IS-GPS-800H：NAVSTAR GPS空间段与用户段L1C接口规范

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本资源文档标题为《NAVSTAR GPS空间段/用户段L1C接口规范》(IS-GPS-800H.pdf)，主要关注于全球定位系统(IS-GPS)的高级技术要求，特别是针对L1C信号频率的接口标准。该文档是在2021年4月13日发布的RFC-00413中，包含了对IRN-IS-800G-001和IRN-IS-800G-002标准的整合，旨在定义与空间段和用户段之间的交互，这些段分别负责GPS卫星系统和接收设备的通信。文档强调了规范的权威性和公开性，指出其已获得PNT Capability Area Integration和Portfolio Architect的批准，并由Anthony Flores撰写，Kevin Cano审核。SAIC(GPS SE&I)作为接口控制承包商，位于加州埃尔塞贡多的办公地址也被列出。该文档详细规定了技术要求，不构成或改变任何受影响合同方之间的采购订单条款。在文档修订历史中，首次发布日期为2003年11月24日，此次修订加入了Phase Variance语言修订、基于覆盖角度的误差处理以及添加了新的描述内容。值得注意的是，除非另有说明，所有数字表示均为十进制形式。主要内容可能包括但不限于： 1. **L1C接口设计**：文档详细阐述了L1C信号如何在空间段（卫星）与用户段（接收机）之间进行传输，包括信号编码、调制方式和数据包结构。 2. **频率管理**：对于L1C频率的使用和分配有明确的规定，确保信号的准确传播和干扰避免。 3. **信号精度**：涉及了如何通过Phase Variance语言修订来提高信号稳定性，以及如何通过覆盖角度规格来减小误差影响。 4. **安全性与认证**：可能讨论了接口的安全机制，包括数据加密和认证方法，以保护用户数据免受未经授权的访问。 5. **兼容性与互操作性**：文档可能涵盖了与其他GPS接收设备及系统版本的兼容性要求，以确保无缝集成。 6. **变更管理和标准化**：对于新修订的条款，可能会介绍如何适应现有系统，并确保符合行业标准和法规。 7. **文档管理和跟踪**：修订记录表明了文档在发布后的持续更新和改进过程，以适应技术进步和需求变化。这份规范对于GPS技术开发者、系统集成商和运营商来说，提供了关于如何实现L1C接口技术性能和安全性的关键指南。理解并遵循IS-GPS-800H的要求，是确保GPS服务质量和系统间有效交互的关键。

IS-GPS-800H

13 Apr 2021

3.2.1.1 Frequency Plan

The carrier frequency for the L1C signal shall be coherently derived from a frequency source common

with other signals within the SV.

The nominal frequency of this source as it appears to an observer on the ground is 10.23 MHz. The

SV carrier frequency and clock rates, as they would appear to an observer located in the SV, are offset

to compensate for relativistic effects. The clock rates are offset by Δf/f = -4.4647E-10, which is

equivalent to a change in the L1C-code chipping rate of 1.023 MHz by a Δf = -4.5674E-4 Hz. This

results in an offset L1C-code chipping rate of 1.02299999954326 MHz. The nominal carrier

frequency (f

) - as it appears to an observer on the ground - shall be 1575.42 MHz.

The requirements specified in this IS shall pertain to the signal contained within a 30.69 MHz

bandwidth centered about the L1 nominal frequency.

The L1C signal shall utilize a modulation technique of BOC (f

, 1) which specifies a subcarrier

frequency of f

× 1.023 MHz and a spreading code chipping rate of 1 × 1.023 MHz = 1.023 MHz.

3.2.1.2 Signal Polarization

The transmitted signal shall be Right-Hand Circularly Polarized (RHCP).

For an angular range of 13.8 degrees from nadir, the L1 ellipticity shall be no worse than 1.8 dB.

3.2.1.3 Carrier Phase Noise

The phase noise spectral density of the unmodulated carrier shall be such that an approximation to the

third order Jaffe-Rechtin phase lock loop, closed-loop transfer function H(f) such that |1 - H(f)|

/(f

+ f

) where f

= 3B

/(5), which has a 10 Hz one-sided loop noise bandwidth, shall be able to

track the carrier to an accuracy of 0.035 radians rms.

3.2.1.4 Spurious Transmissions

In-band spurious transmissions, from the SV, shall be at or below -40 dBc over the band specified in

3.2.1.1. In-band spurious transmissions are defined as transmissions within the band specified in

3.2.1.1 which are not expressly components of the L1 signal.

