]> HTT Consulting

Oak Park MI 48237 USA rgm@labs.htt-consult.com

AX Enterprize

4947 Commercial Drive Yorkville NY 13495 USA stu.card@axenterprize.com

AX Enterprize

4947 Commercial Drive Yorkville NY 13495 USA adam.wiethuechter@axenterprize.com

Intel

2200 Mission College Blvd Santa Clara CA 95054 USA shuai.zhao@ieee.org

Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@sit.fraunhofer.de

Internet DRIP RFC This document describes using the ASTM Broadcast Remote ID (B-RID) specification in a "crowd sourced" smart phone or fixed receiver asset environment to provide much of the ASTM and FAA envisioned Network Remote ID (Net-RID) functionality. This crowd sourced B-RID (CS-RID) data will use multilateration to add a level of reliability in the location data on the Uncrewed Aircraft (UA). The crowd sourced environment will also provide a monitoring coverage map to authorized observers.

Introduction This document defines a mechanism to capture the ASTM Broadcast Remote ID messages (B-RID) on any Internet connected device that receives them and can forward them to the Supplemental Data Service Providers (SDSPs) responsible for the geographic area the UA and receivers are in. This crowd sourced B-RID (CS-RID) will create a ecosystem that will meet most if not all data collection requirements that Civil Aviation Authorities (CAA) are placing on Network Remote ID (Net-RID). These Internet connected B-RID receivers are herein called "Finders", as they find UAs by listening for B-RID messages. The Finders are B-RID forwarding proxies. Their potentially limited spacial view of RID messages could result in bad decisions on what messages to send to the SDSP and which to drop. Thus they will send all received messages and the SDSP will make any filtering decisions in what it forwards into the UAS Traffic Management (UTM). Finders can be smartphones, tablets, connected cars, or any computing platform with Internet connectivity that can meet the requirements defined in this document. It is not expected, nor necessary, that Finders have any information about a UAS beyond the content in the B-RID messages. The SDSPs are similar to, but different from the Surveillance SDSPs in . defines sensors which are predominately radars, similar to . The difference stems from orientation with ICAO Aircraft numbers and aviation radar history of tracking objects in flight and trying to feed that data into UTM. It is likely that this document will act as guidance to for future revisions to better align it with UTM over manned aviation Aircraft Traffic Control (ATC). Finders MAY only need a loose association with the SDSP(s). They may only have the SDSP's Public Key and FQDN. It would use these, along with the Finder's Public Keypair to use Elliptic Curve Integrated Encryption Scheme (ECIES), or other security methods, to send the messages in a secure manner to the SDSP. The SDSP MAY require a stronger relationship to the Finders. This may range from the Finder's Public Key being registered to the SDSP with other information so that the SDSP has some level of trust in the Finders to requiring transmissions be sent over long-lived transport connections like ESP or DTLS . If a 1-way only secure packet forwarding method is used (e.g., not a TCP connection), the Finder SHOULD receive periodic "heartbeats" from the SDSP to inform it that its transmissions are being received. The SDSP sets the rules on when to send these heartbeats as discuss below in .

Role of Supplemental Data Service Provider (SDSP) The DRIP Architecture introduces the basic CS-RID entities including CS-RID Finder and CS-RID SDSP. This document has minimal information about the actions of SDSPs. In general the SDSP is out of scope of this document. That said, the SDSPs should not simply proxy B-RID messages to the UTM(s). They should perform some minimal level of filtering and content checking before forwarding those messages that pass these tests in a secure manner to the UTM(s). The SDSPs are also capable of maintaining a monitoring map, based on location of active Finders. UTMs may use this information to notify authorized observers of where there is and there is not monitoring coverage. They may also use this information of where to place pro-active monitoring coverage. An SDSP should only forward Authenticated B-RID messages like those defined in to the UTM(s). Further, the SDSP SHOULD validate the Remote ID (RID) and the Authentication signature before forwarding anything from the UA, and flagging those RIDs that were not validated. The SDSP MAY forward all B-RID messages to the UTM, leaving all decision making on B-RID messages veracity to the UTM. When 3 or more Finders are reporting to an SDSP on a specific UA, the SDSP is in a unique position to perform multilateration on these messages and compute the Finder's view of the UA location to compare with the UA Location/Vector messages. This check against the UA's location claims is both a validation on the UA's reliability as well as the trustworthiness of the Finders. Other than providing data to allow for multilateration, this SDSP feature is out of scope of this document. This function is limited by the location accuracy for both the Finders and UA.

