ICD
Locata Interface Control Document
GPS Heritage and LocataNet Distinctions
A careful comparison of Locata’s ICD with its GPS equivalent will reveal many of the similarities and differences that exist between the two networks. The following paragraphs summarize some of these similarities and differences both as a convenience to the reader and in order to provide a framework within which to understand the structure and intent of the LocataNet positioning signal interface.
A LocataNet includes a Terrestrial Segment (TS) and a User Segment (US). There is no separate control segment. The TS includes a number of LocataLite transceivers located within or around a defined service area. The US includes any number of fixed or moving Locata user receivers (Rovers) operating within the service area and deriving locations and time within the area using signals emitted by the LocataLites in the TS. LocataNets can span areas as large as several tens of kilometers in extent, being for the most part limited by the availability of adequate line-of-sight geometries between the various elements of the LocataNet. With adequate signal power, working networks have demonstrated LocataLite-Rover operating ranges of up to 50 kilometers.
LocataNets can adopt any convenient coordinate reference system, including WGS-84, or other global, regional, local, or custom grids.
LocataNet’s overall concept derives from the Navstar Global Positioning System (GPS). Many of its underlying elements therefore are similar to GPS. The LocataLites assume the same role as GPS satellites, and the Locata user receiver operates much like a GPS receiver. Position and time calculations for the most part use techniques similar to those of GPS. Given these similarities and the likely familiarity of many readers with GPS, this document presents the LocataNet system interface in the same overall form as used by IS-GPS-200E, Reference 1.
Locata Rovers use the fine time definition supplied by the pseudorandom spreading codes impressed on LocataLite transmitted signals, along with data supplied by a data overlay on those signals, for calculating positions and time using techniques well known to GPS users. The Locata network design also lends itself to integrated carrier phase position determination techniques for high location accuracy.
In several respects the LocataNet position solution is simplified relative to that of GPS. Unlike the GPS satellites, all emitters are fixed, local, ground-based emitters for networks covered by this specification. Hence there is no need to solve for emitter position as a function of time. The position of each emitter in the network is broadcast by that emitter in its “ephemeris” data, part of the data overlay stream on the positioning signal. But transmission of successive, frequently changed data sets of orbital parameters and curve fit coefficients for calculating LocataLite positions is not necessary.
LocataNets can operate their data overlay streams at either 100 bits per second or 50 bits per second. Normally the former is preferred to speed acquisition and information updates. The lower 50 bits per second speed provides more data robustness in the presence of marginal links or interference.
All valid emitters in a given LocataNet are synchronized to a Master station in the network, either directly or indirectly, to within very tight tolerances, using a proprietary TimeLoc time synchronization process. TimeLoc maintains set phase differences among signals emitted by the various LocataLites in a LocataNet. Since all clocks track a master, the TimeLoc process compensates for differences in clock drift and aging among emitters, which therefore are not factors in position solutions. The LocataNet therefore does not need to transmit or use clock drift and aging coefficients.
A LocataNet can operate completely autonomously, using its own relative and independent time reference generated by a designated Master LocataLite in the network. LocataNets can synchronize to any time source providing a 1 pulse-per-second (PPS) time reference, or operate independent of any such reference. Therefore, LocataNets can, for example, optionally synchronize themselves to GPS time, and transfer GPS time to any associated Locata user receiver, to within 100 nanoseconds of a one-1 PPS GPS time base supplied by an appropriate GPS time receiver at the Master LocataLite. How closely this transferred time will track actual GPS time will depend on the quality of the GPS time supplied to the Master LocataLite.
LocataNets operate using a continuous time base, of which GPS time is an example and network option. Rovers provide a UTC conversion for the user.
This edition of the Locata Interface Specification assumes that the LocataLites are stationary devices. It does not contain those data elements needed to support moving LocataLites. But since they do not move, the stationary emitters contribute no Doppler shift to the frequency uncertainty of the received signal. The limited network sizes, specified in the data overlay, also limit relative delay uncertainty among the received signals, so that synchronization to one signal significantly limits the time uncertainty of other signals in the network. Both of these factors reduce the uncertainty space in delay and Doppler over which a receiver must search for other LocataLites in the network.
LocataNet signals only traverse the troposphere, and not the ionosphere. Therefore no ionospheric corrections are needed, and are not accounted for in the specification. However network signals remain subject to troposphere-induced delays due to local tropospheric conditions. The specification supports the dissemination of temperature, pressure, and humidity local to the network for use as input factors to user-furnished models supplying troposphere-induced delay compensation.
Since the LocataNet emitters and the Locata Rovers share the same local geographic area (i.e. within a few kilometers of each other), average received signal strengths are often significantly higher than those for GPS, where in contrast all users are at extreme range (over 20,200 kilometers for most users) relative to the emitters. However strengths of the various network signals within a Rover can span a much wider range than is normally true for GPS. These signal differences can easily exceed the dynamic range available by exploiting the pseudorandom spreading code’s processing gain to supply code division multiple access.
Hence, the positioning signal interface described by this specification introduces a time division multiple access scheme for LocataLite emissions to supplement the code division multiple access. The LocataNet pseudo-random spreading codes, derived from GPS C/A codes, run at ten times the rate of the C/A code in GPS, but with only a ten-percent transmit duty cycle within which an entire code epoch is transmitted. Each code therefore completes the entire code epoch in 100 microseconds, but sends its code sequence in only one time slot in each successive millisecond interval. The added signal orthogonality introduced by assigning different time slots to different emitters, assuming appropriate receiver design, supplies adequate signal discrimination to overcome the significant “near-far” problem local networks can otherwise introduce to their receivers. LocataNet receiver designers should bear in mind the wide dynamic range needed in LocataNet Rovers.
Since the Locata spreading code runs at 10 times the GPS C/A rate, the waveform requires a 10-fold larger bandwidth. The LocataNet’s faster chip rate increases time resolution, but the ten percent duty cycle requires correspondingly greater transmitter power to conserve integrated energy per code epoch. The higher power levels necessary are easily achieved in the relatively short ranges over which LocataNets operate. For most applications, transmit powers of less than one Watt suffices.
The LocataNet broadcasts signals on two frequencies within the 2.4 gigahertz license-free Industrial, Scientific, and Medical (ISM) band. Using a non-GPS band avoids interference issues with GPS. The two S-band frequencies in use provide frequency diversity to aid in multipath mitigation, and a “wide lane” phase difference beat to aid in integrated carrier phase techniques. The specification also supports transmit antenna spatial diversity at each frequency at each LocataLite. There is nothing inherent in the design of the LocataNet that would prohibit using other frequencies if desired.
