Alternate Navigation Architectures and Data Needs Benjamin Peterson

Background Addition/Increase of data capacity on DME/ADS-B signals needs examining requirement – Could design for “max” BW but result in overdesign, etc. Examine design, benefit, limitations Use of data usage – Time/heartbeat, authentication, etc.

Basic Concept DME’s continue to support legacy use. DME’s also transmit heartbeat sync’ed to UTC. – Unique signal so A/C can distinguish heartbeat from normal response. Maybe unique from other DME’s on same channel that can be received at high altitudes. – Use 10 Hz for discussion, could be higher/lower

Basic Concept-2 A/C navigates: – Completely passively (no interrogations) if 3 or more DME’s received (& geometry acceptable) – With 2 DME’s & occasional interrogation on one DME & passive ranging on other(s). Time modulo heartbeat determined on A/C by measurement of offset between receiving heartbeat & response to interrogation. – This enables getting range (not pseudorange) from other DME’s passively. – Interrogation only has to happen often enough to update estimate of receiver clock. – Greatly expands capacity of DME system

Basic Concept-RAIM For navigation solution w/RAIM: – Completely passively (no interrogations) with 4 or more DME’s & acceptable geometry. – With 3 DME’s & occasional interrogation on one DME & passive ranging on others.

Basic Concept-RAIM For navigation solution w/RAIM: – Completely passively (no interrogations) with 4 or more DME’s & acceptable geometry. – With 3 DME’s & occasional interrogation on one DME & passive ranging on others. – Over-determined solution resolves ambiguity You really are here X X

Basic Concept-Authentication Assume no authentication on responses to interrogations Heartbeat signal could include public key authentication – Length of authentication message determines Time to First Fix, & possibly Time to Alarm but not update rate. Individual heartbeats Questions: 1.Can we design an authentication message such that if it is N heartbeats long, once we have authenticated, we can authenticate again using last N heartbeats as opposed to waiting for next complete block of N heartbeats? 2.Otherwise, message would need to fit within some Time to Alert requirement (but not within 1 heartbeat period.) Authentication msg in N heartbeats Can these N heartbeats also be an authentication msg?

Basic Concept-Base Station Time Need source of UTC (to at least modulo some period >= heartbeat, i.e. 1 sec.) that has at least input independent of GPS & can coast thru outages. – Two Way Satellite Time Transfer or dish pointed at WAAS SV (in addition to GPS) – Redundant atomic clocks – Note: This will be necessary for multi-lateration If only for nav purposes, do not need Time of Day, only time modulo heartbeat – If intent is to transmit Time of Day data & source of Time is WAAS, then need WAAS message with Time of Day data. (Should be able to do cold start w/o GPS) Example: If we were to transmit GPS week & Z count/5 every 5 secs, at 10 Hz, we could transmit 1 bit/heartbeat & have 23 of the 50 bits left over for FEC parity, etc.

Utilizing UAT for A/C Navigation A/C Ranging on UAT signals: Unless major mod to MOPS – No direct ranging as in current DME – Passive pseudorange available at 1 Hz (not more) Transmitting A/C position determined by multi-lateration up to A/C – Attractive assuming multi-lateration & TIS-B are done anyway for surveillance purposes; no additional hardware either on ground or in the A/C – Update rate also limited to 1 Hz – Makes navigation dependent on Comms. If you lose Comms, you lose Nav as well. Multiple single points of failure makes demonstration of integrity more difficult. – Not unlimited capacity such as with passive ranging – Spoofing (data sent to A/C) would much easier unless authenticated