Draft Status This draft is still incomplete. New features are being added as capabilities are researched. The actual message formats also still need work.

Terms and Definitions

Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This document uses terms defined in and .

Definitions

B-RID:

Broadcast Remote ID. A method of sending RID messages as 1-way transmissions from the UA to any Observers within radio range.

ECIES:

Elliptic Curve Integrated Encryption Scheme. A hybrid encryption scheme which provides semantic security against an adversary who is allowed to use chosen-plaintext and chosen-ciphertext attacks.

Finder:

In Internet connected device that can receive B-RID messages and forward them to a UTM.

Multilateration:

Multilateration (more completely, pseudo range multilateration) is a navigation and surveillance technique based on measurement of the times of arrival (TOAs) of energy waves (radio, acoustic, seismic, etc.) having a known propagation speed.

Net-RID:

Network Remote ID. A method of sending RID messages via the Internet connection of the UAS directly to the UTM.

Problem Space

Meeting the needs of Network Remote ID The USA Federal Aviation Authority (FAA), in the January 2021 Remote ID Final rule , postponed Network Remote ID (Net-RID) and focused on Broadcast Remote ID. This was in response to the UAS vendors comments that Net-RID places considerable demands on then currently used UAS. However, Net-RID, or equivalent, is necessary for UTM and knowing what soon may be in an airspace. A method that proxies B-RID into UTM can function as an interim approach to Net-RID and continue as a adjunct to Net-RID.

Advantages of Broadcast Remote ID B-RID has its advantages over Net-RID. B-RID can more readily be implemented directly in the UA. Net-RID will more frequently be provided by the GCS or a pilot's Internet connected device. If Command and Control (C2) is bi-directional over a direct radio connection, B-RID could be a straight-forward addition. Small IoT devices can be mounted on UA to provide B-RID. B-RID can also be used by the UA to assist in Detect and Avoid (DAA). B-RID is available to observers even in situations with no Internet like natural disaster situations.

Trustworthiness of Proxied Data When a proxy is introduced in any communication protocol, there is a risk of corrupted data and DOS attacks. The Finders, in their role as proxies for B-RID, are authenticated to the SDSP (see ). The SDSP can compare the information from multiple Finders to isolate a Finder sending fraudulent information. SDSPs can additionally verify authenticated messages that follow . The SPDP can manage the number of Finders in an area (see ) to limit DOS attacks from a group of clustered Finders.

Defense against fraudulent RID Messages The strongest defense against fraudulent RID messages is to focus on conforming messages. Unless this behavior is mandated, SPDPs will have to use assorted algorithms to isolate messages of questionable content.

The Finder - SDSP Security Relationship The SDSP(s) and Finders SHOULD use EdDSA keys as their trusted Identities. The public keys SHOULD be registered DRIP UAS Remote ID and . Other similar methods may be used. During this registration, the Finder gets the SDSP's EdDSA Public Key. These Public Keys allow for the following options for authenticated messaging from the Finder to the SDSP. The SDSP uses some process (out of scope here) to register the Finders and their EdDSA Public Key. During this registration, the Finder gets the SDSP's EdDSA Public Key. These Public Keys allow for the following options for authenticated messaging from the Finder to the SDSP. EdDSA keys are converted to X25519 keys per Curve25519 to use in ECIES. ECIES can be used with a unique nonce to authenticate each message sent from a Finder to the SDSP. ECIES can be used at the start of some period (e.g. day) to establish a shared secret that is then used to authenticate each message sent from a Finder to the SDSP sent during that period. HIP can be used to establish a session secret that is then used with ESP to authenticate each message sent from a Finder to the SDSP. DTLS can be used to establish a secure connection that is then used to authenticate each message sent from a Finder to the SDSP.