3.2.1.5 Correlation Loss

Correlation loss is defined as the difference between the SV power received in the bandwidth defined

in 3.2.1.1 and the signal power recovered in an ideal correlation receiver of the same bandwidth using

an exact replica of the waveform within an ideal sharp-cutoff filter bandwidth centered at L1, whose

bandwidth corresponds to that specified in 3.2.1.1 and whose phase is linear over that bandwidth. The

correlation loss apportionment due to SV modulation and filtering imperfections shall be 0.2 dB

maximum.

IS-GPS-800H

13 Apr 2021

3.2.1.6 Signal Component Phasing

3.2.1.6.1 Phase Relationship

Carriers of the two L1C components defined in Section 3.1 shall be in the same phase within ±100

milliradians. Carriers of the two L1C components shall be in the same phase (within ±100

milliradians) as the P(Y)-code carrier. See IS-GPS-200 for phase relationships to other L1 signals.

3.2.1.6.2 Phase Continuity

While a satellite is broadcasting standard L1C

code and standard L1C

code signals with data which

indicates L1C signal health is OK, there will not be any commanded operation causing an intentional

phase discontinuity. This does not apply to phase discontinuities caused by signal modulation. Phase

discontinuities are subject to the requirements of 3.2.1.6.1.

3.2.1.7 Signal Characteristics

3.2.1.7.1 Signal Coherence

All transmitted signals for a particular SV shall be coherently derived from the same on-board

frequency standard.

On the L1 carrier, the chip transitions of the two modulating signals, L1C

and L1C

, shall be such

that the average time difference between them (i.e. L1C

/L1C

), and between each and the transitions

of L1P(Y) (i.e. L1C

/L1P(Y), L1C

/L1P(Y)), do not exceed 10 nanoseconds.

The variable time difference shall not exceed 1 nanosecond (95% probability), when including

consideration of the temperature and antenna effect changes during a vehicle orbital revolution.

Corrections for the bias components of the time difference are provided to the US in the CNAV-2

message using parameters designated as ISCs (reference paragraph 3.5.3.9.1).

3.2.1.7.2 Signal Distortion

The duration of the “+1 polarity” portions of the BOC (1, 1) code shall equal the duration of the “-1

polarity” portions of the BOC (1, 1) code within 5 nanoseconds as measured at the zero crossing

point.

The duration of the “+1 polarity” portions of the BOC (6, 1) code shall equal the duration of the “-1

polarity” portions of the BOC (6, 1) code within 5 nanoseconds as measured at the zero crossing

point.

IS-GPS-800H

13 Apr 2021

3.2.1.8 Equipment Group Delay

Equipment group delay is defined as the delay between the signal radiated output of a specific SV

(measured at the antenna phase center as observed from the signal’s zero crossings) and the output of

that SV's on-board frequency source; the delay consists of a bias term and an uncertainty. The bias

term is of no concern to the US since it is included in the clock correction parameters relayed in the

navigation data, and is therefore accounted for by the user computations of system time. The

uncertainty (variation) of this delay, as well as the group delay differential, between the reference

signal and the signals of L1C, are defined in the following subsections.

3.2.1.8.1 Group Delay Uncertainty

The effective uncertainty of the group delay shall not exceed 1.5 nanoseconds (95% probability).

3.2.1.8.2 Group Delay Differential

Not applicable. See Sections 3.2.1.7.1 (Signal Coherence) and 3.5.3.9.1 (Inter-Signal Group Delay

Differential Correction).

3.2.1.8.3 Space Service Volume (SSV) Group Delay Differential

The group delay differential for the radiated L1 signal with respect to the Earth Coverage signal for

users of the Space Service Volume are provided in http://www.igs.org/products/ssv

3.2.1.9 Signal Power Levels

The SV shall provide an L1C signal strength at End-of-Life (EOL), worst-case, in order to meet the

minimum effective received signal levels specified in Table 3.2-1. Any combining operation done by

the SV and associated loss is compensated by an increase in SV transmitted power and thus

transparent to the user segment. For terrestrial users, the minimum effective received signal power is

measured at the output of a 3 dBi linearly polarized user receiving antenna (located near ground) at

worst normal orientation, when the SV elevation angle is higher than 5 degrees and assuming 0.5 dB

atmospheric loss. For orbital users, the minimum effective received signal power is measured at the

output of a 0 dBi ideal right-hand circularly polarized (i.e. 0 dB axial ratio) user receiving antenna (in

geosynchronous orbit) at 23.5 degrees off nadir and using 0 dB atmospheric loss. The received signal

levels are observed within the in-band allocation defined in Para. 3.2.1.1

The SV shall provide signals with the following characteristic:

the off-axis relative power (referenced to peak transmitted power) shall not decrease by more than 2

dB from the Edge-of-Earth (EOE) to nadir,

nor more than 10 dB from EOE to 20 degrees off nadir,

and no more than 19.5 dB from EOE to 23.5 degrees off nadir;

IS-GPS-800H

13 Apr 2021

the power drop off between EOE and 23.5 degrees off nadir shall be in a monotonically decreasing

fashion.

