An Integrated Satellite Based Asset Management System. Tarun Soni Torrey Science Corporation 10065 Barnes Canyon Road San Diego, CA 92121*
Abstract This paper describes the essential segments of a satellite based communication and asset management system. The Low Earth Orbit satellite used for such a system is the ORBCOMM low earth orbit (LEO) constellation. The architecture and network structure of the satellite system is described. The satellites themselves are very small and are launched using the Pegasus launch vehicle. This makes the satellites very cost effective. The ground infrastructure for satellite control and messaging is also described. The end to end connectivity using a packet based communication system is explained. The architecture of the mobile subscriber terminal is described. The subscriber terminal for this LEO system is capable of position determination and sensor monitoring and control. The position determination in this terminal is done by using satellite ephemeris data along with the Doppler shift observed during a satellite pass.
1.
Introduction
Recent advances in electronics have made possible a major leap in satellite communications. Satellite communications is now being applied to a number of communication needs ranging from simple voice and telephony to direct broadcast video and Internet data delivery. These new satellite services are expected to reach the market well before their personal communication services(PCS) competitors[1]. They are also likely to liberate users from the requirement of a land line based infrastructure and hence may claim a large share of the wireless market. These mobile satellite systems can be broadly classified into three categories: the geostationary satellites capable of providing large bandwidth, large coverage to vehicle mounted systems; the big LEOs which provide voice, data, fax and other medium bandwidth services and can work with a *
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hand held terminal; and finally the little LEOs which have no voice capability, but provide position location and packet messaging facilities. This paper describes the use of one such little LEO system, the ORBCOMM system, for remote asset management. This system has been developed for dual use with applications for the armed forces[2,3] as well as in the commercial sector. The first two LEOs in this system (Flight Model 1 and 2) were launched on April 3, 1995 and have been tested thoroughly and opened up for intermittent commercial use. The ORBCOMM LEO system permits messaging between subscriber terminals and other subscriber terminals or other networks ranging from Internet email to fax machines and pagers. The basic satellite modem functionality of a ground terminal can be augmented by a number of additonal features. This includes position determination based on the Doppler shift information available from the satellites. This can be used as an alternative to GPS for asset location. The subscriber terminals developed by Torrey Science Corporation (TSC) also include sensor monitoring and control functionality permitting complete remote asset management including control. In all these applications, ground based systems and cell-based wireless solutions exist. Most of these solutions are limited in their coverage due to the constraints place on them by the “tether”. A number of satellite based systems are being planned for similar applications. However almost all the proposed solutions are either too expensive, in communication and power costs, or leave a lot to be desired in their functionality or availability. For example, the next LEO system being planned, GE STARSYS, is scheduled to launch the first two satellites in July 1998.
In this paper we describe a low earth orbit satellite based asset management system. This system is low cost and provides all the functionality required for asset monitoring and control, and with the launch of two of the ORBCOMM LEO satellites, is available today[4,5]. Section 2 describes the ORBCOMM network, including the satellites, the ground infrastructure and the end-to-end networking involved. Section 3 describes the mobile terminal which also functions as a controller for remote sensors.
2. Low Earth Orbit Satellite System The ORBCOMM system is the first LEO satellite based data communication network to be launched. The system plans to provide global wireless data communication and messaging services using a constellation of up to 36 LEOs. The first two of these satellites were launched in April 1995 and are operational today. By Early 1998 ORBCOMM plans to have 26 operational satellites providing continuous coverage to the North American region.
2.1.
System Architecture
The ORBCOMM system consists of three basic segments: the space segment, the ground segment and the subscriber communicators(ST). The interactions between the various segments has been shown in Fig.1. The basic components of the system[6] are: •
Satellites: Routing and queuing functions in orbit, which talk to the Subscriber Terminals over various RF links.
•
Gateway Earth Stations (GESs): These interconnect the satellites to the Network Control Centers.
•
Network Control Center (NCC): The hub of ground operations for the ORBCOMM system. This includes interfacing with the public and private data networks (like X.25 and X.400 as well as the internet) and also satellite operations including monitoring and control. Each region of operation will have an NCC. The master NCC resides in the U.S. and is co-located with the North American regional NCC.