SDSP Heartbeats If a 1-way messaging approach is used (e.g. not TCP-based), the SDSP SHOULD send a heartbeat at some periodicity to the Finders so that they get confirmation that there is a receiver of their transmissions. A simple (see ) message that identifies the SDSP is sent to the Finder per some published policy of the SDSP. For example, at the first reception by the SDSP for the day, then the 1st for the hour. It is NOT recommended for the SDSP to send a heartbeat for every message received, as this is a potential DOS attack against the SDSP.

The Finder Map The Finders are regularly providing their SDSP with their location. This is through the B-RID Proxy Messages and Finder Location Update Messages. With this information, the SDSP can maintain a monitoring map. That is a map of where there Finder coverage.

Managing Finders Finder density will vary over time and space. For example, sidewalks outside an urban train station can be packed with pedestrians at rush hour, either coming or going to their commute trains. An SDSP may want to proactively limit the number of active Finders in such situations. Using the Finder mapping feature, the SDSP can instruct Finders to NOT proxy B-RID messages. These Finders will continue to report their location and through that reporting, the SDSP can instruct them to again take on the proxying role. For example a Finder moving slowly along with dozens of other slow-moving Finders may be instructed to suspend proxying. Whereas a fast-moving Finder at the same location (perhaps a connected car or a pedestrian on a bus) would not be asked to suspend proxying as it will soon be out of the congested area.

UA location via multilateration The SDSP can confirm/correct the UA location provided in the Location/Vector message by using multilateration on data provided by at least 3 Finders that reported a specific Location/Vector message (Note that 4 Finders are needed to get altitude sign correctly). In fact, the SDSP can calculate the UA location from 3 observations of any B-RID message. This is of particular value if the UA is only within reception range of the Finders for messages other than the Location/Vector message. This feature is of particular value when the Finders are fixed assets with highly reliable GPS location, around a high value site like an airport or large public venue.

GPS Inaccuracy Single-band, consumer grade, GPS on small platforms is not accurate, particularly for altitude. Longitude/latitude measurements can easily be off by 3M based on satellite postion and clock accuracy. Altitude accuracy is reported in product spec sheets and actual tests to be 3x less accurate. Altitude accuracy is hindered by ionosphere activity. In fact, there are studies of ionospheric events (e.g. 2015 St. Patrick's day ) as measured by GPS devices at known locations. Thus where a UA reports it is rarely accurate, but may be accurate enough to map to visual sightings of single UA. Smartphones and particulary smartwatches are plagued with the same challenge, though some of these can combine other information like cell tower data to improve location accuracy. FCC E911 accuracy, by FCC rules is NOT available to non-E911 applications due to privacy concerns, but general higher accuracy is found on some smart devices than reported for consumer UA. The SDSP MAY have information on the Finder location accuracy that it can use in calculating the accuracy of a multilaterated location value. When the Finders are fixed assets, the SDSP may have very high trust in their location for trusting the multilateration calculation over the UA reported location.

The CS-RID Messages The CS-RID messages between the Finders and the SDSPs primarily support the proxy role of the Finders in forwarding the B-RID messages. There are also Finder registration and status messages. CS-RID information is represented in CBOR . COSE MAY be used for CS-RID MAC and COAP for the CS-RID protocol. The CDDL specification is used for CS-RID message description. The following is a general representation of the content in the CS-RID messages.

The CS-RID MESSAGE CONTENT varies by MESSAGE TYPE.

CS-RID MESSAGE TYPE The CS-RID MESSAGE TYPE is defined in :

CDDL description for CS-RID message type The overall CS-RID CDDL description is structured in .

info_message, proxy_message => broadcast_rid_proxy_message, finder_registration => finder_registration_message, sdsp_response => sdsp_response_message, location_update => location_update_message, sdsp_heartbeat => sdsp_heartbeat_message, } info_message = { common_message_members, message_content => tstr, } common_message_members = ( message_type => message_types, mac_address => #6.37(bstr), ) message_types = &( Reserved : 0, BRD : 1, Finder-Registration : 2, SDSP-Response : 3, Finder-Location : 4, ) ]]>

The application context rule is defined in for CS-RID application identification and version negotiation.

