of SVN 54;Case ; 10 minutes after the failure of SVN 54; The red region covers whole of Japan; Non precision approach is suddenly stopped nationwide; Such a situation continued for 30 minutes;The red region covers whole of Japan; Non precision approach is suddenly stopped nationwide; Such a situation continued for 30 minutes; There is a case that the red region appears only 10 minutes after the failure.There is a case that the red region appears only 10 minutes after the failure. Snapshot: 10 min after failure

ION ITM Jan Slide 30 Conclusion Current Situation in Japan:Current Situation in Japan: –Use of GPS/RAIM is currently limited to supplemental navigation; –JCAB is now considering to relax the regulation and allow GPS/RAIM as a primary navigation source. Concerns and Investigation Ordered by JCAB:Concerns and Investigation Ordered by JCAB: –ATC controllers have a concern on ‘Missed Alarm Event’; –JCAB decided to observe the situation by a two-year-trial with limited users and to conduct an investigation to estimate the effects. Investigation of each failure case:Investigation of each failure case: –There were 8 cases RAIM prediction affected by GPS satellite failure in Japan for years 2007 to 2009; events/year; –It was confirmed that GPS satellite failure could make GPS/RAIM navigation unavailable without prediction; Affected region and temporal range is not small; –In fact, there are cases that GPS/RAIM navigation is lost for Terminal and Approach phases of flight over the whole of Japan.

ION ITM Jan Slide 31 Backup Statistics on the Number of Available GPS Satellites

ION ITM Jan Slide 32 Source: NANU Information NANU (Notice Advisory to Navstar Users) information:NANU (Notice Advisory to Navstar Users) information: –Information on GPS satellites issued by GPS operator; –Provides information on launch, decommission, maintenance, and failure of each GPS satellite; –Distributed by ; Archive is also available. Scheduled maintenance information:Scheduled maintenance information: –NANU notifies scheduled maintenance by a few days before; –After the maintenance, actual down period is reported; –RAIM prediction is performed based on this ‘FORECAST’ information. Failure information:Failure information: –GPS satellite failure is quickly reported when the event is detected: Latency is a few minutes to a few hours (Not guaranteed: sometimes missed to report); –After repair, actual down period is reported; –Actual availability of GPS/RAIM is estimated based on this ‘ACTUAL’ information. NOTICE ADVISORY TO NAVSTAR USERS (NANU) SUBJ: SVN59 (PRN19) FORECAST OUTAGE JDAY 042/ JDAY 043/ NANU TYPE: FCSTDV NANU NUMBER: NANU NUMBER: NANU DTG: Z FEB 2009 NANU DTG: Z FEB 2009 REFERENCE NANU: N/A REFERENCE NANU: N/A REF NANU DTG: N/A REF NANU DTG: N/A SVN: 59 SVN: 59 PRN: 19 PRN: 19 START JDAY: 042 START JDAY: 042 START TIME ZULU: 2230 START TIME ZULU: 2230 START CALENDAR DATE: 11 FEB 2009 START CALENDAR DATE: 11 FEB 2009 STOP JDAY: 043 STOP JDAY: 043 STOP TIME ZULU: 1230 STOP TIME ZULU: 1230 STOP CALENDAR DATE: 12 FEB 2009 STOP CALENDAR DATE: 12 FEB CONDITION: GPS SATELLITE SVN59 (PRN19) WILL BE UNUSABLE ON JDAY 042 (11 FEB 2009) BEGINNING 2230 ZULU UNTIL JDAY 043 (12 FEB 2009) (11 FEB 2009) BEGINNING 2230 ZULU UNTIL JDAY 043 (12 FEB 2009) ENDING 1230 ZULU. ENDING 1230 ZULU. NANU Sample (NANU ) Scheduled Maintenance Start Date/Time Stop Date/Time

ION ITM Jan Slide 33 Available GPS Satellites # of Satellites in Operation Excluding Outage Period Statistics on the number of available GPS satellites for years 2007 to 2009;Statistics on the number of available GPS satellites for years 2007 to 2009; 29 to 31 satellites are operated; 29 satellites for 8% of the whole period;29 to 31 satellites are operated; 29 satellites for 8% of the whole period; Considering outages due to maintenance and failure shows there is 27 satellites period for 0.04% (10 hours), and 29 or more for 95%.Considering outages due to maintenance and failure shows there is 27 satellites period for 0.04% (10 hours), and 29 or more for 95%.

ION ITM Jan Slide 34 Available GPS Satellites GPS Satellites in Operation Excluded Maintenance and Failure Statistics on the number of available GPS satellites for years 2007 to 2009;Statistics on the number of available GPS satellites for years 2007 to 2009; 29 to 31 satellites are operated;29 to 31 satellites are operated; Up to 3 satellites are down due to maintenance and/or failure.Up to 3 satellites are down due to maintenance and/or failure.

ION ITM Jan Slide 35 Availability and Failure Rate Average of all satellitesMTBF [h]MTTR [h]Availability All Outage Only Maintenance Only Failure Computes MTBO (mean time between outage) and MTTR (mean time to repair) based on NANU information;Computes MTBO (mean time between outage) and MTTR (mean time to repair) based on NANU information; Availability of the system:Availability of the system: Estimation of frequency that a GPS satellite in view fails suddenly: Assuming failure rate of 1 / per hour per satellite, mean number of visible satellites as 7:Estimation of frequency that a GPS satellite in view fails suddenly: Assuming failure rate of 1 / per hour per satellite, mean number of visible satellites as 7: 7 / * 24 * 365 = events/year.