Satellite Data Transmission
Gateway Earth Station
Tracking Terminals Management System Monitor/Display Messaging Terminals
Figure 1: The ORBCOMM Message Delivery System
•
Subscriber Terminal (ST): The small and mobile communication devices available to subscribers for sending and receiving messages on the ORBCOMM system.
A message from the ST is passed via the satellite to a regional gateway earth station(GES). There are four earth stations currently operatonal in the U.S (located in the four corners of the country for maximum coverage of the land area and coastal waters). The GES then passes the message onto the Network Control Center(NCC). The network control center is also sometimes called the network operations center (NOC). This GES to NCC is a fibre land-line. The NCC then checks then parses the message header for appropriate action to be taken, including further routing of the message. The NCC itself is connected to various terrestrial networks ranging from the PSTN (Public Switched Telephone Network) to an X.400 backbone. This permits routing between a landline user and the ST via a number of standard formats, the most popular being email. The outbound message from a user to the ST traverses a similar route. It is also possible for the NCC to route the message back to a GES for downlink to another ST, thus permitting two STs to send messages to each other.
2.2.
The Satellites
The ORBCOMM system uses a number of Low Earth Orbit (LEO) satellites for providing connectivity between the subscriber terminals in the field and the network control center. The satellites are 95 lbs and orbit 775 kms above the earth. The 1st two satellites have been deployed in a near-polar orbit and are used to provide intermittent service today. The coverage provided by these satellites is on the order of 15 minutes of visibility in a single pass and 6-8 passes daily. An additional 26 satellites are planned for launch in four launches, with 2 being planned for early 1997 and three groups of 8 each in the mid to late 1997 time frame. Most (24) of these satellites will be launched using the Pegasus launch vehicle from Orbital Sciences Corporation (OSC). These will be placed in three planes of 8 equidistant satellites in a circular orbit inclined 45 degrees from the equator. The use of LEOs in such a system leads to a number of features unique to the system. Such a
system is capable of providing better connectivity and coverage than a standard cellular system. Further, there is better overall link availability independent of the surrounding terrain. Since the satellite system used is a LEO, there is minimal satellite transmission delays (unlike a geostationary link) as well as lower transmitted power. Since the LEO system implies a relative motion between the satellite and the receivers, there is a Doppler shift in the signal, which can be used for position determination purposes, thus providing a positioning alternative, though not as accurate as GPS technology. The use of OSC’s Pegasus launch vehicle further reduces the cost of the launch making the use of a large number of LEO satellites much more feasible. From a communications perspective, the satellites are very simple elements in a end-toend link between the mobile user and the network control center. They use the 137-138 MHz frequency range for downlink to the subscriber communicator and the 148-150.05 MHz for transmission up to the satellite. Besides the VHF receivers and transmitters, the satellites also have a UHF transmitter (at 400.1 MHz) which transmits a signal which can be used for more accurate Doppler (and hence position) estimation. The data rate on uplink is 2400 baud and the data rate for the downlink is 4800 baud. The satellite communicates with the Gateway Earth Station at 57.6 Kbps.
2.3.
The Ground Infrastructure
The Gateway Earth Stations are a link between the satellite and the network control center. In the continental United States, there are four GESs located in the states of Arizona, Washington, New York and Georgia. Each GES has a fully redundant set of two terminals complete with tracking antennae, VHF modems and RF power amplifiers. The GES and the satellite, in effect, provide transparent access to the subscriber terminal in the field. These gateway stations along with 26 satellites imply a 98 percent coverage in the North America, with a satellite coming into view in a few minutes the remainder of the time. Further development of the ground stations and land infrastructure is currently being pursued by ORBCOMM to provide coverage over other sections of the globe.
transfer between these two ends. The typical end-to-end latency, when a satellite is in view, will be no more than a few seconds, though this is dependent on the communication load of the link.