"DRIP-CSRID", ? version => uint .size(1..2), ) ]]>

The predefined CDDL text string labels (author note: for JSON currently, will move to CBOR uint keys in upcoming versions) used in the specification is listed in .

The CS-RID B-RID Proxy Message The Finders add their own information to the B-RID messages, permitting the SDSP(s) to gain additional knowledge about the UA(s). The RID information is the B-RID message content plus the MAC address. The MAC address is critical, as it is the only field that links a UA's B-RID messages together. Only the ASTM Basic ID Message and possibly the Authentication Message contain the UAS ID field. The Finders add an SDSP assigned ID, a 64 bit timestamp, GPS information, and type of B-RID media to the B-RID message. Both the timestamp and GPS information are for when the B-RID message(s) were received, not forwarded to the SDSP. All this content is MACed using a key shared between the Finder and SDSP. The following is a representation of the content in the CS-RID messages.

CS-RID ID The CS-RID ID is the ID recognized by the SDSP. This may be an HHIT , or any ID used by the SDSP.

CDDL description for CS-RID B-RID Proxy Message The broadcast CS-RID proxy CDDL is defined in

tstr, timestamp => tdate, gps => gps-coordinates, radio_type => radio_types, broadcast_mac_address => #6.37(bstr), broadcast_message => #6.37(bstr), } radio_types = &( EFL : 0, VLF : 1, LF : 2, MF : 3, HF : 4, HF : 5, VHF : 6, UHF : 7, SHF : 8, EHF : 9, ) gps-coordinates = [ latitude : float, longitude: float, altitude : float, ] ]]>

CS-RID Finder Registration The CS-RID Finder MAY use with the SDSP to establish a Security Association and a shared secret to use for the CS-RID MAC generation. In this approach, the HIP mobility functionality and support are not used. When HIP is used as above, the Finder Registration is a SDSP "wake up". It is sent prior to the Finder sending any proxied B-RID messages to ensure that the SDSP is able to receive and process the messages. In this usage, the CS-RID ID is the Finder HIT. If the SDSP has lost state with the Finder, it initiates the HIP exchange with the Finder to reestablish HIP state and a new shared secret for the CS-RID B-RID Proxy Messages. In this case the Finder Registration Message is:

CDDL description for Finder Registration The CDDL for CS-RID Finder Registration is defined in

tstr, timestamp => tdate, gps => gps-coordinates, } gps-coordinates = [ latitude : float, longitude: float, altitude : float, ] ]]>

CS-RID SDSP Response The SDSP MAY respond to any Finder messages to instruct the Finder on its behavior.

The Proxy Status instructs the Finder if it should actively proxy B-RID messages, or suspend proxying and only report its location. The Update Interval is the frequency that the Finder SHOULD notify the SDSP of its current location using the Location Update message.

CDDL description for SDSP Response The CDDL for CS-RID SDSP response is defined in

tstr, rid => tstr, proxy_status_type => proxy_status_types, update_interval => uint, } gps-coordinates = [ latitude : float, longitude: float, altitude : float, ] proxy_status_types = &( 0: "forward", 1: "reverse", 2: "bi-directional", ) ]]>

CS-RID Location Update The Finder SHOULD provide regular location updates to the SDSP. The interval is based on the Update Interval from plus a random slew less than 1 second. The Location Update message is only sent when no other CS-RID messages, containing the Finder's GPS location, have been sent since the Update Interval. If the Finder has not recieved a SDSP Registration Response, a default of 5 minutes is used for the Update Interval.

CDDL description for Location Update The CDDL for CS-RID Location update is defined in .

tstr, timestamp => tdate, gps => gps-coordinates, } gps-coordinates = [ latitude : float, longitude: float, altitude : float, ] ]]>

SDSP Heartbeat The SDSP SHOULD send a heartbeat message at some periodicity to the Finders so that they get confirmation that their is a receiver of their transmissions.