Higher received signal levels than those shown in Table 3.2-1 can be caused by such factors as SV

temperature-induced transmitter power variations, voltage variations and power amplifier variations,

and due to variability in link atmospheric path loss. The terrestrial user’s maximum received signal

power level resulting from these factors is not expected to exceed -154 dBW total for the composite

L1C signal. For purposes of establishing user receiver dynamic range for receiver design and test, the

maximum received signal power level is not expected to exceed -150 dBW total for the composite

L1C signal.

Table 3.2-1. Received Minimum RF Signal Strength

Terrestrial

Orbital

Terrestrial

Orbital

L1C

- 157 dBW

- 182.5 dBW*

L1C

- 158.25 dBW

- 183.75 dBW*

L1C

- 163 dBW

- 188.5 dBW*

*Over 99.5% of the solid angle inside a cone with a 23.5 degree half-angle with its apex at the SV and measured from 0

degrees at the center of the Earth.

3.2.2 PRN Code Characteristics

The characteristics of the L1C

-, L1C

-, and the L1C

-codes are defined below in terms of their

structure and the basic method used for generating them. Figures 3.2-1 and 3.2-2 depict simplified

block diagrams of the scheme for generating the L1C PRN codes.

3.2.2.1 L1C Codes

The PRN ranging codes L1C

(t) and L1C

(t) for PRN signal number i are independent, time

synchronized, and 10 milliseconds in length at a chipping rate of 1.023 Mbps, for total length of

10230 chips. In addition, there is an overlay modulation code L1C

(t) for PRN signal number i

which also is independent, time synchronized, and 18 seconds in length at a rate of 100 bps, for total

length of 1800 bits. The overlay code, L1C

(t), is modulo-2 added to L1C

(t). (See Figure 3.3-2 for

timing relationship.)

Assignment of these code segments by PRN signal number is given in Table 3.2-2 and Table 3.2-3.

The tables provide code segments for 63 PRN numbers that are designated for GPS use.

IS-GPS-800H

13 Apr 2021

3.2.2.1.1 Ranging Code Structure

Both L1C

(t) and L1C

(t) are constructed using the same method. Each ranging code is derived

from a unique length-10223 sequence with a common 7-bit expansion sequence inserted at a PRN

signal number-dependent point.

The unique length-10223 sequence for each ranging code is derived from a single fixed length-10223

sequence called a Legendre sequence L(t), for t = 0, …, 10222. L(t) is defined as,

L(0) = 0 ;

L(t) = 1, if there exists an integer x such that t is congruent to x

modulo 10223;

L(t) = 0, if there exists no integer x such that t is congruent to x

modulo 10223.

Table 6.2-1 in Section 6.2.3 provides the generated sequence of the above defined L(t).

The above Legendre sequence is used to construct the unique length-10223 sequence used for each

ranging code. This sequence, called a Weil-code, is the exclusive-or of L(t) and a shift of L(t). A

Weil-code W

(t ; w) is specified by Weil Index w, ranging from 1 to 5111, which represents the shift

of L(t) and is defined as,

(t ; w) = L(t)  L( (t + w) modulo 10223 ) for t = 0 to 10222.

Assignment of Weil Index, w, by PRN signal number, i, is given in Table 3.2-2.

Finally, the ranging code is constructed by inserting a fixed expansion sequence into the Weil-code.

The expansion sequence is composed of seven bit values 0 1 1 0 1 0 0. The insert point is specified by

Insertion Index p, where p = 1 to 10223. The expansion sequence is inserted before the p

value of

the Weil-code.

Thus the ranging code L1C

(t)/L1C

(t) is defined as,

L1C

(t)/L1C

(t) = W

(t ; w), for t = 0, 1, . . . , p - 2;

L1C

(t)/L1C

(t) = 0, for t = p - 1;

L1C

(t)/L1C

(t) = 1, for t = p;

L1C

(t)/L1C

(t) = 1, for t = p + 1;

L1C

(t)/L1C

(t) = 0, for t = p + 2;

L1C

(t)/L1C

(t) = 1, for t = p + 3;

L1C

(t)/L1C

(t) = 0, for t = p + 4;

L1C

(t)/L1C

(t) = 0, for t = p + 5;

L1C

(t)/L1C

(t) = W

(t - 7 ; w), for t = p + 6, p + 7, . . ., 10229.

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