The network control center is the “brains” of the ORBCOMM network. The connection between the GESs and the NCC is a fibre based land line. The NCC provides basic control functions such as message handling and delivery, network management and diagnostics etc. For the purposes of message delivery, the NCC is also connected to a number of terrestrial networks
RF Communications
There are three basic types of packets that a ST can transmit to or receive from the satellite: a short report, a message or a globalgram. A short
Position Determination
Subscriber Terminal Serial I/O
GPS
Digital Sensors
Analog Sensors
Peripherals Figure 2: Functionality of the ORBCOMM Subscriber Terminal report is limited to a small number of bytes and including the PSTN (public switched telephone has the least overhead for transmission. The network) , X.400 backbones, X.25 as well as the message packet type is designed for longer Internet. As a specific case, message delivery message. The globalgram is a facility designed based on the Internet is done using standard for message transmission when the satellite can email services and hence does not require any see the ST but cannot see the gateway station. In specific hardware or software at the user site. such a case, it is possible for the ST to transmit a Besides the GESs and the NCCs, there is also a globalgram and the satellite does a store, with a single satellite control center[7], owned and forwarding to the GES (and on to the target operated by ORBCOMM. This is the hub of all NCC) when the GES does come into view. satellite telemetry and position control activities. All transmissions can be acknowledged at This is also the point of control for the tracking various levels of the transmission. Thus a of the antennae on the GESs, and is generally transmission from the subscriber terminal can be responsible for the satellite bus activities. This is acknowledged variously by the satellite itself, situated in Dulles, Virginia. the GES, the NCC, or even by the end user computer system (when the path permits it, i.e, 2.4. End to End Communication not when the delivery medium is Internet email), The basic communication protocol for depending on the configuration selected. This communicating over the ORBCOMM satellite leads to efficient usage of the link, based on the system is packet based with time slots for priority and the reliability required by various multiple access. The system is structured so that kinds of data transmissions. the remote subscriber terminal appears as a data node to the user computer connected to the NCC. The intermediate links including the NCC, GES and the satellite itself are transparent to data
3.
The Subscriber Terminal
The subscriber terminal is the mobile part of this messaging system. The design of the ST is crucial for proper use of the capabilities of the ORBCOMM system. The subscriber terminals currently being manufactured by Torrey Science Corporation have, by design, incorporated a number of features to operate as an autonomous device. This includes a number of functions, over and above the basic functionality of an RF modem. Figure 2. shows the various functions currently performed by the subscriber terminal.
3.1.
The RF Communication
Terminals provide two-way packet message communications with regional Gateway Earth Stations via the constellation of LEO satellites. The basic subscriber terminal for the ORBCOMM system is a half-duplex RF modem. It follows the ORBCOMM protocol for communicating with the satellites (and hence the NCC) from a remote site. The unit is capable of transmitting at 2400 baud in the frequency range of 148-150.05 MHz. and uses a physical modulation of DPSK. The typical transmitted power is 5 watts. It can receive at a data rate of 4800 baud at a downlink frequency of 137-138 MHz. Each terminal is assigned an identification tag for uplink and downlink as well as a unique X.400 address as part of its activation scheme. This ID and X.400 uniquely identify the terminal as well as permit verification by ORBCOMM and the NCC. Besides the VHF communication system, the subscriber terminal may also have a UHF band receiver on board for receiving the ORBCOMM UHF transmission. This UHF transmission is meant for removing the effects of ionospheric refraction (somewhat similar to the use of L1 and L2 bands in the GPS system) and improve the accuracy of the position determination system. Messages transmitted by the ORBCOMM system are generated with varying priority and timeliness requirements. The message manager provides these messages to the subscriber terminal, with the highest priority presented first. The terminal manages all processes to deliver those messages to the Network Control Center (NCC). The NCC in turn manages all those processes to deliver those messages to the end destination. All messages are transmitted in a packet format, with integral check sums. No
error correction is provided. Packets are delivered only if the parity check computed at the end receiver agrees with the received parity check. Besides such link layer functions, the terminal also contains all the code required to handle the transport layer protocol of the ORBCOMM system and higher level functions required for “intelligent” responses, sensor monitoring and system control.
3.2.