CDDL description for SDSP Heartbeat The CDDL for CS-RID Heartbeat is defined in .

tstr, timestamp => tdate, } ]]>

The Full CS-RID CDDL specification

; CDDL specification for Crowd source RID ; It specifies a collection of CS message types ; ; ; The CSRID overall data structure CSRID_Object = { application-context, info => info_message, proxy_message => broadcast_rid_proxy_message, finder_registration => finder_registration_message, sdsp_response => sdsp_response_message, location_update => location_update_message, } ; ; Application context: general information about CSRID message application-context = ( application => "DRIP-CSRID", ; TBD: consider CBOR tag ? version => uint .size(1..2), ) ; These members are include in every message common_message_members = ( message_type => message_types, mac_address => #6.37(bstr), ) ; ; CSRID message general information info_message = { common_message_members, message_content => tstr, } broadcast_rid_proxy_message = { common_message_members, rid => tstr, timestamp => tdate, gps => gps-coordinates, radio_type => radio_types, broadcast_mac_address => #6.37(bstr) broadcast_message => #6.37(bstr) } finder_registration_message = { common_message_members, rid => tstr, timestamp => tdate, gps => gps-coordinates, } sdsp_response_message = { common_message_members, sdsp_id => tstr, rid => tstr, proxy_status_type => proxy_status_types, update_interval => uint, } location_update_message = { common_message_members, rid => tstr, timestamp => tdate, gps => gps-coordinates, } ; ; Common rule definition message_types = &( Reserved : 0, BRD : 1, Finder-Registration : 2, SDSP-Response : 3, Finder-Location : 4, ) gps-coordinates = [ lat: float, long: float, alt : float, ] ; Radio types, choose from one of radio_types (required) radio_types = &( EFL : 0, VLF : 1, LF : 2, MF : 3, HF : 4, HF : 5, VHF : 6, UHF : 7, SHF : 8, EHF : 9, ) proxy_status_types = &( 0: "forward", 1: "reverse", 2: "bi", ) ; ; JSON label names application = "application" version = "version" info = "message_info" proxy_message = "proxy_message-type" finder_registration = "finder_registration" sdsp_response = "sdsp_response" location_update = "location_update" rid = "id" message_type = "message_type" mac_address = "mac_address" message_content = "message_content" timestamp = "timestamp" gps = "gps" radio_type = "radio_type" broadcast_mac_address = "broadcast_mac_address" broadcast_message = "broadcast_message" sdsp_id = "sdsp_id" proxy_status_type = "proxy_status_type" update_interval = "update_interval" ]]>

IANA Considerations TBD

Security Considerations TBD

Privacy Concerns TBD

ASTM International ASTM International United States Federal Aviation Administration (FAA)

Using LIDAR for UA location If the Finder has LIDAR or similar detection equipment (e.g. on a connected car) that has full sky coverage, the Finder can use this equipment to locate UAs in its airspace. The Finder would then be able to detect non-participating UAs. A non-participating UA is one that the Finder can "see" with the LIDAR, but not "hear" any B-RID messages. These Finders would then take the LIDAR data, construct appropriate B-RID messages, and forward them to the SPDP as any real B-RID messages. The MAC address for this messages SHOULD be a locally administered, random address. The Finder should make all effort to use the same address for a UA detected in this manner. The UAS ID SHOULD be a UUIDv4 (Type=3). The Finder should make all effort to use the same UUID for a UA detected in this manner. The SDSP would do the work of linking information on a non-participating UA that it has received from multiple Finders with LIDAR detection. In doing so, it would have to select a RemoteID to use. A seemingly non-participating UA may actually be a UA that is beyond range for its B-RID but in the LIDAR range. This would provide valuable information to SDSPs to forward to UTMs on potential at-risk situations. At this time, research on LIDAR and other detection technology is needed. there are full-sky LIDAR for automotive use with ranges varying from 20M to 250M. Would more than UA location information be available? What information can be sent in a CS-RID message for such "unmarked" UAs?

Acknowledgments The Crowd Sourcing idea in this document came from the Apple "Find My Device" presentation at the International Association for Cryptographic Research's Real World Crypto 2020 conference.