The External Interfaces
For the RF modem functionality there is an external asynchronous serial port which can use various standard serial interfaces permitting devices like laptop computers to send data to the terminal. This permits the use of this terminal along with an external computer (or a palmtop) as an proper email messaging system. The terminal is also designed to act as a sensor monitoring and control system. This provides the system with the backend capabilities, over and above that of a simple RF modem. For this purpose a set of four analog lines are provided. These lines permit the monitoring and control of up to four analog devices. The A/D associated with these lines has a resolution of 12 bits and permits a high degree of precision. Similarly four bi-directional digital I/O lines are also provided for communication with simple digital devices. The availability of such digital ports also makes the insertion of the terminal into a serial network of devices, either as a master or a slave, relatively easy. The basic software allows control of all these interfaces over the RF link, permitting the host based user to query these ports, as well as set a “query and report” schedule for these sensors. All these interfaces are housed in a weatherproof NEMA-4 enclosure with other packaging options also available. This along with the use of extended temperature parts in all the components permits the operation of these terminals in extremely hostile environmental conditions. A number of different antenna styles are also available depending on the application and geometric requirements. These include regular whip antennas as well as low profile semi-covert antennas.
3.3.
Position Determination
In addition to the RF communication capability, the subscriber terminal can also determine its
position. This can be done by using the Global Positioning System[8] satellite constellation (GPS) or by using the inherent Doppler positioning capabilities of the ORBCOMM LEO satellites[9]. The terminal is capable of supporting a GPS receiver card. Such a GPS receiver derives its own position based on computing it’s distance from four GPS satellites. With active selective availability, such GPS based position determination can attain an accuracy of 100 meters for a circular error probability(CEP) of 95 percent. Further, once an initial position determination has been done, the GPS system is capable of delivering a new location every second, which leads to the possibility of realtime position location. And the use of a differential GPS station can lead to much higher accuracy in such a system. However this comes at the cost of added hardware and power consumption. Both the size and the power consumption (as well as cost) are prime concerns when using a system like this. The ORBCOMM position location system provides two levels of resolution to the users. This position location system depends on the Doppler frequency shift observed by the receiver during a satellite pass. The terminal can use the time, frequency and satellite position received from the satellite in one of two frequency plans (VHF only, or VHF and UHF jointly) and estimate its position using the Doppler shift observed. Each ORBCOMM satellite contains a GPS receiver on-board providing precise satellite position and timing information. The position information along with a time reference is then packaged by the satellite into a 100 byte message which is inserted into each of the downlinks. Since the 137 MHz downlink is also used for user data, the ephemeris packet is inserted into this downlink based on the message loading and could be as infrequent as once every 16 seconds. In the commercial satellites, the position and time information is will be the only data being downlinked on the 400 MHz link and hence will be available all the time (if the UHF receiver is supported by the equipment). The use of just the 137 MHz frequency leads to a slightly less accurate position estimate due to both the ionospheric effects. Also the infrequency of the ephemeris data packet on this
downlink also leads to a longer amount to time required by the terminal before the position is estimated. The use of both the 400 MHz UHF frequency downlink and the 137 MHz VHF downlink mitigates both of these effects. However this is obtained at the cost of adding a UHF receiver into the terminal.
3.4.
Reporting Modes
Power consumption is a major concern in such remotely located devices. Such a battery operated mobile device has to trade-off size, weight, cost and battery life. To minimize the power drain in the subscriber terminal, the usual mode the of terminal is actually a sleep mode. This is a “complete” shutdown of the terminal, including the power supply, thus permitting a reduction is power over and above that attained by the usual low power techniques of freezing onboard clocks etc. In such a mode, the only “active” element of the system is a real-time clock which can be set to “wake” the terminal up after any given time interval. Besides this timer driven wake-up, the terminal can also be configured to wake-up in the event of an interrupt either due to activity on the sensors (analog or digital) or due to the arrival of data from an external serial interface (for e.g., an external laptop computer using this as an RF modem). Such a default “sleep” mode permits a very long battery life and is ideal for remote monitoring applications. During it’s active state (after the wake-up), the terminal can be configured to perform a number of tasks. These include periodic monitoring of external sensors, periodic position determination (either by Doppler or by an optional GPS receiver) which can all be stored in non-volatile memory for later transmission. This permits the use of the terminal as an event logger which can record events even when the satellites are not in view, collecting all the necessary data for eventual transmission. The use of on-board application specific software also permits data analysis and reduction thus letting the terminal evaluate the severity of various events and transmit only data relevant to the application. This provides on-site intelligence for the sensor control and also uses the RF link very efficiently. For e.g., this permits an end-user to obtain exception reports and alarms rather than just the raw-data from sensors.
Reporting of position and other data, can be driven by one of three criteria: Automatic reporting of specific application defined packets at well defined intervals, event driven reporting and finally request-response type of reporting. In case of the automatic pre-scheduled transmissions, it is possible to provide the terminal with a schedule (for e.g., every 6 hours) and the terminal will wake-up and transmit the requested data (typically position and status) on the schedule. In case of the event driven mode, the terminal can also be programmed to wake-up and transmit if an exception condition (defined by the application) is seen on any of its sensor ports. This is the mode used for generating alarms in case of remote monitoring applications. And finally it is also possible for the terminal to be polled from the host and can reply to the polled message. This permits the terminal to be completely silent, until some data is requested by the host, at which time an appropriate response is sent over the RF link. This polling technique can be used to locate lost assets efficiently, though it should be noted that this polling mode requires the host to have knowledge of the “wake-up” times of the terminal.
number of bits used for the message. DES itself is a secret key algorithm, but by generating a one-time secret key and exchanging that under the highly secure RSA public key based system, it can be used for providing strong encryption.
3.5.
Further issues include key management, which is normally ignored or left up to a piece of software in the case of most simple implementations. In case of a remote subscriber communicator with no concept of an "operator" or a "password" the key has to be built into the embedded software. However this makes the use of a public key rather difficult. Adding to this is the manufacturability (and maintenance) issue for software with individual keys.
Encryption and Privacy
Since all the communication to and from the subscriber communicator is being done on open RF, there is a need to provide some form of privacy, if not robust security on such a public channel[10]. A number of issues differentiate the encryption requirements of such a system from that of the usual PGP and RSA based systems, which have recently evolved on internet delivery systems. First and foremost is the quantity of data involved. Typical internet (or similar) encryption algorithms trade off bandwidth for security. Typical Public Key Cryptography Standards (also known as PKCS) define the encoding for things like public keys, private keys, signatures a short encrypted message, a short signed message and password based encryption. In the typical RSA based scheme, a DES (Data encryption standard) key is encoded by using an RSA based public key and transmitted. This key is typically 56 bits long and with 8 bits of checksum information, making for a 64 bit block. This 56 bit key maps a 64 bit block of data into 64 bits of encrypted data without any increase in the
However, this standard assumes a 64 bit (or 8 byte) block of data available for encoding, with required padding in the absence of such a block. Further, the message transmitted by such a scheme includes a 64 bit DES key (appropriately encoded with a public key) and at least a 64 bit block of data possibly with digital signatures added on. This converts a 6 byte message into 16 or more bytes of data, thus causing an increase in the bandwidth required. This is obviously an expensive increase for data which may be just one 6 byte packet long. Even more extreme are the requirements for the encryption of a position report. A position report is typically just two numbers (the latitude and the longitude) and is transmitted as such. Obviously, padding the data and increasing the number of bytes in such a message is not feasible since this would convert a "position report" packet to a "message" and consequently increase the bandwidth used by such a transmission (and consequently drive up costs).
TSC is currently working on systems and architectures for the encryption of short data bursts which use public key algorithms. Specific algorithms meant for the encryption of position information alone are also being developed. These algorithms are expected to use the same concepts as public key algorithms developed and standardized in most other encryption algorithms. The use of such standards will permit the use of hardware encryption features like the Fortezza card in future implementations.
4.
Summary and Conclusions
Wide area satellite systems are usually evaluated on criteria like communication coverage,
installation cost and operational costs. The ORBCOMM system is the world’s first lowearth orbit satellite system to provide high availability, low cost, low power, ubiquitous two-way mobile communications, position determination and sensor monitoring and control. The availaibility of such a system opens up key applications in transportation, energy and environmental sectors for commercial as well as government usage. Specifically, tracking of high value assets and cargoes in the trucking, rail, heavy equipment and maritime industries; environmental monitoring for data logging and disaster warning; or even animal monitoring and tracking all become feasible with this technology. Over the next few years we expect a number of these applications to be demonstrated using LEO satellite based packet data delivery systems.
5.
References
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